Fine DRX Grains Research Articles

Microstructure control and DRX characteristics of Ni–Co–W superalloys affected by changing deformation direction on [001] columnar grain

Controlling the evolution process of columnar grains is benefits to achieve microstructure regulation during subsequent hot processing in superalloys. In present research, it takes a Ni–Co–W superalloy as an example, aims to clarify the underlying connections between the compression direction and dynamic recrystallization (DRX) behaviors in microstructure evolution. The compression direction (CD) was parallel or perpendicular to [001] columnar has been defined as CD∥[001] and CD⊥[001], respectively. The columnar evolution and DRX characteristics under two sets of experimental during hot deformation were identified deeply. The results show that complete DRX is more easily to occur when CD⊥[001], but fine DRX grains are tendency to form when CD∥[001]. DRX nucleation within CD∥[001] and CD⊥[001] deformed microstructure under dislocation energy was discussed deeply. The critical size of nucleation is decreased while the nucleation density is increased in CD∥[001] with high dislocation density, which benefits to form numerous fine DRX grains along the original columnar boundaries. In addition, according to Taylor factors (TFs) criterion, TFs difference will always existed in CD∥[001], which promotes the necklace structure gradually replaced columnar structure and some of them developed into fine DRX bands. With increasing of trues strain, TFs difference gradually decreased in CD⊥[001], DRX nucleation was inhibited, thus the existed DRX grains further grow and finally coarse DRX grains were obtained. The findings clarified the flow behaviors and DRX characteristics of [001] columnar in two directions, and then proposed a microstructure control mechanism of superalloys with [001] columnar based on deformation vector and evolution decomposition.

Inhomogeneous deformation behavior and recrystallization mechanism of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar LPSO phase

Inhomogeneous plastic deformation and recrystallization behavior of Mg-Gd-Y-Zn-Zr alloy containing only intragranular lamellar long-period stacking ordered (LPSO) phase were investigated by isothermal compression experiments at the temperatures of 350–450 °C, and the strain rates of 0.001 ∼ 10 s−1. Results showed that Mg-Gd-Y-Zn-Zr alloys with varying degrees of bimodal structures were obtained after compression. The lamellar LPSO phase directions in residual coarse grains were aligned in the radial direction (RD). Dynamic recrystallization (DRX) grains induced at grain boundaries, kinked bands, and lamellar shearing regions during compression synergistically promoted coarse grain refinement. Moreover, the condition of high temperature and medium strain rate (450 °C-0.1 s−1) contributed to the uniform extension of "mantle layers" of fine DRX grains. In contrast, the condition of medium temperature and high strain rate (400 °C-10 s−1) could accelerate the deflection of CD-directed (compression direction) LPSO lamellae and the formation of local high-energy regions, facilitating the generation of recrystallization band. In addition, a high strain rate (400 °C-10 s−1) is more conducive to the formation of a heavily bimodal structure than a high temperature (450 °C-0.1 s−1). Only a larger spacing of LPSO lamellae (∼1.4 μm) could meet the spatial requirements of recrystallization. Further analysis revealed that the continuous dynamic recrystallization (CDRX) mechanism characterized by the expansion of "mantle layers" and nucleation along kinked boundaries and lamellar shearing regions was the primary grain refinement mechanism. The discontinuous dynamic recrystallization (DDRX) mechanism only exhibited a limited role due to the inhibition of the highly dense lamellar LPSO phase on grain boundaries bulging. After compression, the orientation of residual coarse grains gradually deviated toward the CD. While the recrystallization with limited proportion and random orientation cannot significantly weaken the basal texture.

Hot deformation behavior of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting

TiAl-based alloys are known for their exceptional high-temperature properties but suffer from low hot workability. The emergence of additive manufacturing (AM) technology significantly decreases the workability requirement, while the microstructure heterogeneities in AMed TiAl are troublesome. This paper explores the hot deformation behavior and microstructure evolution of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting. The Arrhenius constitutive model with strain compensation was established, and hot processing maps were constructed. The microstructure showed that at the initial stage of deformation, a large number of dislocation slips occur preferentially in the soft coarse-grained layers. With the increase of strain, dislocation accumulation in heterogeneous interfaces, and dynamic recrystallization (DRX) occurs. Eventually, the coarse-grained region is constantly occupied by fine DRX grains. A high density of deformation twins at high strain rates further promotes the formation of tiny DRX grains. When the hot compression temperature reaches 1200 °C, the initial alternative fine-grained and coarse-grained hetero-structure transforms into duplex (DP) and near-lamellar (NL) microstructure, and microstructure heterogeneity is eliminated. The results enhance understanding of the hot deformation behavior of heterogeneous microstructure TiAl alloys.

