Effect of Zr addition and cold-rolling deformation on microstructure and mechanical properties of TiNi alloys

The Ti50-0.5xNi50-0.5xZrx series alloys were prepared by vacuum arc melting and subsequent suctioncasting. Metallographic observation and tensile mechanical tests show that adding trace amount of Zr can reduce grain size and increase higher tensile strength and elongation, however, excessive Zr content can easily lead to the formation of brittle intermetallic compound Zr2Ni and reduce its mechanical properties.The multi-pass cold-rolling deformation and annealing treatment on Ti49.9Ni49.9Zr0.2 alloy were carried out.The results show that the tensile strength sigma(b) and the elongation delta of the Ti50-0.5xNi50-0.5xZrx alloys increase with the increase of cold-rolling deformation. Compared with the unrolled alloys, after four-passes cold-rolling deformation, the tensile strength and the elongation after fracture of the alloys increase from561 MPa, 1.14% to 768 MPa , 35.1%, respectively.Microscopic observation of SEM and TEM show that the hard Ti2Ni particles distributed along the grain boundaries during the cold rolling process are sheared and broken, resulting in a discontinuous distribution of refinement. After 700 degrees C annealing process, the deformed microstructure has recovery recrystallization, resulting in the appearance of nanocrystalline and amorphous regions. Moreover, a large number of plate-like nano-twin structures and high-density dislocations appear in the TiNi matrix with the increase of cold-rolling deformation.