Discontinuous DRX Research Articles

Revealing the dynamic recrystallization mechanism, extrusion deformation mechanism, and tensile deformation behavior of Mg–6Al–1Zn–1.1Sc alloy

In this study, extruded deformed billet and extruded rod of Mg–6Al–1Zn–1.1Sc (wt. %) alloy were characterized by optical microscopy (OM) and electron backscatter diffraction (EBSD). The room temperature tensile properties and deformation behavior of extruded alloy were investigated using a universal testing machine and the visco-plastic self-consistent (VPSC) model. The results indicated that the coarse deformed grains (DGs) progressively disappeared and the degree of dynamic recrystallization (DRX) of the alloy increased as the sampling position approached the outlet of the extrusion die. In addition, the grain boundary strengthening induced by DRX increased the corresponding Vickers hardness. DRX mechanisms during extrusion included discontinuous DRX, continuous DRX, twin-induced DRX, and particle-stimulated nucleation (PSN). The EBSD-based in-grain misorientation axes (IGMA) analysis revealed that the slip modes within DGs were dominated by prismatic slip in the early stages of extrusion and by multiple slip modes in the later stages. The yield strength, ultimate tensile strength, and elongation of the extruded alloy were 172.4 MPa, 290.1 MPa, and 26.9%, respectively. The combination of grain boundary strengthening, texture strengthening, and secondary phase strengthening resulted in the high yield strength. The VPSC model well simulated the tensile deformation of the extruded alloy with (0001)//ED (extrusion direction) fiber texture. The superior plasticity of the extruded alloy was ascribed to the high Schmid factor (SF) of the non-basal slip and the activation of the non-basal slip facilitated by the reduction in the ratio of the critical resolved shear stress (CRSS) between the non-basal slip and the basal slip.

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.

Influence of solute Mg and Zener–Hollomon parameter on microstructural evolution of linear-friction-welded Al joints

Linear friction welding (LFW) was performed on face-centered cubic materials, that is, Al alloys (AA1050 and AA5052) with different stacking fault energies (SFEs). The pressure was controlled during the LFW to clarify the effects of temperature, strain, and strain rate on microstructural evolution and dynamic recrystallization (DRX) mechanism. The microstructural evolution during DRX was mainly dependent on the temperature. The grain structures and textural evolution at high temperatures were governed by a continuous DRX, whereas those at low temperatures were primarily controlled by a discontinuous DRX. Various parameters determined the grain structure and textural evolution in different regimes. The evolution of grain structures and texture was dominated by strains and strain rates at temperatures above 0.5Tm (Tm: melting temperature) and by the SFE at temperatures below 0.5Tm. The Zener–Hollomon parameters of the Al alloy joints were investigated. It was found that the slope in AA5052 is larger than that in AA1050 during LFW. At a lower Z value, the grain diameter of the AA5052 joints was larger than that of the AA1050 joints, presumably because of the lower strain rate. Contrastingly, at a higher Z value, the grain diameter of the AA5052 joints was smaller than that of the AA1050 joints, even though the strain rates of the AA5052 joints were considerably lower than those of the AA1050 joints. These results were mainly attributed to the lower SFE of AA5052, which promoted DRX and grain refinement during the LFW. This report reveals the effects of SFE, temperature, strain, and strain rate on the DRX-induced microstructural evolutions during severe plastic deformation.

Effects of dynamic recrystallization mechanisms on texture evolution in Mg-Gd-Y-Zr-Ag alloy during hot compression

In this paper, effects of thermal mechanical parameters on the flow behaviors and microstructure evolution of Mg-7.5Gd-1.5Y-0.4Zr-0.5Ag (wt%) alloy were analyzed by the hot compression test combined with electron backscatter diffraction and transmission electron microscopy. Results demonstrate that increasing temperature or decreasing strain rate can effectively improve the dynamic recrystallization (DRX) degree, decrease the flow stress, and weaken the whole texture intensity. Basal<a>and prismatic<a>slip are the dominant deformation mechanisms during hot compression. At a temperature range of 420–470 °C, discontinuous DRX (DDRX) and continuous DRX (CDRX) synergistically affect the texture evolution. DDRX randomizes the grain orientation and has a strong effect on weakening the basal texture. The limited effect of a single grain induced by CDRX on weakening texture intensity can be strengthened with the increasing number of grains at a high strain. At a high strain rate of 0.1 s−1, orientations of new DRX grains induced by compression twins are scattered, and can also weaken the<0001>//CD texture intensity. Therefore, this study provides the basis for further controlling the DRX behavior and texture characteristics for manufacturing large-scale wrought Mg alloys.

