Microstructure and mechanical properties of fine-grained aluminum matrix composite reinforced with nitinol shape memory alloy particulates produced by underwater friction stir processing

Abstract In this research, a novel nitinol shape memory alloy particulate (NiTi p ) reinforced aluminum matrix composites (AMCs) with fine-grained structure was prepared by underwater friction stir processing (UFSP). Microstructure observation and NiTi p /Al interfacial composition analysis showed that UFSP resulted in an uniform dispersion of NiTi p in the simultaneously created fine-grained 5083Al matrix, with no additional intermetallics formed at well-bonded NiTi p /Al interfaces. The addition of NiTi p could accelerate dynamic recovery (DRV) by increasing matrix deformation and promote dynamic recrystallization (DRX) by particle stimulated nucleation (PSN). Due to lack of serious NiTi p /Al interfacial diffusion deteriorating their shape memory effect, the introduced NiTi p in the resulting AMCs still exhibited a one-stage reversible phase transformation between martensite and austenite. Also, the shape memory effect of NiTi p can be activated through pre-deforming in martensite state and then reheating above austenite transformation temperature, thereby introducing compressive residual stress into the matrix. The fine-grained structure with homogeneous NiTi p distribution and well-bonded NiTi p /Al interfaces in UFSP AMCs, significantly improved strength without adversely affecting the ductility. A detailed analysis on various strengthening mechanisms contributing to the strength of the UFSP AMCs was carried out. Fine grain strengthening, geometrically necessary dislocation strengthening and load transfer effect were demonstrated to be three main contributors to the improved strength of the UFSP AMCs.