The rheological behavior of brittle BMGs of Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7 in the supercooled liquid region

Abstract

Bulk metallic glasses (BMGs) show limited plasticity at room temperature. However, BMGs usually exhibit superplasticity and high plastic forming ability in their supercooled liquid region (SLR). The rheological behavior of BMGs in SLR is vitally important to their thermoplastic forming process. In contrast to the ductile BMGs, thermoplastic deformation behavior of brittle BMGs is rarely reported. In present work, the rheological behavior of two brittle BMGs with high glass forming ability Cu_44.25Ag_14.75Zr₃₆Ti₅ and Ti_32.8Zr_30.2Cu₉Ni_5.3Be_22.7 was investigated on Gleeble3500. The two BMGs can deform homogeneously depending on the temperature and strain rate. According to the high value of m (strain rate sensitivity index), which is the most important mechanical characteristic of a superplastic material, the two BMGs show superplasticity in their SLR with m ≥ 0.3. Based on the free volume model, their activation volumes are calculated as 0.263∼0.486 nm³ and 1.261∼1.650 nm³, indicating the minimum displacement clusters with average 26∼48 and 91∼127 atoms, for Cu_44.25Ag_14.75Zr₃₆Ti₅ and Ti_32.8Zr_30.2Cu₉Ni_5.3Be_22.7, respectively. Thus, the two investigated brittle BMGs can be thermoplastic processed in the SLR and the deformation maps are given. BMG Cu_44.25Ag_14.75Zr₃₆Ti₅ shows better machinable property than Ti_32.8Zr_30.2Cu₉Ni_5.3Be_22.7. Compared to the ductile BMGs, no Newtonian flow is found for the two investigated brittle BMGs.