Effect Of Heat Treatment On Plastic Deformation Behavior Of Zr - Based Amorphous Alloy Vickers

Bulk metallic glasses(BMGs) possess attractive properties including high strength, high hardness, large elastic deformation, excellent wear resistance and corrosion resistance. They are expected to become a new class of structural material. However, its plastic deformation at room temperature is carried by highly localized shear bands due to lack of grain boundary and dislocations. The rapid propagation of shear bands results in an apparently brittle fracture, showing little macroscopic plasticity at room temperature that limits BMGs’ application as structural materials. In other words, plastic deformation of BMGs at room temperature is closely correlated with the nucleation and propagation of shear bands. Large plastic strains can be achieved if the highly localized primary shear bands are effectively hindered and they can be branched into abundant tiny shear bands. Recent investigations have shown that the content of free volume and the secondary phase particles distributed into the amorphous matrix considerably affect the nucleation and propagation of shear bands. The morphologies of shear bands formed during interface indentation are easily observed to investigate the mechanism of plastic deformation of BMGs.In the present work, isothermal annealing treatment was employed to change the content of free volume or obtain secondary phase particles embedded into amorphous matrix in Zr55Al10Ni5Cu30 and Zr65Al7.5Cu12.5Ni10Ag5 BMGs. The isothermal annealing treatment temperatures were selected below glass transition temperatures Tg or in the supercooled regions(defined as the temperature regions between the glass transition temperature Tg and the onset temperature of crystallization Tx). The changes of microstructure were investigated by X-ray diffraction(XRD), differential scanning calorimetry(DSC) and high-resolution transmission electron microscopy(HRTEM). While the measurements of micro-hardness and interface indentation tests were performed by Vickers Indenter. The results show that the micro-hardness of alloys increase due to isothermal annealing below Tg or in the supercooled regions. The morphologies of shear bands due to indentation were investigated by scanning electron microscopy(SEM) to understand the influences of free volume content and the second phase particles on the nucleation and propagation of shear bands. The results indicate that the sizes of shear deformation zones, the densities of shear bands and the radius of shearing decrease due to annealing below Tg, and the tendency becomes evident with the increase of holding time. This is attributed to more homogeneous distribution of free volume and the annihilation of the excess free volume. While the size deformation zones, the densities of shear bands and the radius of shearing of the annealed alloys in the supercooled regions increase initially with increase of crystallization volume fraction, and then decrease after reaching the peak values. The shear bands become smooth and uniform and the number and size of secondary shear bands increases when the density of shear bands increases. A certain degree of flow phenomenon occurs and nearly no secondary shear bands appear when the density of shear bands decrease. The phenomena result from the decrease of free volume content and the precipitation of the secondary phase particles.