||Metallic glasses have attracted considerable attention because of their excellent mechanical properties such as high strength and hardness. Moreover, many researchers note the potential applications in terms of biodegradable and micro-electro-mechanical systems (MEMS) due to the lack of periodic atomic packing, indicating that there is no defect such as grain boundary, dislocation, or plane defect in amorphous alloys. However, the high strength of metallic glasses is often accompanied by a virtually zero plastic strain and can fail in a catastrophic manner. Plastic deformation of metallic glasses is highly localized in shear bands, which usually propagate rapidly through the sample. The brittle problem limits the application of thin film metallic glasses (TFMGs) for MEMS devices. |
To improve the brittle problem, this study has used two types of improvement methods, namely, the multilayered thin film metallic glasses and thin film metallic glass composites. For multilayered thin film metallic glasses, ZrCuTi (ZCT) 50 nm / PdCuSi (PCS) 50 nm TFMGs and ZrCuTi (ZCT) 50 nm / ZrCuTiTa (ZCTTa) 50 nm TFMGs are prepared by sputtering. The mechanical properties and the deformation characteristics of TFMGs are investigated by using expanding cavity model and nanoindentation testing. Utilizing the multilayered structure, the as-formed shear bands can be easily detected by scanning electron microscopy (SEM) and cross section transmission electron microscopy (XTEM). Two kinds of shear bands (radius shear bands and semi-circular shear bands) are observed by XTEM. Comparing the results of deformed microstructure and the load-displacement curves, a transition point from the radius type to semi-circular type of shear band can be found. The deflection phenomenon has been found by microcompression. In addition, the yield strength is enhanced by the multilayered structure.
For improving the ductility of bulk metallic glasses (BMGs), nanocrystals within the amorphous matrix have been frequently and intentionally added. Similarly, to reduce the brittle problem of TFMGs, the MgCuZr TFMGs, with a positive mixing heat between Mg and Zr, are fabricated via co-sputtering, in an attempt to separate the pure Mg nano-particles from the amorphous ZrCu matrix. The nanocrystalline Mg particles are expected to hinder the propagation of shear bands and to affect the mechanical characteristics of TFMGs. The structures of sputtered MgCuZr thin films are found to depend on the composition of Mg. For MgCuZr thin films with Mg content from 48 to 73 at%, the structure is the amorphous matrix with discontinuous Mg particles, and the Mg particle size in these TGMGs is all about 20-50 nm, as measured by TEM. The effects of different microstructures of Mg on mechanical response are investigated and discussed.
From the nanoindentation load-displacement curves, the Mg-based metallic glass composites exhibit smoother nature. It implies that the Mg nano-particle can stop the propagation of shear bands under nanoindentation loading. Meanwhile, the microcompression stress-strain curves also show that the Mg-based metallic glasses composites with Mg contents greater than 65% exhibit smoother and more ductile behavior. In addition, due to separation of Mg particles the Mg-based thin films composites and micropillars possess rather high modulus ~80 GPa and yield stress ~1.5 GPa.
Finally, the nano-tension behavior of various pure metals and TFMGs are explored. The results are discussed and compared with those obtained from the micro-compression. There is tremendous room for future research along this line.