||BMGs (bulk metallic glasses) exhibit many exceptional advantages for engineering applications, such as high strength, good corrosion resistance, etc. Despite of having these excellent properties, the brittle nature of metallic glasses in the bulk and thin film forms inevitably imposes limitation and restricts the wide application of BMGs and TFMGs. Composite concept might be another idea to solve this dilemma. In order to manufacture the bulk metallic glass composites (BMGCs), the approaches are classified into two categories: the intrinsic and extrinsic methods. For the intrinsic method, the in situ process and heat treatment process are two kinds of ways in common uses. Adding reinforcements into the BMGs or TFMGs is extensively used to manufacture composites in the extrinsic method. |
In this study, the deformation behaviors of multilayer (amorphous/nanocrystalline) micropillars are studied by uniaxial microcompression tests at room temperature. The nanocrystalline layer to be coupled with the amorphous layer can be of either face-centered cubic (FCC), hexagonal close-packed (HCP) or body-centered cubic (BCC) in crystal structure. The current study demonstrates that brittle problem of a metallic glass coating can be alleviated by percolating with a nanocrystalline metallic underlayer. The brittle thin film metallic glass can become highly ductile and exhibit a plastic strain over 50% at room temperature. The present study has an important implication for MEMS applications, namely, the life span of a brittle amorphous layer can be significantly improved by using an appropriate metallic underlayer.
The brittle problem of thin film ZrCu metallic glasses was also treated by invoking soft Cu layers with optimum film layer thickness. Such multilayered amorphous/crystalline samples exhibit superplastic-like homogeneous deformation at room temperature. It is found that the deformability of the resultant micropillars depends on the thickness of Cu layers. Microstructural observations and theoretical analysis suggest that the superplastic-like deformation mode is attributed to homogeneous co-deformation of amorphous ZrCu and nanocrystalline Cu layers because the 100 nm-thick Cu layers can provide compatible flow stress and “plastic zone” size well matched with those of ZrCu amorphous layers.
Besides, we also made attempts to investigate the critical sample size below which shear band localization would disappear and the sample can deform homogeneously. In situ TEM compression was conducted on amorphous ZrCu nanopillars to study shear band formation behavior. The nanopillar is 140 nm in diameter and with a taper angle of 3°. Experimental observations and simulations based on a free-volume model both demonstrate that the deformation was localized near the top of the tapered metallic glass pillar.
Eventually, the interface nature of metallic glass amorphous/crystalline was characterized through evaluating its energy and validated by the mechanical response of micropillar with ~45o inclined interface under compression. The calculated results showed that the ZrCu/Zr interface energy resides several joules per meter square, meaning that the Zr/ZrCu interface is inherently strong. The high strong adhesion ability of ZrCu/Zr interface was further confirmed by shear fracture happening rightly within the Zr layers rather than along the interface when compressing the ZrCu/Zr micropillars with 45o inclined interface.