Processing, structure, and properties of in situ formed metallic glass matrix composites

Bulk metallic glasses (BMGs) exhibit unique mechanical properties including high yield stresses and large elastic strains. For BMGs, plastic strain is localized into narrow shear bands which results in fracture with little macroscopic plasticity. Metallic glass matrix composite are a novel class of alloys which can exhibit high yield stresses and large plastic strains. A particularly interesting type of metallic glass matrix composite is in situ formed composites in which the reinforcing-phase is precipitated out during processing.; We have developed a new in situ formed metallic glass matrix composite which consists of micron scale Ta-rich particles dispersed in an amorphous matrix. We have found that the volume fraction of the particles can be controlled simply by the amount of Ta added to the alloy. We have investigated different processing techniques for synthesizing the composite microstructure.; We have examined the crystallization behavior and glass forming ability of the amorphous matrix for the composite alloys. The addition of Ta to the amorphous matrix has a significant effect on the multi-stage crystallization sequence. The non-Ta containing alloys that we have studied form intermetallic crystalline phases via a two-step crystallization process during thermal annealing, while the addition of Ta to the amorphous matrix leads to the formation of icosohedral quasicrystals along with phase separation in the residual glass matrix prior to the formation of crystalline intermetallic phases.; Metallic glass matrix composites can exhibit mechanical properties which combine the salient properties of the metallic glass matrix, with the enhanced plastic strain of the reinforcing-phase. We have investigated the macroscopic mechanical behavior of the composite alloys using uniaxial compressive loading and Finite Element Modeling (FEM). The experimental and FEM results show that the second-phase particles have a significant effect on the initiation and propagation of shear bands in the glass matrix during plastic deformation. We also discuss the effects of the second-phase particles on the yield criterion of the alloys.; Finally, we have studied the micromechanics of deformation for the composite alloys using in situ synchrotron strain measurements and FEM. The results show that the Ta particles experience compressive yielding during loading followed by tensile yielding during unloading. The plastic misfit stress that develops in the glass matrix following particle yielding leads to localized yielding in the glass matrix surrounding the particles at an applied stress lower than the composite yield strength. The yielding in the glass matrix has been examined by measuring the magnitude of the von Mises effective stress in the Ta particles as a function of applied stress. The synchrotron strain measurements and the FEM calculations reveal the importance of the micromechanics of deformation on the macroscopic mechanical behavior of the composite alloys.