Metadynamic Recrystallization Research Articles

Rheological properties of EP742-ID alloy in the context of Integrated Computational Materials Engineering (ICME). Part 1. Results of experimental research

The article covers rheological properties of the EP742-ID alloy in high-temperature compression tests of cylindrical samples with different ratios of similar initial diameters and heights (d0 /h0). The results of experimental research in the temperature range t = 1000÷1150 °C and initial deformation rates e · 0 = 3·10–2÷3·10–4 s–1 have shown that an increase in compression flow stress with the growth of the d0 /h0 ratio is observed at all temperatures and deformation rates with a linear dependence on the e · 0 value and the d0 /h0 ratio. The method is proposed to recalculate deformation resistance indicators to the set ratio of similar sizes. Higher compression flow stress is connected with an increase in the coefficient of rigidity of samples and their specific contact surfaces. The dependence of apparent activation energy of alloy plastic deformation (Qdef), its relationship with the phase structure and conditions of the process of γ solid solution dynamic recrystallization is established. In the temperature conditions of the beginning of γ solid solution dynamic recrystallization process (1000–1050 °C) the Qdef value for d0 /h0 = 0,75 samples is 959 kJ/mol. The highest Qdef values for d0 /h0 = 0,75 samples of 1248 and 1790 kJ/mol are observed in the range of temperatures of intensive grain boundary γ ′-phase dissolution and coagulation (1050–1100 °C). Samples with d0 /h0 = 3,0 in this temperature range have Qdef up to 2277 kJ/mol. The apparent activation energy of plastic deformation decreases to 869 kJ/mol in the range of temperatures of homogeneous γ solid solution with grain-boundary primary and secondary carbides (1100–1150 °C). The paper provides the results of alloy sample compression at single and repeated consecutive loading with various times of pauses between deformations. It is shown that meta dynamic recrystallization under experimental conditions does not occur in the γ + γ ′-range, and occurs inertly in the γ-range.

Rheological Properties of the EP742-ID Alloy in the Context of Integrated Computational Materials Science and Engineering: Part I. Results of Experimental Investigations

Rheological properties of the EP742-ID alloy are investigated during high-temperature compression tests of cylindrical samples with various ratios of homological initial sizes of diameter and height (d0/h0). It is shown by the results of tests in ranges of temperature t = 1000–1150°C and initial deformation rates e0= 3 × 10–2–3 × 10–4 s–1 that an increase in the compression flow tension manifests itself at all temperatures and deformation rates with an increase in ratio d0/h0 with the linear dependence on the magnitude of e0 and ratio d0/h0. The procedure of recalculating characteristics of the deformation resistance for the specified ratio of homological parameters is proposed. An increase in the compression flow tension is associated with an increase in the rigidity coefficient of the samples and their specific contact surfaces. The temperature dependence of the apparent activation energy of the plastic deformation (Qdef) of the alloy and its relation with the phase composition and running conditions of the dynamic recrystallization of the γ-solid solution are established. The magnitude of Qdef in temperature conditions of the development onset of the dynamic recrystallization of the γ-solid solution (1000–1050°C) is 959 kJ/mol for samples with d0/h0 = 0.75. The largest values of Qdef for the samples with d0/h0 = 0.75 equal to 1248 and 1790 kJ/mol are observed in a temperature region of the intense dissolution and coagulation of the grain-boundary γ′-phase (1050–1100°C). The value of Qdef for the samples with d0/h0 = 3.0 increases to 2277 kJ/mol in this temperature range. The apparent activation energy of the plastic deformation lowers to 869 kJ/mol in the temperature region of the γ-solid solution with grain-boundary primary and secondary carbides (1100–1150°C). The results of compression of alloy samples during single and repeated sequential loading with various durations of pauses between deformation actions are presented. It is shown that no metadynamic recrystallization occurs in the experimental conditions in the γ + γ′-region, while it runs slowly in the γ-region.

