Plastic Deformation Induced Amorphous Alloy Crystallization Kinetics Study

In this paper, the microstructure evolution of Zr60Al15Ni25 and Zr65Al7.5Cu12.5Ni10Ag5 bulk metallic glass during rolling at room temperature is investigated by X-ray diffractometer (XRD), differential scanning calorimeter (DSC), high-resolution transmission electron microscopy (HRTEM) and selected area electronic diffraction (SAED).The thickness reduction (ε) of 95% is obtained during rolling of Zr60Al15Ni25 and Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glasses under the low strain rate (1.0×10-1 s-1 ~ 1.0×10-2 s-1) at room temperature. Shear bands are produced in the rolled specimens, showing that they experience an inhomogeneous deformation mode. The varieties of full width at the half maximum (FWHM) in the XRD patterns and the enthalpy of structure relaxation (Er) in the DSC curves with the increasing thickness reduction indicate that the"disordering-ordering-disordering"transition occurs in the Zr60Al15Ni25 and Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glasses during rolling deformation.A few icosahedral clusters was observed near the shear bands in the as-rolled Zr65Al7.5Ni10Cu12.5Ag5 specimen when the thickness reduction is small, which is attributed to the low Ag content in the shear bands and the directional movement of atoms induced by shear stress. With the increasing thickness reduction, the numbers of icosahedral clusters increase gradually so that the continuous ordering zones formed near the shear bands. As preexisting nucleus, icosahedral clusters and continuous ordering zones accelerated the I-phase precipitate in initial stages during scanning heating. However, the elements redistribution, especially Ag, within shear bands is unbenefit of the growth of I-phase, which delay the precipitatioin of I-phase.As is shown in the HRTEM images, some nanocrystals with diameter about 5 ~ 10 nm were observed near the shear bands in the as-rolled Zr60Al15Ni25 specimens with thickness reduction of 80% and 95%. Based on the polyhedral structure model and the shear transformation zone (STZ) theory, the influence of viscous flow, free volume, viscosity and the stress situation on the nucleation behavior of the amorphous alloy during rolling is discussed.