| Bulk metallic glasses (BMGs) have been emerging as a type of novel materials, due to their excellent mechanical, physical and chemical properties compared to crystalline metals. In the present study, the mechanical behaviors of Zr-Cu-Ni-Al-Ti BMG were investigated not only in compression test, but also in tribotest. The elastic moduli and compressive ductility for various BMGs were compiled and analyzed, in order to identify key physical factor controlling the deformation and fracture behavior of BMGs. The soft surface of as-cast Zr-based BMG that induced by high cooling-rate during solidification was machined for the comparison of the compressive behavior with the cast one. The friction and wear behaviors of the Zr-based BMG were studied using a ball-on-flat measurement at different loads, and the effect of crystallinity on the wear performance was also examined in the same equipment. To understand the effect of the oxygen in test environment, other wear tests were conducted in three different atmospheres, i.e. air, oxygen and argon, using a home-built pin-on-disk friction system. Different counterface was employed in the study as well. The microstructures of worn specimens were characterized by focus ion beam and transmission electron microscopy. The following conclusions are drawn:(1) The ductility of metallic glasses has a good correlation with the ratio of bulk modulus to shear modulus, BIG. For individual BMGs, there seems to be a critical BIG ratio for the ductility, such as 2.6 for Fe-based BMGs,3.4 for Zr-based BMGs. Since the shear modulus is very sensitive to the atomic structural change, it may dominate the plastic flow behavior. The smaller shear modulus corresponds to a looser atomic packing, and thus higher plasticity.(2) The as-machined small specimens without the cast-softened surface exhibites highly dense and intersecting shear bands, and extensive plastic deformation, in contrast to the catastrophic failure and low deformability in the cast specimens with the softened surface. More free volume was detected in the small as-fractured specimens, indicating the occurrence of strain softening during the compressive process. Compared to the relatively smooth fracture surface of the small specimens, the large specimens showed more diverse features on the fracture surface due to the graded structures.(3) The friction coefficient of metallic glass with a steel counterpart is in the range of 0.24-0.32. Both surface softening and crystallization occur on the surface of metallic glass during wear, and wear curve is not as stable as the crystalline materials due to the interaction of the two processes. The wear mechanism of metallic glass may change with wear conditions and the crystalline phase content. Fully amorphous material shows an abrasive wear at a low load, then adhesive wear at a high load. Increasing the crystalline phase results in more abrasive wear. The wear behaviors of metallic glass and its crystalline composites do not follow the Archard’s equation. Only a good combination of the hardness and the toughness can the metallic glass be wear resistant.(4) The wear rate of the specimens increased dramatically with increasing oxygen content in the testing environment. A number of cracks and pits were present on the worn surface of the pin tested in the oxygen-containing environments, whilst a relatively smooth worn surface and a mixed layer with a thickness of about 2-10μm were observed in the specimens tested in argon. For the tests in oxygen, abrasive particles induced by oxidation protruded and peeled off from the glassy matrix together with the debris come from the conterface, resulting in a combination of two-body and three-body abrasion, thus indicating an abrasive wear controlled process. In the oxygen-free environment, the plastic flow took place presumably accompanied by work-softening due to the frictional heat and the local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. The wear coefficient K of the pin obtained via Archard’s model implied that a mild sliding wear was dominated.(5) For wear tests against steel, both the pin and the disk showed more roughly worn surface, thus exhibiting a much higher wear rate than that of the pin tested against zirconia. The possible reason may be due to the significantly low hardness and relatively good plasticity of the steel, which leads to the severe adhesion between the disk and the pin. This may induce large shear stress on the wear surface of the pin, resulting in local deformation and fracture. |
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