Using Advanced Characterization to Provide Insights into Mechanisms of Functional Fatigue in Shape Memory Alloys

Advanced characterization methods are providing new insights into the functional fatigue of shape memory alloys (SMAs). One application to be discussed is the pronounced and beneficial effect of nanoscale precipitates on the constant force thermal cycling (CFTC) response of Ni(Ti,Hf) alloys. The structure of the ordered H phase precipitates has been determined using scanning transmission electron microscopy (STEM) based techniques. These precipitates are fully coherent with the austenite matrix, and create significant strains (and stress fields) which can influence the martensitic transformation that occurs locally around the particles. The technique of “4D STEM” has been used to quantify these strain fields, which should also strongly influence the local martensite orientations that form around the particles. Based on these results, finite element models have been developed to help explain how the microstructural scale of the martensite plates, and consequently the transformation strains, can be modified by the aging process. A second fundamental investigation involves the extensive strain that accumulates during CFTC of recrystallized, binary NiTi. In addition to dislocation production, automated crystal orientation TEM (ACOM) after in situ heating of samples indicates that new austenite grains with special orientation relationship to the original grains are produced during CFTC. Specialized in situ high energy X Ray diffraction (HEDM) experiments have been conducted to gain direct insight into the reorientation process during CFTC conditions. The unique insights provided by this innovative technique are discussed along with future work to further clarify the mechanisms of functional fatigue in shape memory alloys.