Microstructure and texture evolution of friction stir welded dissimilar aluminum alloys: AA2024 and AA6061

In this study, the development of the microstructure and crystallographic texture during friction stir welding (FSW) of AA2024 and AA6061 dissimilar joint was investigated. Electron backscattered diffraction (EBSD) technique was employed to characterize the textures and revealed the evolution of microstructures during friction stir welding. Experimental results showed the grain refinement in the stirred zone through continuous and discontinuous recrystallization. The fraction of precipitates in the stirred zone of the retreating side was much higher than that of the advancing side. On the other hand, the extent of continuous dynamic recrystallization in the thermomechanically affected zone (TMAZ) of the advancing side was lower than that of the retreating side and the recrystallized grains were rarely seen on the advancing side. The initial texture components of the sheets mainly consisted of {001}〈100〉 Cube and {011}〈011〉 Rotated Goss. The pole figures became asymmetric due to the inhomogeneous plastic deformation during FSW. The overall texture intensity was weaker on the advancing side and stronger on the retreating side than that in the starting materials. On the advancing side, {011}〈211〉 and {221}〈012〉 texture components created and the γ-fiber (〈111〉//ND) shear texture developed on the retreating side. Correlation between the microstructure and texture proved that the discontinuous static recrystallization and/or meta-dynamic recrystallization occurred on the advancing side. Microhardness profile on the advancing side was almost identical, while the profile revealed three distinguishable regions on the retreating side.

Effects of rolling curve on recrystallization evolution during hot radial-axial ring rolling of super lager alloy steel ring

Hot radial-axial ring rolling is one typical ring rolling method, which is suited to manufacture a large-scale ring at high temperature. Rolling curve describes the changing process of the cross-section of the blank ring to the target ring. For a certain target ring, when the blank ring is determined, different rolling curves can influence the rolling shape and microstructure of the final ring. In this study, effects of rolling curve on recrystallization evolution during hot radial-axial ring rolling of a super lager alloy steel ring were studied. Firstly, based on the material recrystallization mathematical model, a finite element modeling method for recrystallization evolution of large ring rolling process was developed by adopting subroutine in ABAQUS. The dynamic recrystallization, static recrystallization and metadynamic recrystallization were considered. A comparison between simulation and experiment result was made to verify the prediction accuracy. Then, the design method of the rolling curve was given. Finally, the recrystallization evolution including dynamic recrystallization, static recrystallization and meta-dynamic recrystallization during hot radial-axial ring rolling was revealed. Besides, simulations of different rolling curves were made and the effects on recrystallization evolution were analyzed. The results show that a S-type rolling curve is preferred to realize smaller grain size and more uniform grain distribution.

Metadynamic and static recrystallization softening behavior of a bainite steel

The metadynamic recrystallization (MDRX) and static recrystallization (SRX) softening behavior of a bainite steel was investigated by two-pass isothermal compression experiments at temperatures of 1173, 1273, 1373, and 1473 K and strain rates of 0.01, 0.1, 1, and 10 s−1 with inter-pass times of 1, 5, 10, and 30 s on a Gleeble-1500 thermo-mechanical simulator. Kinetic equations were developed to evaluate the softening fractions caused by MDRX and SRX. A comparison between the experimental and predicted softening fractions showed that the proposed kinetic equations can provide a precise estimation of the MDRX and SRX behavior of the studied steel. The results based on the kinetic equations indicated that the MDRX and SRX softening fraction increases with the increase in strain rate, deformation temperature, inter-pass time, and pre-strain; the activation energy of MDRX is much smaller than that of SRX; and the no-recrystallization temperature of the investigated steel is 1179.4 K.

