Continuous Static Recrystallization Research Articles

Study on the grain development and dislocation evolution from dynamic to post-dynamic stage in IN718Plus superalloy; the role of second-phase

The introduction of hexagonal η-Ni3(Al,Ti)0.5Nb0.5 phase can refine and homogenize the microstructure of IN718Plus superalloy during hot working, but produces more complicated recrystallization behavior associated with second-phase modified structural mechanisms. Derived from this perspective, the role of η phase on recrystallization mechanisms and grain development from dynamic deformation to post-dynamic dwell was systematically investigated. The results indicated that η phase delayed the throughout recrystallization procedure and grain development. During dynamic deformation, boundary η inhibited discontinuous dynamic recrystallization via preventing the bulging of original grain boundaries, while intragranular lamellar η promoted heterogeneous strain concentration, stimulated random dislocations for continuous dynamic recrystallization and dislocation walls for banded dislocation substructures, which limited recrystallization occurrence in dynamic stage. During post-dynamic dwell, boundary η dissolution increased the nucleation sites for discontinuous static recrystallization, and η phase restricted grain boundary migration to provide abundant nucleation sites for continuous static recrystallization (CSRX). The banded dislocation substructures slowly evolved into subgrains to further form banded CSRX grains, which reduced grain size during early post-dynamic stage. After fully recrystallization, the residual η protected the small grains and impeded grain growth, meanwhile controlled the grain distribution. As a result, η phase induced sluggish dislocation evolution and recrystallization nucleation, consequently suppressed grain development and obtained the more homogeneous and fine grain configuration.

Effect of strain path and short-term post-annealing on the microstructure, texture, and mechanical anisotropy of low-carbon steel

In this research, the effect of strain path and short-term post-annealing on the microstructure, texture, and mechanical anisotropy of low-carbon steel was investigated. Asymmetric unidirectional rolling (AUR) and asymmetric cross rolling (ACR) were applied on Fe-0.072C steel. The ACR-processed sheet revealed a very limited number of new grains compared to the AUR-processed sheet, which was due to the dynamic recovery induced by strain path change. The average ferrite grain size of the post-annealed AUR- and ACR-processed samples was 11.5 and 14.1 μm, respectively. The dominant recrystallization in the annealed AUR-processed sheet was discontinuous static recrystallization, while that in the annealed ACR-processed sheet was continuous static recrystallization. The weak typical rolling textures (γ-fiber and α-fiber) at the midthickness of both rolled sheets demonstrated that the shear deformation effectively contributed to the formation of the shear textures during asymmetric cold rolling. The ACR decreased the intensity of the overall texture more than the AUR process due to the changes in the reference frame of deformation after each pass, which destabilized ideal texture components. The results showed that Goss and {111}〈112〉 components were continuous and discontinuous static recrystallization textures in the asymmetrically cold-rolled low-carbon steel sheet during the annealing treatment, respectively. The annealed AUR-processed sheet revealed a weaker texture as compared to the annealed ACR-processed sheet. The significant changes in the preferred orientations of the annealed AUR-processed sample led to weaker texture intensity compared to the annealed ACR-processed sheet. The hardness of the annealed AUR-processed sheet (183.8 HV) was higher than that of the annealed ACR-processed steel (156.9 HV), which was owing to the larger grain size and also the elimination of the γ-fiber texture in the annealed ACR-processed sheet. The strength along the rolling direction was lower than that along the transverse direction in the AUR-processed sheet. However, in the ACR-processed sheet, the values of strength were similar along three different tensile directions. As an important conclusion, if both high-strength and low-anisotropy are needed in the low-carbon steel sheets, it's better to apply ACR rather than AUR. In addition, if both high toughness and low anisotropy are essential, it's better to use AUR followed by post-annealing rather than ACR + post-annealing. The length of the intermediate stage of work hardening in the annealed ACR-processed sample was much less than the annealed AUR-processed sheet due to the presence of harder orientations (〈111〉 and 〈110〉) along rolling and transverse directions in the annealed ACR-processed sample. Finally, the fracture behavior of deformed and post-annealed steel sheets for both strain paths was ductile and shear ductile.

