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Hot deformation behavior of the maraging stainless steel is studied in temperature range from 900 to 1150 °C and strain rate from 0.1 to 10 s−1. When the deformation temperature is 900−950 °C, the abnormal stress increase is observed at the end of the flow curves. Transmission electron micrographs reveal that the Laves phase at the interface between prior austenitic (γ) and high‐temperature ferrite (δ) impedes hot deformation. The microstructure analysis shows that the dynamic recrystallization (DRX) mechanism of γ and δ is discontinuous DRX and continuous DRX, respectively. When the specimens are deformed at 950 °C, the extent of DRX at a strain rate of 5 s−1 is higher than at 0.1 s−1. This anomaly is due to adiabatic heating causing the actual deformation temperature at high strain rate (ε.$\overset{.}{\epsilon}$) to be higher than at low ε.$\overset{.}{\epsilon}$, indicating that DRX is more influenced by temperature compared to ε.$\overset{.}{\epsilon}$. The influences of adiabatic heating and friction are corrected. Strain‐dependent constitutive equation is developed based on the revised flow curves, yielding an average absolute relative error of 4.69% and a correlation coefficient of 0.99; the prediction accuracy exceeds 90% when the relative error is within 10%. [ABSTRACT FROM AUTHOR]

To investigate the microstructure evolution of powder metallurgy (P/M) superalloys during high-strain-rate deformation, a P/M nickel-base superalloy is subjected to thermal compression at temperatures ranging from 1000 °C to 1100 °C and strain rates ranging from 1 s−1 to 10 s−1. The analysis regarding the flow behaviors and microstructure observation indicates that discontinuous softening is significant at high strain rates. Moreover, based on experimental measurement and theoretic estimation, the adiabatic heating under high-strain-rate deformation is hardly neglectable, which enhances the dynamic softening via dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. In order to obtain refined and homogeneous grains, deformation at temperature range of 1025–1050 °C with strain rate rang of 1–10 s−1 or at 1000 °C/1 s−1 is suggested for high-strain-rate compression on the P/M superalloy, where DRX plays more dominating roles. • Discontinuous hardening and softening are observed at high-strain-rate compression. • The adiabatic heating under higher strain rate is more significant. • A processing window is preferred to obtain refined grains after high-speed deformation. [ABSTRACT FROM AUTHOR]

The evolution of the flow stress and microstructure for Waspaloy was studied in the 950–1140 °C temperature range under constant true strain rate conditions of 0.001–1 s −1 up to a true strain of 0.83 using isothermal hot compression testing. The impact of friction at the sample/anvil interface and adiabatic heating during deformation on the flow stress evolution was also examined. Mathematical models relating the flow stress to the deformation temperature and strain rate were derived using a power–law relationship. The strain rate sensitivity and the activation energy for hot deformation of Waspaloy were found to be considerably different for deformation in the subsolvus and supersolvus temperature ranges. According to the microstructural investigations, at 950 °C dynamic recovery (DRV) was the main softening mechanism. By contrast, dynamic recrystallization (DRX), partial or complete, occurred at temperatures above 950 °C and resulted in flow softening. [ABSTRACT FROM AUTHOR]

Abstract: The effect of swaging on the Young's modulus of β Ti–33.6Nb–4Sn rods was investigated by X-ray diffraction, thermography, microstructural observations, deformation simulator analysis and cyclic tensile deformation. Stress-induced α″ martensite was stabilized by swaging, dependent on the diameter reduction rate during swaging. Thermography and deformation simulator analysis revealed that swaged rods were adiabatically heated, and consequently, stress-induced α″ underwent reverse transformation. Young's modulus, which was measured by the slope of the initial portion of the stress–strain curve, decreased from 56GPa in the hot-forged/quenched rod to 44GPa in the rapidly swaged rod with a high reduction rate and to 45GPa in the gradually swaged rod with a low reduction rate. The tangent modulus, which was measured by the slope of the tangent to any point on the stress–strain curve, decreased with strain even in the linear range of the stress–strain curve of the hot-forged/quenched rod and the rapidly swaged rod, while the tangent modulus remained unchanged for the gradually swaged rod. It was found that Young's moduli in swaged β Ti–33.6Nb–4Sn rods were affected by stabilized α″ martensite. Low Young's modulus of 45GPa and high strength over 800MPa were obtained when the reverse transformation by adiabatic heating was suppressed and the stress-induced α″ was sufficiently stabilized by gradual swaging to a 75% reduction in cross section area. [Copyright &y& Elsevier]

