Capture Deformation Twinning in Mg during Shock Compression with Ultrafast Synchrotron X-Ray Diffraction

Deformation twinning plays a vital role in accommodating plastic deformation of hexagonal-close-packed (hcp) metals, but its mechanisms are still unsettled under high strain rate shock compression. Here we investigate deformation twinning in shock-compressed Mg as a typical hcp metal with in situ, ultrafast synchrotron x-ray diffraction. Extension twinning occurs upon shock compression along $⟨11\overline{2}0⟩$ and $⟨10\overline{1}0⟩$, but only upon release for loading along $⟨0001⟩$. Such deformation mechanisms are a result of the polarity of deformation twinning, which depends on directionality and relative magnitude of resolved shear stress and may be common for Mg and its alloys in a wide range of strain rates.

Figure 1

Schematic of gas gun loading and subnanosecond x-ray diffraction measurements. (1) gun barrel, (2) sample chamber, (3) scintillator, (4) image intensifier, (5) gated camera(s), (6) polycarbonate sabot, (7) polycarbonate flyer plate, and (8) single-crystal Mg sample. (Top right) Orientation relations between impact LA and crystallographic orientation of single-crystal Mg sample.

Figure 2

Shock compression along $⟨11\overline{2}0⟩$. (a) Measured diffraction pattern 335 ns before impact. (b) Measured diffraction pattern 124 ns after impact showing twinning spots. (c) Simulated diffraction pattern corresponding to (b). The matching font colors of Miller indices refer to ambient (white) and twinning (orange) spots. (d) Schematic showing extension twinning mechanisms for loading along $⟨11\overline{2}0⟩$.

Figure 3

Shock compression along $⟨10\overline{1}0⟩$. (a) Measured diffraction pattern 221 ns before impact. (b) Measured diffraction pattern 85 ns after impact showing twinning spots. (c) Simulated diffraction pattern corresponding to (b). The matching font colors of Miller indices refer to ambient (white) and twinning (orange) spots. (d) Schematic showing extension twinning mechanisms for loading along $⟨10\overline{1}0⟩$.

Figure 4

Possible extension twinning systems for loading along (a) $⟨11\overline{2}0⟩$ and (b) $⟨10\overline{1}0⟩$. Colored and dotted planes are symmetrical with respect to loading direction (red arrows). Only group II twinning systems are activated for both loading directions. See Fig. S6 for complete equivalent twinning systems [24].