Preparation And Mechanical Properties Of In-situ Cu-Zr-based Bulk Metallic Glass Composites

Cu-Zr-based metallic glass has attracted much attention not only the characteristics ofhigh strength, low elastic modulus and high elastic strain limit, but also the ability of easyto precipitate complicated metastable phases in the cooling process. In this paper, thepreparation and mechanical properties of in-situ Cu-Zr-based metallic glass compositeswere investigated. The Cu-Zr-Al-Co metallic glass composite with spherical crystallinephases dispersed was prepared by arc melting and injection casting using copper mold.Phase composition, microstructure, and mechanical properties associated with strain rate ofthe composite were investigated, especially the mechanism of work-hardening effect andthe reason of plastic deformation capacity increased.With the decrease of cooling rate, the organization of the Cu-Zr-Al ternary alloychanges from amorphous structure into a composite structure of the amorphous andcrystalline phase. The volume fraction of crystalline phase in Cu-Zr-Al-Co metallic glasscomposite also gradually increased. Compared with Cu-Zr-Al ternary alloy system, Co canpromote the formation of nucleation core to effectively avoid the phenomenon of large areaof crystallization. There were uniform size spherical B2-ZrCu phases dispersed inCu47Zr46.5Al6Co0.5metallic glass composite with the volume fraction of about10%.For Cu-Zr-Al ternary alloy, when the organization is amorphous structure, sampleunder quasi-static compression showed the typical characteristic of brittle fracture.However, when the organization is a composite structure of the amorphous and crystallinephase, sample showed certain plastic deformation ability. In contrast, Cu-Zr-Al-Co metallicglass composite not only had good capability of plastic deformation, but also showedobvious work hardening phenomenon, the fracture strain were above5%. Among them, thefracture strain of Cu47Zr46.5Al6Co0.5metallic glass composite was up to9.06%, whichattribute to the interaction between crystalline phase and shear band, anddeformation-induced martensitic transformation.The work-hardening effect of Cu-Zr-Al-Co metallic glass composite is mainly due tothe reinforcement of deformation-induced martensitic transformation. In the process ofdeformation, the martensitic transformation took place from B2-ZrCu phase to ZrCumartensitic phase, which began in the elastic deformation stage. With the increase ofdeformation, the volume fraction of ZrCu martensitic phase was also increasing. Comparedto B2-CuZr phase, the hardness of ZrCu martensitic phase increased significantly, which can compensate the strain softening of amorphous matrix.Within the scope of relatively low strain rate, composite showed obvious plasticdeformation behavior. The yield strength increased with the increase of strain rate, whichpresent positive strain rate sensitivity. This is because the small amount of dislocationsinside played a leading role in the deformation, and the generated ZrCu martensitic phaseimproved the strength of composite as a whole. However, within the scope of high strainrate, the fracture strength of composite decreased with the increase of strain rate, whichpresent negative strain rate sensitivity. This is because the shear bands played a leading rolein the deformation. The high strain rate leaded to adiabatic heating inside the shear band.Localized softening even melting occurred in composite, leading to generation of defectssuch as micro-holes or micro-cracks and sudden fracture.