Effects of Surface Modifications on the Fatigue Life of Unconstrained Ni-Mn-Ga Single Crystals in a Rotating Magnetic Field

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Long-term fatigue life during high-cycle magnetic-mechanical actuation is crucial to the application of Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs). It has been reported that long fatigue life can be achieved by both reducing surface damage and constraining Ni-Mn-Ga single crystals to exhibit much lower strain than the theoretical limit. In the present study, the fatigue life of Ni-Mn-Ga single crystal samples treated with various surface modifications was investigated in a rotary fatigue testing instrument. The apparatus minimally constrained the samples and allowed for magnetic-field-induced strain (MFIS) close to the theoretical limit. We first treated the samples with electropolishing, which we found created more surface defects than those of the mechanically polished sample. These defects acted as dispersed pinning sites for twin boundaries and nucleated cracks easily due to the localized stress concentration, resulting in reduced fatigue life. We then studied the introduction of residual compressive stresses imparted by micropeening. Although micropeening increased surface roughness, it produced a uniform surface morphology and a finely twinned structure. We argue that the large groups of dislocations did not pile up and the stress distribution was more homogeneous due to the fine twin structure, lowering the crack nucleation rate. Consequently, the fatigue life of unconstrained Ni-Mn-Ga single crystals with large MFIS was significantly improved by the micropeening treatment.