Early Stages of Microstructure and Texture Evolution during Beta Annealing of Ti-6Al-4V

The early stages of microstructure evolution during annealing of Ti-6Al-4V in the beta phase field were established. For this purpose, a series of short-time heat treatments was performed using sheet samples that had a noticeable degree of alpha-phase microtexture in the as-received condition. Reconstruction of the beta-grain structure from electron-backscatter-diffraction measurements of the room-temperature alpha-phase texture revealed that microstructure evolution at short times was controlled not by general grain growth, but rather by nucleation-and-growth events analogous to discontinuous recrystallization. The nuclei comprised a small subset of beta grains that were highly misoriented relative to those comprising the principal texture component of the beta matrix. From a quantitative standpoint, the transformation kinetics were characterized by an Avrami exponent of approximately unity, thus suggestive of metadynamic recrystallization. The recrystallization process led to the weakening and eventual elimination of the initial beta texture through the growth of a population of highly misoriented grains.

Investigation on strain dependence of metadynamic recrystallization behaviors of GH4169 superalloy

The metadynamic recrystallization (MDRX) behaviors of a solid-solution GH4169 superalloy were studied by two-pass hot compression tests. The effects of pre-strain (deformation degree of the first pass) on MDRX behaviors were analyzed. Results show that the MDRX behaviors intensely depend on the pre-strain. The MDRX fraction first increases and then drops with the increased pre-strain. This is because a relatively large pre-strain can facilitate the nucleation of dynamic recrystallization (DRX) grains, which promotes the subsequent MDRX behaviors. However, due to the decreased stored deformation energy at relatively high pre-strain, the number of DRX grains cannot continuously increases. Thus, the MDRX fraction is reduced with the further increase of pre-strain. The segmented MDRX kinetics models were proposed and validated to describe the significant strain dependence of MDRX behaviors.

In Situ Characterization of Inconel 718 Post-Dynamic Recrystallization within a Scanning Electron Microscope

Microstructure evolution within the post-dynamic regime following hot deformation was investigated in Inconel 718 samples with different dynamically recrystallized volume fractions and under conditions such that no δ-phase particles were present. In situ annealing treatments carried out to mimic post-dynamic conditions inside the Scanning Electron Microscope (SEM) chamber suggest the occurrence of both metadynamic and static recrystallization mechanisms. Static recrystallization was observed in addition to metadynamic recrystallization, only when the initial dynamically recrystallized volume fraction was very small. The initial volume fraction of dynamically recrystallized grains appears to be decisive for subsequent microstructural evolution mechanisms and kinetics. In addition, the formation of annealing twins is observed along with the growth of recrystallized grains, but then the twin density decreases as the material enters the capillarity-driven grain growth regime.

Finite element modelling on microstructure evolution during multi-pass hot compression for AZ31 alloys using incremental method

Based on the principle of piecewise linearization, the incremental forms of microstructure evolution models were integrated into the thermo-mechanical coupled finite element (FE) model to simulate non-linear microstructure evolution during multi-pass hot deformation. This is an unsteady-state deformation where dynamic recrystallization (DRX), meta-dynamic recrystallization (MDRX), static recrystallization (SRX) and grain growth (GG) take place during hot deformation or deformation interval. The distributions of deformation and microstructure for cylindrical AZ31 sample during single-pass and double-pass hot compressions were quantitatively calculated and compared with the metallographic observation. It is shown that both the deformation and microstructure are non-uniformly distributed due to the presence of friction between the die and the flat end of sample. The average grain size and its standard deviation under the double-pass hot compression are slightly smaller than those under single-pass compression. The simulated average grain sizes agree well with the experiments, which validates that the developed FE model on the basis of incremental forms of microstructure evolution models is reasonable.

