Enhancement of the mechanical properties by controlling the superlattice ordering in Ni-42W alloys

Short-range order (SRO) is a local superlattice ordering of elements over spatial dimensions of a few nearest neighbor spacings, which is widely observed in metallic solid solutions. The spatial extent of SRO is usually at sub-nanometer scale. Changing the degree of superlattice ordering provides a way to strengthen the metallic alloys. In this study, the spatial extent of superlattice ordering from sub-nanometer scale to several nanometers is achieved in Ni-W alloys, which is realized by the preferential growth of D1_a subunit cells in the aging process. By controlling the growth kinetics via applying plastic deformation, a significant enhancement in strength and a moderate reduction in total elongation are achieved in the pre-deformed samples under aging at relatively low temperatures. The strengthening from superlattice ordering is evaluated by using the precipitate strengthening model considering the contribution of coherency strengthening, modulus-mismatch strengthening, and ordering strengthening. The coherency strengthening is enlarged in the aged samples, which can be attributed to the increase of the spatial extent and fraction of superlattice ordering. The preferential growth of D1_a cells into extended precipitates causes the anomalous increase in modulus-mismatch strengthening. Our findings provide insights into the SRO strengthening mechanism and offer an idea for changing the SRO to achieve superior mechanical performance.