Investigation of the Cr solubility in the MC phase where M = Ti, Ta

UPTEC K 16 001

Examensarbete 30 hp April 2016

Investigation of the Cr solubility in the MC phase

where M = Ti, Ta

Anna Ehrenborg

Teknisk- naturvetenskaplig fakultet UTH-enheten

Besöksadress:

Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0

Postadress:

Box 536 751 21 Uppsala

Telefon:

018 – 471 30 03

Telefax:

018 – 471 30 00

Hemsida:

http://www.teknat.uu.se/student

Abstract

Investigation of the Cr solubility in the MC phase where M = Ti, Ta

Anna Ehrenborg

In this work the chromium solubility in MC, and M in Cr3C2 and Cr7C3 carbides in the Ti-Cr-C and Ta-Cr-C system have been examined experimentally. Special attention is given to the cubic MC phase due to its frequent use in industrial cemented carbides. A sample series was made where half of the samples were arc-melted and all samples were heat-treated at different temperatures. By

arc-melting some of the samples it was possible to compare the arc-melted and non arc-melted samples to confirm equilibrium. Three phases were expected in each sample. The microstructure was examined by LOM and SEM. The phases were identified by XRD and the amount of Cr in each phase was measured by WDS in FEG-SEM or by microprobe analysis. A higher temperature for the heat-treatment allows more Cr to dissolve in the cubic carbide. Arc-melted samples allow more Cr to dissolve than the same system which has not been arc-melted. The Cr solubility in the cubic carbide in non arc-melted samples at 1400 degree Celsius is 8,1±0,4 at% in (Ti, Cr)C and 7,6±0,3 at% in (Ta, Cr)C. According to the samples the phase diagrams based on thermodynamic calculations are different to experimental data. Therefore, more experimental data should be made to update existing ternary diagrams.

Tryckt av: US-AB

ISSN: 1650-8297, UPTEC K 16 001 Examinator: Erik Lewin

Ämnesgranskare: Ulf Jansson

Handledare: Susanne Norgren

1 Populärvetenskaplig sammanfattning

Undersökning av kroms löslighet i kubisk karbid

De fasdiagram och experiment som finns i dagens litteratur ger olika uppgifter för hur mycket krom som löser sig i kubiska karbider. Många av de fasdiagram som finns idag har gjorts med

termodynamisk modellering med hjälp av datorprogram. Dessa modeller ger en bra uppfattning om vad som borde ske men med bättre data kan en bättre modell göras. Därför har den här studien experimentellt undersökt kroms löslighet för att förbättra data för att kunna skapa en mer verklighetstrogen modell.

Karbider är en av de två komponenterna i hårdmetall. Dessa bidrar till hårdmetallens hårdhet. Den andra komponenten av hårdmetall är bindefasen som bidrar till hårdmetallens seghet och

motståndskraft mot att spricka. Hårdmetaller används i skärande, svarvande och borrande bearbetning av andra metaller eller av berggrunden. För dessa applikationer krävs en metall som är både hård och tålig.

Till hårdmetaller vill man tillsätta krom (Cr) för dess korntillväxtinhiberande egenskaper. I traditionella hårdmetaller (WC-Co) löses Cr in i kobolt (Co) som är huvudbeståndsdelen av

bindefasen. I modernare hårdmetaller som ofta består av flera faser med andra sammansättningar kan kubisk karbid vara en ytterligare fas. När man tillsätter Cr löses den in i både bindefasen och den kubiska karbiden. Genom att veta hur mycket Cr som löses in i karbiden kan man räkna ut hur mycket mer Cr man måste tillsättas för att fortfarande ha önskad halt Cr i bindefasen.

Prover innehållandes (M, Cr)C, där M=Titan (Ti) eller Tantal (Ta), förbereddes både från pulver.

Sammansättningen på proverna har beräknats för att ligga i önskat trefasområde i fasdiagrammet.

Några av proverna smältes i en ljusbågsugn och alla proverna värmebehandlades för att försöka nå jämvikt i strukturen.

Efter värmebehandling undersöktes mikrostrukturen med Ljus Optiskt Mikroskop (LOM) och Svep Elektron Mikroskop (SEM). Antalet faser i provet bestämdes med röntgendiffraktion (XRD) och våglängdsdispersiv spektrometri (WDS). Med WDS bestämdes även Cr innehållet i faserna.

Prov som smälts med ljusbåge kan lösa in mer Cr i strukturen än prover som inte smälts. Det beror på att mer Cr löses in i strukturen när de hade en högre temperatur vid smältningen. Jämvikt nås troligen vid värmebehandling vid 1400˚C och 1500˚C efter 150 timmar. Då har både ljusbågssmälta och bara värmebehandlade prover samma antal faser och liknade sammansättning. Vid 1400˚C är Cr lösligheten i den kubiska karbiden:

8,1±0,4 at% i (Ti, Cr)C 7,6±0,3 at% i (Ta, Cr)C

Enligt de prover som gjort i studien skiljer sig resultaten från de termodynamiskt uträknade diagrammen. Därför måste fler experimentella prover göras för att uppdatera de ternära fasdiagrammen. Resultaten bidrar med kunskap som kan användas i legeringsutveckling och skiktutveckling för framtida skär- och bearbetnings verktyg.

Examensarbete 30 hp på civilingenjörsprogrammet Kemiteknik med materialvetenskap

Uppsala universitet, april 2016

2

Contents

1. Background and Aim ... 4

2. Introduction ... 6

2.1. Cubic carbides ... 6

2.2. Ternary systems ... 6

2.3. Construction of phase diagrams ... 7

2.4. Equilibrium ... 7

3. Literature study ... 8

3.1. Raw-material ... 8

3.2. Arc-melting ... 8

3.3. Heat-treatment: Time and temperature ... 8

3.4. Cooling ... 9

3.5. Analysis ... 9

3.6. Cr solubility in the cubic carbide ... 9

3.7. Discussion ... 10

4. Experimental section ... 11

4.1. Calculations ... 11

4.1.1. Heat-treatment time ... 11

4.1.2. Metal-carbon ratio ... 11

4.1.3. Thermodynamic calculation ... 11

4.2. Sample preparation ... 12

5. Results ... 16

5.1. Samples heat-treated at 1300

C for 250 hours ... 16

5.2. Samples heat-treated at 1400

C for 150 hours ... 18

5.2.1. Sample S3 and S3A ... 18

5.2.2. Sample S9 and S9A ... 20

5.3. Samples heat-treated at 1500

C for 150 hours ... 22

5.4. Summary of results ... 25

5.4.1. Ti – Cr – C system ... 26

5.4.2. Ta - Cr – C system ... 29

6. Discussion ... 34

6.1. Phases ... 34

6.2. Arc-melting and heat-treatment time ... 35

6.3. Phase composition of the cubic carbide ... 36

3

6.3.1. (Ti, Cr)C ... 36

6.3.2. (Ta, Cr)C ... 37

6.4. Ti and Ta solubility in M

C

and M

C

... 37

6.5. Source of error in experimental method ... 38

Electrical short circuit during heat-treatment ... 38

Simplified XRD analysis ... 38

WDS in FEG-SEM and microprobe ... 38

Small phase areas made the WDS analysis difficult ... 38

Inaccurate or bad analyses in WDS ... 38

Error in composition calculation ... 38

7. Conclusions ... 39

8. Future work ... 40

9. Acknowledgement ... 41

References ... 42

4

1. Background and Aim

Hard metals consist of carbides embedded in a softer binder metal. The carbides are hard but brittle.

