Influence of Cu on the Mechanical and Shape Memory Properties of TiNi Alloys.

The significant phase transformation hysteresis in TiNi alloys limits their performance. To address this, copper (Cu) was added as an alloying element to reduce hysteresis. This study synthesized three compositions of TiNiCu (x = 0, 5, 7 at.%) shape memory alloys (SMAs) via vacuum arc melting to optimize the Cu content. The alloys were homogenized through hot rolling to maintain stable mechanical and shape memory properties. The hot-rolled TiNiCu alloy demonstrated excellent shape memory behavior, achieving 100% thermal recovery after one cycle at 4% and 6% strain and 99.2% recovery after six cycles at 4% strain. It also exhibited outstanding mechanical performance, with a tensile strength of 900 MPa and 40% elongation. Microscopic analysis using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) revealed that Cu preferentially segregates at grain boundaries, suppressing the formation of the Ti(Ni,Cu) phase. This moderate segregation, combined with hot rolling, promotes the reprecipitation and uniform distribution of phases, reducing the likelihood of premature fracture caused by stress concentration during deformation. The moderate thickness and uniformly distributed martensite, as well as the Type II twins with strong deformation ability, significantly improved the shape memory properties of TiNiCu. This study provides valuable insights into the microscopic mechanisms influenced by Cu in TiNi alloys and proposes a novel strategy for controlling precipitate phases through adjustments in alloy composition and optimized processing conditions.