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

Cu-based shape memory alloys have specific target demands in various fields due to excellent thermal stability and high damping properties. However, due to the coarse grain size and elastic anisotropy, they exhibit lower mechanical fracture strength and limited plastic deformation capability. Through SPD, notable alterations can be made to the microstructure of the alloy. The process not only results in grain refinement but also facilitates dislocation multiplication, thereby introducing novel mechanical strengthening mechanisms, including dislocation strengthening. Cold rolling, hot rolling, or a combination of both are commonly employed and straightforward methods for achieving significant plastic deformation. These processes are widely employed in the processing of materials, facilitating enhancements in the mechanical properties and functional characteristics of alloys.

The Cu-11.36Al-5Mn-xY (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, mass fraction/%) series alloys were prepared by vacuum arc melting furnace. The solid solution treatment was conducted in a chamber furnace at a solid solution temperature of 900 ℃ for 30 minutes, followed by an annealing treatment at 350 ℃ for 30 minutes.The SPD of the CuAlMn alloy was conducted using a two-roll mill. Initially, hot rolling was performed at a temperature of 850 ℃, with a deformation of 20% in each pass. Following the completion of each pass, the alloy was maintained at 850 ℃ for 15 minutes, after which the next pass was carried out. This process yielded hot rolled and water-quenched samples with deformations of 20%, 40%, 60%, and 80% after one to four passes. Subsequently ,the samples were subjected to an aging treatment at 350 ℃ for 30 minutes. Samples that had undergone hot rolling with a 60% and 80% morphology were selected for cold rolling treatment, with a 10% morphology per pass.After multiple passes, samples with 20%, 40% and 60% cold rolled morphology were obtained. In each case, the alloys were subjected to recrystallization annealing at 600 ℃×60 minutes after each pass. The alloys were subjected to microstructural observation and physical phase calibration using metallographic microscopy, SEM, TEM and XRD, respectively. The phase transition behavior of the alloys was examined using DSC under N protection. The quasi-static tensile mechanical properties of the alloys were tested by a universal material testing machine at room temperature, and the superelasticity of the alloys was investigated by loading-unloading cyclic tensile experiments.

The addition of the Y element resulted in a notable refinement of the grains, as observed through metallographic microscopy, SEM, TEM. 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 α 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 elongation 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.Conclusions: The Cu-11.36Al-5Mn alloy is composed of austenite and a minor quantity of 18R martensite. The incorporation of rare-earth Y elements markedly refines the grains, resulting in the formation of a considerable number of Y-bearing precipitation particles along the crystals, in addition to the emergence of Al-rich precipitation phases. Following hot rolling deformation, the grain continues to refine, accompanied by the appearance of high-density dislocations and dislocation cytosolic structures, a dislocation entangled subcrystalline structure, and a refinement of the martensite slat. Following dual-step rolling, the dislocation density continued to increase, accompanied by the formation of a high-density dislocation entanglement. Subsequent annealing treatment resulted in the precipitation of a substantial number of Cu-rich α phases, with the annealing twins and martensite laths arranged alternately. The incorporation of rare earth Y elements and the application of rolling deformation resulted in a notable enhancement of the mechanical properties of the alloys. The tensile fracture strength exhibited a marked increase from 366.67 MPa to 546.99 MPa at a Y element mass fraction of 0.4%, reaching 879.25 MPa after 80% hot rolled deformation and further elevating to 1025.25 MPa after dual-step rolling (60% hot rolled - 60% cold rolled). The elongation of the alloy increases from 3.05% in the as-received state to 8.38% after dual-step rolling deformation, exhibiting a similar trend as tensile fracture strength. The fracture mode of the alloy undergoes a transformation from brittle to ductile as a consequence of the addition of the Y-element and subsequent rolling deformation. The enhancement of mechanical properties can be attributed to three primary mechanisms: fine grain strengthening, precipitation strengthening, and dislocation strengthening. The Cu-11.36Al-5Mn alloy exhibits enhanced superelasticity in its pristine state, exhibiting a higher superelastic strain at the same strain level. However, this superelastic strain decreases rapidly following rolling deformation. The addition of the Y element and the subsequent rolling deformation resulted in the precipitation of particles on the matrix, accompanied by an evident pinning effect. This led to the proliferation of local dislocations, the emergence of a non-uniform stress field, and the formation of a synergistic effect between the precipitation phase and the dislocations, which collectively made the transformation of martensite to austenite more challenging. Consequently, martensite exhibited a tendency to stabilize. Furthermore, the observed decrease in superelasticity was also attributed to the grain size effect, with smaller grains exhibiting diminished superelasticity.