The binder phase holds the carbides together and makes the structure tough so it does not break. Hard metals are used in metal cutting and mining tools. The most common hard metal consists of hexagonal tungsten carbide (WC) in a cobalt (Co) binder phase. By adding other carbides from other metals or changing the binder phase, different properties can be modified. Hard metals are processed by hot- pressing or sintering. These processes are ways to compact and make the particles react with each other by raising the temperature and/or lowering the pressure without melting the components [1].

Hard metals based on WC-Co systems may be alloyed with chromium (Cr)-based carbides to inhibit the WC grain growth during sintering as well as to get a better corrosion resistance [2]. Titanium (Ti), tantalum (Ta), vanadium (V) and niobium (Nb) are some examples of other growth inhibitors. Cr- based carbides are suited for high-temperature applications for example in cutting and mining tools.

In traditional hard metals, WC-Co systems, the Cr is dissolved in the binder phase (Co-phase). In hard metals consisting of more phases, a cubic carbide (MC) can be an additional phase. The MC-phase is a phase with a cubic crystal structure where the M stands for a metal atom and the C for carbon. A figure of the hard metal microstructure is seen in Figure 1. When adding Cr it diffuses into the binder phase, the WC-phase and into the cubic carbide. By knowing the amount of Cr that is dissolved in the cubic carbide it is possible to know how much Cr that should be added to get the right composition of Cr in the binding phase and in the WC-phase.

Figure 1: A schematic microstructure of a hard metal [3].

Phase diagrams reported in literature differ in the amount of Cr that can be dissolved in cubic carbides.

Often there is a difference in the phase diagrams between theoretical values of the solubility and phase diagrams based on experimental results [4], [5], [6], [7]. For more detailed description of the phase diagrams follow previous work made by He [7]. The solubility of Cr in cubic carbides has been presented in previous work on WC-Co based systems with different carbides [2]. The solubility of Cr in cubic carbides is strongly dependent on which cubic carbide former is present in the system. Cr replaces both Ti and Ta in the cubic carbide.

For this study, the phases to be studied are; M

C

, M

C

and (M, Cr) C –type, where M=Ti or Ta.

Special focus is given to the MC phase, the cubic carbide. Samples made from powder were heat-

treated to reach thermodynamic equilibrium. The solubility of Cr in the cubic carbide was analyzed.

5 By understanding how Cr reacts in the processing of the hard metal, better materials can be made and processing can be improved. The solubility of Cr in the cubic carbide was examined by heat-treating samples and examining the microstructure, the phases and composition. The composition of the samples was prepared to get a certain three phase equilibrium in the ternary phase diagram.

The aim is to measure the solubility of Cr in cubic carbides in (Ti, Cr)C and (Ta, Cr)C systems. This

study should lead to updated phase diagrams and a better understanding of the solubility of Cr in cubic

carbides. The results will give valuable information for continued development of hard metals and

surface coatings.

6

2. Introduction

2.1. Cubic carbides

The systems for this study are (M, Cr)C where M=Ti or Ta. These elements form cubic carbides with NaCl structure. Carbides are compounds that contain carbon and a less electronegative element [8].

Refractory cubic mono-carbides have high melting points, hardness, wear resistance, electrical and thermal conductivity. The chemical bonding of the carbide is mostly covalent between the metal and the carbon atoms. Also ionic and metallic bondings are contributing to the bonding of cubic mono- carbides. As a result of the bonding in the carbides high strength and stability are obtained [9].

In comparison to hexagonal WC, cubic carbides are in general harder, have better thermal conductivity and a lower price but are more brittle. In mono-carbides, such as TiC and TaC, mutual miscibility is possible [6]. It is also possible to dissolve other elements, such as Cr into their structure.

2.2. Ternary systems

Systems with three components can be presented in a ternary phase diagram. The three corners

represent the three components. The phase diagrams are often drawn at a certain temperature. Liquidus and solidus lines from binary diagrams become surfaces in ternary diagrams [10]. Every point in the diagram has a certain composition of the elements.

The average composition of a sample at a certain point in the three phase region is given by the phase fractions and the composition of the phases. The composition of the phase is represented by the corners by the closest surrounding triangle and are fixed as long as the average composition is within the three phases region in a ternary system, see Figure 2a. In the three phase region the phase

composition (x) for each phase is fixed and the total composition is varied only by varying the phase fractions (f).

F

* x

MC

+ f

* x

M3C2

+ f

* x

M7C3

= total Ti content F

* x

MC

+ f

* x

M3C2

+ f

* x

M7C3

= total Cr content F

* x

+ f

* x

+ f

* x

= total C content

However, in the existing thermodynamic description of the Ti-Cr-C and Ta-Cr-C phase diagrams there is a discrepancy on the solubility of Cr with the cubic MC carbide [7], illustrated here for the Ti-Cr-C system. In Figure 2a the Ti-Cr-C phase diagram is given, calculated at 1300˚C using the TCFE7 database available from www.thermocalc.com. The green lines are iso-activity tie lines in the two phase regions and the three phase regions (triangles) are colored in red. The arrows indicate the MC+M

C

+M

C

and MC+M

C

+graphite the three phase regions where M = Ti, Cr. In Figure 2b the same phase diagram, disregarding the binary Ti-Cr phases, is calculated using the database by He [7].

The dark blue area is the MC+M

C

+M

C

three phase region.

7

Figure 2: (a) The Ti-Cr-C phase diagram at 1300

C calculated using the TCFE7 database available from

www.thermocalc.com and (b) using the database by He (disregarding the binary Ti-Cr phases) [7], [17]. The dark blue area illustrates the (Ti, Cr)C+(Ti, Cr)

C

+(Ti, Cr)

C

three phase region.

Comparing these phase diagrams it is clear that the thermodynamic description in Figure 2a gives a much higher solubility of Cr in the MC = (Ti, Cr)C carbide than the description proposed by He [7].

The same is valid for the Ta-Cr-C system. This means that the solubility of Cr differs in

thermodynamic models from calculation to models based on experimental values. In the continuation of this thesis all diagrams calculated using thermodynamic data will not include the binary Ti-Cr phases as they are not regarded in the database. In the phase areas and compositions examined in this study, there will never form any intermetallic phases, MCr

.

