Continuous Grain Size Gradients in Austenitic Incoloy 800H: Design, Processing, and Characterization

Grain size variations are common in thermomechanically processed alloys with non-uniform cold work. A method to produce samples with grain size gradients was developed using Incoloy 800H. Two tensile samples with non-uniform gages were designed with finite element analysis and manufactured. Measured strain profiles were consistent with designs, and maximum von Mises strains of (18.6 ± 6.1) and (13.6 ± 4.6) pct were obtained. After annealing, an area 40 mm× 5 mm was mapped by electron backscatter diffraction. Totals of 2849 and 2569 grains were identified after merging twins. Both samples had duplex grain structures as defined in ASTM E1181. Grains were binned into 4 mm strips to evaluate the spatial grain size distribution. Grain size gradients of 0.0081 and 0.0112 mm^2/mm were obtained. Simulated grain growth of the linear gradient microstructure was consistent with accelerated growth predictions. This new method of making samples will enable laboratory studies of gradient grain size effects in realistic industrial alloy microstructures. Further, samples could be used for parallel, single specimen experiments on phenomena that depend on grain size such as complexion transitions and fatigue. Most importantly, high-throughput parallel testing of microstructures enabled by our method could accelerate materials discovery and qualification in fields such as high-entropy and nuclear alloys.