A grain-size-dependent structure evolution in gradient-structured (GS) Ni under tension

This work outlines an experimental investigation of grain-size-dependent structure evolution under tension in nickel with a grain size gradient. Two opposite and competing processes, grain refinement and coarsening, were examined within one specimen, due to the widely ranging grain size in gradient-structured (GS) Ni. A tension-induced minimum grain size of approximately 280 nm was determined in GS Ni, which is comparable to those obtained by severe plastic deformation processes. The minimum grain size was phenomenologically explained using a dislocation model. Below the minimum grain size, the Ni's grain coarsening ability peaked at approximately 50 nm and progressively decreased with decreasing grain size, showing an inverse grain-size-dependent coarsening tendency. Moreover, this inverse grain coarsening behavior was related to a transition in the deformation mechanism, through which the deformation process was accommodated more by partial dislocation than by full dislocation below a critical grain size. This was confirmed by observation of the microstructure and low temperature tensile testing results. This work demonstrates a high-throughput strategy for exploring the minimum grain size and grain-size-dependent coarsening in metals.