비정질 합금의 기계적 거동에 관한 나노압입연구

Abstract

Due to the excellent mechanical performance such as higher yield strength, elastic limit, and wear-resistance than their crystalline counterparts, bulk metallic glasses (BMGs) are often considered as candidates for advanced structural materials. However, their potential applications as structural materials are generally limited by the nature of their plasticity at room temperature; at ambient temperature, plastic strain is highly localized into very narrow zones, so-called ‘shear bands’. While it is now well established that shear banding behavior causes the softening phenomenon, the role played by the free volume content at the plastic flow is not well known. On the other hand, recently, many interests of amorphous materials have been paid to their possible applications to the micro-electro-mechanical systems (MEMS) components. Since the reliability and life-time prediction of the MEMS are important issue from an industrial viewpoint, the effective application of BMGs to MEMS requires better understanding of the time-dependent mechanical behavior. Keeping this in view, this study is focused on two subjects divided according to deformation types during indentation; the quasi-static deformation and time-dependent deformation (creep). As a first step in examining a new aspect of the quasi-static plastic flow (i.e. softening behavior), the role of free volume on work softening during indentation was investigated. As-cast BMG samples were annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique was employed to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. Results show that the structural relaxation enhances the plasticity with annealing time. Regardless of the annealing condition, the nanohardness of the deformed regions is lower, implying that the prior free volume only changes the hardness but not the extent of strain softening. Additionally, the influence of hydrogen on the softening behavior was analyzed through nanoindentation experiments on the subsurface deformation region produced during macroscopic indentation of the as-cast and hydrogen-charged BMG samples. Results reveal that hydrogen restricts the shear band evolution and thus significantly enhances hardness. The deformed regions of both the as-cast and hydrogen-charged samples exhibit plastic flow softening. From this, it was possible to demonstrate that the hydrogen affects the plastic deformation of BMGs in a manner that is similar to structural relaxation. For the starting point in analyzing the room-temperature time-dependent deformation, as-cast and annealed BMGs are investigated by performing nanoindentation creep experiments through a sharp indenter. Experimental results show that creep is much more pronounced in the as-cast sample than in the annealed one. In both cases, the amount of creep displacement was found to increase with the loading rate. From the results, it could be believed that the influence of structural state and indentation rate strongly influence the creep behavior. Next, the role of imposed initial strain on the room temperature time-dependent deformation behavior was systematically investigated through spherical nanoindentation creep experiments. Results show that creep occurs even at very low strains within elastic regime and, interestingly, precipitous increase in creep rates is found in plastic regime, with the BMG that has a higher free volume exhibiting more pronounced creep rates. Finally, uniaxial compression creep experiments were performed on micro-/nano-sized pillars to investigate the influence of sample size on the time-dependent plastic deformation behavior in amorphous alloys. Experimental results reveal that plastic deformation indeed occurs at ambient temperature, that too at stresses that are well below the nominal quasi-static yield stress in the manner of homogeneous flow. At a given stress, higher total strains accrue in the smaller specimens. The stress exponent obtained from the slope of the linear relation between strain rate and applied stress also shows a strong size effect, which can be rationalized in terms of the amount of free volume creation and the surface-to-volume ratio of the pillar.