Ultralow Functional Fatigue of NiFeGaCo Shape Memory Alloy Enabled by Nanometer-scale Martensitic Twin Spacing

Ferroelastic shape memory alloys (SMAs) with low functional fatigue during cyclic phase transition (PT) are crucial for elastocaloric cooling technology 1-4. However, this property is a long-standing challenge due to severe dislocation accumulation driven by the misfit stress in the boundary layer between the parent and the product (twinned martensite) phases5-7. Here, we find that nanometer-scale martensitic twin spacing can enable ultralow functional fatigue in single-crystalline Ni50Fe19Ga27Co4. It is revealed that the ultrafine twin spacing (1.6 – 3.2 nm) causes an ultrathin stressed boundary layer of 3 – 7 nm thickness, 10–103 times thinner than those of many other SMAs. This nanometer-scale thickness significantly limits the curvature radius of dislocation segments bent from Frank-Read sources, leading to strong line tension to suppress dislocation multiplication. Consequently, the alloy shows stable, large cooling capacity over 107 PT cycles. Our findings suggest a new route to develop ferroelastic materials with high functional stability by engineering nanotwins in the low-symmetry product phase.