The evolution and effects of second phase particles during hot extrusion and re-extrusion of a NiTi shape memory alloy

Highlights: • Ti₄Ni₂O_x (with x ≤ 1) are the particles that precipitate in the austenite matrix. • Increasing the extrusion ratio changes the size of SPPs from 0.3∼16.0 to 0.3–24.3 μm. • Non-deforming SPPs affect greatly the surrounding austenite microstructure. Second phase particles (SPPs) that are formed during melting process have a significant effect on the hot extrusion and re-extrusion processes of NiTi. In the present work, the evolution of Ti₄Ni₂O_x (with x ≤ 1) particles during hot extrusion and re-extrusion of near equiatomic Ti-50.6 at.% Ni billets, and their effects on the surrounding matrix microstructure were studied using OM, SEM, TEM and EBSD. The analyses indicate that Ti₄Ni₂O_x (with x ≤ 1) are the particles that precipitate in the austenite matrix. After hot extrusion and re-extrusion, some particles are elongated and preferentially oriented along the ED. Eventually, these elongated particles are located at the grain boundaries. The size distribution of the SPPs is non-uniform after the first hot extrusion, and this non-uniformity increases after re-extrusion. The presence of non-deforming SPPs produces inhomogeneous deformation with a sharp increase in dislocation density in the surrounding matrix. This is shown to exert a profound effect on the processes of recovery and recrystallization for the surrounding matrix.