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The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts. [ABSTRACT FROM AUTHOR]

The microstructural variation and high-temperature flow features of a Ti-55511 alloy in the β region are studied by utilizing double-stage compression with a stepped strain rate. The results demonstrate that the stresses in the latter stage of hot compression markedly reduce as the strain at the previous stage or the strain rate at the previous/latter stage drops. Moreover, the annihilation/interaction of substructures is promoted, and the distinct refinement of the dynamic recrystallization (DRX) in the β grain can be found. However, the coarsening of the β grain and the consumption of dislocation substructures are accelerated at high temperatures. Furthermore, the principal DRX nucleation mechanism of the Ti-55511 alloy during double-stage compression with a stepped strain rate in the β region is discontinuous DRX. Additionally, by using the microstructural variation characteristics related to the forming parameters, a physical mechanism equation is modeled to forecast the forming features, the DRX fraction, and the size of the β grain in the investigated alloy. The forecasted results are in accordance with the tested results, indicating that the established model can accurately forecast the microstructure variation and flow features of the studied alloy. [ABSTRACT FROM AUTHOR]

The flow characteristics and microstructure evolution of an aged nickel-based superalloy are researched by thermal compression experiments at different temperatures (1223–1283 K) and strain rates (0.001–0.1 s−1). Based on the theoretical analysis, the dynamic recrystallization kinetics and the evolution of grain size are quantitatively modeled. A unified dislocation density-based model is developed to characterize the flow features of the researched superalloy. In the developed model, the work hardening effect is taken into account by involving the coupled effects between the dislocations and substructures/grain boundaries/δ phases. Especially, the initial contents/morphologies of δ phase are reasonably considered. The dynamic softening mechanisms including dynamic recrystallization and dynamic recovery are considered. The comparisons between the predicted and experimental data indicate that the developed unified model can make accurate predictions for the flow stresses of the aged nickel-based superalloy at constant or variable strain rates. Also, it is found that the morphology of δ phase gradually transforms from spherical to short-rod-like or needle-like with the increased aging time. The dissolution and bend of δ phase induced by dislocation movement easily occur. Besides, the DRX behavior is accelerated, and the grains are refined as the aging time is increased. • Flow characteristics and microstructure evolution of an aged nickel-based superalloy are researched. • A unified dislocation density-based model is developed to describe the flow features of the studied alloy. • The coupled effects between the dislocations and substructures/grain boundaries/δ phases are considered. • The δ phase gradually transforms from spherical to short-rod-like or needle-like with increasing aging time. • The dissolution and bend of δ phase induced by dislocation movement usually occur. [ABSTRACT FROM AUTHOR]

Isothermal compressive tests of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are performed at diverse deformation amounts, strain rates, and elevated temperatures. The impacts of hot compression parameters on the morphological evolution and dissolution behavior of γ' phase are studied. Results show that the morphology of γ′ phase changes from the various initial shapes (irregular, split-off branch, dendrite, butterfly-like, and cauliflower) to spherical, due to the overlap of the γ'/γ interface energy and the coherent strain energy related to lattice misfit. Besides, the dislocation accumulation leads to the increased local stress and enhances the spheroidization behavior of γ' phase. Increasing temperature/true strain or decreasing strain rate, the dynamic dissolution behavior of γ' phase is obvious. It is because the dislocation/ vacancies-assisted element diffusion is rapid at high deformation temperatures/true strains, and the low strain rate can provide the enough time for element diffusion. The substantial dissolution of γ' phase at 1140 °C and 1170 °C is also related to the deformation temperature approaching and exceeding the γ' solvus temperature (1150 °C), respectively. Additionally, the decreased fraction of γ' phase is also related to the high γ'/γ interfacial energy. To quantitatively describe the dissolution behavior of γ′ phase, a dynamic dissolution kinetics model is proposed, and the correlation coefficient between the predicted and experimental values is 0.990. It indicates that the proposed model can precisely depict the dissolution behavior of γ′ phase. The findings in this work can provide the theoretical guidance to optimize deformation parameters, as well as control the morphology and fraction of γ′ phases. • Dissolution and spheroidization mechanisms of γ′ phase in a novel HIPed P/M superalloy during hot deformation are studied. • The morphology of γ′ phase changes from the various initial shapes (irregular, split-off branch, et al.) to spherical. • The spheroidization of γ' phase mainly results from lattice misfit and the interaction between dislocation and γ' phase. • The dynamic dissolution behavior of γ' phase is obvious with increasing temperature/strain or decreasing strain rate. • The proposed dynamic dissolution kinetics model can precisely depict the dissolution behavior of γ′ phase. [ABSTRACT FROM AUTHOR]

