Multi-modal three-dimensional observation of unknown morphology of Ni4Ti3 and NiTi2 precipitates in NiTi shape memory alloy

Abstract

Characterizing the 3D morphology of nano- and micro-scale precipitates in metallic materials remains challenging. By combining 3D focused ion beam imaging with nondestructive synchrotron x-ray ptychographic tomography, this study pioneers a multi-modal imaging approach that unveils the 3D morphology of NiTi2 precipitates and a previously unknown cross-linked network of Ni4Ti3 precipitates in the nickel–titanium alloys, achieving a spatial resolution of 52 nm. Key discoveries challenge long-standing assumptions: Ni4Ti3 precipitates can form a network rather than isolated ellipsoids through three distinct cross-linking modes. Their shapes are not perfectly lenticular due to overlapping stress fields and loss of coherency. The 3D morphology of NiTi2 precipitates shows that they are primarily spherical and governed by interfacial energy minimization. The competitive growth mechanisms are captured via phase field simulation. These insights deepen the understanding of precipitate growth in NiTi alloys and establish a new paradigm for 3D microstructural imaging.