Coupled plastic strain- and stress-induced phase transformations and microstructure evolution in Fe-7%Mn alloy in a dynamic rotational diamond anvil cell

Abstract

Studies of severe plastic deformation (SPD), microstructure evolution, and plastic strain-induced phase transformations (PTs) are crucial for various fundamental and applied disciplines and phenomena. However, they are performed either quasi-statically or at low strain rates and pressures. Here, in situ experiments in a dynamic rotational diamond anvil cell (dRDAC) on SPD and BCC<->HCP PTs at pressures up to 27.6 GPa, rotation rates up to 1,500 RPM, and strain rates up to 2,299/s are performed, considering Fe-7%Mn alloy as an example. Strong plastic straining leads to a unique mechanism and kinetics with simultaneous direct and reverse PTs, which has not been studied for any material. For quasi-static loading, we determine the kinetic parameters for strain-induced direct-reverse PTs and the stationary volume fraction $c$ versus pressure. During torsion at 1,000 and 1,500 RPM, $c$ does not change. After torsion stops, 51 minutes later, it increases by 31% at 1,000 RPM and $c$->0, 7 minutes later at 1,500 RPM. These findings contradict the general wisdom that strain-induced PTs occur only during straining and are governed by strain, independent of time; they reveal an alternative mechanism. It is revealed that the crystallite size of ~28(6) nm, microstrain ~0.0035(8), and dislocation density ~1.3(6)x$10^{15}$/$m^2$ in the HCP phase are steady during static compression and dynamic torsion, during and after the PT, and after torsion. These parameters are independent of pressure, plastic strain tensor, its path, strain rates, and $c$. The results obtained open fundamental research on combined strain- and stress-induced PTs and microstructure evolution under dynamic SPD and high pressure, with various important applications.