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1. Effect of initial microstructure on its evolution and α→ω phase transition in Zr under hydrostatic loading.

   Pandey, K.K. ; Levitas, Valery I. ; Park, C ; Shen, G ( February 2024 , arXivorg)

   The first study of the effect of the initial microstructure on its evolution under hydrostatic compression before, during, and after the irreversible α → ω phase transformation and during pressure release in Zr using in situ x-ray diffraction is presented. Two samples were studied: one is plastically pre-deformed Zr with saturated hardness and the other is annealed. Phase transformation α → ω initiates at lower pressure for pre-deformed sample but above volume fraction of ω Zr c = 0.7, larger volume fraction is observed for the annealed sample. This implies that the general theory based on the proportionality between the athermal resistance to the transformation and the yield strength must be essentially advanced. The crystal domain size significantly reduces, and microstrain and dislocation density increase during loading for both α and ω phases in their single-phase regions. For the α phase, domain sizes are much smaller for prestrained Zr, while microstrain and dislocation densities are much higher. For the cold-rolled sample at 5.9 GPa (just before initiation of transformation), domain size in α Zr decreased to ∼ 45 nm and dislocation density increased to 1.1 × 1015 lines/m2 , values similar to those after severe plastic deformation under high pressure. Despite the generally accepted concept that hydrostatic pressure does not cause plastic straining, it does and is estimated. During transformation, the first rule was found: The average domain size, microstrain, and dislocation density in ω Zr for c < 0.8 are functions of the volume fraction of ω Zr only, which are independent of the plastic strain tensor prior to transformation and pressure. The microstructure is not inherited during phase transformation. Surprisingly, for the annealed sample, the final dislocation density and average microstrain after pressure release in the ω phase are larger than for the severely pre-deformed sample. The significant evolution of the microstructure and its effect on phase transformation demonstrates that their postmortem evaluation does not represent the actual conditions during loading. A simple model for the initiation of the phase transformation involving microstrain is suggested. The results suggest that an extended experimental basis is required for the predictive models for the combined pressure-induced phase transformations and microstructure evolutions. 

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2. Laws of high-pressure phase and nanostructure evolution and severe plastic flow

   Levitas, Valery ; Lin, Feng ; Pandey, Krishan ; Yesudhas, Sorb ; Park, Changyong ( September 2022 , Research square)

   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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3. Rough diamond anvils: Steady microstructure, yield surface, and transformation kinetics in Zr

   Lin, Feng ; Levitas, Valery I. ; Pandey, K. K. ; Yesudhas, Sorb ; Park, Changyong. ( August 2022 , ArXivorg)

   Study of the plastic flow and strain-induced phase transformations (PTs) under high pressure with diamond anvils is important for material and geophysics. We introduce rough diamond anvils and apply them to Zr, which drastically change the plastic flow, microstructure, and PTs. Multiple steady microstructures independent of pressure, plastic strain, and strain path are reached. Maximum friction equal to the yield strength in shear is achieved. This allows determination of the pressure-dependence of the yield strength and proves that ω-Zr behaves like perfectly plastic, isotropic, and strain path-independent immediately after PT. Record minimum pressure for α-ω PT was identified. Kinetics of strain-induced PT depends on plastic strain and time. Crystallite size and dislocation density in ω-Zr during PT depend solely on the volume fraction of ω-Zr. 

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4. Recent In Situ Experimental and Theoretical Advances in Severe Plastic Deformations, Strain-Induced Phase Transformations, and Microstructure Evolution under High Pressure

   https://doi.org/10.2320/matertrans.MT-MF2022055  

   Levitas, Valery I. ( January 2023 , MATERIALS TRANSACTIONS)

   Severe plastic deformations (SPD) under high pressure, mostly by high-pressure torsion, are employed for producing nanostructured materials and stable or metastable high-pressure phases. However, they were studied postmortem after pressure release. Here, we review recent in situ experimental and theoretical studies of coupled SPD, strain-induced phase transformations (PTs), and microstructure evolution under high pressure obtained under compression in diamond anvil cell or compression and torsion in rotational diamond anvil cell. The utilization of x-ray diffraction with synchrotron radiation allows one to determine the radial distribution of volume fraction of phases, pressure, dislocation density, and crystallite size in each phase and the main laws of their evolution and interaction. Coupling with the finite element simulations of the sample behavior allows the determination of fields of all components of the stress and plastic strain tensors and volume fraction of high-pressure phase and provides a better understanding of ways to control occurring processes. Atomistic, nanoscale and scale-free phase-field simulations allow elucidation of the main physical mechanisms of the plastic strain-induced drastic reduction in phase transformation pressure (by one to two orders of magnitude), the appearance of new phases, and strain-controlled PT kinetics in comparison with hydrostatic loading. Combining in situ experiments with multiscale theory potentially leads to the formulation of methods to control strain-induced PT and microstructure evolution and designing economic synthetic paths for the defect-induced synthesis of desired high-pressure phases, nanostructures, and nanocomposites. 

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5. Rules of plastic strain-induced phase transformations and nanostructure evolution under high-pressure and severe plastic flow.

   Lin, Feng ; Levitas, Valery I. ; Pandey, K.K. ; Yesudhas, Sorb ; Park, C ( May 2023 , arXivorg)

   Severe plastic deformations under high pressure are used to produce nanostructured materials but were studied ex-situ. We introduce rough diamond anvils to reach maximum friction equal to yield strength in shear and perform the first in-situ study of the evolution of the pressure-dependent yield strength and nanostructural parameters for severely pre-deformed Zr. ω-Zr behaves like perfectly plastic, isotropic, and strain-path-independent. This is related to reaching steady values of the crystallite size and dislocation density, which are pressure-, strain- and strain-path-independent. However, steady states for α-Zr obtained with smooth and rough anvils are different, which causes major challenge in plasticity theory. 

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