PRECIPITATION IN Ni-RICH Ti-Zr-Ni SHAPE MEMORY ALLOYS BY ISOTHERMAL AGEING

We have reported that isothermal ageing of Ni-rich Ti-Zr-Ni alloys leads to a development of diffuse scatterings in electron diffraction patterns (SADP) followed by fine precipitation. Ti-6.0mol%Zr-52mol%Ni and Ti-6.0mol%Zr-53mol%Ni ingots were prepared by tri-arc melting. Isothermal ageing was carried out for temperatures ranging from 673 to 823 K with maintaining times between 3.6 and 360 ks. The samples aged at 673 K for up to 216 ks revealed diffuse scattering rings surrounding the matrix B2 spots in the SADP. However, fine spherical nano-sized precipitates (similar to 5 nm) were seen in the samples treated for 360 ks at 673 K. Lenticular precipitates with a diameter of similar to 35 rim and approximately 7 nm thick were observed for ageing at 773 K for 144 ks. The < 001 > B-2 SADP exhibits sharp superlattice reflections at 1/3 < 110 > B-2 and diffuse superlattices at 1/4 < 210 > B-2 elongated in < 100 > B-2 directions. HRTEM investigations revealed that sharp superlattice spots are given by the precipitates with the habit plane normal to the electron beam. The superlattice spots positions change concomitantly with the matrix reflections during martensitic transformation. Their Zr content is 15.7 +/- 1.4 mol%, while Ti content stands at 31 +/- 1.2 mol%. Ni concentration was slightly higher than that of the matrix.