Enhancement of superelastic property in Ti–Zr–Ni–Cu alloy by using glass alloy precursor with high glass forming ability

Abstract

The glass forming ability (GFA), crystallization behavior and subsequent superelastic property after crystallization in Ti50-xZrxNi35Cu15 (x = 5, 10, 15, 20 at%) alloys have been investigated with an aim to develop glassy alloy precursor for shape memory alloy having both high glass forming ability and superior superelastic property. The addition of Zr is effective in improving the GFA exhibiting a critical thickness larger than 100 μm by increasing both liquid stability and resistance to crystallization. The glassy Ti50-xZrxNi35Cu15 alloys are polymorphically crystallized into B2 structure. With increasing Zr content, the grain size after crystallization decreases significantly by accelerated nucleation process. Thus, the stability of austenite increases, resulting in the decrease of Ms. The critical stress for slip deformation gradually increases with alloying Zr up to 15 at%, improving the superelastic recovery. The Ti35Zr15Ni35Cu15 alloy exhibits the highest remnant depth ratio value of 7.2%. By contrast, the superelasticity is significantly deteriorated in the Ti30Zr20Ni35Cu15 nanocrystalline alloy due to the difficulty in stress-induced martensitic transformation which is caused by extremely small grain size and irregular morphology of grain boundary. Considering both glass forming ability and superelastic property, the Ti35Zr15Ni35Cu15 alloy is confirmed to be the most suitable alloy satisfying wide supercooled liquid region (∼40 K), large maximum thickness for amorphous alloy formation (∼100 μm) and excellent superelasticity after crystallization.