| 其他摘要 | Application of metallic glasses with high strength, large elastic limit, good corrosion resistance, is always the prime target pursuited by researchers in this field, and the only way for using bulk metallic glasses widely is plasticity improvement at ambient temperature by understanding the deformation mechanism. Here, by scanning electron microscope (SEM) and transmission electron microscope (TEM), systematic research was carried out on the propagation of shear bands and deformation processes of the entire sample in a Zr52.5Cu17.9Al10Ni14.6Ti5 metallic glass under various loading conditions, revealing the fundamental deformation mechanisms in microscopic and macroscopic scales at room temperature.
At ambient temperature plasticity in monolithic bulk metallic glass is very limited, and the plastic deformation is heavily localized in shear bands, which propogate rapidly after initiation, resulting in the catastrophic fracture instantly. So making clear what happens in primary shear band is very helpful for understanding the fracture of metallic glass. To obtain the sample with primary shear band, the rod sample was first compressed with a cylindrical stopper. There was very limited room for shearing on the plane of maximum shear stress due to the restraint of the stopper, and the unfractured specimen with dominant shear band was achieved. The offsets in about 300 µm clearly pointed out the position where shearing operated on cross-section of the splited sample. In TEM, preferentially ion-milled region with a width of 10 µm could be observed on the position of primary shear band, which revealed that the structure of the shear band had been changed locally. Such region should be caused by the combination of shear band and its heat affected zone, the width of which was approximately consistent with the thickness of heat affected zone estimated based on the isothermal model. This meant that the shear band evolution was not an adiabatic process, and shear band should keep on being a certain high temperature for a long time. Based on the isothermal model, it was found that not only the thickness of heat affected zone depended on the shearing time, but also the shear band thickness increased as shearing rather than keeping constant.
Bulk metallic glasses usually display brittle fracture, but the compressive plasticity could be realized in fully amorphous sample by changing its geometry. For instance, heavy deformation could be obtained in specimen with low aspect ratio under compressive loading, even no fracture would occur. Although it is impossible to improve plasticity at the source by this method, this provides an opportunity to observe how metallic glass deforms. In the compressive test on box-shaped bulk sample, by comparing the SEM morphologies of the same specimen at different strains, it was found that the amount of shear bands increased with the strain while the distribution of shear bands was uneven. At large strain, the multiplication of shear bands mainly came from the propagation and branching of the formed shear bands. In contrast with the previous viewpoint that the plastic deformation should concentrate in shear bands at room temperature for metallic glasses, the plastic deformation took place at some regions out of shear bands as well. No shear bands were born at such deformed region, which demonstrated that homogeneous flow happened. The homogeneous flow regions were enclosed or half-enclosed by shear bands with spacing in the tens of microns, where the stress state is complicated.
The above investigation seems to hint that the fracture mode of a small-volume sample may be different from the bulk one, and it is worthy of consideration how the metallic glasses deform when the specimen dimension is close to the thickness of shear band on order of 10 nm. Therefore in-situ tensile tests were carried out in TEM on sub-micron scaled samples, which were fabricated from a bulk metallic glass by focused ion beam technology. The result shows that the obvious size effect happened on sub-micron scale and the tensile plasticity reached as high as 23-45%, including homogeneous elongation beyond 10%. The homogeneous plastic deformation in metallic glasses did not depend on shear band any more, and atomic-scaled shear events took place through the entire sample. After the occurrence of localized deformation, shearing was not the only choice, and necking, the typical fracture mode for ductile materials, could happen. Also, the localized deformation presented as a controllable process. The experimental evidence demonstrated that small-volume metallic glass had ability to deform with large plastic strain inherently.
In all test samples, heat/deformation induced crystallization in the shear bands or the heat affected zone was not observed by select area diffraction and high resolution TEM. No difference between the homogeneous flow region and the undeformed sample was detected by the new developed fluctuation electron microscopy (FEM), because the structure change was too small for FEM. While the medium-range order in shear band and heat affected zone was reduced, which meant that the atomic arrangement was more disorder.
In conclusions, the uniform deformation could occur in Zr52.5Cu17.9Al10Ni14.6Ti5 monolithic metallic glass at room temperature, without any crystallization and shear bands involved. But the homogeneous flow was limited in the small-volume sample or the small restricted region. The homogeneous flow and shear band operated at the same time and accommodated all the plastic deformation in macroscopic scale. When the rapid propagation of shear bands could not be confined effectively, the appearance of a primary shear band would lead to the catastrophic fracture. |
修改评论