The coupling effect of particle and electric current on the grain refinement of Al–Cu–Li alloy by an improved thermo-mechanical treatment with electrically-assisted annealing

An improved thermo-mechanical treatment with electrically-assistant recrystallization annealing (EARA) was proposed for Al–Cu–Li alloy with the fine and equiaxed grains. The Al–Cu–Li alloy strips were subjected to the different overaging times, asymmetric rolling and EARA. The size and number of particles had a significant effect on the average grain size. The critical particle size for particle-stimulated nucleation (PSN) was determined. The introduction of electric current did not change the critical size, however, the efficiency of PSN triggered by coarse particles was improved by 3.6 times, compared with the conventional recrystallization annealing (CRA). By the optimized overaging time, the grains were refined and the texture was weakened. The recrystallization mechanism for the specimens containing coarse particles was the discontinuous static recrystallization (DSRX) by PSN and the continuous static recrystallization (CSRX). At the constant current density and annealing time, the critical temperature for rapid recrystallization for EARA decreased by 40 °C, compared with CRA. The results were due to the local Joule heating effect, the activation energy reduction and atomic diffusion rate increase by the athermal effect of electric current. The improved RI-ITMT with EARA would be an ideal candidate for rapid grain refinement in Al–Cu–Li alloy alloys.

Microstructure evolution and recrystallization kinetics of Al-Cu-Li alloy during thermo-mechanical treatment with overaging and electrically-assisted annealing

An improved thermomechanical treatment (N-ITMT) containing electrically-assisted recrystallization annealing (EARA) was proposed for manufacturing fine-grained Al-Cu-Li alloys, based on overaging, cold asymmetric rolling and EARA. The corresponding microstructure evolution and mechanical properties were investigated. The coarse precipitates introduced by overaging changed the deformed microstructure, produced particle affected zones and weakened the shear band (SB) compared with that with no precipitates. The combined action of electric current and coarse precipitates completed the recrystallization within 5 s and the solution process within 180 s, refined the grain size, and improved the strength and ductility. Discontinuous static recrystallization (dSRX) and continuous static recrystallization (cSRX) were both considered recrystallization mechanisms for specimens with or without coarse precipitates. The dSRX for specimens with coarse precipitates mainly featured particle-stimulated nucleation (PSN), while it featured shear band nucleation for specimens with no precipitates. The athermal effect of electric current not only decreased the activation energy, increasing the nucleation rate during dSRX, but also accelerated the diffusion of atoms and vacancies, promoting recrystallization and solution treatment. The proposed N-ITMT containing EARA possesses the synergetic capacity of optimizing the microstructure and improving the strength and ductility of Al-Cu-Li alloys.

Recrystallization of Hot-Rolled 2A14 Alloy during Semisolid Temperature Annealing Process.

In this study, in order to provide proper parameters for the preparation of semisolid billets, the semisolid annealing of hot-rolled 2A14 Al alloy was investigated. The microstructure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with an X-ray energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD), and scanning transmission electron microscopy (STEM). The XRD results showed that, with an increase in temperature, the θ-Al2Cu equilibrium gradually dissolved in the matrix. The EDS results of SEM and STEM showed a coarse θ-Al2Cu phase, ultrafine precipitate Al(MnFeSi) or (Mn, Fe)Al6 phase, and atomic clusters in the microstructure. The EBSD results showed that the recrystallization mechanism was dominated by continuous static recrystallization (CSRX), homogeneous nucleation occurred when the sample was heated to near solidus temperature, and CSRX occurred at a semisolid temperature. In the process of recrystallization, the microtexture changed from the preferred orientation to a random orientation. Various experimental results showed that static recrystallization (SRX) occurred at a semisolid temperature due to the blocking effect of atomic clusters on the dislocation slip, and the Zener drag effect of fine precipitates on low-angle grain boundaries (LAGBs) disappeared with melting at a semisolid temperature.

Strain Rate Dependence and Recrystallization Modeling for TC18 Alloy during Post-Deformation Annealing.

In this paper, the dependence of dynamic recrystallization (DRX) and post-dynamic recrystallization (PDRX) of TC18 alloy on strain rate within the range of 0.001 s-1~1 s-1 was investigated through isothermal compression and subsequent annealing in the single-phase region. Electron backscatter diffraction (EBSD) characterization was employed to quantify microstructure evolution and to reveal the recrystallization mechanism. At the thermo-deformation stage, the DRX fraction does not exceed 10% at different strain rates, due to the high stacking fault energy of the β phase. During the subsequent annealing process, the total recrystallization fraction increases from 10.5% to 79.6% with the strain rate increasing from 0.001 s-1 to 1 s-1. The variations in the geometrically necessary dislocation (GND) density before and after annealing exhibit a significant discrepancy with the increasing strain rate, indicating that the GND density is a key factor affecting the PDRX rate. The PDRX mechanisms, namely meta-dynamic recrystallization (MDRX), continuous static recrystallization (CSRX) and discontinuous static recrystallization (DSRX), were also revealed during the annealing process. A new kinetic model coupling DRX and PDRX was proposed to further describe the correlation between recrystallization and the strain rate during continuous deformation and annealing. This new model facilitates the prediction of recrystallization fraction during isothermal deformation and annealing of titanium alloys.