Zn–0.8Mn alloy for degradable structural applications: Hot compression behaviors, four dynamic recrystallization mechanisms, and better elevated-temperature strength

• Hot deformation behavior of Zn-0.8Mn alloy are revealed. • Constitutive equation and processing map of the alloy are established. • Four DRX mechanisms, i.e., DDRX, CDRX, TDRX and PSN, are activated. • The alloy exhibits excellent elevated-temperature strengths. Environmentally degradable Zn-0.8Mn alloy is highly ductile, which lays the foundation for developing high-performance Zn-Mn-based alloys. However, not only constitutive equation of this alloy is unknown, but also its dynamic recrystallization (DRX) behavior is unclear, which makes optimization of hot processing parameters of this alloy almost dependent on trial-and-error. This work aims to tackle these problems. The constitutive equation was deduced to be ε ˙ = 1.38 × 10 12 × [sin h (0.009 σ )] 8 exp(–135150/ RT ). A processing map of the alloy was obtained for the first time, which shows that it has excellent hot formability with narrow instability zones. At a final true strain of 0.8, the volume fraction of DRX grains increased from 37% to 79% with temperature increasing from 150 °C to 350 °C and strain rate decreasing from 10 s –1 to 10 –3 s –1 . Discontinuous DRX (DDRX), continuous DRX (CDRX), twinning-induced DRX (TDRX), and particle stimulated nucleation (PSN) were activated during hot compressions. DDRX was always the main mechanism. TDRX was completely suppressed at 300 °C and above. PSN arose from dispersed MnZn 13 particles. Furthermore, Zn-0.8Mn alloy exhibited elevated-temperature strengths better than pure Zn and Zn-Al-based alloys. At 300 °C and 0.1 s –1 , its peak stress was 1.8 times of pure Zn, owing to MnZn 13 particles of 277 ± 79 nm impeding the motion of grain boundaries and dislocations.

Strain rate and temperature sensitivity on the flow behaviour of a duplex stainless steel during hot deformation

ABSTRACT We elucidate here the temperature and strain rate sensitivity of a duplex stainless steel through the development of deformation maps and by studying the flow behaviour during hot compression tests in the temperature range of 1223–1473 K and at strain rates between 0.01 and 30 s−1. The flow curves exhibited softening after attaining a peak condition and the flow stress increased with the decrease of temperature and increase of strain rate. The observed change in the activation energy is attributed to strain partitioning between the two constituent phases and the deformation maps were characterised by two regions of flow instability. On the other hand, the microstructural evolution indicated that the strain rate and deformation temperature had a significant impact on the flow behaviour. In the flow stability region, the continuous dynamic recrystallisation (DRX) of ferrite was strongly activated, while in the flow instability region, both the continuous dynamic recrystallisation in ferrite and discontinuous DRX in austenite were weak.

Thermal Deformation Behavior and Microstructure Evolution of GH4151 Alloy

Herein, an as‐cast GH4151 alloy is subjected to thermal compression at different temperatures and strain rates by a thermal simulation testing machine. The constitutive equation, dynamic recrystallization (DRX) model, and hot processing map are obtained from the stress–strain curves. The results show that the optimal heat processing conditions are 1100–1120 °C, 0.001–0.01 s−1 and 1100–1135 °C, 0.001–0.1 s−1. Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) show that the recrystallization mechanism of GH4151 alloy is mainly discontinuous DRX (DDRX) and is supplemented by continuous DRX (CDRX). Deformation dislocations are eliminated by the phagocytosis mechanism of dislocations during recrystallization, and the subgrains and low‐angle grain boundaries are transformed into high‐angle grain boundaries, which finally form equiaxed grains. In addition, due to the pinning effect of γ′ phase on grain boundaries, the redissolution of γ′ phase during DRX greatly affects grain growth.

