[Paper Review] Rules of plastic strain-induced phase transformations and nanostructure evolution under high-pressure and severe plastic flow

Lin Feng, Valery I. Levitas+3 more|arXiv (Cornell University)|May 25, 2023

High-pressure geophysics and materials4 citations

TL;DR

This study reveals that rough diamond anvils (rough-DA) in a diamond anvil cell (DAC) enable strain-induced α→ω phase transformation in pre-deformed zirconium at a record-low pressure of 0.67 GPa by promoting a steady nanostructure with reduced crystallite size and elevated dislocation density. The transformation kinetics unexpectedly depend on time, and the process becomes independent of strain path and plastic strain tensor when normalized by ω-phase volume fraction.

ABSTRACT

Rough diamond anvils (rough-DA) are introduced to intensify all occurring processes during an in-situ study of heterogeneous compression of strongly pre-deformed Zr in diamond anvil cell (DAC). Crystallite size and dislocation density of Zr are getting pressure-, plastic strain tensor- and strain-path-independent during α-ω phase transformation (PT) and depend solely on the volume fraction of ω-Zr. Rough-DA produce a steady nanostructure in α-Zr with lower crystallite size and larger dislocation density than smooth-DA, leading to a two-time reduction in a minimum pressure for α-ω PT to a record value 0.67 GPa. The kinetics of strain-induced PT unexpectedly depends on time.

Motivation & Objective

• To investigate the role of rough diamond anvils (rough-DA) in enhancing plastic strain-induced phase transformations under high pressure.
• To determine how nanostructure evolution—specifically crystallite size and dislocation density—depends on pressure and strain during α→ω phase transformation in zirconium.
• To identify whether the transformation kinetics are influenced by time, strain path, or plastic strain tensor.
• To establish the conditions under which phase transformation becomes independent of processing history and depends solely on the volume fraction of the ω-phase.
• To achieve a record reduction in the minimum pressure required for α→ω phase transformation in zirconium using in-situ DAC experiments.

Proposed method

• Employed in-situ high-pressure experiments in a diamond anvil cell (DAC) with both smooth and rough diamond anvils (smooth-DA and rough-DA) to study phase transformations in pre-deformed zirconium.
• Used rough-DA to intensify plastic flow and strain localization, promoting more homogeneous nanostructure evolution during compression.
• Measured crystallite size and dislocation density via X-ray diffraction (XRD) analysis to quantify nanostructural evolution under varying pressure and strain.
• Tracked the volume fraction of ω-phase as a function of pressure and time to assess phase transformation kinetics.
• Compared results between smooth-DA and rough-DA to isolate the effect of anvil surface roughness on phase transformation thresholds and nanostructure stability.
• Analyzed the dependence of phase transformation on strain path and plastic strain tensor, finding independence when normalized by ω-phase volume fraction.

Experimental results

Research questions

• RQ1 How does the use of rough diamond anvils (rough-DA) affect the minimum pressure required for α→ω phase transformation in zirconium?
• RQ2 To what extent do crystallite size and dislocation density in α-Zr depend on pressure, plastic strain tensor, and strain path during phase transformation?
• RQ3 Does the kinetics of strain-induced phase transformation exhibit time dependence under high-pressure and severe plastic flow?
• RQ4 Can the phase transformation be decoupled from strain path and plastic strain tensor by normalizing with the volume fraction of ω-phase?
• RQ5 What is the role of nanostructure evolution (crystallite size and dislocation density) in enabling lower transformation pressures?

Key findings

• The minimum pressure for α→ω phase transformation in zirconium was reduced to 0.67 GPa using rough-DA, a record-low value.
• Rough-DA produced a steady nanostructure in α-Zr with smaller crystallite size and higher dislocation density compared to smooth-DA.
• Crystallite size and dislocation density became independent of pressure, plastic strain tensor, and strain path when normalized by the volume fraction of ω-phase.
• The kinetics of strain-induced phase transformation were found to depend on time, contrary to expected instantaneous behavior.
• The transformation process became fully governed by the volume fraction of ω-phase, indicating a universal scaling behavior.
• The study demonstrates that surface roughness of diamond anvils is a critical factor in enabling low-pressure phase transformations through enhanced strain localization and nanostructure control.