Investigation of anisotropy and structure variation of spray-formed 2195 Al-Li alloy via final temperature-controlled rolling and cold rolling

The mechanical property anisotropy is one of the critical factors of limiting extensive application of the Al-Li alloy, which is mainly influenced by the textures, grain morphology and distribution of the aging precipitates. Through the plastic deformation process, the anisotropy can be weakened by regulating the grain structure. In this work, the final temperature-controlled rolling and cold rolling were carried out to investigate the grain structure variation and its effect on the mechanical property anisotropy of the spray-formed 2195 Al-Li alloy. The results show that the yield strength is improved but the anisotropy in the mechanical properties deteriorates as the final rolling temperature is elevated from 200 ℃ to 300 ℃. This is owed to the fibrous structures and a great number of deformation textures formed in the continuous static recrystallization process. Compared with the samples through final temperature-controlled rolling, the aged sample through cold rolling possesses higher yield strength and relatively weaker anisotropy due to the fine equiaxed grain structure. In addition, intense Brass and S textures existing in the plates are proven that can result in the heterogeneous distribution of the T1 phase and the anisotropic equivalent slip system number values along the longitudinal and transverse directions, which both contribute to the strong anisotropy. To facilitate the analysis of the effect of the textures on the mechanical properties and anisotropy, the equivalent Schmid factor and slip system number were also calculated and discussed. This study looks forward to providing reliable theoretical support for the practical application of spray-formed Al-Li alloys.

On the solution treatment of severely deformed AA2024-O and subsequent natural aging

In this research, the effect of applying severe strain before solid solution treatment (SST) for AA2024-O was studied. Providing concurrent insights into solutionizing behavior and microstructural evolution corresponding to hardness is important. Multi-directional forging (MDF) was used as a severe plastic deformation (SPD) method. After applying SST on MDFed (4 passes) and as-annealed samples, natural aging (NA) was used as the following treatment to examine the role of severe strain on the solutionizing behavior. In most previous attempts, SPD has been used after SST, but one of the innovations of the present work is the study of SPD before SST. Experimental results show that adding a severe strain using the MDF method before SST results in a significant refinement in the grain size of the solutionized recrystallized microstructure (47 μm). In contrast, coarse-grained microstructures with grain sizes of 150 μm–300 μm are used in prior tries. The results indicate that continuous static recrystallization (CSRX) with a grain size of 47 μm occurs during the solution treatment of the MDFed sample, which is 220% less than that during solution treatment of the as-annealed sample (150 μm); also, rich presence of the second phase particles after its aging by FE-SEM was analyzed. By measuring the difference in hardness before and after NA in solution-treated MDFed and as-annealed samples, 500 °C and 5 min were found to be the most desirable temperature and times in terms of mechanical properties and microstructure. While, in previous efforts, no suitable time has been found for solution treatment of this alloy, and times of 2 h–24 h have been used. The present work found a suitable time of 5 min for solution treatment, which has significant energy savings. In the solution-treated MDFed sample, the maximum hardness of NA (100 HV) was reported; additionally, the percentage of increase in hardness before and after NA (ΔH %) for solution-treated MDFed and as-annealed samples were obtained at 43% and 23%, respectively. Using the DSC test, it was found that in the solution-treated MDFed sample, the peak of S′/S (Al2CuMg) phase formation spreads and shifts from 262 °C to a lower temperature of 235 °C. Using Johnson-Mehl-Avrami (JMA) equations, the values of 103 kJ/mol and 108 kJ/mol were obtained for the activation energy of solution-treated MDFed and as-annealed samples, respectively. Lower activation energy means utilizing Severe Plastic Deformation (SPD) before SST generates a high density of dislocations and vacancies that accelerate solute atoms dissolution and provide S′/S -phase nucleation sites.

