High-cycle fatigue of 10M Ni–Mn–Ga magnetic shape memory alloy in reversed mechanicalloading

Abstract

Application of Ni–Mn–Ga magnetic shape memory alloys in magnetic-field-induced actuationrelies on their performance in long-term high-cycle fatigue. In this paper the performance andchanges in the microstructure of a Ni–Mn–Ga 10M martensite single crystal material arereported in a long-term mechanically induced shape change cycling. The longest test was run for2 × 109 cycles at a frequency of 250 Hz and a strain amplitude of ± 1%. After the test a clear increase of the dynamic stiffness of the material was detected. Threespecimens out of ten were cycled until fracture occurred and their fracture mechanism wasstudied. It was observed that the macroscopic crack growth took place roughly at a45° angle with respect to the loading direction that was along the⟨100⟩ crystallographic direction of the sample. The macroscopic fracture plane seemed tocorrespond roughly to the {111} crystal planes. On a microscopic scale the fracturepropagated in a step-like manner at least partly along crystallographic planes. The steps atthe fracture plane correspond to the {101} twin planes, with the height of steps along the⟨101⟩ direction. The final fracture of the samples occurred in a brittle manner after the criticalstress was exceeded.