Microstructure Design of Multiphase Compositionally Complex Alloys I: Effects of Strength Contrast and Strain Hardening

Abstract

AbstractThis work addresses the question, “Which microstructure yields the highest “toughness” for a two phase, compositionally complex alloy (CCA) composed of a ductile, face centered cubic (FCC) matrix phase and a stronger, intermetallic reinforcing phase?” A polycrystal plasticity model is used, which takes as input the phase fraction, grain sizes, and parameters describing the mechanical properties (stiffness, strength and strain hardening behavior) of each phase. In addition to assuming an elasto-viscoplastic matrix, the reinforcing phase is also assumed to be elasto-viscoplastic, albeit with a higher strength and lower strain hardening rate. The results reveal that, if the reinforcement remains elastic, the addition of such a reinforcement leads to an increase in strain hardening rate, in addition to the obvious increase in yield strength. If the reinforcement yields, the hardening rate sharply decreases, leading to earlier tensile instability. Therefore, when the strength contrast between the phases is high, addition of reinforcement is beneficial to toughness as well as strength. For lower contrast cases, the optimal toughness case may be the unreinforced matrix material. More specifically, only if the FCC matrix exhibits sustained linear hardening-type behavior typical of TWIP or TRIP effects (which correlate with very low stacking fault energies achievable in CCAs), will it exhibit an increase in toughness, as well as an increase in strength. Finally, the hard reinforcement is under a lower stress triaxiality due to pressure imposed by the ductile matrix, and this causes it to yield at a lower uniaxial stress than it would in isolation. The findings of this work provide a mechanistic understanding of the dependence of toughness on the microstructure of two-phase aggregates, and the approach is equally applicable to multiphase alloys.