Hardening by Point Defects and Solutes in B2 Intermetallics
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Hardening by Point Defects and Solutes in B2 Intermetallics L. M. Pike*, Y. A. Chang**, C. T. Liu***, and I. M. Anderson*** * Engineering and Technology, Haynes International, Kokomo, IN **Department of Materials Science and Engineering, Univ. of Wisconsin, Madison, WI ***Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN ABSTRACT This paper provides a review of recent progress on point defect and solute hardening in binary and ternary B2 intermetallics. As is the case for disordered metallic solutions, the presence of point defects and solute atoms in ordered intermetallic compounds results in solid solution hardening (SSH). However, factors unique to ordered systems are often responsible for unusual hardening effects. Binary compounds with identical crystal structures can exhibit significantly different hardness behavior. Ternary solute additions to ordered compounds can give rise to apparent solid solution softening as well as unexpectedly rapid hardening. These effects arise from the interaction of multiple defect types as well as the presence of multiple sublattice sites available for solute occupation. Therefore, before the SSH behavior of ordered intermetallics can be properly studied, it is necessary to develop an understanding of the types and quantities of the point defects which are present. Three recent studies by the authors are reviewed. Much of the work was done on NiAl and FeAl in binary form as well as with ternary additions. Defect concentrations over wide ranges in alloy composition and quenching temperature were determined using the ALCHEMI (atom location by channeling enhanced microanalysis) technique combined with vacancy measurements. Hardness values were also measured. It was found that most of the observed SSH effects could be rationalized on the basis of the measured point defect concentrations. INTRODUCTION As with traditional alloys, it is likely that solid solution hardening will have an important role in the design of intermetallic alloys. However, due to the complexity of the point defect structure and the availability of multiple sublattice sites for solute occupation the effects of these solute additions on the hardness of intermetallic alloys are not as straightforward. Before discussing the SSH behavior of B2 compounds, a brief description of their point defect structures will be provided. A more detailed description can be found in the review by Chang and Neumann [1]. The unit cell of the B2 intermetallic compound is ordered bcc with one type of atom on the cube corner sites and another type on the cube center sites. In the following discussion the different type atoms will be referred to as A and B, with the B atoms having the larger atomic radius. In a perfectly ordered crystal all of the A atoms would be on one sublattice, while all of the B atoms would be on the other sublattice. The sublattices normally occupied by A and B atoms will be referred to as the α and β sublattices, respectively. A perfectly ordered crystal can exist only at absolute
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