Improving Mechanical Properties Of Bulk Metallic Glasses By Approaches Of In Situ Composites And Thin Films

Bulk-metallic glasses (BMGs) exhibits lots of unique properties, such as, high strengths, high hardness, high specific strengths, superior elastic limits, high corrosion resistance, etc. However, the applications of BMGs are still quite limited due to their intrinsic brittleness and low ductility at room temperature. Many efforts have been conducted to improve the plasticity of BMGs, in which metallic-glass-matrix composites (MGMCs) and thin-film metallic glasses (TFMGs) are two popular and effective approaches. Nevertheless, the deformation mechanisms for the improved plasticity of MGMCs and TFMGs are still far from satisfactory understanding, which will be investigated using both experimental and simulation methods in the present work. For the MGMCs, in situ high-energy synchrotron X-ray diffraction experiments and micromechanics-based finite element simulations have been conducted to examine their lattice strain evolution. The entire lattice-strain evolution curves can be divided into elastic-elastic (denoting deformation behavior of matrix and inclusion, respectively), elastic-plastic, and plastic-plastic stages. Characteristics of these three stages are governed by the constitutive laws of the two phases (modeled by free-volume theory and crystal plasticity) and geometric information (crystalline phase morphology and distribution). The deformation behavior, especially the fatigue behavior, of TFMG materials has been investigated on the some substrates, including 316L stainless steel, BMG, etc. The results show that the four-point-bending fatigue life of the substrates is greatly improved by Zr- and Cu-based TFMGs, while Fe-based TFMG, TiN, and pure-Cu films are not so beneficial in extending the fatigue life of 316L stainless steel. However, quite limited work is reported on the fatigue behavior of TFMG coated on the BMG substrate, which can be a very interesting topic. Moreover, a synergistic experimental/theoretical study are conducted to investigate the micro-mechanisms of the fatigue behavior of TFMGs adhered to BMG substrates. Furthermore, shear-band initiation and propagation under deformation are investigated using the Rudnicki-Rice instability theory and the free-volume models employing finite-element simulations.