Strengthening of Ni-Mn-Cu-Al-Co steel by nanoscale Cu and β-NiAl co-precipitated couples

Qingdong Liu ^a,b,*, Hui Song ^c, Jing Zhang ^a, Jiaxin Ding ^d, Yihua Chen ^a, Jianfeng Gu ^a,**

a Institute of Materials Modification and Modelling, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
b Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiaotong University, Shanghai 200240, PR China
c Electron Microscopy Center of Chongqing University, Chongqing University, Chongqing 400044, PR China
d Instrumental Analysis Center, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

ABSTRACT

A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500◦C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well.