Microstructure evolution during the heavy warm rolling of a nickel aluminum bronze

Abstract

Nickel aluminum bronze (NAB) alloys quenched after rolling to different reduction ratios were used to investigate the microstructure evolution during the heavy warm rolling (HWR) process. The results demonstrate that the average grain size continuously declines as rolling progresses while the dislocation density steadily increases. As a result of Widmanstatten and eutectoid transformation, the β phases are almost entirely transformed into α phases. Simultaneously, different types of k phases also rapidly precipitate from the matrix. After HWR, the microstructure comprises coarse-grained Widmanstatten α phases, ultrafine-grained eutectoid structures, and many types of k phases dispersedly distributed on the matrix. There is no noticeable texture due to the formation of numerous equiaxed α grains generated by eutectoid transformation. As the reduction ratio increases, the increasing dislocation lines gradually become entangled, eventually forming many dislocation cells. The precise statistics of the precipitates' size, position, and area fraction manifest that the newly generated k phases during rolling are primarily distributed in the region of eutectoid structures, and their size is typically below 50 nm. The improvement of the mechanical properties is attributed to the increase of dislocation density and nano precipitates as well as the continuous microstructure refinement. Based on the accurate characterization of microstructures, the phase transformation kinetics during rolling influenced by combined thermo-mechanical effects are holistically described.