Modeling the effects of initial grain size, martensitic transformation induced dynamic grain refinement, phases, and texture on strength of a high entropy alloy using crystal plasticity

Abstract

Microstructurally flexible high entropy alloys (HEAs) can be tailored for strength via dual-phase strengthening mechanisms, which originate from the strain-induced phase transformation microstructural phenomena. One such alloy is a recently synthesized Fe42Mn28Co10Cr15Si5 (at%) HEA consisting of metastable gamma austenite (γ), stable epsilon martensite (ε), and stable sigma (σ) phases. In this work, a crystal plasticity homogenization incorporating a physically based phase transformation and hardening models is used to interpret and better understand the effects of strain induced phase transformation phenomena on the overall hardening response of the alloy. The transformation model is conceived based on the grain-scale stress sensitive motion of partial dislocations forming shear bands, while the hardening model is an extended Kocks-Mecking-type dislocation density-based formulation sensitive to grain size and shape. The deformation of constituent grains per phase in the alloy is modeled as a combination of anisotropic elasticity, crystallographic glide, and γ→ε phase transformation. Parameters of the hardening and transformation models within the homogenization are calibrated and validated on a suite of data including flow stress curves and phase fractions measured under compression, while the corresponding texture evolution data is used solely for verification. Good predictions of the model elucidate that the transformation induced dynamic Hall-Petch-type barrier effect is the primary origin of strain hardening along with the increase in the ε-phase fraction and dislocation density, while the individual strength of the phases gives rise to the overall strength. The modeling framework described in the present work is expected to be applicable to other HEA systems.

Keywords

• High entropy alloys
• Phase transformations
• Crystal plasticity
• Texture
• Hardening