Cooperative Shear in Bulk Metallic Glass Composites Containing Metastable $\beta$-Ti Dendrites

We report a novel plastic deformation mechanism of bulk metallic glass composites (BMGCs) containing metastable $\beta \text{−}\mathrm{Ti}$ dendrites. Plastic deformation of the BMGCs beyond the ultimate tensile strength is mediated by cooperative shear events, which comprise a shear band in the glassy matrix and a continuous $\omega \text{−}\mathrm{Ti}$ band with a thickness of $\sim 10\mathrm{nm}$ in the $\beta \text{−}\mathrm{Ti}$ dendrite. The cooperative shear leads to serrated shear avalanches. The formation of narrow $\omega \text{−}\mathrm{Ti}$ bands is caused by high local strain rates during the cooperative shear. The cooperative shear mechanism enriches the deformation mechanisms of BMGCs and also deepens the understanding of $\omega \text{−}\mathrm{Ti}$ formation.

Figure 1

(a) SEM micrograph and (b) TEM micrograph of the as-cast $\varphi 14\mathrm{mm}$ ${\mathrm{Ti}}_{45.7}{\mathrm{Zr}}_{33}{\mathrm{Cu}}_{5.8}{\mathrm{Co}}_3{\mathrm{Be}}_{12.5}$ BMGC rod, with insets of the XRD pattern of the BMGC and the SAED pattern from the crystal, respectively. (c) Engineering stress-strain curves under tension with an inset showing the plots of stress and strain vs time.

Figure 2

(a) TEM micrograph and SEM micrograph (left inset) of the tension-fractured ${\mathrm{Ti}}_{45.7}{\mathrm{Zr}}_{33}{\mathrm{Cu}}_{5.8}{\mathrm{Co}}_3{\mathrm{Be}}_{12.5}$ BMGC. The right inset in (a) is an SAED pattern from the circle region. (b) HRTEM image from the squared region in (a). (c) and (d) are FFT images from region $c$ and region $d$, respectively, in (b).

Figure 3

(a) Plot of the cumulative distribution of stress drops $P(>\mathrm{\Delta }\sigma )$ vs stress drop magnitude $\mathrm{\Delta }\sigma$. (b) Shear strain rate $\dot{\gamma }$ as a function of time.

Figure 4

(a) Transition of $\beta \text{−}\mathrm{Ti}$ to $\omega \text{−}\mathrm{Ti}$ via the dissociation of a $\mathbf{b}=1/2{⟨\overline{1}11⟩}_{\beta }$ dislocation into three partials: ${\mathbf{b}}_1={\mathbf{b}}_3=1/12{⟨\overline{1}11⟩}_{\beta }$ and ${\mathbf{b}}_2=1/3{⟨\overline{1}11⟩}_{\beta }$, gliding on three consecutive ${\{\overline{1}1\overline{2}\}}_{\beta }$ planes. (b) Glide of a perfect $\mathbf{b}=1/2{⟨\overline{1}11⟩}_{\beta }$ dislocation further contributes to the shear strain. (c)–(e) Schematic views of the plastic deformation stages of the ${\mathrm{Ti}}_{45.7}{\mathrm{Zr}}_{33}{\mathrm{Cu}}_{5.8}{\mathrm{Co}}_3{\mathrm{Be}}_{12.5}$ BMGC with increasing tensile strain.