Mg-based bulk metallic glass has the advantages of low density, high specific strength and high elastic limits, which have made it superior in comparing with its crystalline counterparts. However, one of the disadvantages of BMG is the highly inhomogeneous deformation mode by shear bands below glass transition temperature. As a result, at room temperature monolithic Mg-based metallic glass is one of the most brittle BMGs and always breaks into pieces by catastrophic fracture, which has excluded it from being a candidate engineering material.
In the thesis, the micro-mechanism of deformation and fracture process of metallic glass was studied, and the mechanical properties were improved by appropriate alloy addition and composite synthesis.
A striation structure with nano-scale spacing in the mirror area of fracture surface of brittle Mg-based bulk metallic glass was observed using high resolution SEM. The origin of the striation structure can be predicted using Taylor instability analysis based on a grease model combined with microvoid formation mechanism of typical ductile fracture. The formation mechanism of the structure implies a competition between two effects, i.e. the tendency to extend crack by means of viscous flow to release free energy, and the constraining effect impeding a fast crack growth by the formation of ductile microvoids ahead of the crack tip. But unfortunately, such a plastic fracture process is highly localized and contributes little to the global plasticity.
The effect of Ni and Zr addition to the thermal and mechanical properties in Mg-based BMG was investigated. The improvement of thermal stability and compressive fracture stress of the alloy with Ni addition is contributed to the high elastic modulus of Ni. However, the highμ/B ratio of Ni decreases the resistance to dilatation in the region of a crack tip and deteriorates the plasticity. Zr addition induces the precipitation of minor crystallite in Mg-based BMG matrix due to the positive mixing heat between Zr and some constituents of metallic glass. The existence of the minor crystallite disturbs the propagation of shear bands and in turn contributes to the appreciable yielding. The brittle crystallite has very limited effect on plasticity and is at the cost of glass forming ability.
Mg-based BMG composites (BMGC) reinforced with ductile Nb and Fe particles were successfully synthesized. The ductile particles effectively impede shear band propagation and upon yielding, the ductile particle firstly deforms and the load is transferred to the surrounding glassy matrix to promote the initiation and branching of secondary shear bands. As a result, the composite shows high strength and large plasticity, which is comparable to some best Zr- or Ti-based BMGC. Reinforcing metallic glass with ductile particles is proved to be a simple and effective method. The results imply that by appropriate alloy design, Mg-based BMGC has great potential in serving as an engineering material.
修改评论