Deformation mechanisms of bulk metallic glass matrix composites

Abstract

Bulk metallic glasses (BMGs) possess a unique set of mechanical properties that make them attractive structural materials: yield strength > 2 GPa, fracture toughness ~20 MPa.m1/2 and elastic strain limit ~2%. BMGs can also be cast into intricate shapes which retain their dimensional integrity and require no further machining. Unfortunately, monolithic BMGs fail catastrophically under unconstrained loading by forming shear bands. To overcome this problem, BMG matrix composites with fiber and dendritic reinforcements were proposed. The former type includes metallic fibers of Ta, Mo and stainless steel. The latter composites develop precipitates during casting and are thus called in-situ composites. Here, the reinforcements form an interpenetrating dendritic structure and enhance the ductility of the composite. This study investigated the deformation behavior of these two types of BMG composites. Loading measurements were performed during neutron or high-energy X-ray diffraction to determine lattice strains in the crystalline reinforcements. The diffraction data were then combined with finite element and self-consistent modeling to deduce the behavior of the amorphous matrix, as well as to understand the effective deformation mechanisms in the composite. The deformation of the wire composites was studied using an integrated neutron diffraction and finite element (FE) approach. The FE model yielded a reasonable version of in-situ stress-strain plots for both reinforcements and the matrix. It was found that the reinforcements yielded first and started transferring load to the matrix which remained elastic throughout the whole loading experiment. The reinforcements were seen to possess yield strengths lower than their monolithic forms, likely due to annealing during processing. After optimizing material properties to fit experimental data, the FE model developed was reasonably successful in describing both the macroscopic composite deformation and the lattice strain evolution in the reinforcements. In the case of the in-situ composites, a detailed neutron and high energy X-ray diffraction study was conducted combined with a self-consistent deformation model. The compressive behavior of the composite and the second phase (in its monolithic form) were investigated. It was shown that the ductile second phase yields first upon loading the composite followed by multiple shear band formation in the BMG matrix, a process which enhances the ductility of the composite. It was also discovered that the mechanical properties of the reinforcements, and indirectly the composite, are highly variable and quite sensitive to processing conditions. This resulted from the unstable nature of the BCC beta phase reinforcements which tend to transform into an ordered phase leading to significant stiffening, but also loss of ductility. An additional heat treatment study confirmed this phase evolution. The overall conclusion of this study is that BMG composites with high ductility require reinforcements that yield first and induce multiple shear bands in the amorphous matrix, which in turn enhances the latter’s ductility. To also retain a high yield point, the reinforcements need to be stiff. These two properties can best be optimized in beta phase composites via a judicious combination of microstructure control and heat treatment.