Strengthening Mechanisms of Ni–Co–Cr Alloys via Nanotwins and Nanophases

Abstract

The ultra-high strength of multiphase (MP) alloys for fastener applications is endowed by cold deformation, which induces deformation twinning or phase transformation in the materials. In the present work, MP159 alloy as well as NiCoCr medium-entropy alloy are investigated to assess the effectiveness of a torsional pre-straining as well as surface mechanical grinding treatments to further improve their strength. For the pre-torsion route, microstructural analysis shows that with the activation of different twinning systems and stacking faults, the sequential torsion and tension tests lead to the observed hierarchical microstructure, which gives rise to the significant increase in the yield strength in the investigated alloys while retaining a good ductility. Further studies reveal that aging treatment of the pre-torsion bars provides additional strengthening to MP159 alloys, with the synergistic strengthening of nanotwins and nanoprecipitates. After the surface mechanical grinding treatment of NiCoCr alloys, a nanocrystalline structure is formed in a region extending about 150 μm from the edge. Considering the high applied strains, a high density of deformation twins is expected in the nanocrystalline region. Micropillar compression of single crystal and nanocrystalline NiCoCr alloy shows that with the refinement of grain size from 200 μm to 40 nm, the yield strength could increase from 900 to 2500 MPa. This reveals that the refinement of grain sizes can significantly increase the yield strength of NiCoCr alloys with a low stacking fault energy.