Experimental study on tensile deformation behaviors under room and elevated temperatures induced by microstructure inhomogeneity of ZK60 Mg with a longitudinal weld

The tensile tests of the extruded ZK60 Mg containing a longitudinal weld seam were carried out at room and elevated temperatures, and the effects of induced microstructure inhomogeneity on tensile deformation behavior was clarified. The results show that the deformation mode, dynamic recrystallization (DRX), texture evolution and mechanical properties are strongly affected by the longitudinal weld seam, temperature, and loading direction. The room temperature (RT) deformation of welding zone is controlled by the dislocation slips with the association of some twins, while twinning plays significant roles in the accommodation of c-axis strain of the coarse grains on matrix zone. The deformation at RT stretched along extrusion direction (ED) and transverse direction (TD) are controlled by basal slip/twinning and basal slip/prismatic slip/twinning, respectively. During high temperature tension, the dislocation cross slip of pyramidal slip is activated, and grain boundary sliding occurred in welding zone, leading to the superplastic behavior. With the increase of tensile temperature, the predominant DRX mode is transformed from continuous DRX to discontinuous DRX. Moreover, the basal poles of the grains spread from TD towards ED with the decrease of maximum pole intensity when stretched along ED, while non-basal textures are transformed to 〈10–10〉 fiber texture when stretched along TD. The slip-dominated flow is seen during RT tension along ED, while twinning becomes predominant during RT tension along TD. The fine grain structure causes the superior RT tensile properties along ED of welding zone with ultimate tensile strength of 315 MPa and elongation to failure of 13.8%. With the increase of tensile temperature, the slipping-dominated deformation is transformed into twinning-dominated, causing the decrease of strength and increase of elongation.

The Effect of Rotary-Die Equal-Channel Angular Pressing Process on the Microstructure, the Mechanical and Friction Properties of GW103 Alloy.

In this study, the effect of rotary-die equal-channel angular pressing (RD-ECAP) on the microstructure and texture evolution of GW103 alloy is studied. RD-ECAP processes were carried out for 1, 4 and 12 passes at 450 °C. The mechanical properties and friction behavior of RD-ECAP-processed Mg-10Gd-3Y (wt%) alloy (GW103) are discussed. The results reveal that the size of dynamic recrystallized grains and second-phase particles are significantly refined to about 1.3 μm and 1 μm, respectively. The texture evolution of the processed samples is studied by X-ray diffraction and electron backscattered diffraction techniques. The multiple texture components formed are not observed after the conventional ECAP process. Moreover, different dynamic recrystallization (DRX) mechanisms are systemically analyzed and discussed in view of the texture evolution of ECAP processed samples. The final textures obtained after 12 passes are identified as two types: The C-texture type induced by continuous and discontinuous DRX, and the random texture components induced by reorientation of the initial &lt;101¯0&gt; fiber. Based on the grain refinement, precipitate strengthening and texture weakening mechanisms, a high-performance ternary alloy of Mg-Gd-Y was firstly obtained through 12 passes RD-ECAP processing, with a combination of high yield strength of 312 MPa and a high ductility of 22%. In addition, the friction behaviors are also studied. The multi-pass-processed samples exhibit a relatively lower friction coefficient under a load of 10 N at room temperature.

Effect of twin nucleation and twin on the recrystallization of a nickel-based superalloy during hot compression

The nucleation mechanism of deformation twin (DT) and annealing twin (AT), and the effect of twins on recrystallization were studied comprehensively for Nickel-based superalloys during dynamic recrystallization (DRX). The aim is to improve the understanding of microstructure evolution during hot deformation. Hot compression tests were carried out at the temperature of 900-1150 °C with the strain rate of 0.01-1 s -1 for a Nickel-base superalloy. The results show that the DT and the Ʃ3 boundary exist simultaneously in the DRX stage. DTs are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. The DTs hinder the movement of dislocations in deformed grains and promote the nucleation of DRX grains. Ʃ3 boundaries associated with stacking faults are formed at recrystallized grain boundaries. In the condition of continuous DRX (CDRX), the Ʃ3 boundary changes the grain orientation and restrain the CDRX nucleation. While for the discontinuous DRX (DDRX), the Ʃ3 boundary provides an additional nucleation site and increase the nucleation rate. In addition, the comprehensive effect of DTs and ATs can promote the DRX nucleation, for which increase at first and then decrease with nucleation mechanical transform from CDRX to DDRX. • Deformation twins are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. • The quantity of deformation twins decreasing with the increase of DRX volume fraction. • Deformation twins hinder the movement of dislocations and promote the nucleation of CDRX. • The Ʃ3 boundaries hinder the CDRX nucleation and accumulate the DDRX nucleation process.