2.3. Construction of phase diagrams

Phase diagrams are made from a variety of analyses to get as reliable diagrams as possible. Phase field boundaries are drawn from chemical analysis, thermal analysis (TA) and electron probe microanalysis (EPMA) where the composition of every phase is measured. In the TA the change of the material temperature is plotted against time. Terraces in the curve show where the material starts to crystallize and other phase transformations occur. Several samples with different compositions are prepared and analyzed to draw the whole phase diagram. To increase sensitivity different thermal analyses are made and the results are compared to a reference material that did not undergo a phase transformation. From x-ray diffraction methods the crystal structure and lattice parameters are determined. Physical

properties as hardness, volume, electrical and magnetic properties are often changed in phase transformation. By plotting these properties against temperature or composition they can indicate phase boundaries. Metallographic methods, looking at the microstructure, are also preformed. By quenching the specimens the microstructure at the temperature of interest is preserved.

Thermodynamic modeling constructs phase diagrams based on thermodynamic relationships, Gibbs free energy, between phases. Modeling alone does not give reliable diagrams but based on

experimental data they are a powerful tool [10].

2.4. Equilibrium

At equilibrium the Gibbs free energy has as low value as possible. It is important that the samples

contain the same phases and have reached equilibrium to compare the Cr solubility. To ensure that

equilibrium has been reached no macroscopic gradients should be seen between the sample surface

and the center, and each phase should have uniform chemical composition [2]. To be even more

certain of being in the equilibrium, another set of samples could be made with the same components

and treatment but made from different raw materials. When the samples reach equilibrium the

8 composition of the phase and the number of phases should be the same. The microstructure can still be different.

3. Literature study

Many investigations on ternary systems with cubic carbides and Cr are described in literature. The authors study the ternary system aiming to find eutectic compositions and equilibrium at different alloy compositions. Only a few articles focus on the solubility of Cr in cubic carbides. The articles give good examples of how the experiments can be designed. In this project the focus has been on published bulk studies on systems involving the cubic carbide (M, Cr)C where M= Ti, Ta, Zr, Nb or V.

This literature study focuses on the experimental design. It describes the raw-material used, if and how the arc-melting is preformed, the time and temperature for the heat-treatment, how cooling is done, how the analysis is made and how the authors ensure equilibrium. The different systems of elements are compared and the reliability of the articles is discussed.

3.1. Raw-material

To prepare the samples for the experiment powders of pure elements and carbide powders were mixed.

The compositions of the samples can be calculated from ideal stoichiometric composition and theoretical density of each desired phase [7]. Powders are milled for several hours to get a good mixture [2]. The powders can also be mixed in an agate mortar and by adding alcohol to the mixture it becomes a paste that more easily can form a green-body for following heat-treatment [5]. The raw- material can also be mixed without any additives [7]. Other ways to form samples are cold-compacting the raw-material [11] or by arc-melting [5], [12], [13], [14].

3.2. Arc-melting

To confirm a homogenous mixture some of the authors arc-melted the specimens several times and turned the samples over between the melting [5], [12], [13], [14]. The arc-melting was carried out in an argon atmosphere. The specimens were put on a water-cooled copper crucible and a tungsten electrode was used. Some of the samples were crushed, grounded and arc-melted several times [13], [14]. Some articles analyzed their samples from this step [13] but other authors had a heat-treatment to ensure equilibrium [5], [6], [11], [12], [14].

3.3. Heat-treatment: Time and temperature

The heat-treatment properties, time and temperature differ in the literature. In one study the green bodies were prepared by hot-pressing the powder mixtures at 1300°C or 1500°C for 40 hours before analyzing the specimens [4]. The hot-pressing was carried out in a tungsten mesh furnace with an atmosphere of helium with the pressure of 1 atm. A five-component system with a binder phase from Norgren et al. was sintered at 1410°C for 24 hours. The atmosphere during sintering was vacuum [2].

Dovbenko et al. annealed the samples in 10-40°C below the solidus temperature for 30 minutes [13].

Alloys were annealed for 30 minutes in the temperature interval 1630-1950°C [14]. In another

experiment the annealing was carried out at 1300°C for 120 hours [6]. Heat-treatment was also made

in temperatures ranging from 1000-2200°C [11]. Federov et al. multistage annealed specimens in the

V-Cr-C systems. The stages were: 16 hours at 1900-1800°C, 25 hours at 1700-1500°C, 37 hours at

1300-1100°C and finally 200 hours at 1000°C [5].

9 In the experimental method from the previous work by He used in this survey, the heat-treatments were carried out at 1300, 1400 and 1500°C for 100 hours. These specimens were compared to arc- melted samples with the same heat-treatment. Only samples at 1500°C for the Ti system were analyzed. Samples at other temperatures and systems were only prepared up to hot-pressing [7].

3.4. Cooling

Federov et al. and Booker et al. cooled their samples as quick as possible by quenching the samples in water [4], [5]. Other experimentalists cooled down their samples in room temperature [7]. Cooling could also be carried out by a controlled cooling rate [2], [13], [14].

3.5. Analysis

The samples were examined by light optical microscope (LOM) [5], [6], [7], [11], [12], x-ray

diffraction (XRD) [4], [5], [6], [11], [12], scanning electron microscope (SEM) [7], energy dispersive spectroscopy (EDS) [11], [12], wavelength dispersive spectroscopy (WDS) [7], electron probe microanalysis (EPMA) [11], [12] and metallographic techniques [4], [5], [6], [11], [12]. For the metallographic techniques samples were etched and microstructure was examined in LOM or SEM.

Guha et al. examined their specimen by x-ray diffraction and metallographic techniques. To confirm the results, the samples were examined by an EPMA [11]. Federov et al. examined the microstructure by etching in concentrated HNO

and HF to confirm the change in the microstructure [6]. In studies with the purpose to get a better understanding of the system differential thermal analysis (DTA) and melting studies were made [4]. The carbon content was determined by combustion and conductometric methods [11].

3.6. Cr solubility in the cubic carbide

The measured solubility of Cr in cubic carbides and the different properties in cubic carbides system are given in Table 1 with respective reference. Only systems with cubic carbide are described. The table explains heat-treatment temperature and time, solubility of Cr and which cubic carbide studied.

The solubility of Cr in the (Ta, Cr)C system is 6 at% at 1000°C [5]. The solubility of Cr in (Ti, Cr)C given by Booker are 20,72 at% for the equilibrium MC, M

C

and M

C

[4]. In the equilibrium MC, M

C

and M

C

the Cr solubility is 13,05 at% at 1500°C for the system (Ti, Cr)C [7].

Table 1: Summary of the Cr solubility in cubic carbides from the literature study.

System Carbide Phases Cr solubility Temperature [˚C] Heat-treatment Refrence

in MC[at%] time [h]

Ta-Cr-C TaC not given 6 1000 500 [5]

V-Cr-C VC M

C

, M

C

, MC 30 1000 500 [5]

Nb-Cr-C NbC not given 5 1050 500 [5]

Zr-Cr-C ZrC not given 6 1300 120 [6]

Ti-Cr-C TiC MC, M

C

, graphite 22,79 1500 40 [4]

Ti-Cr-C TiC M

C

, M

C

, MC 20,72 1500 40 [4]

Ti-Cr-C TiC M

C

, M

C

, MC 13,09 1500 100 [7]

Ti-Cr-C TiC M

C

, MC, graphite 16,79 1500 100 [7]

10

3.7. Discussion

In the Guha study the Cr solubility of (Nb, Cr)C evaluated from the XRD was thought to be misleading [12]. According to Guha et al. it was difficult to measure with high accuracy. They also pointed out that some Cr was lost due to evaporation because of the high temperature. In addition Guha article from 1973 does not give any annealing time for homogenization [11].

