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1. The Effect of Secondary‐Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates: 2. Mechanical Behavior

   https://doi.org/10.1029/2022JB025724  

   Wiesman, Harison S.; Zimmerman, Mark E.; Kohlstedt, David L. (April 2023, Journal of Geophysical Research: Solid Earth)

   Abstract To study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (f_Per) between 0.1 and 0.8. Each sample was deformed in torsion atT = 1523 K,P = 300 MPa at a constant strain rate to a final shear strain ofγ = 6 to 7. The stress‐strain data and calculated values of the stress exponent,n, indicate that Ol in our samples deformed by dislocation‐accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains ofγ < 1, the strengths of samples withf_Per > 0.5 match model predictions for both phases deforming at the same stress, the lower‐strength bound for two‐phase materials, while the strengths of samples withf_Per < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper‐strength bound. These observations suggest a transition from a weak‐phase supported to a strong‐phase supported regime with decreasingf_Per. Aboveγ = 4, however, the strength of all two‐phase samples is greater than those predicted by either the uniform‐stress or the uniform‐strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two‐phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries. 

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2. Texture Development and Stress–Strain Partitioning in Periclase + Halite Aggregates

   https://doi.org/10.3390/min9110679  

   Lin, F.; Giannetta, M.; Jugle, M.; Couper, S.; Dunleavy, B.; Miyagi, L. (November 2019, Minerals)

   Multiphase materials are widely applied in engineering due to desirable mechanical properties and are of interest to geoscience as rocks are multiphase. High-pressure mechanical behavior is important for understanding the deep Earth where rocks deform at extreme pressure and temperature. In order to systematically study the underlying physics of multiphase deformation at high pressure, we perform diamond anvil cell deformation experiments on MgO + NaCl aggregates with varying phase proportions. Lattice strain and texture evolution are recorded using in-situ synchrotron x-ray diffraction and are modeled using two-phase elasto-viscoplastic self-consistent (EVPSC) simulations to deduce stress, strain, and deformation mechanisms in individual phases and the aggregate. Texture development of MgO and NaCl are affected by phase proportions. In NaCl, a (100) compression texture is observed when small amounts of MgO are present. In contrast, when deformed as a single phase or when large amounts of MgO are present, NaCl develops a (110) texture. Stress and strain evolution in MgO and NaCl also show different trends with varying phase proportions. Based on the results from this study, we construct a general scheme of stress evolution as a function of phase proportion for individual phases and the aggregate. 

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3. Simulations of multivariant Si I to Si II phase transformation in polycrystalline silicon with finite-strain scale-free phase-field approach

   https://doi.org/10.1016/j.actamat.2023.118996  

   Babaei, H; Pratoori, R; Levitas, V.I. (May 2023, Acta Materialia)

   Scale-free phase-field approach and corresponding finite element method simulations for multivariant martensitic phase transformation from cubic Si I to tetragonal Si II in a polycrystalline aggregate are presented. Important features of the model are large and very anisotropic transformation strain tensor 𝜺𝑡 = {0.1753; 0.1753; −0.447} and stress-tensor dependent athermal dissipative threshold for transformation, which produce essential challenges for computations. 3D polycrystals with stochastically oriented grains are subjected to uniaxial strain- and stress-controlled loadings under periodic boundary conditions and zero averaged lateral strains. Coupled evolution of discrete martensitic microstructure, volume fractions of martensitic variants and Si II, stress and transformation strain tensors, and texture are presented and analyzed. Macroscopic variables effectively representing multivariant transformational behavior are introduced. Macroscopic stress–strain and transformational behavior for 55 and 910 grains are close. Large transformation strains and grain boundaries lead to huge internal stresses of tens GPa, which affect microstructure evolution and macroscopic behavior. In contrast to a single crystal, the local mechanical instabilities due to phase transformation and negative local tangent modulus are stabilized at the macroscale by arresting/slowing the growth of Si II regions by the grain boundaries. This leads to increasing stress during transformation. The developed methodology can be used for studying similar phase transformations with large transformation strains and for further development by including plastic strain and strain-induced transformations. 

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4. Soft elasticity optimises dissipation in 3D-printed liquid crystal elastomers

   https://doi.org/10.1038/s41467-021-27013-0  

   Mistry, D.; Traugutt, N. A.; Sanborn, B.; Volpe, R. H.; Chatham, L. S.; Zhou, R.; Song, B.; Yu, K.; Long, K. N.; Yakacki, C. M. (December 2021, Nature Communications)

   Abstract Soft-elasticity in monodomain liquid crystal elastomers (LCEs) is promising for impact-absorbing applications where strain energy is ideally absorbed at constant stress. Conventionally, compressive and impact studies on LCEs have not been performed given the notorious difficulty synthesizing sufficiently large monodomain devices. Here, we use direct-ink writing 3D printing to fabricate bulk (>cm 3 ) monodomain LCE devices and study their compressive soft-elasticity over 8 decades of strain rate. At quasi-static rates, the monodomain soft-elastic LCE dissipated 45% of strain energy while comparator materials dissipated less than 20%. At strain rates up to 3000 s −1 , our soft-elastic monodomain LCE consistently performed closest to an ideal-impact absorber. Drop testing reveals soft-elasticity as a likely mechanism for effectively reducing the severity of impacts – with soft elastic LCEs offering a Gadd Severity Index 40% lower than a comparable isotropic elastomer. Lastly, we demonstrate tailoring deformation and buckling behavior in monodomain LCEs via the printed director orientation. 

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5. Tensorial stress-plastic strain fields in α - ω Zr mixture, transformation kinetics, and friction in diamond-anvil cell

   https://doi.org/10.1038/s41467-023-41680-1  

   Levitas, Valery I.; Dhar, Achyut; Pandey, K. K. (September 2023, Nature Communications)

   Abstract Various phenomena (phase transformations (PTs), chemical reactions, microstructure evolution, strength, and friction) under high pressures in diamond-anvil cell are strongly affected by fields of stress and plastic strain tensors. However, they could not be measured. Here, we suggest coupled experimental-analytical-computational approaches utilizing synchrotron X-ray diffraction, to solve an inverse problem and find fields of all components of stress and plastic strain tensors and friction rules before, during, and after α-ω PT in strongly plastically predeformed Zr. Results are in good correspondence with each other and experiments. Due to advanced characterization, the minimum pressure for the strain-induced α-ω PT is changed from 1.36 to 2.7 GPa. It is independent of the plastic strain before PT and compression-shear path. The theoretically predicted plastic strain-controlled kinetic equation is verified and quantified. Obtained results open opportunities for developing quantitative high-pressure/stress science, including mechanochemistry, synthesis of new nanostructured materials, geophysics, astrogeology, and tribology. 

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