Suppression of the thermal embrittlement induced by sulfur segregation to grain boundary in Ni-based electrodeposits

Electrodeposited Ni and its alloys typically contain S due to crucial reagents used in the electrodeposition processes. The relative S concentration at the grain boundary (GB) increases when grain sizes are increased by thermal treatments, resulting in a transition from ductile to brittle failure. In this study, we theoretically explored effective solute elements for suppressing thermal embrittlement induced by S segregation to the GBs and experimentally verified the effect of a candidate solute. The GB segregation energies, interaction energy between S and solute, and embrittling potency energy were calculated from first-principles calculations for 57 Ni-based solid solutions. The results of these calculations are discussed based on the McLean and Rice-Wang models, which are used to identify effective elements to counteract the GB-decohesion by S segregation, such as B and W. For verification, electrodeposited Ni, Ni-B, and Ni-W alloys, containing the same S content, were prepared and their tensile behavior after annealing was examined. Although thermal embrittlement occurred in all electrodeposited samples, microstructure observation and fracture surface analysis indicated differences in grain size and S concentration at the GBs, resulting in embrittlement. These results revealed that W alloying in Ni-based electrodeposits has a high ability to suppress thermal embrittlement.