The article written by Booker has not given any measured data explicitly. There are no proofs that the samples are in equilibrium, such as images of the microstructure or a second data set. It is also confusing which measurements have been made by the author or from earlier studies. Booker have high values on the solubility of Cr in (Ti, Cr)C, 20,72 at% (MC, M

C

and M

C

) and 22,79 at% ( MC, M

C

and graphite) [4]. The study of He comment on the inadequate data from Booker. The heat- treatment time of 40 hours may be too short to allow for equilibrium or the value for the solubility of Cr in the carbide is due to erroneous measurement. Cr solubility of 16,79 at% (MC, M

C

and graphite) and 13,05 at% (MC, M

C

and M

C

) at 1500°C are more reliable values according to He, who used a considerably longer heat-treatment time (100 hours) [7]. The solubility values from Booker are too high, most certain the samples have not reached equilibrium yet. According to

calculations in the following chapter, a reasonable time for heat-treatment is at least 139 hours to allow Cr diffusion of 10 µm. Indicating that the study of He is more reliable than that of Booker.

A critical question for all the studies is whether their samples have reached equilibrium. If not the reported solubility of Cr may be inaccurately. To investigate if the equilibrium is reached the samples are examined in XRD, EPMA or by metallographic methods. These analysis methods examine the phase composition or lattice parameters of the specimen and the composition of Cr is determined. But how can one be sure that equilibrium is really reached? Most of the authors have chosen compositions in a certain phase region in the ternary phase diagram available up to this point. In this area certain phases are expected. However, observing these phases in a sample is not a sufficient proof of having reached equilibrium.

One way to determine whether equilibrium has been reached or not, is to make two samples from different raw-materials but having the same element composition. After heat-treatment the composition of the phases and the number of phases should be the same for both sample sets at equilibrium. There are several ways to confirm equilibrium but no author has confirmed equilibrium.

Although, a high temperature and a sufficient time for diffusion to occur should lead to equilibrium.

11

4. Experimental section 4.1. Calculations

4.1.1. Heat-treatment time

The time should be sufficient for the diffusion to occur between two grains in the structure. The diffusion constant for Cr in carbides could not be found in the literature survey. Therefore, the

diffusion constant was roughly calculated with Einstein’s formula, see Equation 1, and compared with existing data for other compounds [15].

L = √(2 ∗ D ∗ t) “Mean diffusion over a distance” (Equation 1) Where L is the mean distance of diffusion [m], D the diffusion constant [m

/s] and t is time [s] [16].

Calculations made using Equation 1 were done to find the diffusion constant for Cr in carbides. The time used was 100 hours and the grain size (used as distance of diffusion) was assumed to be 10 µm, values taken from previous experiments [7]. This gave a diffusion constant of 1,39*10

m

/s. The diffusion constant for carbon in Fe

C is D=6*10

from Brizes et al. [15]. By using the diffusion constant in the range of 10

the diffusion time was calculated to 139 hours. To assure the system reach equilibrium the time was rounded up to 150 hours. Heat-treatment time at 150 hours was chosen for temperatures 1400˚C and 1500˚C. For heat-treating samples at 1300˚C a longer time was given to allow sufficient diffusion to reach equilibrium.

4.1.2. Metal-carbon ratio

From the data gathered form the WDS analysis a metal-carbon ratio was calculated to assure the found phases. The ratio was calculated by adding Cr and Ti/Ta compositions and dividing them with the composition of C, Table 2.

Table 2: The theoretical metal-carbon ratios for each expected phase.

4.1.3. Thermodynamic calculation

The sample compositions were selected to be in the three phase region of MC-M

C

-M

C

and MC- M

C

-graphite based on the ternary phase diagrams by He [7]. The ternary diagrams based on the work by He were calculated using Thermo-Calc software [18], with the aim to investigate the accuracy of the thermodynamic prediction. Five diagrams were calculated, one for each temperature and

systems with focus on the three phase carbide regions.

The sample compositions measured by chemical analysis after heat-treatment are given in the calculated in the diagrams, presented in Figure 21, 22, 24, 25 and 26. The ternary diagrams are extrapolated from binary systems. Note that the description of all laves –phases from Ti-Cr and Ta-Cr are incomplete in this database [19], [20], [21], [22], [23]. Thus, note that in the Ti-Cr binary the intermetallic phases were disregarded and are thus not present in the extrapolations to the ternary systems either.

Metal-carbon theoretical ratios

MC 1,00

M

C

1,50

M

C

2,33

12

4.2. Sample preparation

Samples that had been previously prepared by He [7]. The sample compositions were all designed to target two different three phase regions based on thermodynamic calculations. All investigated compositions targeted three phase equilibrium involving the MC phase: the MC-M

C

-M

C

or MC- M

C

-graphite three phase regions, where M = Cr and Ti or Ta, indicated by the arrows in Figure 2a.

The samples were made from hot-pressed powders and alloys of Cr

C

, Cr, C, TiC or TaC, see Table 3. Hot-pressing was carried out at three different temperatures: 1300, 1400 and 1500

C. To this point He had prepared the samples. Two of the samples had already been heat-treated and the composition was analyzed by He. A graphite sheet was applied between the press tool and the sample in the hot- pressing procedure. The coating was later removed by grinding.

Table 3: A summary of the samples: raw-materials, temperature and time for the heat-treatment. Samples with a capital letter A have been arc-melted.

Arc-melting

Some of the samples from the hot-pressing were arc-melted, see Table 3. The arc-melting was carried out in a furnace on a water-cooled copper crucible in argon atmosphere using a tungsten electrode. The samples were only arc-melted once and where not turned over during the process because the

Ti system

Name TiC [at%] Cr

C

[at%] Cr [at%] C [at%] Temperature [˚C] Time [hours]

for heat-treatment for heat-treatment

S1A 26,09 69,54 4,37 1300 250

S1 26,09 69,54 4,37 1300 250

S2A 25,85 68,84 5,30 1400 150

S2A-2 25,85 68,84 5,30 1400 150

S2 25,85 68,84 5,30 1400 150

S3A 32,66 32,02 35,33 0,07 1400 150

S3 32,66 32,02 35,33 0,07 1400 150

S4A 34,10 31,75 34,15 0,06 1400 150

S5A 40,43 59,57 1,03 1400 150

Ta system

Name TaC [at%] Cr

C

[at%] Cr [at%] C [at%] Temperature [˚C] Time [hours]

for heat-treatment for heat-treatment

S6A 30,00 44,23 25,77 1300 250

S6 30,00 44,23 25,77 1300 250

S6-2 30,00 44,23 25,77 1300 250

S7A 27,46 62,79 9,74 1400 150

S8 26,94 62,98 10,07 1400 150

S8-2 26,94 62,98 10,07 1400 150

S9A 28,94 44,51 26,56 1400 150

S9 28,94 44,51 26,56 1400 150

S10A 26,42 63,17 10,40 1500 150

S10 26,42 63,17 10,40 1500 150

S11A 28,39 44,65 26,97 1500 150

S11 28,39 44,65 26,97 1500 150

13 components were expected to be in a good mixture. Parts of non-melted sample on the melted sample, if any, were removed by grinding.

