Grain growth behavior in Fe 3 Al alloys fabricated by different methods
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Grain sizes were measured after various heat treatments in three Fe3Al alloys having similar composition that were fabricated using the techniques of ingot metallurgy (cast and wrought), hot extrusion of prealloyed powder, and hot isostatic pressing (HIP) of elemental powders. The ingot metallurgy (I/M) material exhibited normal grain growth behavior at temperatures above 750 °C, in agreement with previous observations. Both powder metallurgy (P/M) materials displayed unusual resistance to grain growth compared to the I/M alloy. In the case of the prealloyed P/M material, the initial (recrystallized) grain size was larger than the initial grain size of the I/M material, although little grain growth was observed for heat-treatment temperatures up to 1100 °C. At higher temperatures grain growth occurred at a rate comparable to that observed to the I/M alloy. The elemental powder P/M material exhibited similar grain growth behavior to the prealloyed P/M material, although the initial (as-HIPed) grain size was considerably smaller. Transmission electron microscopy (TEM) indicated that the grain growth resistance of the P/M materials could be attributed to grain boundary pinning by oxide particles presumed to originate from the powder particle surfaces. The difference in the stable grain size between the prealloyed and elemental powder P/M materials was attributed to the nature of the particle dispersions resulting from processing.
I. INTRODUCTION Among the benefits frequently attributed to P/M processed materials are their homogeneous microstructures and small grain sizes that can lead to improved properties and performance compared to conventionally processed I/M products. In addition, observations have also been made suggesting that some P/M products exhibit superior microstructural stability compared to I/M materials, particularly with regard to grain coarsening at elevated temperatures. Grain growth resistance is significant in that property improvements resulting from microstructural refinement could be maintained to higher temperatures, potentially extending the service temperature of advanced alloys. Theories of normal grain growth in single phase polycrystalline materials are well developed and have recently been reviewed.1 It is widely recognized that grain growth can be inhibited by many factors, including impurities or solute atoms,2 second phase particles or pores,3-4 texture, 56 and free surfaces.7 Several other factors can also influence grain growth, such as grain size distribution, grain shape, grain boundary misorientation and structure, grain boundary energy anisotropy, etc., although much less is known abut these effects.8"11 While a tremendous number of theoretical and experimental studies of grain growth phenomena have been conducted, it remains extremely difficult to explain the coarsening 1384
J. Mater. Res., Vol. 9, No. 6, Jun 1994
http://journals.cambridge.org
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behavior observed in many complex materials systems of practical interest. The factors affecting grain grow
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