Design of new metallic glass composites and nanostructured alloys with improved mechanical properties

In this thesis two series of alloys were developed to obtain a combination of high strength, high ductility and work hardening. A new composition design strategy was proposed to create bulk metallic glass composites (BMGC) with high strength, large ductility and excellent work hardening from the brittle MgZnCa bulk metallic glass system. The volume fraction, size of both the dendrites and amorphous matrix can be effectively tuned by varying of the composition, and the yield strength, fracture strength and ductility also varies accordingly. The increase in alloying elements results in an increase in the volume fraction of amorphous matrix and a decrease in the dendrite size, which leads to higher yielding strength but lower ductility. The mechanical properties of the current Mg alloy can be interpreted by considering the BMGCs as a combination of the nanometer scale metallic glass matrix with a ductile dendritic structure. The high strength, large ductility and excellent work hardening observed in the Mg_(91.5 ) Zn_(7.5 ) Ca_1 can be attributed to the homogeneous deformation of nanometre scale amorphous matrices, which delays the formation and rapid propagation of microcracks from the interface.
In addition, a series of in-situ-cast nanostructured CuZrTi alloys were successfully designed by appropriate choice of alloying elements and compositions. XRD and TEM analysis shows that the alloys consist of softer Cu solid solution and harder nano-scale Cu_51 Zr_(14 )matrix embedded with retained Cu_5 Zr_ .The Cu_90.5 Zr_(7.5 ) Ti_2alloy exhibited a yield strength of 787MPa, a fracture strength of 1221MPa and room temperature uniform tensile elongation of 5.16%, exhibiting simultaneous ultrahigh strength and large uniform elongation. During tensile tests, the relatively softer (larger) primary Cu dendrites with numerous intragranular nanoprecipiates are believed to yield first, leading to substantial dislocation accumulation due to their relatively large grain size and the uniform distribution of numerous intragranular nanoprecipiates. With further increase in loading, the ultrafine Cu solid solution in the ultrafine clusters starts yielding and dislocation multiplication commences in this ultrafine Cu. Meanwhile, the formation of deformation bands are believed to start in the primary Cu dendrites due to the already existing high dislocation density, both of which further contribute to the work hardening and uniform plastic deformation. Finally, the hard Cu_51 Zr_(14 ) matrix commences plastic deformation and upon further loading, cracks start to form from the interface, leading to the final failure.