Effects of surface modifications on the fatigue life of unconstrained Ni-Mn-Ga single crystals in a rotating magnetic field
Effects of surface modifications on the fatigue life of unconstrained Ni-Mn-Ga single crystals in a rotating magnetic field
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旋转磁场中表面改性对无约束Ni-Mn-Ga单晶疲劳寿命的影响
DOI：
10.1016/j.actamat.2018.05.070
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发表时间：
2018
期刊：
影响因子：
9.4
通讯作者：
Muellner Peter
中科院分区：
文献类型：
--
作者：
Zhang Hu;Armstrong Andrew;Muellner Peter
Ni-Mn-Ga铁磁形状记忆合金在高周磁力驱动下的长期疲劳寿命是其应用的关键。据报道，通过减少表面损伤和限制Ni-Mn-Ga单晶表现出比理论极限低得多的应变，可以实现长的疲劳寿命。在本研究中，在旋转疲劳试验机上研究了不同表面改性处理的Ni-Mn-Ga单晶样品的疲劳寿命。该装置最低限度地限制样品，并允许磁场诱导应变（MFIS）接近理论极限。我们首先用电解抛光处理样品，我们发现这比机械抛光的样品产生更多的表面缺陷。这些缺陷作为分散的钉扎位置的孪晶界和形核裂纹容易由于局部应力集中，导致疲劳寿命降低。然后，我们研究了引入残余压应力赋予微喷丸。虽然微喷丸增加了表面粗糙度，但它产生了均匀的表面形态和精细的孪晶结构。我们认为，由于细小的孪晶结构，大群位错不会堆积，应力分布更加均匀，从而降低了裂纹形核率。结果表明，微冲击处理显著提高了具有大MFIS的无约束Ni-Mn-Ga单晶的疲劳寿命。
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.