Effects of precipitate size and surface normal orientation on micro-identification and compressive fatigue of NiTi shape memory alloys
Juntunen, Kevin K.
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This work seeks to determine the evolution of the indentation and fatigue response of heat-treated Nickel-Titanium shape memory alloys over a range of precipitate sizes and surface normal orientations. Various tests were used to determine the impact of aging treatments on particle size, coherence, and phase transition temperatures. Through instrumented sharp microindentation and cyclic compression testing, the varying mechanical response of the NITI materials given different aging treatments was determined. Small precipitate sizes were found to strongly resist dislocation motion and the martensitic transformation. Intermediate precipitate sizes had lower resistance to both mechanisms. For the larger precipitates, the resistance to dislocation motion continued to lessen, while the resistance to martensitic transformation increased. During indentation, the 100 orientation tended to exhibit the greatest resistance to dislocation motion, while 221 showed the least. An orientation dependence of resistance to transformation was not obvious in the indentation tests. Cyclic compression tests showed that orientation and heat treatment combinations that resisted dislocation motion while allowing martensitic transformation showed the least residual strain accumulation and cyclic degradation. Those specimens that resisted the transformation or allowed significant dislocation motion saw a considerable reduction in the compressive hysteresis over 100 cycles.
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