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Study of the plastic flow, strain-induced phase transformations (PTs), and nanostructure evolution under high pressure is important for producing new nanostructured phases and understanding physical processes. However, these processes depend on an unlimited combination of five plastic strain components and an entire strain path with no hope of fully comprehending. Here, we introduce the rough diamond anvils (rough-DA) to reach maximum friction equal to the yield strength in shear, which allows determination of pressure-dependent yield strength. We apply rough-DA to compression of severely pre-deformed Zr. We found in situ that after severe straining, crystallite size and dislocation density of α and ω-Zr are getting pressure-, strain- and strain-path-independent, reach steady values before and after PT, and depend solely on the volume fraction of ω-Zr during PT. Immediately after completing PT, ω-Zr behaves like perfectly plastic, isotropic, and strain-path-independent. Rough-DA produces a steady nanostructure in α-Zr with lower crystallite size and larger dislocation density than smooth diamonds. This leads to a record minimum pressure (0.67 GPa) for α-ω PT. Kinetics of strain-induced PT, in addition to plastic strain, unexpectedly depends on time. The obtained results significantly enrich the fundamental understanding of plasticity, PTs, and nanostructure, and create new opportunities in material design, synthesis, and processing of nanostructured materials by coupling severe plastic deformations and PT at low pressure.
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Rules for the crystallite size and dislocation density evolution in phases during α-ω transformation in Zr under high-pressure and severe plastic flow
The first in-situ X-ray diffraction (XRD) study of the evolution of the crystallite size and dislocation density in phases during plastic strain-induced phase transformation (PT) is performed utilizing α-ω PT in strongly pre-deformed commercially pure Zr as an important example. Rough diamond anvils (rough-DA) are introduced to intensify all occurring processes during heterogeneous compression of Zr in a diamond anvil cell (DAC). The main rule is found that during α-ω PT, the crystallite size and dislocation density in ω-Zr depend solely on the volume fraction of ω-Zr and are independent of pressure, plastic strain tensor, its path, and initial nanostructure. Crystallite size in ω-Zr increases from 10 to 60 nm during the PT, while dislocation density reduces from 1.83 ×1015/m^2 to 0.6 ×1015/m^2. Rough-DA produce a steady nanostructure in α-Zr before PT with smaller crystallite size and larger dislocation density than smooth-DA, leading to a reduction of the minimum pressure for α-ω PT to a record value 0.67 GPa, 9 times smaller than under hydrostatic loading and 5.1 times lower than the phase equilibrium pressure. In addition to strain, the kinetics of strain-induced PT unexpectedly depends on time. Also, strain-controlled part of kinetics is zero order, in contrast to the first-order kinetics with smooth-DA. The obtained results open a new window for understanding the mutual effects of nanostructure evolution and PT during severe plastic flow in various technological and natural processes. They may bring up economic strategies of producing nanocomposites and single-phase nanostructured materials with optimal properties.
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- PAR ID:
- 10696665
- Publisher / Repository:
- Acta Materialia Inc.
- Date Published:
- Journal Name:
- Acta Materialia
- Volume:
- 295
- Issue:
- C
- ISSN:
- 1359-6454
- Page Range / eLocation ID:
- 121151
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.actamat.2025.121151
