Strength coupling in mixed phases under high pressure

The strength of a material can be altered by temperature, pressure, grain size, and orientation distributions. At the microscale, neighboring grains often play important roles in the elastic and plastic deformation process. By applying high pressure to a mixture of germanium and gold powder in the vicinity of the germanium phase transition pressure, we found that the deformation behavior of gold largely correlates with that of the surrounding germanium. The deviatoric strain and compressibility of Au behaves anomalously when Ge undergoes a diamond to β-tin structure transition, accompanying a large volume and strength drop. The results demonstrate that the intrinsic strength of a mixed phase could be largely controlled by the other surrounding phase, which is fundamentally important in understanding the deformation mechanism of multiphase materials, especially when one phase undergoes dramatic changes in strength under high pressure conditions.

Figure 1

(a) Axial XRD patterns of the Ge-Au mixture under various pressures. (b) A Rietveld refinement on the powder XRD pattern of the Au-Ge mixture at 11.7 GPa. Both Ge-I and Ge-II phases coexist and are fitted simultaneously with Au.

Figure 2

Deviatoric strain of semiconducting diamond phase Ge (a), metallic β-tin phase Ge (b), and fcc Au (c) vs pressure.

Figure 3

(a) A schematic illustration of the setup of XRD experiments. (b) The ratio of the interplanar spacing of different lattice plane for Au vs pressure. The uncertainty is smaller than the symbol size. (c) The IPFs of gold at 11.9 GPa. (ND: normal direction; RD: rolling direction; TD: transverse direction.) The degree of lattice preferred orientation (texture strength) is expressed as multiples of random distribution (m.r.d.), where m.r.d. $=1$ denotes random distribution and a higher m.r.d. number represents stronger texture.

Figure 4

The SEM images of the sample quenched from 10.65 GPa. The white parts are gold grains and the others are germanium grains.