Correlation between local mechanical properties and corresponding microstructures in a friction stir processed Monel alloy

Friction stir processing (FSP), an innovative local microstructural modification technique for bulk material, has been used to process Monel alloy. A detailed microstructural analysis was conducted by acquiring high-resolution electron backscatter diffraction (EBSD). The local tensile properties and hardness nanoindentation were obtained for the different FSP area zones. The coarse equiaxed grain structure of 18.9 μm average grain size at the base material (BM) has been dynamically recrystallized refined equiaxed grains of an average grain size of 3.6 μm in the stir zone (SZ) where the thermomechanically affected zone (TMAZ) has an average grain size of 10.7 μm. The grain structure at the interface of the SZ and TMAZ consisted of mixed fine recrystallized grains and coarse deformed grains. The SZ is dominated by significantly refined grains, with the highest fraction of high-angle grain boundaries (HAGBs) of about 71%. The point-to-point misorientation measured by Kernel Average Misorientation (KAM) indicates low values in the SZ compared with the highest values at the interface between SZ and TMAZ. In terms of tensile properties, the yield strength of TMAZ and SZ improved to 380 and 440 MPa, respectively. The ultimate tensile strength (UTS) also increased by 18% from 499 to 593 MPa in TMAZ and 37% from 499 to 699 MPa in the SZ. The maximum nano hardness found in the TMAZ of about 7.13 ± 1.92 GPa due to the high density of the substructure. However, the SZ slightly increased to 3.88 ± 0.22 GPa from 3.33 ± 0.17 GPa of the BM.