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EROSION BEHAVIOR OF Fe-Al INTERMETALLIC ALLOYS
Y.-S. Kim, J.-H. Song* and Y.W. Chang”
Department of Metallurgy and Materials Engineering
Kookmin University, 86 l-l Joengneung-dong, Sungbuk-ku, Seoul, Korea 136-702
*Materials Division, Research Institute of Industrial Science and Technology, San 32, Hyoja-dong, Pohang, Korea 790-330
**Center for Advanced Aerospace Materials,
Pohang .University of Science and Technology, San 3 1, Hyoja-dong, Pohang, Korea 790-784 (Received August 27,1996)
(Accepted November 8, 1996)
Introduction
The Fe-rich Fe-Al intermetallics have generated some interest, especially during the last decade, due to their excellent resistance for oxidation and sulfidation, high specific strength, and low material cost.
The aluminide is therefore considered as one of the promising candidates for high-temperature structural materials in a corrosive atmosphere. Research effort has been focused mainly on process, development, and enhancement of room-temperature ductility together with the characterization of physical properties such as mechanical properties, oxidation, corrosion, and abrasive wear behavior [l-6]. However, there have been only a few works reported to date in regard to the erosion characteristics of the alloy, one of the most important material property of this ordered intermetallic alloy for the use in a fossil-fuel plant. The solid-particle erosion is a removal process of materials from the surface by repeated dynamic impacts of moving solid particles resulting into a complex surface damaging process. The physical properties of impacted materials as well as the eroding particles are known to affect the erosion behavior [7-91.
In this study, the solid-particle erosion behavior of the Fe-Al intermetallic alloys containing the various aluminum contents ranging from 25 to 30 at.% has been investigated to clarify the effect of aluminum content and different ordered structures, viz. DO3 and B2, on the erosion behavior. An attempt has been made to correlate the erosion behavior of these intermetallics to their mechanical properties by carrying out tensile tests together with SEM observation of the eroded surfaces.
Experimental Procedures
Iron-aluminide ingots containing 25, 28, and 30 at.% of aluminum were first prepared by a vacuum induction mehing. The ingots were then hot rolled into a 3mm thick plate with the start and finish temperatures of 1250°C and 650°C, respectively. The erosion and tensile-test specimens were consequently machined from this hot-rolled plate. The tensile specimen had a gauge section of
829
3 x 6.25 x 32mm. The erosion-test specimens, having a rectangular shape with the dimensions of 3Ox40x3mm, as well as the tensile specimens were heat treated to have the two different ordered structures, viz. DO3 and B2 structures. The heat-treatment conditions were determined based on the phase diagram by Allen and Cahn [lo]. The B2 structure was obtained by annealing the specimens at 650°C for one hour followed by oil quenching, while the ordered DO3 phase was acquired by holding the specimens at 450°C for 48 hours before the subsequent oil quenching. The structures of heat treated aluminides were then analyzed by a TEM and an XRD. The erosion tests were performed in the air at room temperature by using angular SiO;! particles with an average size of 360pm as an eroding particle. The velocity of impacting particles was around Slm/sec. The erosion rate was calculated by dividing the weight loss of a specimen by the weight of eroding particles used. The eroded specimen surface was also examined by using an SEM. Tensile tests were carried out at room temperature on an Instron machine at a strain rate of 1 x 1U3/s. All the tests were carried out in an ordinary atmosphere.
Results and Discussion
Figures 1 (a)-(c) are the XRD data and TEM photo micrographs, for specimens of Fe-30AI subjected to the B2 and DO3 heat treatments. The figures show that high degree of B2 and DOS order have been produced by the heat treatments. The erosion-test results of the aluminides having DO3 and B2 structures are plotted as the erosion rate vs. impingement angle of the eroding particle in Figures 2 and 3, respectively. The erosion rates are shown to decrease as the impingement angle increases in both figures indicating a typical ductile erosion behavior. It can also be observed from the figures that the erosion rate of the aluminides decreases as the aluminum content increases. The erosion rates at high impact angles do not show clearly the rate difference among the aluminides, while the rates under low- impact-angle conditions evidently show that the erosion rate decreases with the increase in aluminum content. The aluminides with the DO3 phase seem to show a slightly higher erosion resistance than those with B2 structure. However, the erosion-rate difference between the two ordered phases is minimal under the given test condition.
The SEM photo micrographs of eroded surfaces are shown in Figure 4 for the three DO3 aluminides impacted by SiOz particles at the incident angle of 30’. The micrographs show that erosion wear of the aluminides proceeds by the formation of surface platelets and subsequent detachment of them from the surface. The eroded surfaces do not show any sharply delineated cleavage faces or the cracks of brittle nature. Some evidence of microcutting and microploughing processes can also be observed. The formation of platelets on the eroding surface is another distinctive feature of a ductile erosion, confirming the ductile erosion behavior of iron aluminides. The SEM pictures also show that the platelet size becomes smaller as the aluminum content of the aluminide increases, though the difference is not very noticeable. The measured size of the platelet in the Fe-25Al aluminide is about 7l.tm, while the aluminides with 28 and 30 at.% Al have the platelet size of about 5.7pm and 5.6p,m, respectively.
