Martensitic transformation behavior of a shape memory Ti-40.5Ni-10Cu alloy affected by the C11(b)-type precipitates

Effect of aging at 873 K on the two-step B2-->B19-->B19' transformation of a shape memory Ti-40.5Ni-10Cu alloy was studied. It was found that the start temperature of the B2-->B19 transformation and that of the B19-->B19' transformation increase with increasing aging time. The start temperatures of the reverse transformations also increase. The temperature range of the two-step transformation decreases with increasing aging time. The aged Ti-40.5Ni-1OCu alloy is composed of the matrix with B2-type structure (a(c)=0.3029 nm) and precipitates with C11(b)-type structure (a(t)=0.3132 nm, c(t)=0.7991 nm). The precipitates are partly coherent with the matrix and are plate-like in shape having a orientation relationship [100](c)//[100](t) and [001](c)//[001](t), and a habit plane {001}(c). The increase of transformation temperature is caused by the precipitation through aging. The precipitation increases the Ti content of the matrix, and the precipitates act as a nucleation sites of B19 martensite due to strain field around them preferable for the B19 martensite.