Effect Of Ceramic Particles And Nano-crystallines On The Strength And Plasticity Of Zr-Cu Based Bulk Metallic Glasses

Bulk metallic glasses (BMGs) have attracted considerable attention over the past decade due to their scientific significance and potential engineering applications. Unfortunately, monolithic BMGs exhibit only limited macroscopic plastic flow due to the formation of highly localized shear bands before the catastrophic failure. Efforts have been taken to improve the plasticity of BMGs. One effective way is to develop bulk metallic glass matrix composites, including introducing a reinforcement phase into the melt through either ex-situ or in-situ methods. The methods for synthesis of the BMG matrix composites can be roughly divided into two categories according to the means of the reinforcing phases introduced, i.e., ex-situ and in-situ methods. In the former, the reinforcement, either in the form of particle or fiber, was priorly mechanically blended with the matrix material before the final casting process. In the latter, however, the reinforcement is produced through reaction synthesis or precipitation from the melt. For instance, Kato et al. produced a ZrC reinforced glassy composite by adding the elemental graphite and Zr in a Zr₅₅Cu₃₀Al₁₀Ni₅ bulk glass through an in-situ reaction. The strength and ductility were reported to have increased to 2060MPa and 4.5%, respectively, when the ZrC content was 15%. Besides, nanocrystallites produced both through annealing or deformation are considered rather idealized in improving the plasticity of the amorphous alloys by retarding the sudden propagation of localized shear bands and generating multiple shear bands. However, a high degree of crystallization could lead to embrittlement.It is well known that the wettability of a second phase by a liquid metal is of practical importance in the fabrication of metal matrix composites using a liquid casting route. The mechanical properties of the composites depend not only on the properties of the matrix and the reinforcement but also on the interfacial microstructure. An optimized interfacial microstructure is essential to improving the mechanical properties.Nevertheless, to our knowledge, only limited work has been done on the wetting behavior between the molten BMG alloys and their reinforcements in the fabrication of the glass metallic matrix composites. In this study, the wetting behavior in the reactive Zr₅₅Cu₃₀Al₁₀Ni₅ alloy/TiC system was examined in order to provide a guidance for the fabrication of the composite. Then, the Zr-based glassy composites reinforced with the in-situ ZrC particles produced through the reaction between Zr and TiC were prepared. The microstructures and the mechanical properties of the composites were investigated. Moreover, we report an ZrCuAlNi BMG, which exhibits superior ductility (～25%) in our experiments at room temperature. However, the outstanding plasticity results from nanocrystallization during the compressive deformation process, which restrains the propagation of localized shear bands and branches the nucleation ones.(1) The wettability of TiC by the molten Zr55Al10Ni5Cu30 alloy is quite good, favoring the composite preparation using a liquid casting route. On the other hand, a (Zr,Ti)C reaction layer forms at the interface. The Zr-based BMG composites reinforced by the in-situ produced ZrC particles up to 19vol.% were successfully fabricated. The ZrC particles considerably enhance not only the fracture strength but also the plasticity. The compressive fracture strength increases from 1680 MPa in the monolithic glass to 2078MPa in the 19vol.% ZrC glassy composite, the plastic strain from 0 to～2%, and the hardness increased from 500Hv to 590Hv.(2) The excellent wettability of SiC by the molten alloy makes the infiltration process of the matrix alloy into the particles in synthesizing composites much easier. An evident TiC reaction layer formed between the surface of SiC and the Cu₄₇Ti₃₄Zr₁₁Ni₈ alloy, and the interfacial reaction benefits the synthesis of the in-situ composites. Furthermore, the good wettability and the absence of significant interfacial reaction between the Cu47Ti33Zr11Ni8 alloy and TiC enable the fabrication of the TiC reinforced composites. The Cu-based BMGMCs reinforced by the in-situ TiC particles were successfully fabricated. The TiC particles considerably enhance not only the fracture strength but also the plasticity. The compressive fracture strength increases from 2000 MPa in the monolithic glass to 2159 MPa in the 5.6vol.% TiC glassy composite, the plastic strain from 0 to 2.5%, and and the hardness increased from 566Hv to 648Hv. However, the ex-situ TiC reinforced Cu-based BMGMCs show little improvement of the compression properties because of the aggregation of the particles.(3) Good ductility was achieved in the Zr₆₅Cu₁₅Al₁₀Ni₁₀ BMG alloy by deformation-induced nanocrystallization during compression at room temperature. The nanocrystallites restrict the rapid propagation of the local shear band and promote the generation of multiple shear bands, leading to the enhanced plasticity.