Rapid Grain Refinement and Compositional Homogenization in a Cast Cu50ni Alloy Achieved by Friction Stir Processing

Abstract

Friction stir processing (FSP) has been successfully used in the past to achieve grain refinement, compositional homogenization, and densification of cast microstructures. However, the effect of single or double pass FSP on microstructural homogenization in miscible binary Cu-Ni alloys has not been explored. Therefore, this study uses a binary Cu – 50at.% Ni alloy to clarify the effect of single and double pass FSP, starting from a coarse grained and compositionally heterogeneous cast microstructure. High energy synchrotron X-ray diffraction, electron backscatter diffraction, and nanoindentation are used to clarify the microstructural evolution due to FSP. The process of compositional homogenization of as-cast segregations is studied by energy dispersive spectroscopy and atom probe tomography. Our results show that a single fast FSP pass at 30 mm.s-1 produces a 100µm deep layer of submicrometric and hall-petch hardened CuNi grains. The initial cast compositional heterogeneities in a micrometric scale is rapidly transformed to nano-sized domains, mainly confined at grain boundaries. Double pass FSP increases the penetration depth of the processed layer and leads to ~2.9x grain growth relative to single pass FSP. These results highlight the value of FSP for ultrafast surface modification and reveal the mechanisms of grain fragmentation, discontinuous dynamic recrystallization, grain growth, and twinning.

Keywords: Discontinuous Dynamic Recrystallization, Friction Stir Processing, Miscible alloys, Synchrotron X-ray diffraction