Heat-treatment

Before heat-treatment the samples were cleaned for 10 minutes with ethanol in an ultrasonic bath.

Samples were put in crucibles made of Al

O

and for the graphite sample a graphite crucible was used.

The sample crucible was cleaned the same way as the samples. The heat-treatment was carried out in a furnace with an inner cover of graphite and a stationary atmosphere of argon to avoid oxygen

contamination. All samples, with and without arc-melting, were heat-treated in a furnace at 1300

C for 250 hours, at 1400

C for 150 hours or at 1500

C for 150 hours. The samples at 1300˚C were given a longer time in the furnace to have enough time for diffusion. The furnace was a HKT-8, Gero. After the heat-treatment the furnace was turned off and the samples cooled down to room temperature. The experimental procedure is seen in Figure 3.

Figure 3: Overview of how the samples were treated. The dotted line divides the sample into two pieces. One of the pieces is arc-melted and both of them heat-treated in the furnace in the same way.

LOM and SEM

The samples were molded in a conductive dresin and polished with oil-diamond suspension, 9 and 1 µm diamond, on a paper until the surface was free from scratches. The specimens were examined in light optical microscope at 20 and 100 times magnification.

The specimens were also examined in SEM with an acceleration voltage of 10kV to see the

microstructure and to find the three expected phases. Photos with a magnification of 15000 times were taken with both secondary and backscatter electrons.

XRD

The samples were then examined in XRD, using a Bruker D8 Discover instrument. The x-ray source was a Cr Kα and the beam was focused by a polycapillary glass fiber. The samples were placed in a 5 axis Euler cradle. The detector was a Lynx eye detector with a silicon strip, where 200 channels were detected at the same time. The diffractograms were examined in HighScorePlus from PANalytical.

The peaks were compared against different databases to identify the phases. The references used were

Cr, C, Cr

C

, Cr

C

, Cr

C

, TiC and TaC, see Table 4.

14

Table 4: The table shows which references that were used to find the equivalent peaks in the diffractogram. The references are taken from the database: PDF-2, Release 2000, ICDD (International Centre for Diffraction Data).

WDS in SEM and in Microprobe

To make a quantitative examination of the content in each phase, the samples were examined in Field Emission Gun Scanning Electron Microscope (FEG-SEM) with a Wavelength Dispersive Spectrum (WDS) module or with WDS in a microprobe analyzer. The FEG-SEM used was a JEOL 7000f. The certified standards used were pure metallic Ti, Ta and Cr. In addition, single phase standards of Cr

C

, TiC and TaC were used. The crystals used were LSM80N, LIF200 and TAP. Which crystals used for what element is seen in Table 5. Signals from the Kα or Lα shell were examined. The acceleration voltage was 20kV and the probe current was set to 100nA. The correction used was ”Phi–rho-z”.

The microprobe used was a Jeol JXA8530F. The same Cr

C

standard was used for Cr and C analysis and certified standards were used for Ti and Ta. The crystals used were LIFE and LDEGH, see Table 5. The acceleration voltage was 20kV and the probe current was set to 50nA. The correction was ZAF.

Table 5: Crystals used and signal detected for the WDS analysis from the FEG-SEM and the microprobe.

The samples analyzed in FEG-SEM were first demagnetized to reduce possible magnetic field from the samples. For each sample the composition of C, Cr and Ti/Ta were examined in every phase. The WDS analysis was performed on single-phase areas as large as possible. The measuring point was taken in the middle of the single-phase region to minimize signals from neighboring phases and far away from pores to avoid contamination from the sample preparation that might been collected in the pores. Five measurements were made in each phase. If the values differed a lot additional

measurements were made.

Chemical analysis: Semi-quantitative analysis with XRF

For sample preparation the samples were taken out of the dresin and were crushed in a mortar consisting of WC and 6% Co. Then the samples were sieved in a 180 µm sieve. An amount of 0,1g from each sample was taken out for oxidation. 1g of lithium tetraborate and 2g of sodium carbonate were added to the sample and the mixture was kept at 800˚C over night for oxidation to occur. The sample holder was made of platinum and gold. The sample was melted in approximately 1000˚C and additional 5g of lithium tetraborat was added as well as a tablet of ammonium iodide acting as a realizing agent, so the sample would not stick to the sample holder. When the sample solidified it had a glass structure.

Compound Ref code Space group (no) a b c α b g

Graphite 41-1487 P6₃/mmc(194) 2.470 2.470 6.724 90 90 120

Cr 6-0694 Im-3m(229)cubic 2,884 2,884 2,884 90 90 90

Cr3C2 35-0804 Pnam(62)orthorhombic 5,527 11,488 2,829 90 90 90

Cr₇C₃ 36-1482 Pmcm(62)hexagonal 7,015 12,153 4,532 90 90 90

Cr23C6 35-0783 Fm-3m(225)cubic 10,66 10,66 10,66 90 90 90

TaC 35-0801 Fm-3m(225) 4.455 4.455 4.455 90 90 90

TiC 32-1383 Fm-3m(225) 4.33 4.33 4.33 90 90 90

FEG-SEM Microprobe

Element Crystal Signal Element Crystal Signal

C LSM80N Kα C LDE6H Kα

Cr LIF200 Kα Cr LIFH Kα

Ta LIF200 Lα Ta LIFH Lα

Ti LIF200 Kα Ti LIFH Kα

Al TAP Kα

15

A semi-quantitative analysis with XRF was made on the samples. The machine used was an Axiox

max advanced from PANalytical, equipped with a 4kW rhodium x-ray tube. Ti and Cr were examined

from the Kα signal and Ta with its Lα signal. The estimated accuracy was ±1-2wt%.

16

5. Results

To make it easier for the reader the most important samples and their corresponding results are found in this chapter. The complete results are found in Appendix. In this section four samples are taken from the temperature 1400

C and two samples are taken from each of the temperatures 1500

C and 1300

C. A summary of the results are found in the end of this chapter.

5.1. Samples heat-treated at 1300

C for 250 hours

Figure 4 gives the microstructures of the two samples S1 and S1A. Sample S1 consist of three phases while only two phases were discovered in S1A. Additional studies with XRD and WDS, however, confirms that both samples consist of three phases: MC, M

C

and M

C

, where M = Ti, Cr. A diffractogram from sample S1 is seen in Figure 5. The diffractogram from S1A is found in appendix.

Figure 4: (a) Sample S1, not arc-melted. Light grey phase (M

C

), medium grey (M

C

) and dark grey (MC). (b) Sample S1A,

arc-melted. The light grey phase (M

C

) and the dark grey (MC). Photos taken in SEM.

