Influence of Inclusions on Localized Stress/Strain Distributions

Two primary inclusion types in Nitinol, TiC and Ti₄Ni₂O, whose presence in the alloy is dependent upon the melting technique and purity of raw material are studied using Finite Element Analysis (FEA). Each inclusion type exhibits a unique morphology and mechanical behavior in medical-grade Nitinol. Inclusions are routinely observed at the fatigue fracture origin in both laboratory experiments and in returned commercial products. The mechanical behavior of each type of inclusion was characterized by in situ Berkovich-tip SEM nano-indentation. Using the indenter response, the elastic modulus of each inclusion type was calculated, statistically evaluated, and average values were used for FEA. A superelastic Nitinol diamond surrogate geometry was modeled using FEA under cyclic loading conditions and the resulting fatigue stress/strain domain was studied. The diamond surrogate was then “implanted” with an inclusion. The characteristics of the inclusion were varied and its effects on changes in the resulting stress/strain fields were studied. These characteristics include orientation, location, and inclusion-to-inclusion interactions.