Microstructure evolution and constitutive model for a Ni-Mo-Cr base alloy in double-stages hot compression with step-strain rates

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.

Study on thermal compression deformation behavior and constitutive model of homogenized Mg–5Zn–1Mn alloy

The thermal compression deformation behavior of homogenized Mg–5Zn–1Mn (wt.%) alloy was systematically investigated over the temperature and strain rate ranges of 285–345 °C and 0.001–1 s−1, respectively. The constitutive model and thermal processing diagrams were established. The microstructure evolution process and dynamic recrystallization (DRX) mechanisms were analyzed. The results demonstrate that the flow stress curves of the ZM51 alloy exhibit prominent DRX characteristics. The peak stress can be improved by decreasing the deformation temperature or increasing the strain rate. A finite element simulation is performed, and the Arrhenius constitutive model can accurately depict the plastic flow process of the alloy with strain correction, and the average activation energy is about 143.821 kJ/mol. Both the percentage DRX and the DRX grain size are found to be sensitive to the applied deformation conditions, including temperature, strain rate, and true strain. The formation of DRX grains promotes the weakness of the texture and the refinement of the grains. Grain boundaries nucleation (including discontinuous and continuous DRX) and particle-stimulated nucleation (PSN) are the main DRX mechanisms. Based on the processing map, ZM51 alloy is suitable for large plastic deformation by multi-pass forging. The optimum processing temperature is 345 °C, and as the degree of plastic deformation increases, the strain rate can be suitably increased. Local plastic flow and microcracking at grain boundaries are the primary instability mechanisms of the alloy.

High-temperature deformation behavior and microstructural evolution of NbZrTiTa refractory high entropy alloy

The hot deformation behaviors and microstructural evolution of an equiatomic NbZrTiTa refractory high entropy alloy (RHEA) were studied by using isothermal compression tests in a range of temperatures (900 °C∼1200 °C) and strain rates (10−3 s−1∼1 s−1). A category of Arrhenius power law relationship was constructed and the activation energy was calculated to be 336–403 kJ mol−1 throughout the strain. The dislocation piling-up and kink bands found in local microstructural areas at low temperatures and high strain rates contributed to intense strain hardening and subsequent strain softening. As the temperature increased and the strain rate decreased, a sharp drop in flow stress was observed after the peak stress. This may be explained by dislocations pinned by short-range ordering (SRO) structures being unlocked. The higher dislocation density and newly emerging fine grains near deformed grain boundaries suggested that the dynamic recrystallization (DRX) was responsible for the continuous flow softening. Discontinuous DRX (DDRX) was the main DRX mechanism in this work, however, the isolated equiaxed grains in initial grains and large cumulative misorientation along the interior of the deformed grains were also found at the high temperatures and low strain rates, which demonstrated that the continuous DRX (CDRX) occurred.

Synergistic effect of Mn + Sc additions on the microstructure evolution and mechanical properties of Mg-Gd-Y-Mn-Sc alloy

Mg-Gd-Y-Mn-Sc alloys with different Mn + Sc contents were prepared by hot extrusion and ageing treatment. The effect of Mn + Sc additions on the microstructures, tensile properties, and corresponding strengthening mechanisms of the as-extruded and peak-aged Mg-Gd-Y-Mn-Sc alloys were systematically investigated. Mn + Sc additions had outstanding grain refinement effect on the homogenized alloy and promoted the supersaturation of the Gd and Y atoms in α-Mg matrix, facilitating the dynamic recrystallization (DRX) and dynamic precipitation during hot extrusion. The DRX mainly occurs via discontinuous DRX (D-DRX) and continuous DRX (C-DRX) mechanisms. The as-extruded alloy with higher Mn + Sc contents exhibits better strength-ductility balance with a tensile yield strength of 337 MPa, ultimate tensile strength of 406 MPa and elongation to failure of 10.8%, which is mainly attributed to higher DRX fraction and denser dynamically precipitated particles. The Mn + Sc additions benefits the age-hardening response and notably improves the tensile yield strength to be 484 MPa for the peak-aged alloys by ageing precipitation and GB solute segregation, which is superior than previously reported Mg-RE-Mn alloys.

