The Role of Parent Phase Compliance on the Fatigue Lifetime of Ni-Ti

It has been previously suggested that the fatigue lifetime of superelastic Ni-Ti might be improved if the R-phase were the parent to martensite rather than austenite. This body of work tests that hypothesis in two separate side-by-side fatigue tests both carefully constructed to match the superelastic properties in the two study arms. Both experiments show the R-phase parent to be more durable than the more commonly considered austenitic parent phase. The first experiment considers straight wire specimens fabricated from standard purity material, in a tension-tension fatigue test to 107 cycles, at mean strain ranging of 0.5-5.8% and strain amplitudes of 0.15-0.45%. The second experiment considers formed wire specimens in bending fatigue, more representative of realistic medical components, with a maximum mean strain of 1.2%, and maximum strain amplitudes ranging from 0.72 to 1.64%. Compared with the austenitic parent material, the R-phase material tolerated 0.1-0.3% higher strain amplitudes.