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

In this research, a novel nitinol shape memory alloy particulate (NiTip) reinforced aluminum matrix composites (AMCs) with fine-grained structure was prepared by underwater friction stir processing (UFSP). Microstructure observation and NiTip/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 NiTip could accelerate dynamic recovery (DRV) by increasing matrix deformation and promote dynamic recrystallization (DRX) by particle stimulated nucleation (PSN). Due to lack of serious NiTip/Al interfacial diffusion deteriorating their shape memory effect, the introduced NiTip in the resulting AMCs still exhibited a one-stage reversible phase transformation between martensite and austenite. Also, the shape memory effect of NiTip 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 NiTip distribution and well-bonded NiTip/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. (C) 2019 Elsevier B.V. All rights reserved.