Comparison of Single-Pass Differential Speed Rolling (DSR) and Conventional Rolling (CR) on the Microstructure and Mechanical Properties of Mg5Zn

Background: The primary hot rolling method implemented is differential speed rolling (DSR). The material is rolled and grains are strained, producing fine dynamic recrystallization (DRX) grains that improve material strength and ductility. Objective: The material introduced and under investigation in this paper is an Mg-based alloy, Mg5Zn (wt. %), whose microstructure is enhanced through a combination of heat treatments with proper temperature and holding time and subsequent plastic deformation through hot rolling to evaluate the effect on mechanical properties Methods: The method involves preheating the material to various temperatures in a range from 250ºC to 350ºC and rolling to various thickness reductions to analyze the effect of single-pass differential speed rolling (DSR) and conventional rolling (CR) on the DRX process and its influence on mechanical properties. Results: The effect of single-pass differential speed rolling (DSR) and conventional rolling (CR) on the DRX process shows that the process produces increasing amounts of finer DRX grains at higher rolling reductions, thereby improving the strength and ductility of the material. Conclusion: This investigation demonstrated that single-pass DSR can improve the mechanical properties and formability of Mg5Zn more effectively than CR in terms of grain refinement analyzed through OM, SEM, and EBSD resulting in enhanced tensile strength and ductility [1].

Dual influences of deformation-induced W precipitates on dynamic recrystallization and fracture mechanism of the hot-extruded Mg-Y-Zn alloys: An experimental and phase field study

Understanding the relationship between nano-sized precipitates and dynamic recrystallization (DRX) during thermo-mechanical processing is vital for developing high-performance Mg-based alloys. This work uses experiment and phase field model to investigate the dynamic precipitation of W particles and DRX grains in the hot-extruded Mg98.5Y1Zn0.5 alloys at 360, 380 and 400 °C and clarifies the fracture mechanisms in detail. The results reveal that under all temperatures, the hot-extruded Mg98.5Y1Zn0.5 alloys are composed of block 18R long-period stacking order (LPSO) phase, lamellar 14H-LPSO phase, W particles and a typical bimodal structure containing fine DRX grains and coarse non-DRX grains. Noted that the W particles not only promote the DRX nucleation by hindering dislocation motion to increase elastic energy, but also inhibit the growth of DRX grains by pinning grain boundaries during deformation. In addition, the strength decreases while the plasticity increases with increasing temperature for the hot-extruded Mg98.5Y1Zn0.5 alloys, but an excellent strength-plasticity balance is achieved at 400 °C. More importantly, analyzing the crack extension in non-DRX, DRX and non-DRX/DRX coexistence regions shows that a ductile-brittle fracture mechanism dominates in the non-DRX regions due to the difficulty of basal slip and low fracture toughness. But a hole-joining fracture mechanism dominates in the DRX regions. The fracture mechanism in the non-DRX/DRX coexistence region is mainly determined by the DRX volume fraction. These results can provide insights and valuable references for developing high-performance Mg-based alloys.

Impact of uneven distribution of grain characteristics on yield strength and martensitic transformation of as-hot-rolled medium-entropy alloys

