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

TL;DR

This study investigates the complex phase transformations and microstructure evolution in Fe-7%Mn alloy under dynamic high-pressure and high-strain-rate conditions using a novel experimental setup, revealing new insights into strain- and stress-induced phase kinetics.

Contribution

It provides the first experimental analysis of coupled strain- and stress-induced phase transformations in Fe-Mn alloy under dynamic conditions, challenging existing theories.

Findings

Phase transformation initiates at different pressures under hydrostatic and plastic loading.

Strain rate influences phase volume fraction and transformation kinetics.

Microstructural parameters remain steady across various conditions.

Abstract

The first experiments in dynamic rotational diamond anvil cell (dRDAC) on severe plastic deformation (SPD) and BCC<->HCP phase transformation (PT) at pressure up to 27.6 GPa, rotation rates up to 1,500 RPM, and strain rates up to 2,094 /s are performed considering Fe-%7Mn alloy as an example. The BCC-HCP PT initiates at 11.4 GPa under hydrostatic loading while it is 3.2 GPa under plastic compression. Strong effect of plastic straining leads to unique kinetics with simultaneous direct and reverse PTs, not studied for any material. For quasi-static loading, parameters in the kinetics for the strain-induced direct-reverse PTs and stationary volume fraction versus pressure are found. During torsion with 1,000 and 1,500 RPM, volume fraction of the HCP phase does not change. After torsion stops, it increases by 30% within a few minutes after 1,000 RPM and HCP phase disappears after 1,500 RPM. These findings contradict general wisdom that strain-induced PTs occur only during straining, time is not a governing parameter, and kinetics is determined by plastic strain instead of time. Thus, nuclei of the HCP phase are generated during straining at high strain rate, but growth/disappearance occur under stresses at much longer time scales. Consequently, a new theory of combined strain- and stress-induced PTs is required. The following important rule is revealed: crystallite size of 30 nm, microstrain ~0.004, and dislocation density ~1.1 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 volume fraction of the HCP phase. Obtained results open fundamental research on combined strain- and stress-induced PTs and microstructure evolution under dynamic SPD and high pressure.