The objective of the study described here is to evaluate the effect of temperature, strain rate, and strain on the microstructure of dynamically recrystallized nickel–chromium alloy (800H) subjected to hot compression over a wide range of strain rates. The microstructural evolution was studied by electron backscattered diffraction (EBSD) and the effect of adiabatic heating on hot deformation was analyzed to correct the flow curves at high strains. The grain orientation spread (GOS) approach was used to distinguish the dynamic recrystallization (DRX) grains from the deformed matrix. The nucleation mechanism of DRX and the role of Σ3 n CSL boundaries during DRX were explored. Additionally, the influence of carbides on the DRX behavior was studied within the temperature of 850–950°C. The results indicated that the DRX can be stimulated by adiabatic heating and strong dislocation–dislocation interaction occurring with increase in the strain rate in the range of 1–30s−1. The threshold value of GOS (1.2°) separated the DRX grains from the deformed matrix. The recrystallized grains nucleated at pre-existing grain boundaries by extensive bulging associated with grain fragmentation. The Σ3 n CSL boundaries play an important role in DRX and they can be generated through interaction among them after the initiation of DRX. The precipitation of Cr23C6 and Ti(C, N) at the parent grain boundary could restrain or even inhibit the occurrence of DRX in the temperature range of 850–950°C. [ABSTRACT FROM AUTHOR]

Abstract: The hot-compression behavior and the microstructure evolution of SUS303 free-cutting steel were investigated in detail by carrying out compression tests in the temperature range of 1073–1323K and at strain rates ranging from 0.01 to 30s−1. From the true stress–true strain curves, dynamic recovery-type curve at low-temperature/high-strain rate and dynamic recrystallization-type curve at low-strain rate/high temperature were observed. During the low-temperature/low-strain rate hot-compression process, a large fraction of non-refined grains with large numbers of subgrains in their interiors were observed. These subgrains are thought to transform into new grains with further straining. During the high-temperature/low-strain rate hot compression process, discontinuous dynamic recrystallization was observed frequently. During the low-temperature/high-strain rate hot-compression process, only a few refined grains were observed near the grain boundaries, whereas at higher temperatures, a fully refined microstructure was obtained through an “explosive” static recrystallization during subsequent cooling process. [Copyright &y& Elsevier]

Abstract: Systematic experiments on laboratory-grown polycrystalline granular ice and columnar S2 ice loaded triaxially under a high degree of confinement at T =−10°C to T =−40°C at applied strain rates to show the mechanical response and microstructural evolution leading to terminal failure. Terminal failure is characterized by a sudden brittle-like drop in load, localized heating and the development of a narrow shear band, consisting of recrystallized grains, oriented on a plane of maximum shear. This mode of failure, termed plastic (P) faulting, is consistent with the idea of adiabatic heating leading to localized mechanical instability and shear deformation. The microstructural state of the material, including the development of dynamic recrystallization and any prior loading history, does not have a significant affect on the character of shear localization or the levels of deformation required to generate P-faulting. [Copyright &y& Elsevier]

Strain induced phase transformation in metastable 301LN stainless steel generates a heterogeneous multiphase microstructure with a capability to achieve excellent strain hardening. The microstructural deformation mechanisms, prior deformation history and their dependency on strain rate and temperature determine much of the desired dynamically evolving strength of the material. To analyze microscale deformation of the material and obtain suitable computational tools to aid material development, this work formulates a crystal plasticity model involving a phase transformation mechanism together with dislocation slip in parent austenite and child martensite. The model is used to investigate microstructural deformation with computational polycrystalline aggregates. In this context, material's strain hardening and phase transformation characteristics are analyzed in a range of quasi-static and dynamic strain rates. Adiabatic heating effects are accounted for in the model framework to elucidate the role of grain level heating under the assumption of fully adiabatic conditions. The model's temperature dependency is analyzed. The modeling results show good agreement with experimental findings. [ABSTRACT FROM AUTHOR]

In this work, the hot workability of a spray formed 7055 aluminum alloy was successfully investigated through advanced Gleeble thermo-mechanical simulation technology, and the EBSD technology was adopted to analyze the evolution of deformation microstructure comprehensively. From the results, the occurrence of the adiabatic heating phenomenon is confirmed, and an abnormal stress drop caused by which is then corrected by the extrapolation method. Moreover, it is noteworthy that with rising deformation parameters (including deformation temperature T from 350 °C to 450 °C, strain rate ε ̇ from 1 s−1 to 20 s−1 and true strain ε from 0.1 to 0.8), the grains become finer, and the distributions of crystal orientation tend to be more homogeneous. Besides, it is found that both the Continuous Dynamic Recrystallization (CDRX) and Discontinuous Dynamic Recrystallization (DDRX) mechanisms coexist during hot deformation. Moreover, the increase of the three deformation parameters enlarges the misorientation angle inside the grains, which facilitates the DRX progress. • The hot workability of a spray formed 7055 alloy was investigated in detail. • Stress drops caused by adiabatic heating were corrected by extrapolation method. • The grains become finer and more homogenous with the promotion of T , ε ̇ and ε. • The fraction of HAGBs increases with the rising T , ε ̇ and ε. • CDRX and DDRX mechanisms coexist during the hot deformation of the 7055 alloy. [ABSTRACT FROM AUTHOR]