Effect of rare earth Y element on grain refinement and mechanical properties of CuAlMn shape memory alloys

CuAlMn shape memory alloys have specific target demands in various fields due to excellent thermal stability and high damping properties.However， the mechanical property of the alloy is severely weakened due to the intergranular fracture arising from the coarse grain size. To improve the mechanical properties， the Cu-11.36Al-5Mn-xY （x=0-3， mass fraction/%，the same below） alloys were prepared by using a vacuum arc melting furnace by introducing rare earth Y element. The microstructure of the as-cast CuAlMn alloy was subsequently tailored and homogenized through solid solution and aging treatment. The phase transformation， phase composition， and microstructure of the alloy were respectively characterized by DSC， XRD， metallography and SEM observations. The hardness and mechanical properties of the alloy were tested by using a microhardness tester and a universal materials testing machine. Results show that the addition of Y element effectively refines the CuAlMn alloy grain， and the grain size even reduces from several hundred μm to around 10 μm. The grain refinement is mainly associated with the increased grain nucleation areas and the inhibition of grain growth during the cooling process. Moreover， quite numbers of Y-containing precipitates with the network structure are formed and distributed along the grain boundaries. The hardness of the alloy increases with the enhancement of Y element addition， which is associated with the precipitation of a large amount of hard and brittle containing Y phases.The hardness of the solution-aged sample is higher than that of the cast sample， due to the precipitate distribution throughout the entire matrix and higher volume fraction of precipitates for the former sample.In addition， the compressive and tensile fracture strength of the alloy are significantly improved when the Y content is in the range of 0.1%-0.4%. The strengthening mechanism can be understood by grain refinement strengthening， precipitation strengthening and solution strengthening. The compressive fracture strain of the alloy reaches its maximum when the Y content is 0.4%， while the elongation after fracture exhibits the maximum value with Y content of 0.1%. The changing trend is closely related to the coupling effect between grain refinement and precipitation phases.