Phase boundary, the interface between neighboring physical phases in matter, hosts various emergent phenomena and has been a pivotal subject of research interest for decades. Conventional techniques for the spatial resolution of phase boundaries rely on the interaction between the sample and certain probing particles, in which a pristine surface after stringent sample preparation and high vacuum are usually mandatory. Here we show that phase boundaries in metallic alloys can be resolved by lock-in thermography (LIT) in an ambient environment, even in the presence of a coating layer. An alternating charge current is applied through the sample, and the phase boundaries are revealed by the Peltier effect therein. We employ the LIT to visualize the propagation of phase boundaries in superelastic NiTi wires during the martensitic phase transition and demonstrate its capability to detect the martensite phase in a dental rotary file during mechanical bending. Furthermore, imaging of the phase boundaries in FeRh is successfully achieved using the same method, thereby demonstrating its broad applicability. This work offers a general technique for phase-boundary visualization in metallic materials, enabling versatile in situ nondestructive defectoscopy of phase-transition alloys in service.

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• Magnetic phase transitions
• Martensitic phase transition
• Medical devices
• Peltier effect
• Alloys
• Shape-memory materials
• Infrared techniques
• Thermal techniques