Microstructural changes in laser hardened Zr-2.5Nb alloy

Abstract

Laser surface treatment of metals is a process in which heating or melting of the surface of the metal by laser irradiation is followed by a rapid cooling via self-quenching. This process can produce metallurgical structures with improved wear and corrosion resistance. For zirconium alloys, laser melting (glazing) has been used to modify the microstructure of the surface layers of Zircaloy-2 and -4 tubing, and Zr-2.5Nb pressure tube alloy (1-4) without affecting the bulk properties. A supersaturated [alpha][prime]-martensitic structure was produced by the laser-glazing process. In addition, laser glazing produced a rough surface finish, which was attributed to rippling effects during the rapid solidification. In the present work the authors have investigated laser surface heating because such a treatment is expected to produce a similar microstructure as laser glazing, but with a better surface finish. Laser irradiation could raise the specimen's surface temperature into the [beta]-phase region and the subsequent rapid cooling should be sufficient to achieve the martensitic phase transformation, without requiring surface melting. The microstructure produced by laser surface heating was characterized by transmission electron microscopy. The depth profiles of the laser surface heated layers were obtained using microindentation. This paper reports the results obtained in this investigationmore » of microstructural changes produced in the surface layers of Zr-2.5Nb alloy by laser surface heating.« less