17

Figure 5: Sample S1A. The sample contains the phases MC, M

C

and M

C

according to the XRD analysis. Peaks below 40˚ 2 theta originating from the fapsa-mold.

The diffractogram in Figure 5 represents the presence of the three equilibrium phases MC, M

C

and M

C

as expected from the phase diagram in Figure 6.

Figure 6: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ti-Cr-C system at 1300˚C.

In Table 6, the average phase compositions of the three phases are given. Five data points were taken in each phase in the WDS analysis. For the MC analysis only two measurements were chosen because of difficulties to find phase areas big enough. The phases were identified by their analyzed metal- carbon ratio. The arc-melted sample has a higher amount of Cr dissolved in the MC carbide

(32,8±11,0 at%) than the non arc-melted sample (1,3±1,1 at%). All data from the composition analysis are found in appendix.

Position [°2Theta] (Chromium (Cr))

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Counts

0 2000 4000 6000 8000

ru3226sample17

Residue

96-100-9020; Cr28.00 C12.00 00-032-1383; Ti C 00-035-0804; Cr3 C2

18

Table 6: Average phase composition from the microprobe analysis of sample S1 (non arc-melted) and S1A (arc- melted) annealed at 1300 ˚C.

5.2. Samples heat-treated at 1400

C for 150 hours

5.2.1. Sample S3 and S3A

Figure 7 gives the microstructures of the two samples S3 and S3A. Three phases were detected in the samples. Additional studies with XRD and WDS confirm the presence of the three phases:

MC, M

C

and M

C

, where M = Ti, Cr. A typical diffractogram from sample S3 is seen in Figure 8. The diffractogram from S3A is found in appendix.

Sample Equilibrium Phases Composition

Cr [at%] Ti [at%] C [at%] M/C ratio

S1 1300˚C MC 1,3±1,1 49,4±0,9 49,4±1,0 1,02

MC, M

C

, M

C

M

C

57,9±0,5 1,3±0,3 40,8±0,2 1,45 M

C

67,2±0,5 1,5±0,4 31,2±0,4 2,20

S1A 1300˚C MC 32,8±11,0 20,8±7,5 46,5±3,5 1,15

MC, M

C

, M

C

M

C

57,7±0,2 2,1±0,1 40,2±0,1 1,49 M

C

68,6±0,2 1,1±0,1 30,3±0,2 2,31

Figure 7: (a) Sample S3, not arc-melted. (b) Sample S3A, arc-melted. The light grey phase (M

C

) and the medium grey (M

C

)

and the dark grey (MC). Photos taken in SEM.

19

Figure 8: Sample S3. The sample contains the phases MC, M

C

and M

C

according to the XRD analysis. Peaks below 40 ˚ 2 theta originating from the fapsa-mold.

The diffractogram in Figure 8 represents the presence of the three equilibrium phases MC, M

C

and M

C

as expected from the phase diagram in Figure 9.

Figure 9: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ti-Cr-C system at 1400˚C.

In Table 7, the average phase compositions of the three phases are given. Five data points were taken in each phase in the WDS analysis. The phases were identified by their analyzed metal-carbon ratio.

The arc-melted sample has a higher amount of Cr dissolved in the MC carbide (8,7±0,8 at%) than the non arc-melted sample (4,4±0,7 at%). All data from the composition analysis are found in appendix.

Position [°2Theta] (Chromium (Cr))

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Counts

0 2000 4000 6000 8000

ru3233sample2L

Residue

96-100-9020; Cr28.00 C12.00 00-032-1383; Ti C 00-035-0804; Cr3 C2

20

Table 7: Average phase composition from the microprobe analysis of sample S3 (non arc-melted) and S3A (arc- melted) annealed at 1400 ˚C. Grey colored data are values that are not too reliable due to small phases areas.

5.2.2. Sample S9 and S9A

Figure 10 gives the microstructures of the two samples S9 and S9A. Three phases were detected in the samples S9 and S9A. Additional studies with XRD and WDS confirm the presence of the three phases: MC, M

C

and M

C

, where M = Ta, Cr. A typical diffractogram from sample S9 is seen in Figure 11. The diffractogram from S9A is found in appendix.

Sample Equilibrium Phases Composition

Cr [at%] Ti [at%] C [at%] M/C ratio

S3 1400˚C MC 4,4±0,7 43,9±1,9 51,8±2,5 0,93

MC, M

C

, M

C

M

C

57,0±0,4 2,7±0,3 40,3±0,4 1,48 M

C

67,9±0,9 1,7±1,1 30,4±0,2 2,29

S3A 1400˚C MC 8,7±0,8 41,9±0,8 49,4±0,1 1,03

MC, M

C

, M

C

M

C

56,1±0,7 3,3±0,6 40,6±0,2 1,46 M

C

67,9±0,3 2,1±0,3 30,1±0,1 2,33

Figure 10: (a) Sample S9. (b) Samples S9A. White phase (MC), medium grey to the left (M

C

) and dark grey (M

C

). Photos taken in

SEM.

21

Figure 11: Sample S9. The sample contains the phases MC, M

C

and M

C

according to the XRD analysis. Peaks below 40˚ 2 theta originating from the fapsa-mold.

The diffractogram in Figure 11 represents the presence of the three equilibrium phases MC, M

C

and M

C

as expected from the phase diagram in Figure 12.

Figure 12: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ta-Cr-C system at 1400˚C.

In Table 8, the average phase compositions of the three phases are given. Five data points were taken in each phase in the WDS analysis. The phases were identified by their analyzed metal-carbon ratio.

The arc-melted sample has a higher amount of Cr dissolved in the MC carbide (11,7±0,4 at%) than the non arc-melted sample (7,6±0,3 at%). All data from the composition analysis are found in appendix.

Position [°2Theta] (Chromium (Cr))

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Counts

0 2000 4000 6000

ru3226no6bis

Residue

96-100-9020; Cr28.00 C12.00 00-035-0801; Ta C 00-035-0804; Cr3 C2

22

Table 8: Average phase composition from the microprobe analysis of sample S9 (non arc-melted) and S9A (arc- melted) annealed at 1400 ˚C.

5.3. Samples heat-treated at 1500

C for 150 hours

Figure 13 gives the microstructures of the two samples S10 and S10A. Only two phases were detected in the samples. Additional studies with XRD and WDS confirm the presence of the two phases: MC and M

C

, where M = Ta, Cr. The diffractogram from sample S10A is seen in Figure 14. The diffractogram from S10 is found in appendix.

Sample Equilibrium Phases Composition

Cr [at%] Ta [at%] C [at%] M/C ratio

S9 1400˚C MC 7,6±0,3 48,8±0,4 43,7±0,3 1,29

MC, M

C

, M

C

M

C

57,3±2,2 0,2±0,02 42,5±2,1 1,35 M

C

67,7±1,0 0,1±0,01 32,1±1,0 2,11

S9A 1400˚C MC 11,7±0,4 45,3±0,4 43,1±0,4 1,32

MC, M

C

, M

C

M

C

58,6±0,1 0,5±0,04 40,9±0,1 1,44 M

C

69,4±0,1 0,09±0,02 30,5±0,1 2,27

Figure 13: (a) Sample S10. (b) Samples S10A. Only two phases were found. White phase (MC), medium grey (M

C

). Photos taken in

SEM.