As-hot-rolled medium-entropy alloys (MEAs) with unevenly distributed grain sizes of face-centered cubic grains exhibit better yield strength without uniform elongation loss compared to cold-rolled and annealed ones. Successive operation of dynamic recrystallization (DRX) during several hot rolling passes leads to a wide range of grain sizes from submicrons to tens of micrometers due to the grain growth after nucleation: early recrystallized grains are coarser than recently recrystallized ones. Not only the grain size but internal dislocation density of the recently recrystallized grain is low. During the tensile deformation of the hot-rolled MEAs at –196 °C, dislocation pile-ups in the relatively soft and fine DRX grains enhance yield stress and hetero-deformation-induced strain hardening. Thanks to the enhanced yield stress of the as-hot-rolled MEAs, stress-induced martensitic transformation easily occurs. Notably, partially DRXed MEAs hot-rolled at 800 °C have lower yield stress than fully DRXed ones, hot-rolled at 900 and 1000 °C. This is attributed to the softening effect of the stress-induced body-centered cubic martensitic transformation in unrecrystallized coarse grains prior to the yielding, which lowers the yield stress of the partially DRXed ones. After yielding, the martensitic transformation facilitates strain hardening and early necking is precluded. This study presents a fresh outlook on the uneven distribution of grain sizes by hot rolling beneficial to mechanical responses of uniform elongation of ∼45% despite the as-rolled states with an advantage of simplified thermo-mechanical processes.

Deformation-induced dynamic precipitation of 14H-LPSO structure and its effect on dynamic recrystallization in hot-extruded Mg-Y-Zn alloys

Understanding dynamic precipitation in thermo-mechanical processing can shed light on the manufacturing of high-performance Mg-based alloys. This study investigates the dynamic precipitation of long-period stacking order (LPSO) phase with 14H structure and its effect on dynamic recrystallization (DRX) in the hot-extruded Mg98.5Y1Zn0.5, Mg97Y2Zn1 and Mg94Y4Zn2 (at. %) alloys. First, the simultaneous increase of Y and Zn atoms not only promotes the formation of interdendritic block 18R-LPSO phases during solidification but also facilitates the dynamic precipitation of intragranular lamellar and thin 14H-LPSO phases in coarse non-DRX and fine DRX grains, respectively, during hot extrusion. Second, the increased block 18R volume fraction and lamellar 14H number density significantly delays DRX. However, as the deformation strain increases, broken 18R fragments accelerate the DRX particle-stimulated nucleation (PSN) mechanism and fluctuant α-Mg/18R interfaces initiate the discontinuous DRX mechanism. In particular, the deformation kinking and dissolving of lamellar 14H activate the continuous DRX mechanism. Finally, the mechanical strength increases but ductility decreases with increasing the content of Y and Zn atoms for the hot-extruded Mg-Y-Zn alloys. The high strength of the hot-extruded Mg94Y4Zn2 alloy is attributed to fine DRX grain size, strong un-DRX grains, major block 18R and lamellar/thin 14H phases, and deformation kinking, while the high ductility of the hot-extruded Mg98.5Y1Zn0.5 alloy is responsible for large DRX volume fraction, high activity of deformation twins, and easy mobility of basal slip.

The Effects of Pre-Consolidation Heat Treatment on the Tensile and Fracture Toughness Behavior of the Rapidly Solidified Mg–Zn–Y–Al Alloys

This study is aimed to optimize the tensile properties and fracture toughness of rapidly solidified (RS) ribbon-consolidated Mg–0.85Zn–2.05Y–0.35Al (at%) alloys by changing the pre-consolidation heat-treatment temperature. The alloys prepared from RS ribbons heat-treated below 673 K consisting of bimodal α-Mg grains; coarse-worked grains (∼2.8 µm) with high Kernel average misorientation (KAM) values (∼1.8°), and ultrafine dynamically recrystallized (DRX) grains (∼0.68 µm) with intermediate KAM values (∼1.1°). The DRX grains involve cluster arranged layers (CALs) and thin plate-shaped long-period stacking ordered (LPSO) phase precipitation. The fine grain structure strengthens the alloy, significantly while they have little beneficial effects in retarding the crack propagation, thus resulting in low fracture toughness. The alloys that were heat-treated above 723 K prior to consolidation possessed a three kind of α-Mg grain structure; it consisted of fine DRX grains (∼2.3 µm) with low KAM values (∼0.5°) in addition to the coarse-worked grains and ultrafine DRX grains. The fine DRX grains improved strain hardening and ductility, resulting in fracture toughness increase. Furthermore, high-temperature pre-consolidation heat treatment produces block-shaped LPSO phase grains associated with α-Mg. This LPSO phase appears to work as an effective feature to toughen the materials, due to the frequent formation of microcracks in the phase and promotion of crack deflection.

