Solid Solution Hardening of Intermetallic Compounds
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SOLID SOLUTION HARDENING OF INTERMETALLIC COMPOUNDS ROBERT L. FLEISCHER
Rensselaer Polytechnic Institute, Troy, NY 12180-3590 (formerly General Electric Research & Development Center, Schenectady, NY 12301) ABSTRACT Solid solution hardening is sought and observed in two B2(cP2) CsCl and two C15(cF24) Cu2Mg type structures. New data are presented for the hightemperature compounds AlCo, AIRu, and Cr2Zr; prior data of Livingston on Cu 2 Mg are analyzed. From lattice parameters and specific gravities, cell occupancy numbers have been measured and used to infer likely defect types and *concentrations. The effects of constitutional defects in binary compounds correlate with traditional substitutional solution hardening from elastic interactions due to size differences and modulus differences. The hardening from ternary solutes is similar in magnitude, but the defect structures are complicated and not yet adequately understood. INTRODUCTION Hardness and strength can be attained in pure metals by solid-solution hardening. Although the primary mechanisms are thought to be known in dilute metal alloys, the mechanisms in intermetallic compounds are not established for most structure types. The mere existence of solution hardening is not widely demonstrated. Prior work however, includes data for the B2 compound AgMg [1], the L12 structure Ni3 Al [2], and preliminary results for the C15 cubic Laves phase Cu2Mg [3,4]. The new work that is summarized here is on the B2 compounds AlCo [5] and AJRu [6], and a C15 phase, Cr 2 Zr [7]. We wish to improve understanding of solid-solution hardening in intermetallics (in contrast to dilute alloys) by testing whether the same elastic interactions between solute atoms and dislocations dominate, but with the effective solute concentration increased in intermetallics by the substitutions or vacancies that are created by deviations from stoichiometry. To make this test, we wish first to identify the sites of constitutional solutes in off-stoichiometric intermetallics, and then to quantify the elastic interactions with dislocations of these solutes and of ternary additions. Data needed are the changes in lattice parameter, density, and shear modulus that accompany alloying and offstoichiometry. Knowledge of the density and lattice parameter indicate whether the excess of one constituent is accommodated by substitution in the other sublattice or by creation of vacancies. A special feature and complication, of intemetallics is that off-stoichiometry leads to constitutional defects--substitutions or vacancies that are created by deviations from stoichiometry. Thus, in order to understand hardening, we need to identify the sites of constitutional solutes in off-stoichiometric intermetallics and to quantify the elastic interactions with dislocations both of these solutes and of ternary additions.
Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
166
Hardening by substitutional solute is generally of what is called the gradual hardening type, usually due to two mechanisms: Ato
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