Hot deformation behavior and microstructural evolution of the Al-Cu-Li alloy: A study with processing map

The 2195 Al-Cu-Li alloy is widely utilized for the fabrication of key aerospace vehicle components; hence, an in-depth understanding of its hot deformation behavior is essential for consistently achieving high-performance products. In this study, isothermal compression tests were performed on 2195 Al-Cu-Li alloy ingots in the temperature and strain rate ranges of 250–500 °C and 10−3-1 s−1, respectively. A processing map was established to predict the workability under different conditions. The corresponding microstructures were examined to reveal their hot deformation mechanism. Results demonstrated that the processing map was divided into four distinct domains. The unstable zone was located in the 250–375 °C/10−1.5-1 s−1 and 250–275 °C/10−2.5–10−1.5 s−1, temperature/strain rate regimes, in which deformation bands and flow localization were observed. The deformation bands blocked dislocations and compromised the dynamic recrystallization (DRX) behavior, hence damaging the microstructure. Further, large coarse T1 phases precipitated in the 325–410 °C/10−2.5–10−3 s−1 regime, which led to the formation of heterogeneities in the microstructure, corresponding to a minimum workability in the processing map. The optimal hot-working domain for the 2195 alloy was determined to be 460–500 °C/10−3–10−2 s−1, and the dominating hot deformation mechanisms were DRX and dynamic recovery (DRV). Specifically, continuous and discontinuous DRX behaviors dominated in the 300–400 °C/10−3 s−1 and 450–500 °C/10−3 s−1 regimes, respectively. DRX gradually weakened as the strain rate increased at 500 °C. Further, the weakening of DRX was only minimally affected by temperature at 400–500 °C during high-speed deformation (1 s−1). The results reported herein can provide valuable insights into the hot working of Al-Cu–Li alloys.

Modeling and characterization of dynamic recrystallization under variable deformation states

Flexible rolling with variable deformation states has brought new challenges to dynamic recrystallization (DRX) theory based on conventional constant deformation state. Modeling and characterization of DRX are investigated, inspired by the successful application of constitutive models and EBSD technique in the field of characterizing the recrystallization of metallic materials, combined with the characteristics of flow stress. Two constitutive models for constant and transient deformation states were considered. The dynamic recovery effect was integrated into the model for predicting the flow stress and DRX volume fraction. A hot processing map was established to predict the instability region. Various DRX synergistic regulation mechanisms and crystallographic orientation distribution under a transient deformation state were explored. The results suggest that DRX is the main softening characteristic under various deformation states. The predicted flow stress of the constant and transient models are in good agreement with the experimental values. The regulation mechanism of DRX transforms from continuous DRX to discontinuous DRX when the strain rate dynamic decreases. DRX grains display a random orientation distribution, and the crystallographic orientation is toward the 〈111〉 fiber parallel to the normal direction of the compression axis when the DRX volume fraction is high. These findings provide insight into the DRX constitutive description and microstructure evolution under variable deformation states.

Dynamic recrystallization behavior and strengthening mechanism of Mg–Gd–Y–Zn–Zr alloy during isothermal MDF process

As one of severe plastic deformation (SPD) methods, multi-directional forging (MDF) technology is an effective way to refine grains. The microstructure, texture evolution and mechanical properties of homogenized Mg–13Gd–4Y–2Zn–0.5Zr (wt. %) alloy during isothermal MDF process were synthetically investigated in the paper. The results showed that the mean grain size of the alloy decreased gradually with the MDF passes increased. After 4 passes, the average grain size of the alloy was refined to 2.7 μm, and the microstructure distribution was uniform. On the one hand, with the increasing of the MDF passes, the massive intergranular LPSO phases are gradually broken up and formed a deformation zone. The plate-like Mg5(Gd,Y) precipitates were located interior the grains. On the other hand, the spherical Mg5(Gd,Y) phases were randomly distributed with the increasing of MDF passes. In addition, the maximum texture intensity of the MDFed alloy on (0001) basal plane gradually was weaken from 15.993 of the 1MDFed alloy to 4.245 of the 4MDFed alloy, which was attributed to the increasing of volume fraction of dynamic recrystallized (DRXed) grains via MDF method. And the dynamic recrystallization (DRX) mechanisms were continuous DRX (CDRX), discontinuous DRX (DDRX), particle stimulated nucleation (PSN) and kink induced DRX. The maximum ultimate tensile strength (UTS) and tensile yield strength (TYS) after 4 passes were 385 MPa and 360 MPa respectively. The excellent mechanical properties of the alloy were owing to the grain boundary strengthening, dislocation strengthening, texture strengthening and precipitation strengthening. In addition, the second phase particles such as lamellar LPSO phases, broken block-shaped LPSO phases, spherical Mg5(Gd,Y) phases and plate-like Mg5(Gd,Y) phases were demonstrated to be the strengthening phases.

