Microstructure Transformation in Different Regions of Zr61.4Cu27.8Al4Ni6Y0.8 Bulk Metallic Glass Induced by Rapid Laser Welding

Abstract

An experimental study is performed on the microstructure transformation induced by rapid laser welding in the different regions of Zr61.4Cu27.8Al4Ni6Y0.8 bulk metallic glass (BMG). The laser input energy has a remarkable influence on the microstructure and microhardness of the Zr-based BMGs. The microstructures in different regions of as-welded joints are diverse. Crystallization happens in heat-affected zone (HAZ) with crystalline dendritic phase of CuZr, which results in the deterioration of the microhardness to 434.9 ± 17.1 HV in HAZ, much lower than that of parent material as 517 ± 5.0 HV. Meanwhile, the element profiles indicate that intragranular segregation of Cu element occurs during the crystallization process. In the center of molten zone (MZ), the structure is fully amorphous and its hardness is close to that of parent material. In the region transiting from MZ to HAZ, some nano-grains with an average size of 20–50 nm are identified. In addition, several flower-like nanostructured grains of Y2O3 phase with a size ranging from 50 to 80 nm are formed in MZ’s matrix. The nanocrystallization is believed to be responsible for the enhancement of the Vickers hardness up to 560.1 ± 8.4 HV in this region. Rapid laser processing induces three kinds of microstructures in Zr61.4Cu27.8Al4Ni6Y0.8 bulk metallic glasses, i.e., the severe crystallization of CuZr phase in heat affected zones (HAZ), the nano-grains in the region transiting from molten zone (MZ) to HAZ, and the fully amorphous structure in the center of MZ, which consequently results in the diverse microhardness in spatial distribution.