Cryogenic Mechanical Properties and Microstructure of CrCoNi Medium Entropy Alloy Processed by Selective Laser Melting

Abstract

The present study investigated the cryogenic mechanical properties and microstructure of CrCoNi medium entropy alloy (MEA) fabricated by selective laser melting (SLM) with hot isostatic pressing (HIP). The SLM processing parameters were optimized with respect to the relative density and micro-hardness of as-built CrCoNi specimens. The microstructure and mechanical properties of CrCoNi MEA fabricated with the optimal SLM processing parameters were characterized by uniaxial tensile test, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at room (298 K) and cryogenic (150 K) temperatures. At 150 K, both strength and ductility of SLM CrCoNi MEA are noticeably increased compared to room temperature (yield strength increased by 6.5% and ductility by 10.0% for as-built specimen, those of HIP treated specimen increased by 22.7% and 6.6%, respectively). The HIP treated CrCoNi reveals the considerable improvement of ductility at both temperatures compared to the as-built one (90.0% and 84.1% at room and cryogenic temperatures, respectively). The high cooling rate of 3.2 × 106 K/s obtained from the simulation may result in the fine microstructure and high dislocation density, accordingly the high yield strength of SLM CrCoNi MEA. The high ductility of SLM CrCoNi MEA is attributed to the deformation-induced nano-twinning as revealed by the TEM measurement. Furthermore, the pronounced dislocation activity and deformation twinning are responsible for the steady work hardening behavior, which postpones the onset of necking instability. It reveals the potential possibility of HIP in additive manufacturing of CrCoNi MEA with the excellent mechanical properties at cryogenic temperature.