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1. Athermal resistance to phase interface motion due to precipitates: A phase field study

   Javanbakht, Mahdi; Levitas, Valery I. (June 2022, ArXivorg)

   Athermal resistance to the motion of a phase interface due to a precipitate is investigated. The coupled phase field and elasticity equations are solved for the phase transformation (PT). The volumetric misfit strain due to the precipitate is included using the error and rectangular functions. Due to the presence of precipitates, the critical thermal driving forces remarkably differ between the direct and reverse PTs, resulting in a hysteresis behavior. For the precipitate radius small compared to the interface width, the misfit strain does not practically show any effect on the critical thermal driving force. Also, the critical thermal driving force value nonlinearly increases vs. the precipitate concentration for both the direct and reverse PTs. Change in the precipitate surface energy significantly changes the PT morphology and the critical thermal driving forces. The critical thermal driving force shows dependence on the misfit strain for large precipitate sizes compared to the interface width. For both the constant surface energy (CSE) and variable surface energy (VSE) boundary conditions (BCs) at the precipitate surface, the critical thermal driving force linearly increases vs. the misfit strain coefficient for the direct PT while it is almost independent of it for the reverse PT. For larger precipitates, the critical thermal driving force nonlinearly increases vs. the precipitate concentration for the direct PT. For the reverse PT, its value for the CSE BCs linearly increases vs. the precipitate concentration while it is independent of the precipitate concentration for the VSE BCs. Also, for any concentration, the VSE BCs result in higher thermal critical driving forces, a smaller hysteresis range, and a larger transformation rate. The critical microstructure and thermal driving forces are validated using the thermodynamic phase equilibrium condition for stationary interfaces. 

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2. Unexpected plastic strain-induced phase transformation phenomena in silicon

   https://doi.org/10.21203/rs.3.rs-4014429/v1  

   Yesudhas, Sorb; Levitas, Valery I.; Lin, Feng; Pandey, K.K.; Smith, Jesse (March 2024, Research square)

   Pressure-induced phase transformations (PTs) in Si, the most important electronic material, have been broadly studied. However, strain-induced PTs in Si were never studied in situ. Here, we revealed in situ various important plastic strain-induced PT phenomena. A correlation between the particle size's direct and inverse Hall-Petch effect on yield strength and pressure for strain-induced PT is found. For 100 nm particles, strain-induced PT Si-I³Si-II initiates at 0.3 GPa versus 16.2 GPa under hydrostatic conditions; Si-I³Si-III PT starts at 0.6 GPa and does not occur under hydrostatic pressure. Pressure in small Si-II and Si-III regions is ~5-7 GPa higher than in Si-I. Retaining Si-II and single-phase Si-III at ambient pressure and obtaining reverse Si-II³Si-I PT demonstrates the possibilities of manipulating different synthetic paths. The obtained results corroborate the elaborated dislocation pileup-based mechanism and have numerous applications for developing economic defect-induced synthesis of nanostructured materials, surface treatment (polishing, turning, etc.), and friction. 

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3. Plastic strain-induced phase transformations in silicon: drastic reduction of transformation pressures, change in transformation sequence, and particle size effect

   Yesudhas, Sorb; Levitas, Valery I.; Lin, Feng; Pandey, K. K.; Smith, Jesse (March 2023, arXivorg)

   Pressure-induced phase transformations (PTs) between numerous phases of Si, the most important electronic material, have been studied for decades. This is not the case for plastic strain-induced PTs. Here, we revealed in-situ various unexpected plastic strain-induced PT phenomena. Thus, for 100 nm Si, strain-induced PT Si-I to Si-II (and Si-I to Si-III) initiates at 0.4 GPa (0.6 GPa) versus 16.2 GPa (∞, since it does not occur) under hydrostatic conditions; for 30 nm Si, it is 6.1 GPa versus ∞. The predicted theoretical correlation between the direct and inverse Hall-Petch effect of the grain size on the yield strength and the minimum pressure for strain-induced PT is confirmed for the appearance of Si-II. Retaining Si-II at ambient pressure and obtaining reverse Si-II to Si-I PT are achieved, demonstrating the possibilities of manipulating different synthetic paths. 

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4. Investigating the Precipitation Kinetics and Hardening Effects of γ” in Inconel 625 Using a Combination of Meso-Scale Phase-Field Simulations and Macro-Scale Precipitate Strengthening Calculations

   https://doi.org/10.1115/IMECE2020-23328  

   Yenusah, Caleb O.; Ji, Yanzhou; Liu, Yucheng; Stone, Tonya W.; Horstemeyer, Mark F.; Chen, Lei (November 2020, Proceedings of the ASME 2020 International Mechanical Engineering Congress and Exposition)

   null (Ed.)

   Abstract Precipitation strengthening of alloys by the formation of secondary particles (precipitates) in the matrix is one of the techniques used for increasing the mechanical strength of metals. Understanding the precipitation kinetics such as nucleation, growth, and coarsening of these precipitates is critical for evaluating their hardening effects and improving the yield strength of the alloy during heat treatment. To optimize the heat treatment strategy and accelerate alloy design, predicting precipitate hardening effects via numerical methods is a promising complement to trial-and-error-based experiments and the physics-based phase-field method stands out with the significant potential to accurately predict the precipitate morphology and kinetics. In this study, we present a phase-field model that captures the nucleation, growth, and coarsening kinetics of precipitates during isothermal heat treatment conditions. Thermodynamic data, diffusion coefficients, and misfit strain data from experimental or lower length-scale calculations are used as input parameters for the phase-field model. Classical nucleation theory is implemented to capture the nucleation kinetics. As a case study, we apply the model to investigate γ″ precipitation kinetics in Inconel 625. The simulated mean particle length, aspect ratio, and volume fraction evolution are in agreement with experimental data for simulations at 600 °C and 650 °C during isothermal heat treatment. Utilizing the meso-scale results from the phase-field simulations as input parameters to a macro-scale coherency strengthening model, the evolution of the yield strength during heat treatment was predicted. In a broader context, we believe the current study can provide practical guidance for applying the phase-field approach as a link in the multiscale modeling of material properties. 

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5. Property and microstructure of Ni50.3Ti29.7Hf20 high-temperature shape memory alloys with different aging conditions

   https://doi.org/10.1016/j.actamat.2023.119642  

   Dong, Jiaqi; Demblon, Alexander; Umale, Tejas; Zhao, Dexin; Valentino, Gianna; Karaman, Ibrahim; Xie, Kelvin Y. (February 2024, Acta Materialia)

   NiTiHf is a class of promising high-temperature shape memory alloys (SMAs) that find many applications. However, their complex martensitic microstructure and attendant thermomechanical properties are not well understood. In this work, we used solution-treated (precipitate-free) and aged (precipitate-bearing) Ni50.3Ti29.7Hf20 (at.%) SMAs as a model system. We observed that the presence of precipitates refines the martensite plates, reduces the number of martensite variants, and changes the orientation relationship between the martensite plates compared with the solution-treated counterpart. Furthermore, the aged samples exhibited higher transformation temperatures, narrower phase transformation temperature windows, improved thermal stability, and retained or even improved actuation strain. The improved thermomechanical properties observed in the aged samples are attributed in part to the reduction of the number of martensite variants and the change in martensite and twin interface characteristics, both of which are induced by the presence of precipitates. The findings of this study offer new information on the processing-property-microstructure relationship in NiTiHf-based SMAs. These insights can guide future materials design efforts, facilitating the development of advanced SMAs tailored for specific high-temperature applications. 

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