Investigation on metadynamic recrystallization behavior in SA508-Ш steel during hot deformation

The metadynamic recrystallization (MDRX) behavior of SA508-Ш steel was investigated by isothermal double-hit hot compression tests at forming temperatures of 950–1250°C, strain rates of 0.001–0.1s−1 and the inter-stage delay time of 1–300s. Experimental results show that the effects of forming temperature and the strain rate are significant, while the pre-strain and initial grain size are less on metadynamic recrystallization behavior of SA508-Ш steel. Based on the experimental results, the kinetic equations and the grain size model for metadynamic recrystallization of SA508-Ш steel were established. Comparisons between the experimental and predicted results were carried out. The predicted results agree well with the experimental data, which indicates that the proposed kinetic equations can give a reasonable estimation of the softening behavior for this steel. Furthermore, by implementing the established kinetics and microstructural evolution equations of MDRX into commercial FE software, the hot forging process of SA508-Ш steel can be designed and optimized through numerical simulation.

Effects of Different Forging Processes on Microstructure Evolution for 316LN Austenitic Stainless Steel

Forging experiments were designed and carried out on a 3150 kN hydraulic press to investigate the effects of different processes on the microstructure evolution for 316LN steel. The forging processes included single-pass (upsetting) and multipass (stretching) deformations, and the experimental results indicated that the average grain size varied with forging processes. Moreover, the size had distinct differences at different positions in the workpiece. Meanwhile, numerical simulations were implemented to study the influence of temperature, strain, and strain rate on microstructure evolution. The results of experiments and simulations comprehensively demonstrated that dynamic, static, and meta-dynamic recrystallization could coexist in the hot forging process and that the recrystallization process could easily occur under the conditions of higher temperature, larger strain, and higher strain rate. Moreover, the temperature had more significant influence on both recrystallization and grain growth. A higher temperature could not only promote the recrystallization but also speed up the grain growth. Therefore, a lower temperature is beneficial to obtain refinement grains on the premise that the recrystallization can occur completely.

The effect of Nb solute and NbC precipitates on dynamic and metadynamic recrystallisation in Ni-30Fe-Nb-C model alloys

Dynamic and metadynamic recrystallisation behaviour of Ni-30Fe-Nb-C model alloys during plane strain compression was investigated by optical and electron microscopy. The dynamically recrystallised grains were primarily located at the pre-existing grain boundaries with few additional new recrystallised grains in the prior grain interior. A limited nucleation of recrystallised grains at >400nm size NbC particles was also evident. On the other hand, smaller semi-coherent particles (~10–150nm) severely inhibit the dislocations rearrangement and subgrain boundary mobility, thus leading to sluggish or even suppressed dynamic and metadynamic recrystallisation. These smaller NbC particles maintained cube-on-cube orientation relationship with austenite matrix (001)NbC||(001)γ, [001]NbC||[001]γ. The shape of the NbC particles changes from nearly ellipsoidal to octahedral followed by hexagonal and tetra-kai-decahedral by truncation of {111} facets by {001} ones.

Investigation of recrystallization behavior of large sized Nb–V microalloyed steel rod during thermomechanical controlled processing

Abstract

Metadynamic recrystallization of 300M steel after isothermal compression

Isothermal compression tests of 300M steel were performed on a Gleeble-1500 thermo-mechanical simulator at deformation temperatures ranging from 1173 to 1373 K, strain rates ranging from 0.1 to 5.0 s−1, and a strain of 0.69. Metadynamic recrystallization and grain growth after complete metadynamic recrystallization were investigated by isothermal compression with different inter-pass times. It was found that the inter-pass time, deformation temperature and strain rate markedly affected the austenite grains size of metadynamic recrystallization. The austenite grain size model and grain growth model of metadynamic recrystallization were determined based on the results of quantitative grain size. A good agreement between the predicted and measured austenite grain size and grain growth of metadynamic recrystallization was obtained, and the present models were effective to predict the austenite grain size and grain growth of metadynamic recrystallization in the isothermal compression of 300M steel.