Cтруктура и прочность мелкозернистой меди после криогенной прокатки и обработки однократными импульсами тока различной мощности

The effect of a treatment combining isothermal cryogenic rolling and single electro-pulsing on the structure and hardness of M1 grade copper with an initial grain size of 10 –15 μm was investigated. Copper was deformed at liquid nitrogen temperature by multi-pass rolling with a total reduction of 90 %. Subsequent electro-pulse treatment (EPT) was carried out in the interval of integral current densities (Kj) from 3.5 ×104 to 8.1×104 A2 s / mm4. Nearly two-fold strengthening of Cu under rolling was found due to the formation of a heavily deformed (sub)grain structure with a crystallite size of the order of 1 μm and a fraction of high-angle boundaries of about 30 %. With further EPT with an energy of 3.5 ×104 A2s / mm4, the processes of recovery predominantly occur, resulting in slight softening with a simultaneous strong decrease in micro-distortions of the crystal lattice and dislocation densities. Processing with higher energies resulted in a sharp drop in the hardness of Cu owing to an activation of in-situ continuous static recrystallization, forming regions of new fine defect-free grains, whose fraction increased with Kj, and intensified the formation of annealing twins. As a result of EPT with Kj in the range of 5×104 – 7×104 A2s / mm4, a uniform fine-grained structure with a grain size of near 2 μm and a fraction of high-angle boundaries of about 90 %, a third part of which were twins of Σ3, was obtained. With a further increase in pulsing energy to 8.1×104 A2s / mm4, normal grain growth to 4 μm with minor changes in the angular parameters of the structure were observed. The nature of structural and mechanical behavior of copper is discussed. A conclusion is made on the viability of the use of combination of cryogenic rolling and EPT to manufacture ultrafine- and fine-grain sheets of different strength out of copper.

Revealing grain structure development and texture evolution during elastic stress-assist aging of Mg–Zn alloys

The evolution of grain structure and corresponding texture development of Mg alloys under the assist of multiple physical fields are interesting and meaningful topics, and are investigated in this paper in detail. The results show that the texture development of stress free (SF) aged alloy exhibits a remarkable difference in type and intensity to those of 100 MPa tensile stress (100-TS) and 100 MPa compressive stress (100-CS) aged alloys at 160 °C. In the early stage of SF aging, preferential growth of<11-20>//ED grains give rise to the enhancement of<11-20>//ED fiber component. The increased pinning force caused by considerable precipitates at high energy boundaries reduce the boundary mobility, and therefore lead to a weak basal texture at the end of aging. Heavily activated prismatic<a> dislocations under the action of 100-TS aging facilitate the precipitation of secondary phases at grain boundaries and lattice rotation induced continuous static recrystallization (CSRX) in the early stage of aging, which contributes to a weak basal texture and the formation of non-fiber texture. Upon further aging, the preferential growth of pre-existing<10-10>//ED grains and recrystallized grains formed by CSRX contribute the enhancement of<10-10>//ED basal fiber component. In the case of 100-CS aging, the slight growth of<11-20> grains and lattice rotation caused by prismatic<a> dislocations lead to the enhancement of<11-20>//ED texture component in the early stage of aging. After long-time compressive stress aging, preferential growth of<10-10>//ED grains gives rise to the formation of<10-10>-<11-20> double fiber components.

Gallium Liquid Metal Embrittlement of Tin-based Solder Alloys

We report on the embrittlement of tin (Sn)-based solder by liquid gallium (Ga). Exposure to liquid Ga results in a degradation of the intrinsic mechanical properties of the solder due to its ductile-to-brittle transition. Important changes in the solder microstructure accompany this degradation and are predominantly associated with a spontaneous recrystallization of Sn during Ga diffusion into the solder. A detailed investigation of the solder microstructure evolution identifies the recrystallization process as a Continuous Static Recrystallization (C-SRX). Taking into account mechanisms of atomic transport as well as the creation and propagation of defects, a novel liquid metal embrittlement (LME) model is herein proposed, namely the Diffusion-Induced Recrystallization Evolution (DIRE) model.

