Micromechanisms of deformation and fracture in ordered intermetallic alloys: 1, Strengthening mechanisms. [Ni/sub 3/Al and CuZn

Abstract

The stability and mobility of active slip and twin modes in superlattice structures, for both cubic and noncubic crystals, are theoretically investigated based on the energetics and kinetics of dislocation dissociations. The main concept of the force couplet model for the positive temperature dependence of yield and flow stresses is introduced. Two sources of the glide resistance in ordered lattices are the fault dragging mechanism and the cross-slip pinning mechanism. The effective fault energy consists of two terms related to the chemical and mechanical instability of a shear fault (antiphase boundary, superlattice intrinsic stacking fault, or microtwin). Dependence of the yield stress on the orientation and the sense of applied stress stems from the signs and magnitudes of the glide and nonglide stresses. As the effective fault energy is altered by solute segregation and/or high nonglide stress, the two glide resistance mechanisms are affected differently. In Ni/sub 3/Al and ..beta..-CuZn, the major aspects of anomalous yield strength, strain rate sensitivity, in situ deformation transmission electron microscopy observations, microtwinning, and nonstoichiometry effect are discussed in view of the present model. In addition, the order twinning-slip conjugate relationship is identified, in all the superlattice structures considered, which will influence the deformation behaviormore » by viscous glide at high temperatures. 106 refs., 17 figs., 5 tabs.« less