DAC and dynamic RDAC with rough diamonds: new tools for discovering rules of severe plastic flow, strain-induced phase transformations, and microstructure evolution

To increase contact friction, we introduced rough diamond anvils (rough-DA) with increased asperities and implemented them for compression in diamond anvil cell (DAC) and torsion in dynamic rotational DAC (dRDAC). With rough-DA, we drastically intensify the plastic flow, microstructure evolution, and strain-induced phase transformations (PTs) and find various new phenomena. Maximum friction shear stress equal to the yield strength in shear is achieved. This allows determination of the pressure-dependence of the yield strength for ω-Zr and proves that ω-Zr behaves like perfectly plastic, isotropic, and strain path-independent immediately after PT. With traditional smooth anvils, friction stress is ~2 times smaller than the yield strength in shear, and significant contact sliding occurs. Multiple steady microstructures independent of pressure, plastic strain, and strain path are reached. Record minimum pressure for α-ω PT was identified. The kinetics of strain-induced PT depends not only on the plastic strain but also on time. Crystallite size and dislocation density in ω-Zr during PT depend solely on the volume fraction of ω-Zr. With rough-DA, the dislocation density in α-Zr is ~2 times larger, and the crystallite size and minimum pressure for α-ω PT are ~2 times smaller than with smooth anvils. Important results have been obtained on PTs olivine-spinel and multiple PTs in Si under compression and torsion in dRDAC.