Evolution of surface and subsurface properties of Zr-4 alloy under the combined effects of laser heat treatment and laser gas nitriding: Microscopic morphology, chemical composition, and mechanical behavior

Abstract

Zr-4 alloy, extensively employed as cladding in nuclear reactor fuel rods, stands as the foremost line of defense for reactor safety. Given its susceptibility to defects such as micro-pores during casting and its long-term exposure to extreme conditions during practical service processes, enhancing the surface and subsurface properties of Zr-4 alloy is essential. Herein, a nanosecond pulsed laser is used to irradiate the Zr-4 alloy in a nitrogen environment to investigate the evolution of its surface and subsurface properties under the combined effects of laser heat treatment and laser gas nitriding. The experimental results reveal a remarkable augmentation in surface hardness and scratch resistance of the Zr-4 alloy after laser irradiation. Notably, the hardness of the laser-irradiated surface obtained with a laser power of 23.0 W and a scanning speed of 150 mm/s reaches 19.10 GPa, which is more than 7 times higher than that of the untreated surface (2.37 GPa). These improvements are attributed to the synergistic effects of the introduction of hard ZrN phase, the reduction of subsurface porosity, and grain refinement. This study demonstrates a promising approach for significantly improving the mechanical properties of Zr-4 alloy, holding considerable practical significance for its applications in the nuclear energy industry and other relevant fields.