The dynamic recrystallization (DRX) phenomena occurring in different thermo-mechanical processing (TMP) conditions for various metallic materials are reviewed. Several types of DRX are described: ...discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX). The terminologies used in this field are summarized, together with the key factors influencing the DRX processes including stacking fault energy, initial grain size, TMP conditions and second-phase particles. Both standard and advanced experimental techniques used to characterize DRX processes are examined. The focus is placed on the mechanisms of these three types of DRX, and the related numerical models.
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•Three types of dynamic recrystallization processes occurring during hot deformation are reviewed.•The mechanisms of these three types of dynamic recrystallization processes are discussed in detail.•Physically based numerical models for all the three dynamic recrystallization process are reviewed.•Topics for further investigation on dynamic recrystallization are recommended.
AlxCoCrFeNi high entropy alloys (HEAs) with Al molar ratios of 0.3, 0.6 and 0.9 were subjected to hot plane strain compression testing at varying strain rates and strains in the temperature range of ...930–1030 °C. A detailed analysis of the flow curves and microstructural evolution of these alloys was conducted under different hot working conditions. Very slow dynamic recrystallization kinetics and a relatively high activation energy of hot deformation (Q=549 kJ/mol) were found for the Al0.3CoCrFeNi FCC HEA. The main restoration mechanism for this alloy was the formation of new grains through strain-induced migration of the grain boundaries. Increasing the Al content to 0.6 and 0.9 molar ratio changed the microstructure to a duplex one comprising of FCC and BCC phases. A complex restoration behaviour was observed for the FCC phase in the duplex structures consisting of discontinuous dynamic recrystallization at the vicinity of the interphases, and a gradual evolution of substructures resembling continuous dynamic recrystallization within the grains in regions far from interphases. The main softening mechanism within the BCC phase in the duplex microstructures was continuous dynamic recrystallization, characterised by a progressive conversion of low-misoriented subgrains into grains bounded by a mixture of low and high angle grain boundaries.
Dynamic recrystallization (DRX) often takes place during hot deformation of metallic materials, which then exerts significant influence on the final microstructures and mechanical properties of the ...formed components. A considerable number of published papers related to DRX, however, suffer from non-negligible flaws originating from inappropriate experimental design. In this paper, the sources of these flaws are critically assessed, including the misinterpretation of DRX mechanisms, the strain localization on tested samples, the neglected post dynamic recrystallization and possible phase transformations which mask the real hot deformation microstructure. Solutions to eliminate or quantify these disturbing factors during DRX studies are suggested, yielding more accurate approaches to investigate DRX behaviour of metallic materials.
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•Common dynamic recrystallization (DRX) mechanisms are reviewed.•The disturbing effect of strain localization on DRX is discussed.•The neglected post DRX after hot deformation is examined.•The possible phase transformation during and after hot deformation is considered.
•Hot deformation behaviour of cast AZ31 and AZ80 alloys is compared at 400 °C.•Dynamically recrystallized (DRX) microstructure and texture is studied using EBSD.•Solid solution Al content is found to ...have a significant effect on the DRX behaviour.•AZ80 alloy showed a higher DRX grain size and percentage compared to the AZ31 alloy.
The present research studies and compares the hot deformation behaviour of the AZ31 and AZ80 magnesium alloys. Uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulation testing system were conducted at 400 °C and constant true strain rates in the range of 0.001 s−1 to 0.1 s−1. Samples were deformed to various levels of strain to assess the progression of dynamic recrystallization. Detailed microstructure and texture characterization were performed using optical microscopy, XRD macrotexture measurements, and electron backscatter diffraction (EBSD) microtexture measurements. Both the starting materials showed coarse dendritic microstructures and random texture. Both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occurred in both the alloys during deformation, although the AZ31 alloy showed a higher tendency towards CDRX compared to the AZ80 alloy, while reverse was true for DDRX. The AZ80 alloy showed initiation of DDRX at lower deformation strain levels, had coarser DDRX grain size and a higher DDRX% compared to the AZ31 alloy. These features resulted in the development of a homogeneous fine-grained microstructure in the AZ80 alloy for deformation to an equivalent strain of 1.0, while the microstructure of the AZ31 alloy sample became bimodal. The choice of the alloy also affected the texture development during the deformation, with the AZ31 alloy showing DDRX texture randomization, while the AZ80 alloy showing deformation texture preservation by the DDRXed grains. The study highlights the substantial effect the Al content in the α-Mg solid solution has on the microstructure and texture development during hot deformation of the AZ alloys.
