The Effect of Alloy Formulation, Cold Work and Inclusion Content on Micro-Void Formation in NiTi Alloy Fine Wires

Inclusion and micro-void content are important for the performance of devices made from nickel-titanium alloys.  This is particularly important for fatigue life. Hot rolled NiTi coils formulated for superelasticity (As = -25oC) and shape memory (As = +95oC) are cold drawn with intermediate anneals to reductions of up to 99.3% in fine wire. Inclusions in the colder alloy are mostly carbides. Inclusions in the warmer alloy are mostly oxides. Size and area fraction of voids are correlated to total cold work, area fraction of carbides and area fraction of oxides. Micro-void formation increases with cold work. Size and area fraction of voids increased and then stabilized through intermediate cold drawing steps as shown below.  Area fractions were reduced in the later drawing steps. The effect of inclusion content and inclusion species will be correlated to the amount of micro-void formation. Mechanisms for void closure will be investigated.  The implications for the optimization of alloy formulation and the design of drawing processes for microstructure and performance will be discussed.