Structural aspects of the manufacture of semiproducts made from titanium nickelide-based alloys

The effects of the charge composition, casting method, and metal forming method on the structure and shape-memory-effect (SME) and superelasticity characteristics of titanium nickelide-based alloys are studied. The shape recovery temperatures of semiproducts are shown to depend substantially on the volume fraction of the Ti₂Ni intermetallic phase, whose formation is stimulated by the oxygen present in a charge or absorbed during casting. An increase in the volume fraction of Ti₂Ni in an alloy leads to nickel enrichment of the B2 phase and a decrease in the shape recovery temperatures. Subsequent metal forming at the stage of semiproduct manufacture only weakly affects the volume fraction of Ti₂Ni and favors the formation of its equiaxed shape and a more uniform Ti₂Ni distribution in the B2 matrix. In alloys where the B2 phase contains more than 56.5 wt % Ni, quenching from temperatures above 600°C and aging in the temperature range 400–500°C result in the dissolution and precipitation, respectively, of the nickel-rich Ti₃Ni₄ and Ti₂Ni₃ intermetallics. Therefore, the shape recovery temperatures of semiproducts and finished products can be controlled. Moreover, as the aging temperature changes, the volume fraction and size of nickel-rich intermetallic particles, the slip stresses, and the SME force characteristics change. For example, to increase the compression forces for osteosynthesis fixation devices, one has to use a titanium nickelide-based alloy with a high nickel content in the B2 phase and to perform aging at low temperatures (400–450°C).