Room-temperature tensile tests of the aluminides have also been carried out to carelate the erosion- behavior with their mechanical properties. The tensile-test results are summarized in Table 1 listing 0.2% offset yield strength, ultimate tensile strength, failure strain, and strain hardening exponent. The strain hardening exponent, n, was estimated as usual by plotting the flow stress and plastic strain data within uniform strain hardening region in a double log scale. Table 1 shows that the Fe-25Al aluminide has a higher strength than the other two ahuninides. The yield strength as well as the ultimate tensile strength of the aluminides can be seen to decrease as the aluminum content increases, which is consistent with the previously reported result by other workers [ 1 I]. It is also noted that the Fe-25Al
(4
DOI
1000 *heat-treated
B2 + heat-treated
0 20 30 40 50 60 70 80 90 100
degree 2 Cl
Figure 1. X-ray difiactograms and electron micrographs of Fe-30AI alloys: (a) XRD of DO3 heat-treated(450°C/48hr/0.Q.) and B2 heat-treated(650Wlhr/O.Q.) alloys. t DO1, 0 B2IDOh (b) Bright field electron micrograph of B2 heat-treated alloy with [Ol : ] zone-axis diffraction pattern, (c) Dark-field electron micrograph( 111 g vector) of DO, heat-treated alloy with [Ol l]
zone-axis diffraction pattern.
0.61 0 8 I # I I I
30 40 50 60 70 80 so
Impingement Angle
Figure 2. Variation of erosion rates as a function of impingement angle of impacting particles for Fe-25, 28, and 30 at.% Al alloys having DO3 phase.
1.6 I ,
- 1.4
P I
Impingement Angle
Figure 3. Variation of erosion rates as a function of impingement angle of impacting particles for Fe-25, 28, and 30 at.% Al alloys having B2 phase.
alloy has the smallest strain hardening exponent, while the ahuninides with 28 and 30 at.% Al have higher but similar hardening exponents.
It has previously been proposed that the strength of a material governs the erosion behavior suggesting the strength as the major parameter of erosion behavior [8]. The erosion resistance is seen to increase with increased aluminum content from Figures 2 and 3, which in turn reduces the strength.
It is therefore evident that the material strength does not seem to explain the different erosion behavior of the aluminides. Hutchings has introduced a model to characterize a ductile erosion using the concept of critical fracture strain [ 121. The model assumes that the erosion failure occurs when a unit material volume reaches at a critical fracture strain, at which is related to the tensile failure strain. It is,
b
a
Figure 4. SEM micrographs of eroded surfaces of (a) Fe-25A1, (b) Fe-28AI, (c) Fe-30AI, all having the DO3 structure and eroded by SiO2 particles at the impact angle of 30°.
TABLE 1
Tensile Properties of Fe-25,28, and 30 at.% Al Aluminides
however, difficult to find any direct relationship between the erosion rate (or resistance) and the failure strain of the aluminides. It is then apparent from the present study that either the strength or the failure strain could not be the characteristic parameter for a ductile erosion.
Following the Hutchings’ critical strain approach, the concept of a deformation zone, formed beneath the eroding surface, has been introduced to correlate the erosion resistance of the ahnninides to the zone thickness. The deformation zone is defined as the material volume beneath the specimen surface, where plastic deformation occurs by the solid-particle impaction. If the plastic deformation beneath the specimen surface is confined within a thin zone, the volume to reach the critical strain would be small, resulting into the formation of thin platelets to be detached during an erosion process.
On the other hand, if the plastic deformation occurs over a thicker zone, larger material volume would experience the critical strain to form larger platelets. Consequently, the erosion resistance would be higher for a material with a thinner plastic zone. The strain hardening behavior is, in the above regard, considered to play an important role on a ductile erosion process. If the strain hardening rate of a material is high, then the material surface tends to work harden rapidly to confine the deformation within a shallow region. The erosion rate of such a material would be low, since a smaller volume would reach at a critical strain required for detachment within a given time. When the strain hardening rate is low on the other hand, the deformation zone depth would be deeper resulting into a higher erosion rate. It can be seen from Table 1 that the aluminide with a higher aluminum content has a higher strain hardening exponent despite the lower strength compared to the low-aluminum-content aluminide. The higher exponent seems to justify the better erosion resistance of the alloy, which is also supported by SEM micrographs shown in Figure 4. The smaller platelet size in an aluminide with a higher aluminum content indicates indirectly that the deformed and hardened layer becomes thinner as the aluminum-content increases. It has previously been reported that the APB(NNN APB)energy of DO3 structure ‘decreases, while the APB(NN APB) energy of B2 structure increases as the aluminum content increases [ 131. The increased NN APB energy should make the superlattice dislocations to move more easily leading into a lower yield strength. On the other hand, the decreased APB energy associated with the DO3 structure seems to increase the strain hardening rate. It can therefore be suggested that aluminum in Fe-Al intermetallics plays an important role of decreasing yield strength and increasing strain hardening rate to provide a better erosion-resistance.
Summary:
The room-temperature erosion behavior of iron-ahuninides containing 25 to 30 at.% of aluminum has been investigated in the air using angular SiOZ as the eroding particles. The aluminides showed a typical ductile erosion behavior characterized by an increased erosion rate at smaller impingement
angles. The formation and subsequent detachment of platelets from the eroding surface was found to be the basic erosion mechanism of these aluminide alloys. The erosion resistance of the aluminides tends to increase with the increase in aluminum contents. The erosion-rate difference between the DO3 and B2 structures was not significant. The erosion rate difference due to the various aluminum contents appears to be attributable to their different strain hardening rates, rather than the previously suggested criterion of material strength or failure strain.
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