Microstructure evolution and dynamic recrystallisation behaviour in hot deformation of Haynes 214 superalloy

The microstructure evolution and dynamic recrystallisation (DRX) behaviour of Haynes 214 superalloy were investigated in the deformation temperature range of 900 ~ 1180 ℃ and strain rate range of 0.1 ~ 10 s−1 during isothermal compression tests. According to the true stress-strain curves that were obtained from the tests, it was found that temperature and strain rate significantly influence the hot deformation behaviour of Haynes 214 alloy. The fraction and grain size of the dynamic recrystallisation both increase as deformation temperature increases. Fine and uniform DRX grains are obtained at low and high strain rates, whereas at 1 s−1, DRX grain size and fraction decrease. In addition, both continuous dynamic recrystallisation (CDRX) and discontinuous dynamic recrystallisation (DDRX) occur during hot deformation, both being affected by strain rate and deformation temperature. At the same time, the fraction of ∑3 n (n = 1, 2, 3) twin boundary increases as temperature increases and is higher at a lower strain rate (0.1 s−1) and higher strain rate (5, 10 s−1), but lower at the intermediate strain rate of 1 s−1.

Influencing Mechanisms of Prior Cold Deformation on Mixed Grain Boundary Network in the Thermal Deformation of Ni80A Superalloy.

Within the grain boundary engineering (GBE) of alloys, a mixed grain boundary network with random grain boundaries interrupted by twin boundaries, contributes to enhancing the overall grain boundary-related properties. The higher density of twin boundaries is pursued herein. Furthermore, a two-stage deformation method, i.e., prior cold deformation followed by thermal deformation, was proposed for improving the mixed grain boundary network in the thermal deformation of Ni80A superalloy. The influence of prior cold deformation on the mixed grain boundary network was investigated through a series of two-stage deformation experiments. The analysis of the stress–strain curves shows that the critical strain for dynamic recrystallization (DRX) and peak strains decrease significantly under the effect of prior cold deformation. In comparison to the necklace-like microstructures that occur after a single thermal deformation, the microstructures apparent after a two-stage deformation are characterized by finer DRX grains with abundant Σ3n twin boundaries, with a significantly improved density of the Σ3n twin boundaries (BLDΣ3n) by a factor of around nine. With increasing prior cold strain, the grain size, after a two-stage deformation, decreases continuously, while the BLDΣ3n increases firstly and then decreases. The mechanisms for improving the mixed grain boundary network via two-stage deformation were uncovered. The sub-grain boundaries formed in prior cold deformation stimulate the nucleation of DRX grains and twins; meanwhile, the driving force for grain boundary migration is enhanced due to prior stored energy. Then, DRX is activated in advance and occurs more completely, thereby promoting the formation of Σ3n twin boundaries.

Microstructural evolution of Al-7.3Zn-2.2Mg-2Cu (Al7068) alloy in T6 condition during isothermal compression using 3-dimensional processing map

Elevated temperature isothermal compression of Al-7.3Zn-2.2Mg-2Cu (Al7068) alloy in T6 condition was studied till a true strain of 0.69 in the working temperature and deformation rate range of 250–450 °C and 0.001–1 s−1, respectively. The rheological behaviour of the specimens showed a decrease in the flow stress values at the higher temperature and lower strain rates. The mean deformation activation energy at peak stress was estimated as 266 kJ/mol. 3D processing map suggested two safe working zones (I and II) for the current alloy. Zone I, with a peak efficiency of 57% located in the domain of 288–312 °C at 0.001 s−1, and zone II with an efficiency ≥ 48% located in the range of 400–450 °C and 0.001–0.044 s−1. The EBSD IPF micrograph of the compression specimens showed a dominance of deformed grains with few fine DRX grains near the grain boundary regions. The analysis showed that the microstructural evolution during hot compression progressed through the CDRX mechanism. TEM analysis confirmed precipitate coarsening with increase in processing temperature and decrease in the deformation rate.

