Plastic Deformation Of Monolithic Bulk Metallic Glasses

Bulk metallic glasses (BMGs) possess unique mechanical properties, such as high strength, high hardness and large elastic limits (2%). However, plastic deformation of BMGs is highly localized into shear bands, which usually rapidly propagate across the sample due to strain softening and/or thermal softening and initiate a crack, resulting in a limited plastic strain (less than 2%) and catastrophic failure at room temperature. To circumvent this drawback, the micrometer or nanometer-sized structural heterogeneities were introduced to prevent the unstable propagation of shear band. And the bulk metallic glass composites with enhanced plasticity were fabricated. However the plastic monolithic BMG was rarely reported, even though it is scientifically important and of potential practical application. In this thesis the approaches to improve the plasticity of monolithic BMGs were developed. The underlying mechanism for the enhancement of plasticity was also discussed. The main results were summarized below. (1) We report the improvement of plasticity in a ternary monolithic BMG caused by large amount of randomly-distributed free volume induced during solidification using high cooling rate. (2) We present a plastic monolithic Cu45Zr46Al7Ti2 BMG. It exhibits high strength and superior compression plastic deformation of up to 32.5% at room temperature. The outstanding intrinsic plastic deformability is attributed to randomly distributed free volume, inducing by minor alloying, which results in extensive shear band formation, branching, interaction and self-healing of minor cracks. (3) Large macroscopic compressive plastic deformation (over 15%) and work-hardening-like behavior were achieved in a monolithic BMG through tailoring loading stress distribution experimentally. Numerical analysis was also carried out to investigate the stress distribution under the same mechanical condition. It is shown that loading induced stress gradient is responsible for the achievement. (4) The applied stress significantly affects the nominal hardness of metallic glass. The nominal hardness decreases with applied tensile stress and increases with applied compressive stress. In contrast to the case in crystalline alloy, the applied stress can also cause the change of real hardness in metallic glass. Pile-up and sink-in are strongly correlates with the applied stress, wich can be characterized by the difference between nominal hardness and real hardness (ΔH). In the stress range we studied,ΔH linearly changes with the applied stress. The finite element analysis results show that the strong hardness dependence on stress results from the large elastic limit of BMGs. Since the nominal hardness andΔH are sensitive to the applied stress, they can be measured by indentation experiments to characterize the residual stress in metallic glass. (5) We conducted stress relaxation experiment, in-situ heating HRXRD test, dilitation and RMC simulation to study the origin of the exothermic peak before glass transition temperature. The results show that the exothermic peak before glass transition mainly results from the free volume annihilation.