Influence of strain rate on dynamic and post-dynamic recrystallization kinetics of Inconel 718 is investigated by performing hot compression tests at constant strain rate in the range 0.001;1s−1 in ...the δ -subsolvus domain, with or without post-deformation holding at the deformation temperature. Dynamically and post-dynamically recrystallized grains are distinguished based on their internal misorientations, using EBSD data with enhanced angular resolution. For the applied deformation conditions (T=980°C and ε=0.7), dynamic recrystallization is inhibited at ε˙>0.1s−1. On the other hand, very fast post-dynamic recrystallization is promoted by high strain rates, with characteristic times which can be as short as few seconds to achieve full recrystallization. Most of previous works on the effect of strain rate on dynamic recrystallization kinetics were done by quenching samples right after deformation, without discriminating dynamically and post-dynamically recrystallized grains. Those works led to the conclusion that increasing strain rate beyond a critical value leads to an increase in dynamic recrystallization kinetics. Experimental quenching delays cannot be shorter that few seconds, which is shown here to be sufficient to get a significant increase in recrystallized fraction by post-dynamic mechanisms. Based on the present work, post-dynamic evolutions are actually very likely to be responsible for the apparent increase in dynamic recrystallization kinetics at high strain rates which has often been reported previously.
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Dynamic microstructural evolution and recrystallization mechanism during hot deformation of intermetallic-hardened duplex Fe-9Al-10.8Mn-4.5Ni-0.7C (wt.%) lightweight steel have been comprehensively ...examined at various deformation temperatures at a fixed strain rate of 0.001 s−1. The flow curves are predicted employing Avrami exponent obtained from the strain dependent Johnson-Mehl-Avrami-Kolmogorov relation, which is further corroborated with the dynamic microstructural response. A detailed analysis of the intermetallic precipitates and elemental partitioning in both the ferrite and austenite phases are performed. The ferrite matrix having uniformly distributed nano-sized B2 (NiAl) precipitates has a higher micro-hardness as compared to the austenite matrix, which corroborates the strain partitioning in the austenite phase during hot deformation. Two distinct restoration mechanisms are observed in this alloy viz. continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) following hot deformation. The CDRX mechanism in the ferrite and austenite phase is characterised by the progressive misorientation development of subgrains into high-angle boundary during straining. The ferrite phase is associated with CDRX mechanism at all the deformation temperatures (1223–1423 K) albeit DDRX-like mechanism, facilitated by austenite/ferrite interphase is found to be an assisting mechanism towards the higher temperatures (1323–1423 K). The austenite phase, on the other hand, exhibits DDRX mechanism during the initial stage and dominant CDRX at the later stage of deformation at lower temperature (1223–1323 K). With the increasing deformation temperature to 1423 K, the dissolution of boundary-B2 precipitates in austenite facilitates the boundary migration, thus promoting the DDRX accompanied by the twinning in this phase.
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•Ferrite with nano-sized B2 precipitates has higher microhardness than austenite.•The flow curves in the steel are predicted employing the JMAK model.•Ferrite phase exhibits both CDRX and DDRX-like mechanism at 1323–1423 K.•CDRX is dominant mechanism in austenite in later stage of deformation at 1223 K.•Dissolution of B2 in austenite phase triggers DDRX and grain coarsening at 1423 K.
The 2195 aluminum alloy has been widely utilized in the aerospace field, of which dynamic recrystallization microstructures have a substantial effect on the mechanical properties of aerospace parts. ...In this study, 2195 aluminum alloy was compressed at 300–520 °C using a Gleeble 3500-GTC thermo-mechanical testing system. The discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) behavior of 2195 aluminum alloy at medium/high temperature was studied. The results demonstrated that during medium temperatures (300–360 °C) deformation the main softening mechanism was DDRX, and at high temperatures (420–520 °C), it was CDRX. CDRX of 2195 aluminum alloy involved three types of subgrain-forming mechanisms: dislocation tangling to form subgrains, microscopic shear bands to form subgrains, and the coalescence of two small subgrains to form larger subgrains. In addition, several recrystallized grains underwent geometric dynamic recrystallization (GDRX) at high temperature and extensive deformation (480 °C-80% or 520 °C-60%).