A new dynamic recrystallization mechanism in adiabatic shear band of an α/β dual phase titanium alloy: Composition redistribution

In this paper, the composition distribution characteristics in adiabatic shear bands (ASBs) of an α/β dual phase Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy and its influence on dynamic recrystallization (DRX) were investigated via atomic probe tomography. The results showed that in the ASB transition region, a clear grain boundary (GB) in α/β interface was observed. However, in the ASB central region, a “fuzzy GB” was formed in α/β interface, and abundant dislocations accumulated in the “fuzzy GB”. In addition, β stabilizing elements (Mo/Cr) became rather sparse in some local regions of β grains to form finer α grains, while in some regions of α grains, local β stabilizing elements were enriched to form finer β grains, thus separating the original grains into finer DRX grains. The unique composition distribution characteristics in α/β grains and α/β interface were defined as composition redistribution, which was the combined result of localized plastic deformation and adiabatic temperature rise.

Microstructure Evolution and Thermal Stability of AZ31 Alloy Deformed by Multi-Pass DSR

Experiments were conducted to investigate the microstructural evolution of AZ31 alloy when deformed by multi-pass differential speed rolling (DSR) using the simplest modification of route A passing sequence. Microstructural observations were taken on all three planes of the sample to obtain a three-dimensional view. Based on quantitative microstructural analysis, the counts of the fine DRX grains increased with increasing number of DSR passes, while the fine DRX grains’ size was consistent in each pass. These results signified that the refinery process was attributed to dynamic recrystallization. Thermal stability was perceived by examining the evolution in microstructure and microhardness value of the 4-pass DSR sample during isochronal annealing over a range of temperature from 423 to 723 K. The increase in distortion energy with multi-pass DSR resulted in the decrease in critical temperature for the initiation of recrystallization upon annealing at 423 K, which induced an immediate drop in microhardness.

Fragmentation of 18R LPSO phases through multi-pass equal channel angular pressing and its impact on rollability of Mg97Y2Zn1 (at%) alloy

In this study, the fragmentation process of 18R long period stacking ordered (LPSO) phase was investigated by comparing the microstructure of as-cast Mg97Y2Zn1 (at%) alloy and 32-pass equal channel angular pressing (ECAP) processed alloy. Results show that two fragmentation modes occurred either singly or in combination during ECAP, i.e., delamination of 18R phase parallel to its basal plane and rupture of 18R perpendicular to its basal plane. Due to these two modes of fragmentation, fine LPSO laths and particles, as well as tiny dynamic recrystallization (DRX) grains were generated in the ECAP alloy, which contributed to its moderate strength and good ductility. In addition, both as-cast and ECAP alloys were subjected to hot rolling to explore the differences of LPSO morphology on rollability of the alloys. The rollability of as-cast alloy was very poor because the coarse microstructure was easy to entangle dislocations and generate microcracks during rolling. However, the rollability of ECAP alloy was significantly improved owing to the additional fragmentation action of band-shaped and kinked LPSO phases, as well as dislocations pinned by LPSO particles or coordinated by more boundaries from fine LPSO particles and DRX grains. Compared with ECAP alloy, the ECAP-rolled alloy exhibited increased strength and moderate ductility with ultimate tensile strength of 384 MPa and fracture elongation of 8.1%.

Microstructure and mechanical properties of friction-stir-welded high-Mg-alloyed Al–Mg alloy plates at different rotating rates

Hot-rolled high-Mg-alloyed Al–Mg alloy (Al–9.2Mg–0.8Mn–0.2Zr–0.15Ti, labeled as 5A12) plates were successfully friction stir welded at rotating rates ranging from 750 to 1500 r·min−1 at a constant welding speed of 50 mm·min−1. The joints were characterized by optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron-dispersive spectroscopy (EDS), and tensile testing. All the joints are volume defect-free and exhibit fine, equiaxed dynamic recrystallization (DRX) grains with high-angle grain boundaries (HAGBs) fractions of 88.6%–93.3% in the nugget zones (NZs). The DRX grain size and the second-phase particle size in the NZs have a parabolic relation with the rotating rate. Furthermore, among the joints tested, the joint prepared at 1000 r·min−1, which has the highest ultimate tensile strength ((478 ± 3) MPa) and the largest elongation to rupture (22.5% ± 1.4%)—approximately 87.5% and 145.2% those of the base metal, respectively, exhibits the smallest grain size of 2.93 μm, as well as the smallest particle size in the NZs. These excellent mechanical properties can be ascribed to the combined effects of the fine DRX grains with high fraction of HAGBs and the fine second-phase particles with a uniform distribution.

