Enhanced Fatigue Behavior of Ni-Mn-Ga Magnetic Shape Memory Alloys with Tailor-made Fine Twin Microstructure

摘要

The fatigue life of Ni-Mn-Ga ferromagnetic shape memory alloys (FSMA) was reported earlier to be short for material with coarse twins but long for material with self-accommodated fine twins. Here we study the fatigue performance of FSMA single crystal transducers in which stable fine twins were tailor-made using a method based on surface modification. The magnetic cycling was performed in magnetic actuator up to 220 million cycles. The initial 3-5% magnetic field-induced strain of the transducers started decreasing significantly after 10-100 million cycles, indicating structural and functional fatigue. The structural fatigue was further evaluated using optical and SEM observations. None of the 19 studied transducers with tailor-made stable fine twins exhibited complete structural failure and only occasional single macrocracks were found in contrast with the reference transducers with coarse twins, which exhibited frequently a large amount of macrocracks or complete structural failure. The origins of the enhanced fatigue performance are discussed in the article.