An Investigation on the Softening Mechanism of 5754 Aluminum Alloy during Multistage Hot Deformation

Isothermal interrupted hot compression tests of 5754 aluminum were conducted on a Gleeble-3500 thermo-mechanical simulator at temperatures of 350 °C and 450 °C, and strain rates of 0.1 s−1 and 1 s−1. To investigate the metadynamic recrystallization behavior, a range of inter-pass delay times (5–60 s) was employed. These tests simulated flat rolling to investigate how softening behaviors respond to controlled parameters, such as deformation temperature, strain rate, and delay times. These data allowed the parameters for the hot rolling process to be optimized. The dynamic softening at each pass and the effect of metadynamic recrystallization on flow properties and microstructural evolution were analyzed in detail. An offset yield strength of 0.2% was employed to calculate the softening fraction undergoing metadynamic recrystallization. A kinetic model was developed to describe the metadynamic recrystallization behaviors of the hot-deformed 5754 aluminum alloy. Furthermore, the time constant for 50% recrystallization was expressed as functions related to the temperature and the strain rate. The experimental and calculated results were found to be in close agreement, which verified the developed model.

The kinetics and microstructural evolution during metadynamic recrystallization of medium carbon Cr–Ni–Mo alloyed steel

Abstract

2D cellular automaton simulation of hot deformation behavior in a Ni-based superalloy under varying thermal-mechanical conditions

A 2D cellular automaton model (CA) is developed to predict the hot deformation behavior of a Ni-based superalloy over a wide range of thermal-mechanical conditions. Relationships between the model parameters (work hardening parameter, dynamic recovery parameter, nucleation rate, and grain boundary mobility) and the thermal-mechanical conditions are established. The developed CA model is further employed to study the hot deformation behavior of the studied superalloy under varying thermal-mechanical conditions. These varying conditions include the sudden change of strain rate and gradual change of deformation temperature. The evolutions of flow stress, average grain size and fraction of dynamic recrystallization (DRX) under the varying thermal-mechanical conditions are predicted. The reliability of the simulated results is verified by experimental results. It is found that the predicted results under varying thermal-mechanical conditions gradually approach those under constant hot deformed conditions. i.e., constant deformation temperature and strain rate. The varying thermal-mechanical condition makes the complex evolution of dislocation density, fraction of DRX, and average grain size. Additionally, a pseudo-metadynamic recrystallization is found. The pseudo-metadynamic recrystallization is a temporary suspension of dynamic recrystallization during a large and rapid increase of strain rate as a consequence of nucleation inhibition. It resembles metadynamic recrystallization, except that the pseudo-metadynamic recrystallization occurs when the hot deformation is still underway.

Particle stimulated nucleation during dynamic and metadynamic recrystallisation of Ni-30%Fe-Nb-C alloy

For the first time, a combination of scanning transmission electron microscopy and electron back-scattering diffraction is used to elucidate the early stages of particle stimulated recrystallisation at Nb carbides in Ni-30wt.%Fe alloy subjected to high temperature plane strain compression. While particles with sizes even below 1μm were found to induce dynamic or metadynamic recrystallisation, only a small fraction of coarse particles served as nucleation sites.

Metadynamic recrystallization of Nb–V microalloyed steel during hot deformation

Abstract

Influence of strain induced grain boundary migration on grain growth of 300M during static and meta-dynamic recrystallization

Abstract The microstructural evolution of 300M steel during the isothermal treatment between deformations is studied. Firstly, interrupt compressions of 300M steel at temperature of 1273K and strain rate of 0.01s -1 are conducted to figure out the kinetics of static recrystallization (SRX) and meta-dynamic recrystallization (MDRX), subsequently an in situ heat treatment experiment is designed to measure the amount of strain induced grain boundary migration (SIGBM) and compared with interrupt compressions. The results show that MDRX is verified to have a lower nucleation rate than SRX in 300M steel. In addition, SIGBMs mainly appear at the triple junctions, and the amount of SIGBMs during SRX and MDRX both reach the maximum value between the steady state time and the time of 50% fraction softening. Finally, the grain growth driven by SIGBMs dominates the subsequent isothermal treatment after the new round of nucleation induced by MDRX and SRX.