•DDRX occurred in 2195 aluminum alloy at medium temperature.•CDRX occurred in the alloy at high temperature.•Continuous dynamic recrystallization is based on 3 subgrain forming mechanisms.•GDRX occurs during large deformation at high temperatures for 2195 aluminum alloy.
Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform ...microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.
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•PDRX evolution during hot deformation in a Ni-base superalloy was evaluated.•The role of grain boundary carbides on the evolution of DRX and PDRX was analyzed.•Carbides have inhibited the DRX kinetics in the initial stage, i.e., at ε<0.40.•At higher strains, carbides inhibit the grain growth of recrystallized grains alone.
In this work, we studied the effect of initial grain size on the hot compression characteristics of super-304H austenitic stainless steel in a range of strain rates (0.001–1 s−1) at a fixed ...temperature of 1223 K. Analysis of the flow curves reveals that the flow stress is inversely proportional to the average sub-grain diameter in both coarse and fine-grained specimens at higher strain (≥0.5) levels. Further, the fine-grained specimen following deformation at a low strain rate (0.001 s−1) reveals the occurrence of both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms. In this condition, the CDRX is characterized by a progressive increase of boundary misorientation, while the DDRX is characterized by bulging and strain-induced boundary migration. In contrast, the CDRX characterized by the formation of microbands is majorly responsible for the grain refinement in the fine-grained specimen at higher strain rates (∼1 s−1) and in the coarse-grained specimen at all strain rates (0.001–1 s−1). The superposition of different DRX mechanisms leads to significant variations in grain refinement kinetics with strain rates and initial grain sizes. The findings of this investigation provide a foundation for the accurate control of the microstructures in the studied alloy with different initial grain sizes during the hot working at various strain rates.
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•Large strain hot compression in super-304H is conducted at 1223 K and 0.001–1 s−1.•Avrami exponent is weakly dependent on strain rate but varies with initial grain size.•Fine-grained specimen at 0.001 s−1 reveals both DDRX and CDRX mechanisms.•Fine and coarse-grained specimens at 1 s−1 reveal CDRX assisted by microbanding.•Superposition of DRX mechanisms leads to variations in grain refinement kinetics.
The high temperature deformation behavior of a X30CrMoN15-1 high-nitrogen stainless steel has been investigated using uniaxial compression test in the temperature range of 850–1250 ℃ and strain rate ...of 0.001–10 s−1. An Arrhenius-based hyperbolic sine equation was used to establish the flow stress constitutive model of the alloy at high temperatures, and the activation energy was 385.5 kJ/mol. Processing maps based on the dynamic material model were developed for true strains of 0.2, 0.4, 0.6 and 0.8. The domain of the safe region was in two parts: first, a strain rate range of 1–10 s−1 and temperature range of 1000–1100 ℃, and second, a strain rate range of 0.3–0.001 s−1 and temperature range of 1100–1250 ℃. The deformed microstructure at 1050 °C was characterized at different strain rates, strong< 100 >fiber textures parallel to the compression axis developed during high temperature deformation, and the strength gradually increased and became more concentrated with decreasing strain rate. The maximum dynamic recrystallization fraction and geometrically necessary dislocation densities were recorded at a medium strain rate (ε̇=0.1s−1), which was more conducive than a low strain rate to the continuous dynamic recrystallization process. Discontinuous and continuous dynamic recrystallization both contributed to the microstructural evolution of the columnar grains studied in this research, but discontinuous dynamic recrystallization was the dominant mechanism.
•We established the constitutive model ofX30CrMoN15-1 HNSS : ε̇=4.16384×1016sinh0.0064σ6.4499exp(−385.52998.3145T).•Processing maps were developed at strains of 0.2, 0.4, 0.6 and 0.8, and provided a guideline for post processing.•The intensity of< 100 >fifiber texture gradually increased and became more concentrated with the decrease of strain rate.•We found an abnormal microstructure evolution during deformation at 1050 ℃, and CDRX dominated at a medium strain rate.
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