Correlation between disorder-order transformations in a Ni-based alloy and its mechanical properties

It is demonstrated that the disorder-order transformations in a commercial Ni-Mo alloy with minor concentrations of Fe and Cr and the corresponding effect on mechanical strength are sensitive functions of the exact chemical composition. A slight change in composition within the specified limits of the alloy is found to change the nature of the ordered phases as well as the kinetics of the transformation and the mechanical strength. The results show that while the increase in tensile yield strength due to atomic order is accompanied by moderate decrease in tensile ductility, additional loss of ductility can occur at constant yield strength due to changes in morphology of the ordered microstructure during isothermal aging. It is also shown that the state of short-range order present in an otherwise disordered solid-solution acts as a pre-existing nucleus for the transformation, which accelerates the respective kinetics. However, elimination of the short-range order by cold work is found to decelerate the kinetics of the transformation. It is concluded that although the disorder-order transformations in the alloy investigated have detrimental effects on mechanical strength, some beneficial effects of technological importance can be realized.