23

Figure 14: Sample S10A. The sample contains the phases MC and M

C

according to the XRD analysis. Peaks below 40 ˚ 2 theta originating from the fapsa-mold.

The diffractogram in Figure 14 represents the presence of the phases MC and M

C

. In phase diagram in Figure 15 the expected phases are MC, M

C

and M

C

.

Figure 15: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ta-Cr-C system at 1500˚C.

In Table 9, the average phase compositions of the two phases are given. Five data points were taken in each phase in the WDS analysis. The phases were identified by their analyzed metal-carbon ratio. The arc-melted sample has a higher amount of Cr dissolved in the MC carbide (9,6±2,6 at%) than the non arc-melted sample (7,6±1,5 at%). All data from the composition analysis are found in appendix.

Position [°2Theta] (Chromium (Cr))

30 40 50 60 70 80 90 100 110 120 130 140 150 160

Counts

0 5000 10000

ru3226sample18

Residue

96-100-9020; Cr28.00 C12.00 00-035-0801; Ta C

24

Table 9: Average phase composition from the microprobe analysis of sample S10 (non arc-melted) and S10A (arc- melted) annealed at 1500 ˚C.

Sample Equilibrium Phases Composition

Cr [at%] Ta [at%] C [at%] M/C ratio

S10 1500˚C MC 7,6±1,5 48,0±1,3 44,4±0,7 1,25

MC, M

C

M

C

69,6±0,2 0,2±0,02 30,2±0,2 2,31

S10A 1500˚C MC 9,6±2,6 47,2±1,2 43,2±1,5 1,32

MC, M

C

M

C

69,4±0,3 0,2±0,1 30,4±0,2 2,29

25

5.4. Summary of results

Table 10 gives a summary of the results concerning expected and detected phases. The phases were determined using WDS (microprobe analysis and FEG-SEM) and XRD. The summary of results is divided into two parts, one describing the Ti – Cr – C system and one the Ta – Cr – C system.

Table 10: Expected phases and found phases from the different analysis are given.

(*) S1A: Only two phases were detected in SEM due to low contrast.

(**) S6-2: The sample holder was made of ZrO

. (***) The samples are from the study of He [4].

Sample 1300˚C Ti/Ta Expected phases Phases SEM Found phases WDS Found phases XRD S1A* Ti MC, M

C

, M

C

2 MC, M

C

, M

C

MC, M

C

, M

C

S1 Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S6A Ta MC, M

C

, M

C

2 MC, M

C

MC, M

C

S6 Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S6-2** Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

Sample 1400˚C Ti/Ta Expected phases Phases SEM Found phases WDS Found phases XRD

S2A Ti MC, M

C

, M

C

2 MC, M

C

Too small sample

S2A-2 Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S2 Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S3A Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S3 Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S4A Ti MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S5A Ti MC, M

C

, C 3 MC, M

C

, M

C

, C MC, M

C

, M

C

, C S7A Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S8-2 Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S9A Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

S9 Ta MC, M

C

, M

C

3 MC, M

C

, M

C

MC, M

C

, M

C

Sample 1500˚C Ti/Ta Expected phases Phases SEM Found phases WDS Found phases XRD

ZH2*** Ti MC, M

C

, M

C

3 MC, M

C

, M

C

not made ZH2#*** Ti MC, M

C

, M

C

3 MC, M

C

, M

C

not made

S10A Ta MC, M

C

, M

C

2 MC, M

C

MC, M

C

S10 Ta MC, M

C

, M

C

2 MC, M

C

MC, M

C

S11A Ta MC, M

C

, M

C

2 MC, M

C

MC, M

C

S11 Ta MC, M

C

, M

C

2 MC, M

C

MC, M

C

26 5.4.1. Ti – Cr – C system

In Table 11 and Table 12 the compositions for each phase are given for non arc-melted and melted samples. The sample S2 and S4A have only been heat-treated in 146 hours instead of 150 hours as the other samples. Three phases were seen in all samples except in S5A where four phases were found.

Results for the S5A sample are found in Appendix. Some of the samples had a more porous structure and were therefore removed, for example S3. The samples at 1500˚C are taken from He’s report [4].

Table 11: Composition of Ti samples, non arc-melted. The results are average values from the WDS.

Table 12: Composition of Ti samples, arc-melted. The results are average values from the WDS.

Ti sample, Equilibrium Phases Composition

Non arc-melted Cr [at%] Ti/Ta [at%] C [at%]

1300˚C

S1 MC 1,3±1,1 49,4±0,9 49,4±1,0

MC, M

C

, M

C

M

C

57,9±0,5 1,3±0,3 40,8±0,2 M

C

67,2±0,5 1,5±0,4 31,2±0,4 1400˚C

S2 MC 8,1±0,4 46,4±0,9 45,6±0,9

MC, M

C

, M

C

M

C

60,2±0,3 2,0±0,1 37,8±0,4 M

C

69,9±0,1 0,9±0,2 29,1±0,1 1500˚C

ZH2 MC 8,2±2,2 44,1±2,7 47,8±1,0

MC, M

C

, M

C

M

C

57,0±0,6 2,8±0,6 40,2±0,1 M

C

68,5±0,4 1,9±0,5 29,7±0,3

Ti sample, Equilibrium Phases Composition

Arc-melted Cr [at%] Ti/Ta [at%] C [at%]

1300˚C

MC 32,8±11,0 20,8±7,5 46,5±3,5 S1A MC, M

C

, M

C

M

C

57,7±0,2 2,1±0,1 40,2±0,1 M

C

68,6±0,2 1,1±0,1 30,3±0,2 1400˚C

S4A MC 11,7±0,9 43,3±1,0 45,1±0,5

MC, M

C

, M

C

M

C

58,7±0,8 2,4±0,2 38,9±0,7 M

C

68,3±1,7 1,1±0,2 30,5±1,8

S3A MC 8,7±0,8 41,9±0,8 49,4±0,1

MC, M

C

, M

C

M

C

56,1±0,7 3,3±0,6 40,6±0,2 M

C

67,9±0,3 2,1±0,3 30,1±0,1 1500˚C

ZH#2 MC 13,1±0,5 40,9±0,5 46,0±0,2

MC, M

C

, M

C

M

C

56,2±1,7 3,6±1,7 40,2±0,6

M

C

68,3±0,4 1,6±0,2 30,1±0,4

27 In Figure 16 the solubility of Cr in MC is given for the equilibrium MC, M

C

and M

C

. The samples at 1500˚C are taken from the experimental study of He [4]. Filled markers are arc-melted samples and non filled are non arc-melted. The related values are found in Table 13.

Figure 16: The solubility of Cr in the Ti system for the phase equilibrium of MC, M

C

and M

C

. Dark markers are arc-melted samples.

Table 13: Dissolved Cr into MC for each temperature. Values are plotted in Figure 16.