Dynamic precipitation behavior and mechanical properties of hot-extruded Mg89Y4Zn2Li5 alloys with different extrusion ratio and speed

Dynamic recrystallization (DRX), dynamic precipitation and mechanical properties of the hot-extruded Mg89Y4Zn2Li5 (at. %) alloys with different extrusion ratio and speed are systematically investigated in this work. The results reveal the formation details of recrystallized grains in the presence of long-period stacking ordered (LPSO) phases during extrusion at 350 °C. In a low extrusion ratio of 4, the recrystallized grains start to form discontinuously along the fluctuant 18R/α-Mg interface and continuously along the 14H kink boundaries with high misorientation angles. As increasing of extrusion ratio from 4 to 16, besides discontinuous and continuous DRX, particle-stimulated nucleation (PSN) of recrystallized grains is accelerated by small 18R and 14H fragments. Moreover, various thin LPSO lamellae including 14H, 18R and 24H as well as γ′ phase are dynamically precipitated inside the DRX grains. Under extrusion with a high ratio of 25, the DRX is mainly produced by PSN mechanism, leading to an almost completely recrystallized microstructure. Also, most of the 18R blocks are broken down to small fragments while the 14H lamellae in the original α-Mg matrix are dissolved at the large deformation strain. In addition, decreasing extrusion speed from 1 to 0.1 mm/s at this extrusion ratio effectively limits the growth of DRX grains and facilitates the dynamic precipitation of thin 14H lamellae within the fine DRX grains. Consequently, the full and fine DRX microstructure, small 18R fragment and deformation kinking, abundant thin 14H lamellae in fine DRX grains result in the excellent strength-ductility balance through hot extrusion with ratio of 25 and speed of 0.1 mm/s.

Rolling parameters, microstructure control, and mechanical properties of powder metallurgy Ti–44Al–3Nb-(Mo, V, Y) alloy: The impact of rolling temperatures

Flow softening behavior, microstructure evolution and hot working parameters of powder metallurgy Ti–44Al–3Nb-(Mo, V, Y) alloy were systematically studied by isothermal hot compression. Based on the analysis of processing maps and the corresponding microstructure, power dissipation efficiency peak occurred at two domains, which were associated with dynamic recrystallization (DRX) and super-plasticity. Pile-up of high density dislocation and a large number of twins were the primary deformation mechanisms at low temperature and high strain rate. As the temperature increases, fine DRX grains promote the movement and rotation of grain boundary, the softening of β phase and the transformation of β→α/α2+γ above 1200 °C improve the deformation capacity of the alloy significantly. Crack-free TiAl sheets were successfully obtained by direct hot rolling process at 1080 °C and 1220 °C. The corresponding microstructure and mechanical properties were analyzed. The sheets rolled at 1080 °C was characterized by near γ (NG)+β0 microstructure with a streamline along the rolling direction, while duplex (DP)+β0 microstructure appeared to be the main microstructure at rolling temperature of 1220 °C. The as-rolled sheet exhibits excellent mechanical properties at 800 °C. The sheet rolled at 1080 °C exhibits 320 MPa yield strength and 468 MPa ultimate tensile strength with 142% elongation. The yield strength, ultimate tensile strength, and elongation of the as-rolled sheet are 446 MPa, 586 MPa, and 3.0%, respectively, when rolled at 1220 °C. The excellent ductility of the sample rolled at 1080 °C is attributed to formation of fine DRX grains due to consumption of twins and substructure under tensile stress, which promote grain boundary slip. Lamellar colonies with fine interlamellar spacing and α2 laths formed at 1220 °C can effectively prevent dislocation motion, resulting in a significant increase in the strength of the alloy.

