Study On The Mechanical Properties Of Bulk Metallic Glasses With Various Glass Transition Temperature

Bulk metallic glasses (BMGs) have gained significant interest due to their unique properties, which has put these materials at the cutting edge of materials research. However, comparing to crystalline materials, the correlation between mechanical properties and its intrinsic mechanism at room temperature still remain unclear and require a systematic investigation urgently in BMGs. Creep behavior of BMGs at room temperature has been attracting considerable attention and systematically investigation on creep behavior is also deficient.By measuring some fundamental parameters and macroscopic bending properties, we found that (1-Cgg)* Vm is a good indicator of open-volume content. The higher the value of (1-Cg)* Vm, the lower the Tg and young’s modulus E. Such correlation helps to understand the glass transformation and elastic behavior in BMGs. It was also confirmed that larger Poisson’s ratio o corresponds to enhanced plasticity well, while no clear tendency of (1-CCg)* Vm with v was seen. So the open-volume may not be the only factor controlling the plasticity of BMGs. Moreover, structural relaxation decreased the value of (1-Cg)* Vm, and resulted in the enhancement of modulus and of decline of v and plasticity, indicating that open-volume content can exert influence on properties of BMGs to some extent.Using nanoindentation technique, we studied the creep behaviors of BMGs with various glass transformation temperature, Tg. The statistical distribution of the first pop-in events in nanoindentation curves were further analyzed by nucleation theory of shear transformation zone (STZs) and probability theory of stochastic variables to deduce information on the activation of STZs. In aspect of various BMGs and a certain composition also, this study successfully constructs the correlation between nanoindentation creep behaviors at room temperature and cooperative shearing of STZs, which is greatly helpful to understand the creep mechanism of BMGs.It was revealed that for various BMGs, the apparent steady creep strain rate normalized by applied maximum load, εs/Pmax, was a good indicator for describing nanoindentation creep resistance, which was demonstrated to strongly depend on Tg. Intrinsically, it was the smaller nucleation site density, m, and the higher activation energy upon yielding, WSTZ, that hindered the activation and cooperative shearing of STZs and caused better creep resistance in harder BMGs with higher Tg.For each composition, the influence that structural heterogeneity exerted on local properties and the activation of STZs was also discussed:with the increase in critical load at first pop-in, P1, both mean contact pressure, Pm, and shear yield strain, yy, linearly increased, while the young’s modulus, Es, remained almost constant, suggesting that the strength and elastic strain were more sensitive than the modulus to structural heterogeneity. No evident variation of creep resistance with P1 for each BMG was seen as a result of the competition between the deteriorated creep resistance, caused by the decrease of WSTZ with P1, and enhanced creep resistance caused by the reduction in m with P1.