Cryogenic tensile behavior of CoNiCrMo medium entropy alloy

Abstract

The microstructure and mechanical properties of 35Co-34Cr-24Ni-7Mo medium entropy alloy, known as MP35N, were investigated at different cryogenic temperatures (77 K and 195 K) and compared with those at room temperature (295 K). The microstructural features at the mentioned three distinct temperatures were characterized by X-Ray diffraction, electron back skater diffraction analysis and transmission electron microscopy. It was found that, compared to room temperature, cryogenic temperatures significantly increased the yield strength and tensile strength from 330 MPa and 1300 MPa at 295 K to 711 MPa and 1890 MPa at 77 K, respectively, without a noticeable change in elongation. Microstructural investigation revealed that lower deformation temperatures resulted in higher dislocation density, higher stacking fault probability, more mechanical twins, and a more homogeneous dislocation structure. Up to a strain of 0.4, no sign of fcc to hcp phase transformation was observed in EBSD, TEM images, or SAED patterns, even at 77 K. The higher work hardening due to mechanical twinning, increased dislocation density, and lower stacking fault energy, in addition to a more homogeneous dislocation structure, postponed the formation of geometric instabilities and maintained a ductile fracture mechanism at cryogenic temperatures.