A Comprehensive Study on Texture Development and Twin-Related Domain Evolution Following Hot Compression in a Super Austenitic Stainless Steel

Uniaxial hot deformation was carried out in a wide range of temperatures (1173 K to 1423 K) and strain rates (0.001 to 10 s−1) in a super austenitic stainless steel employing a Gleeble thermomechanical simulator. The evolution of fine grains due to dynamic recrystallization (DRX) with near-random texture was evident following deformation at strain rates 0.001 to 10 s−1 and temperatures 1173 K to 1273 K. The deformed grains in this domain exhibited the typical formation of 〈110〉//ND or α fiber, and its volume fraction gradually increased when the resistance to deformation increases. Weak deformation texture was persisted in the fine DRX grains (≤ 3 µm) which gradually converted to a strong 〈001〉//ND fiber following grain growth. The deformation texture was simulated by employing a crystal plasticity finite element method (CPFEM). The texture inhomogeneity was observed in the typical compressed specimen which was associated with variation in stress/strain paths at different locations of the hot compressed specimen. Further, the ∑3 twin boundary evolution was analyzed employing Pande’s relationship and twin-related domains (TRDs) analysis. The growth accident was identified as the major mechanism of ∑3 twin boundary evolution during DRX in this alloy. Moreover, the ∑3 regeneration was also evident in a small domain of hot working (1323 K/0.001 s−1) which was substantiated by texture randomization and formation of relatively larger TRDs in the microstructure.

Finite Element Analysis of Dynamic Recrystallization of Casting Slabs during Hot-Core Heavy Reduction Rolling Process

Hot-core heavy reduction rolling (HHR2) is an innovative technology, where a two-high rolling mill is installed after the solidification end of a strand, which can significantly eliminate the core defects of the slab. The mill exhibits a heavy reduction ratio, which promotes the dynamic recrystallization (DRX) of the slab. This study aims to optimize the parameters of the HHR2 process considering the effect of DRX on microstructure homogeneity. The secondary development of commercial software DEFORM-3D is conducted to calculate the deformation and DRX behavior of HHR2 for different reduction ratios. The parameters of DRX volume fraction and DRX grain size are compared, and finer DRX grains are obtained when the greater reduction ratios are conducted in HHR2. Then, corresponding to the deformation conditions in the HHR2, the thermal–mechanical simulations are conducted on the Gleeble3800 to obtain the average grain sizes before and after this process. When the reduction amount increases from 20 mm to 50 mm, the difference of average grain size between the core and the surface reduces by 52%. In other words, appropriately enhancing the reduction ratio is helpful to reduce the average austenite grain and promote the microstructure uniformity of the slab. These results provide some valuable information on the design of deformation parameters for HHR2.

Unique high-temperature deformation dominated by grain boundary sliding in heterogeneous necklace structure formed by dynamic recrystallization in HfNbTaTiZr BCC refractory high entropy alloy

Microstructural evolution of dynamically recrystallized (DRX) grains and grain boundary sliding (GBS) in the heterogeneous necklace structure of HfNbTaTiZr refractory high entropy alloy (RHEA) was studied systematically during high temperature deformation. Uniaxial compression testing was carried out to different strains at 1000 °C and a strain rate 10−3 s−1. Significant bulging of grain boundaries led initially to the formation of DRX grains. The fraction of DRX grains increased with strain, and typical necklace structures of fine (d ≤ 1.5 µm) DRX grains formed at strain ε ≥ 0.3. The DRX grains showed very limited grain growth, and heterogeneous microstructures composed of coarse unrecrystallized regions surrounded by the characteristic DRX necklace structure were formed at larger strains. Interrupted testing with marker grids revealed that DRX grains deformed by a GBS mechanism. The DRX necklace regions connected mesoscopically and also displayed diamond network morphologies, with unique “Y-shaped”, “T-shaped” and “X-shaped” junctions. The formation of different types of junctions were rationalized on the basis of GBS accommodated by local dislocation slip in unrecrystallized regions. The unrecrystallized regions showed preferred <001>/<111> micro-texture, consistent with conventional dislocation slip in the BCC crystals. On the other hand, the newly formed DRX grains initially had similar orientations to those of the parent grains, but they displayed a random texture with increasing strain, as expected from GBS. The randomized texture of DRX grains and the stability of DRX grains size represented GBS in the DRX necklace regions.