Influence of Inclusions On Localized Stress/Strain Distributions

There exist two primary inclusion types in Nitinol, TiC and Ti₄Ni₂O, whose presence in the alloy is dependent upon the melting technique.  Each inclusion type exhibits a unique morphology and mechanical behavior.  Inclusions are routinely observed at the fatigue fracture origin in both laboratory experiments and in returned commercial products.  In this two phase study we i) characterize the mechanics of each inclusion type, and ii) model the mechanical response surrounding the inclusions, in an effort to gain insight into the influence of these inclusions as a localized stress/strain disturbance and potential as a fatigue crack nucleation site.  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 derived, statistically evaluated, and average values were used for finite element modeling.  A superelastic Nitinol simple beam geometry was modeled by FEA that was “implanted” with either a single inclusion or an array of inclusions.  This beam was then subjected to various basic loading modes – tension, compression, bending, torsion and shear. The localized stress/strain distributions were modeled surrounding each inclusion type and deformation mode.  The results from the computational model were used to calculate a stress/strain concentration factor as a function of inclusion type, size, and loading mode.