Figure 17 and Figure 18 compare the compositions from the raw-materials and the chemical analysis for the Ti system at 1300˚C and 1400˚C. The ternary diagrams are extrapolated from binary systems [23], [24], [25], [26], [27]. In Table 14 the compositions from the two analyses are given.

0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00

1250 1300 1350 1400 1450 1500 1550

Cr[at%]

Temperature [˚C]

Cr[at%] in (Ti, Cr)C

Ti-Cr-C

Sample Arc-melted Temperature [˚C] Cr [at% ] in (Cr, Ti)C

S1A yes 1300 32,75

S1 no 1300 1,25

S2 no 1400 8,06

S3A yes 1400 8,72

S4A yes 1400 11,69

ZH#2 yes 1500 13,09

ZH2 no 1500 8,18

28

Figure 17: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ti system at 1300˚C.

Figure 18: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ti system at 1400˚C.

Table 14: Chemical compositions from the raw-material and from the semi quantitative analysis.

Ti Chemical composition Chemical composition

from raw-materials from chemical analysis, normalized Sample Ti [wt%] Cr [wt%] C [wt%] Ti [wt%] Cr [wt%] C [wt%]

S1A 8,73 77,41 13,87 Too small amount of sample

S1 8,73 77,41 13,87 9 78 13

S2A 8,70 77,48 13,82 Too small amount of sample

S2A-2 8,70 77,48 13,82 9 78 13

S2 8,70 77,48 13,82 9 78 13

S3A 16,22 70,89 12,89 16 72 13

S3 16,22 70,89 12,89 16 72 13

S4A 17,00 70,07 12,94 17 71 12

S5A 28,63 45,83 25,54 32 42 26

29 5.4.2. Ta - Cr – C system

In Table 15 and Table 16 the compositions for each phase are given for non arc-melted and melted samples. Only two phases, MC and M

C

, were detected in the arc-melted sample at 1300˚C and in the samples at 1500˚C. In the other samples the expected phases were detected.

Table 15: Composition of Ta samples, non arc-melted. The results are average values from the WDS.

Ta sample, Equilibrium Phases Composition

Non arc-melted Cr [at%] Ti/Ta [at%] C [at%]

1300˚C

S6 MC 1,3±1,1 49,4±0,5 49,4±1,0

MC, M

C

, M

C

M

C

58,6±0,2 0,2±0,03 41,2±0,2 M

C

67,4±2,0 0,2±0,03 32,5±2,0 1400˚C

S9 MC 7,6±0,3 48,8±0,4 43,7±0,3

MC, M

C

, M

C

M

C

57,3±2,2 0,2±0,02 42,5±2,1 M

C

67,7±1,0 0,1±0,01 32,1±1,0 1500˚C

S10 MC 7,6±1,5 48,0±1,3 44,4±0,7

MC, M

C

M

C

69,6±0,2 0,2±0,02 30,2±0,2

S11 MC 6,1±0,9 50,0±0,5 43,8±0,5

MC, M

C

M

C

69,5±0,1 0,2±0,01 30,5±0,1

30

Table 16: Composition of Ta samples, arc-melted. The results are average values from the WDS.

In Figure 19 the solubility of Cr in MC is given for the equilibrium MC, M

C

and M

C

. Filled markers are arc-melted samples and non filled are non arc-melted. Their corresponding data values are found in Table 17. The samples at 1500˚C and the arc-melted samples at 1300˚C in the (Ta, Cr)C system have a two phase equilibrium and not a three phase equilibrium as expected.

Ta sample, Equilibrium Phases Composition

Arc-melted Cr [at%] Ti/Ta [at%] C [at%]

1300˚C

S6A MC 9,7±1,2 47,0±1,3 43,3±0,8

MC, M

C

, M

C

M

C

M

C

69,5±0,1 0,1±0,02 30,4±0,1 1400˚C

S7A MC 10,3±1,5 48,2±2,3 41,5±1,7

MC, M

C

, M

C

M

C

60,8±0,3 0,4±0,2 38,8±0,1 M

C

70,2±0,3 0,03±0,01 29,8±0,3

S9A MC 11,7±0,4 45,3±0,4 43,1±0,4

MC, M

C

, M

C

M

C

58,6±0,1 0,5±0,04 40,9±0,1 M

C

69,4±0,1 0,09±0,02 30,5±0,1 1500˚C

S10A MC 9,6±2,6 47,2±1,2 43,2±1,5

MC, M

C

M

C

69,4±0,3 0,2±0,1 30,4±0,2

S11A MC 9,6±0,3 46,6±0,4 44,0±0,6

MC, M

C

M

C

69,2±0,1 0,2±0,02 30,6±0,1

31

Figure 19: The Cr solubility in Ta system. Darker markers are arc-melted samples.

Table 17: Dissolved Cr into the MC system for each temperature. Values are plotted in Figure 19.

Figure 20-22 compare the compositions from the raw-materials and the chemical analysis for the Ta system at 1300˚C, 1400˚C and 1500˚C in ternary phase diagrams. The ternary diagrams are

extrapolated from binary systems [23], [24], [25], [26], [27]. In Table 18 the compositions in weight percent from the two composition analyses are given. In samples at 1500˚C only two phases are observed the MC and M

C

phase. According to the phase diagram the 1500˚C samples should have three phases because their compositions are in three phase equilibrium.

0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00

1250 1300 1350 1400 1450 1500 1550

Cr[at%]

Temperature [˚C]

Cr[at%] in (Ta, Cr)C

Ta-Cr-C

Sample Arc-melted Temperature [˚C] Cr [at% ] in (Cr, Ta)C

S6A yes 1300 35,97

S6 no 1300 1,25

S7A yes 1400 10,29

S9A yes 1400 11,65

S9 no 1400 7,57

S10A yes 1500 9,59

S10 no 1500 7,59

S11A yes 1500 9,55

S11 no 1500 6,13

32

Figure 20: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ta system at 1300˚C.

Figure 21: The compositions from the raw-materials, empty markers, are compared to the composition from the chemical analysis, filled markers, after heat-treatment. Ta system at 1400˚C.

Figure 22: The compositions from the raw-materials, empty markers, are compared to the composition from the

chemical analysis, filled markers, after heat-treatment. Ta system at 1500˚C.

33

Table 18: Chemical compositions from the raw-material and from the semi quantitative analysis.

Ta Chemical composition Chemical composition

from rawmaterials from chemical analysis, normalized

Sample Ta[wt%] Cr[wt%] C[wt%] Ta[wt%] Cr[wt%] C[wt%]

S6A 35,97 54,60 9,43 39 52 9

S6 35,97 54,60 9,43 38 52 10

S6-2 35,97 54,60 9,43 38 52 11

S7A 29,04 60,20 10,75 33 56 10

S8 28,57 60,65 10,77 31 58 11

S8-2 28,57 60,65 10,77 Too small amount of sample

S9A 34,95 55,58 9,47 38 53 9

S9 34,95 55,58 9,47 38 53 10

S10A 28,10 61,10 10,79 31 58 11

S10 28,10 61,10 10,79 31 59 10

S11A 34,43 56,09 9,48 Too small amount of sample

S11 34,43 56,09 9,48 37 54 10