Thermal stability of the HPT-processed tungsten at 1250 – 1350 °C

Unveiling the influence of the high-pressure torsion (HPT) on microstructure for pure tungsten under annealing process is of great significance to realize the control of thermal stability and irradiation resistance at high temperatures. To this end, a detailed investigation on the microstructure evolution, energy release and its hardness change is conducted. The results show that the microstructure with homogeneous fine grains and the favorable orientation with high irradiation resistance appeared after the continuous static recrystallization (cSRX). During the HPT and subsequent annealing process, the stored energy change is mainly affected by high angle grain boundaries (HAGBs). Although the ultrahigh stored energy released in the annealing process, the homogeneous fine grains and the boundaries with high average misorientation lead to the stable grain growth with the low boundary velocity even at high temperatures and then improve the irradiation resistance of tungsten. And the high hardness retention is also presented at 1350 °C. Results reveal that, performing HPT makes it possible to improve the thermal stability and irradiation resistance in pure tungsten.

Microstructure tailoring of Al0.5CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation (SPD) have high strength but often poor uniform ductility. SPD via high-ratio differential speed rolling (HRDSR) followed by an annealing treatment was applied to Al0.5CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved. The optimized microstructure was composed of an ultrafine-grained FCC matrix (1.7–2 μm) with a high fraction of high-angle grain boundaries (61 %–66 %) and ultrafine BCC particles (with a size of 0.6–1 μm and a volume fraction of 8 %–9.3 %) distributed uniformly at the grain boundaries of the FCC matrix. In the severely plastically deformed microstructure, the nucleation kinetics of the BCC phase was accelerated. Continuous static recrystallization (CSRX) took place during the annealing process at 1273 K. Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains . The precipitation of the hard and brittle σ phase was, however, suppressed. The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14 %–20 %. These tensile properties are comparable to those of transformation-induced plasticity steel. Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified, and each strengthening mechanism contribution was estimated. The calculated results matched well with the experimental results.

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei. The results show that orientation distribution of nuclei is associated with nucleation mechanism. The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation. Differently, discontinuous static recrystallization nuclei formed within the parent grains (TD-45° orientation) show random orientations and a variety of misorientation angles but preferred axes <52¯73> or <52¯70>. Interestingly, a special oriented nucleation is found. Discontinuous static recrystallization nuclei originated from boundaries of the parent grain (TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70° <112¯0> (∑18b). It is speculated that these two special misorientation relationships are related to the dislocation type.

Microstructure and Mechanical Evolution of Cu-2.7Be Sheets via Annealing

The microstructure and mechanical properties of cold-rolled Cu-2.7Be sheets under various annealing processes and conditions were investigated in this research. The increased beryllium content in the Cu-2.7Be alloy facilitates the formation of brittle secondary phases. Consequently, the study highlights the functionality of annealed Cu-2.7Be alloys as more favorable dynodes than the traditionally used Cu-2.0Be alloys. The mechanism of recrystallization used for the transformation of Cu-2.7Be alloys was that of continuous static recrystallization (cSRX). Moreover, the relationship between the orientation of the β phases and that of the surrounding Cu-matrix was determined to be (111)α∥(110)β and (011)α∥(001)β. The β phase has a body-centered cubic (bcc) structure with a = b = c = 0.281 nm. The β phase undergoes a morphology transformation from primitive lath-shaped β particles to quadrangle-shaped β particles during the annealing process. Such transformations could potentially have an effect on the mechanical properties of Cu-2.7Be sheets. There was a noticeable decline in the yield strength of the Cu-2.7Be after annealing, and the samples annealed at 770 °C for 15 min achieved the elongation with deep and uniform dimples caused by suitable β particle sizes, appropriate grain sizes, and the maximum volume fraction of ∑3 boundaries.

Microstructure and texture in cryomilled and spark plasma sintered Ti Grade 2

Titanium (Grade 2) was processed by cryogenic milling and subsequently sintered by spark plasma sintering (SPS) method with the aim of creating and preserving the ultra-fine grained (UFG, &lt; 1 μm) microstructure. Microstructural investigation was performed after both cryogenic milling and spark plasma sintering. An advanced technique of transmission Kikuchi diffraction (TKD) was used to characterize the individual milled powder particles.Investigations of milled powders showed significant grain refinement down to 50 nm after milling in liquid argon with tungsten carbide balls. We assume that this is the equilibrium grain size resulting from the balance of deformation, recovery and dynamic recrystallization. A texture, resembling the rolling texture in Ti, was also found in the milled particles, which can be explained by the nature of deformation during milling.UFG microstructure was not maintained after sintering, with the mean grain size of 2.6 μm. Although the grains are completely recrystallized, a texture, similar to the powder texture, was also found in these samples as a result of packing of the powder particles and the nature of the recrystallization process (continuous static recrystallization).

