Microstructural Evolution and Deformation Behavior of Ni-Si- and Co-Si-Containing Metastable High Entropy Alloys
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INTRODUCTION
THE advent of nonequiatomic high entropy alloys (HEAs) from the traditional approach that focuses on single-phase equiatomic HEAs has created enormous scope for designing compositions with tailored metastability of phases.[1–6] The fundamental basis in the quest for enhanced mechanical properties in these nonequiatomic HEAs is focused on exploiting the benefits of previously established deformation mechanisms such as transformation-induced plasticity (TRIP) and twinning-induced plasticity.[1,4–6] Li and Raabe[3] explained that the activation of multiple deformation mechanisms is a key to the design of nonequiatomic HEAs that exhibit adequate strength and ductility. The broad objective of the present research effort is to design nonequiatomic HEAs, the microstructure and deformation response of which can be tailored through their chemistry and processing, to incorporate the effect of various mechanisms like phase transformation, twinning, dislocation, and precipitation strengthening, while
S. SINHA, S.S. NENE, M. FRANK, K. LIU, and R.S. MISHRA are with the Center for Friction Stir Processing, Department of Materials Science and Engineering, University of North Texas, Denton, TX 76207. Contact e-mail: [email protected] B.A. MCWILLIAMS and K.C. CHO are with the Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Grounds, Aberdeen, MD 21005. Manuscript submitted August 7, 2018. Article published online October 23, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
maintaining the core effects of HEAs.[7–9] Friction stir processing (FSP) of these metastable HEAs enables microstructural refinement with some control over temperature, strain, and strain rate.[10] Previously, Nene et al.[11,12] discussed FSP-engineered dual-phase HEAs where enhanced strength and ductility were the result of pronounced TRIP in the material. Subsequently, a remarkable strength–ductility combination was achieved,[13] wherein the effects of transformation and twinning were incorporated successfully. Those previous studies highlighted the advantages that can be obtained by martensitic transformation in terms of strength–ductility tradeoff, generating interest in studying various aspects of the transformation like its dependence on alloy chemistry, fundamental understanding of the e (h.c.p.) martensite phase in HEAs, and the kinetics of martensite formation. Therefore, it is imperative to be able to control the transformation driving force and kinetics so that h.c.p. phase formation can be promoted or inhibited where necessary. In line with this, the current paper is not about achieving superior mechanical properties by changing the alloy chemistry. Instead, it seeks to explain the selective predominance of strain accommodation mechanisms based on the difference in the phase stability in the newly developed HEAs. This study outlines the dependence of martensitic transformation on alloy chemistry by specifically comparing two friction stir processed HEAs to illustrate the effect of Co or Ni on the c (
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