Hot workability and dynamic recrystallization mechanisms of pure nickel N6

The hot workability and dynamic recrystallization (DRX) mechanisms of pure nickel N6 were systematically investigated using hot compression tests. Based on hot compression data, the constitutive equation of N6 was developed and its reliability was verified. Its hot processing map was constructed, and combined with microstructural observations, a semi-quantitative response relationship between hot deformation parameters and microstructure was established. The DRX process of N6 is a thermally activated process and particularly sensitive to the strain rate. The optimal hot processing parameters for N6 were determined to be 950-1050 °C and 0.1-1 s-1. Furthermore, it was proven that the dominant nucleation mechanism is discontinuous DRX characterized by grain boundary bulging and twins assisting nucleation, while the continuous DRX characterized by subgrains combined with rotation is an inactive nucleation mechanism.

Investigation of isothermal continuous hot deformation behavior of the new as-cast alumina-forming austenitic alloy

Multi-pass isothermal continuous hot deformation experiments were designed for homogenization and recrystallization in the as-cast alumina-forming austenite (AFA) heat-resistant alloy. Flow curves and microstructure characterization were used to study the grain evolution with different deformation parameters in each pass. In particular, attention was paid to the dendrites and segregated phase. Under different hot deformation conditions, the alloys show significant work-hardening characteristics, which result from the strengthening effect of the primary segregated MC-type carbide. The increase in deformation temperature contributes significantly to dynamic recrystallization (DRX), and the main mechanism of DRX is discontinuous DRX (DDRX). The continuous DRX (CDRX) is less occurring. Static recrystallization (SRX) occurs during the isothermal holding time between deformation passes. SRX becomes more and more apparent with temperature and the holding time. The increase in the deformation passes leads to a gradual disappearance of the alloy dendrites and an increase in the recrystallization volume fraction. Meanwhile, the segregated phase gradually decomposes into particles and partially dissolves. However, the segregated phase particles eventually show band-like distribution. SRX grains around the segregated phase will exhibit similar orientations due to particle-stimulated nucleation (PSN) mechanisms. Until the grain size increases to the point that it can escape from particle restraint, it will transform to random orientation. The above study confirms that multi-pass isothermal continuous hot deformation can improve the as-cast microstructure and guide practical industrial production.

Microstructure and mechanical properties of Mg97Y2Zn1 alloy sheets processed by hot rolling

The microstructure and mechanical properties of Mg97Y2Zn1 alloys during rolling at 450 °C and 500 °C have been investigated in this work. The microstructure of pre-rolled alloy mainly consisted of the α-Mg matrix, block 18R LPSO phase distributed at the grain boundary, and lamellar 14H LPSO phase distributed in grain interior. After rolling, the 18R LPSO phase and 14H LPSO phase were kinks obviously at both temperatures of alloys. The alloy rolled at 500 °C produced serious DRX, and the occurrence of DRX is related to the 14H LPSO phase: (i) the interface between 14H LPSO phase and α-Mg matrix could act as a nuclear site of DRX grain and further refine grain by discontinuous DRX mechanism. (ii) The kink boundary of 14H LPSO phase evolved into a grain boundary through the continuous DRX mechanism and promotes the generation of DRX. Tensile testing indicated that the alloy rolled at 450 °C possessed the higher yield strength, and the alloy rolled at 500 °C had the higher ultimate tensile strength. The dominant deformation mechanism of the 450 °C rolled alloy was a prismatic slip and prismatic kink, and the formation of a strong rolling basal texture resulted in the improvement of yield strength. For the alloy rolled at 500 °C, the formation of rolling texture is mainly related to twinning. In addition, the severe DRX degree with refined DRX grains contributed to the improvement of ultimate strength and ductility for the alloy rolled at 500 °C.