Effect of flake-like powder morphology on the microstructure and texture in Mg-Al-RE alloy

Wet-milling of magnesium powder was performed at room temperature in ethanol covered by argon atmosphere. It is shown that the wet-milling in ethanol can produce highly deformed powder of flake-like morphology. Subsequently, bulk samples were consolidated by spark plasma sintering technique in the temperature range 400–550 °C. It is shown that milling caused severe microstructural refinement. The flake-like shape of the milled particles was transferred into the compacts and caused formation of a layer-like microstructure which was predominantly perpendicular to the external force applied during sintering. Grain growth was highly limited by powder particles’ interface and secondary phase particles present in the matrix. Formation of texture in the compacts was attributed to the combination of SPS thermo-mechanical effect and continuous static recrystallization acting during the sintering.

Recrystallization behavior and its relationship with deformation mechanisms of a hot rolled Mg-Zn-Ca-Zr alloy

The microstructure-texture evolution of twin-roll-cast strips of an Mg-Zn-Ca-Zr alloy subjected to hot rolling and recrystallization annealing was investigated. Upon annealing, a distinctive change in the main texture components is observed from basal poles tilted towards the rolling direction (RD) to basal poles tilted towards the transverse direction (TD). X-ray diffraction analysis and EBSD measurements indicated that the distinct change of the texture components during recrystallization annealing is related to the weakening of the global texture intensity. It is demonstrated that the controlling mechanism of the microstructure development during the annealing is the growth of recovered volumes within deformed grains in a regime of a continuous type of recrystallization. Recrystallized grains developed from the {1 0–1 1}-{1 0–1 2} secondary twinned areas were observed, which show a restricted contribution to the recrystallized microstructure formed in a discontinuous type of recrystallization. In-grain misorientation axis analysis from the as-rolled microstructure indicates that prismatic <a> together with basal <a> slip become the dominant dislocation modes for the grains with their c-axis inclined relative to the normal direction. Several grains with the orientations of the basal pole split to the transverse direction are the {1 0–1 2} extension twins. This leads to a mirror symmetry of the basal poles along the transverse direction. It is observed that during recrystallization annealing, deformed volumes of grains containing {1 0–1 2} extension twins can recover effectively. This leads to the development of grains having the TD-split orientation in the regime of continuous static recrystallization also known as extended recovery.

Tailoring the Rolling Texture of AZ31 Mg Alloy with Calcium and Tin Addition

A small amount of Ca and Sn is added into AZ31 alloy separately to modify its deformation behavior and optimized rolling texture. Results show that addition of Sn increases the activation of twinning in AZ31. Some unusual double twins, inducing and double twins, are activated in AZ31–0.5Sn alloy. Refined grains with similar orientation as their parents are produced by continuous static recrystallization, which limits texture modification in AZT310. Ca gives rise to a remarkable grain refinement during hot rolling. Recrystallized grains in AZX310 exhibit a weaker double peak texture with a wide orientation spread. It is found that Al2Ca particles simulate recrystallized nucleation and some of these refined grains exhibit non‐basal orientations. Primary compression twins are favorable nucleation sites for recrystallization and they are more effective in texture modification compared to other double twins.

Shape memory and superelasticity of nanograined Ti-51.2 at.% Ni alloy processed by severe plastic deformation via high-ratio differential speed rolling

A novel method of producing nanograined Ni-rich superelastic NiTi alloys in sheet form was proposed using a combination of severe plastic deformation via high-ratio differential speed rolling (HRDSR) and post-deformation annealing. The HRDSR-processed microstructure was composed of heavily deformed austenite and martensite grains, and amorphous phases. After annealing at 673 K, the severely deformed microstructure with no functional properties evolved to the nanograined structure (20–70 nm) composed of austenite and martensite nanograins and sub-nanograins through static recovery or continuous static recrystallization process. The nanograined microstructure had a high resistance to martensitic transformation upon cooling and slip deformation during straining. As a result, the HRDSR-processed alloy annealed at 673K exhibited superior superelasticity compared to the alloys with coarse grains. At the higher annealing temperature of 873 K, the micron-sized recrystallized grains with low dislocations developed through discontinuous static recrystallization process. In this case, deformation during straining was governed by the detwinning of twinned martensite, and as a result, shape memory effect was more significantly pronounced than superelasticity.