Effects of Y element and dual-step rolling on microstructure and mechanical properties of CuAlMn alloy

Cu-based shape memory alloys exhibit lower mechanical fracture strength and plastic deformation capability due to the coarse grain size and elastic anisotropy. A series of Cu-11.36Al-5Mn alloys were prepared by adding trace amounts of the rare earth element Y, and the microstructure control was achieved through hot rolling and dual-step rolling (hot rolling + cold rolling). It is revealed that the CuAlMn alloy is composed of austenite and a small amount of 18R martensite. A significant refinement of grains was obtained after the addition of Y element. Besides, Y-containing precipitates and Al-rich phases were observed to distribute along the grain boundary within the matrix. After hot rolling deformation, the alloy grains further refined and high-density dislocations and dislocation cell structures appeared. The dislocation density continued to increase accompanied by the emergence of high-density dislocation tangles after dual-step rolling and subsequent annealing. Abundant Cu-rich　precipitates precipitated along the grain boundaries, with alternating arrangements of twins and martensite laths. Tensile mechanical property tests show that the mechanical properties are significantly enhanced due to the inclusion of the rare earth element Y, with further improvement after rolling deformation. The tensile fracture strength increases from 366.67 MPa (as-received condition) → 546.99 MPa (0.4% Y addition)→ 879.25 MPa (80% hot rolling) → 1025.25 MPa (60% hot rolling + 60% cold rolling). The elongation of the alloy increases from 3.05% in the as-received state to 8.38% after dual-step rolling deformation with the similar trend as tensile fracture strength. The maximum superelastic strain increases with the addition of the Y element, but under the same strain, the as-received CuAlMn alloy exhibits higher superelasticity. Furthermore, the superelastic strain rapidly decreases after undergoing rolling deformation. In end, the microstructural origins of the improved mechanical properties were discussed in detail.