The flow characteristics of a Ni-Mo-Cr base alloy are researched by the double-stages hot compression with step-strain rates. Electron backscatter diffraction (EBSD) as well as transmission electron microscopy (TEM) are adopted to systematically comprehend the influences of compressed parameters on microstructural changes. To forecast the hot compressed behaviors, a physically-based constitutive model with considering the evolution characteristics of dislocation density, subgrain and the grain structure is developed. Experimental results indicate that the elevated compressed temperature can exacerbate the dislocation consumption, subgrain rotation/interaction and dynamic recrystallization (DRX) grains coarsening. However, the prominent dislocation accumulation, subgrain generation and the refinement of DRX grains come up as the strain rates at the first or second stage of hot compression are ascended. The discontinuous DRX characterized by the bulging/serrated of grain boundaries is the major DRX nucleation mechanism of the researched alloy. The forecasted compressed stress, grain size and DRX fraction well match the tested results, which indicates the constructed physically-based constitutive equations can be employed to model the high-temperature features and microstructural changes of the studied Ni-Mo-Cr base alloy in double-stages hot compression with step-strain rates. • The flow characteristics of a Ni-Mo-Cr base alloy in double-stages hot compression with step-strain rates are revealed. • With increasing strain of first stage, high density substructures are formed and the bulging of DRX grain boundary is inhibited. • The consumption of substructures and coarsening of DRX grains are enhanced with raising of compressed temperature. • The major DRX nucleation mechanism of the researched alloy in double-stages hot compression with step-strain rates is DDRX. • The developed physically-based constitutive model can precisely reproduce the flow behaviors and microstructural changes. [ABSTRACT FROM AUTHOR]

The spheroidization behavior of γ′ phase and dynamic recrystallization (DRX) mechanisms of a novel HIPed powder metallurgy (P/M) superalloy deformed at high temperatures (1080−1170 ℃) and different strain rates (1–0.001 s−1) are investigated. The results show that the spheroidization behavior of γ' phase and DRX mechanisms are greatly influenced by deformation condition. The coarse γ′ Ⅰ phase promotes the DRX process, while the fine spherical γ' Ⅲ phase results in the formation of sub-grain boundaries. Thus, the grains are refined. During hot deformation, some coarse γ′ Ⅰ and γ′ ⅠⅠ phases transform into the fine spherical γ' Ⅲ phases, while some initial γ' Ⅲ phases gradually diminish. Besides, the spheroidization rates of γ′ Ⅰ , γ′ Ⅱ and γ' Ⅲ phases increase with the increase of strain rate. The low DRX volume fraction and refined grains with the average size of 2.27 µm are obtained at 1110 ℃/1 s−1. It is attributed to the fact that the DRX nucleation is obviously enhanced and the pinning effects of spherical γ′ Ⅲ phases on dislocations accelerate the formation of sub-grains. In addition, the discontinuous DRX is the main DRX mechanism of this novel P/M superalloy. • Spheroidization and DRX mechanisms of a novel HIPed P/M superalloy during hot deformation is studied. • The coarse γ′ Ⅰ phase promotes the DRX by inducing the dislocation accumulation. • The fine spherical γ' Ⅲ phases result in the formation of sub-grain boundaries and the grains are refined. • Some coarse γ′ Ⅰ and γ′ ⅠⅠ phases transform into the fine spherical γ' Ⅲ phases during hot deformation. • The discontinuous DRX is the main DRX mechanism of this novel P/M superalloy. [ABSTRACT FROM AUTHOR]