Post-dynamic Recrystallization Research Articles

Dynamic and Postdynamic Recrystallization Behavior of GWZ Magnesium Alloy Under Double‐Hit Hot Compression

Herein, dynamic and postdynamic recrystallization behaviors of GWZ magnesium are investigated. Toward this end, the single‐hit and double‐hit hot compression tests are conducted under strain rate of 0.001 s−1 at 400 °C. The prestrains of 0.1 and 0.5 are considered to investigate the effect of interpass time (5–300 s) on the compressive strength level. At the low strain level of 0.1, the contribution of Hall–Petch effect is considerable due to the occurrence of static recrystallization. In addition, the rare earth texture component is eliminated during interpass annealing. This causes increasing the strength of the material during second pass of hot compression. In contrast, at higher imposed strain, the strength level decreases with increasing the interpass time of annealing. The high amount of strain is completely consumed and the remaining stored energy is not high enough to trigger the occurrence of static recrystallization. The occurrence of metadynamic recrystallization and subsequent growth are characterized. In addition, the texture does not change in respect of the intensity or numbers/types of components. Accordingly, the observed decreasing trend of the strength is justified relying on the occurrence grain growth.

Supersolvus Recrystallization and Grain Growth Kinetics for the Fine Tuning of Grain Size in VDM Alloy 780 Forgings

VDM Alloy 780 is a new polycrystalline nickel-based superalloy developed for aeronautical applications. In most of the targeted applications, grain size after forging must be precisely controlled to meet the targeted mechanical properties and in-service life requirements. Grain size in forgings is the direct consequence of the recrystallization and grain growth kinetics which are addressed in this paper at high temperatures, above the solvus temperature of γ′ and η/δ phases. The dynamic and post-dynamic recrystallization kinetics as well as the grain growth kinetics of VDM Alloy 780 are detailed over a range of thermomechanical conditions. Dynamic recrystallization appears to be limited, with only 30 pct recrystallized at quite high strain of 1.7 applied at 1050 °C and 0.01 s−1 for instance, but this is compensated by fast post-dynamic evolution. Within the investigated thermomechanical range, recrystallization is completed with 5 minutes of post-deformation hold in VDM Alloy 780 independent of the prior strain, strain rate and dynamic recrystallization fraction. For a strain as low as 0.08, an isothermal annealing of 30 minutes at 1050 °C generates a homogenous and fully recrystallized microstructure. Capillarity driven grain growth following recrystallization is also relatively slow, for instance an exposure at 1050 °C (50 °C above the solvus temperature) for 2 hours results in an increase in average grain size from 20 to 70 μm. This opens the possibility to fine tune the grain sizes by subsequent heat treatments within a time scale that is compatible with industrial conditions. The high cobalt content (25 pct) is suspected to play a role in the control of microstructure evolution kinetics. It is noteworthy that VDM Alloy 780 is shown here to not undergo the heterogeneous grain growth phenomenon reported in low strain regions for other nickel-based superalloys, which is also an asset for applications requiring strict control of grain sizes and grain size distributions.

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.

Microstructure evolution in GH4742 superalloy by combining hot deformation and heat treatment

The microstructure evolution of GH4742 superalloy under the coupling action of hot deformation and heat treatment was studied by isothermal compression and then solvus heat treatment (SHT), with particular attention focused on the interaction mechanism between γ′ precipitates and dynamic recrystallization (DRX) grains. The deformation of material below the γ′ solvus temperature (1020–1050 °C) by DRX was retarded because of the strong pinning effect of γ′ precipitates. At a strain of 0.6 and all strain rates (0.01–1 s−1), a necklace-like microstructure formed with fine DRXed grains (∼2 μm) and a small DRXed fraction (<20%). The γ′ precipitates dissolved with the migration of DRX grain boundaries (GBs), and there were no secondary γ′ precipitates within DRXed grains. During sub-solvus heat treatment (sub-SHT) of pre-deformed samples below the γ′ solvus temperature, the nucleation rate of post dynamic recrystallization (PDRX) increased significantly (30–50%), and the recrystallized grains were small (∼10 μm) due to a large amount of energy stored after thermal deformation. Rod-shaped γ′ precipitates appeared on the DRXed GBs, while coarse γ′ precipitates (3–5 μm) appeared on the PDRXed GBs and formed an approximate γ+γ′ duplex structure. When the strain was 0.6, regardless of changes in the strain rate (0.01–1 s−1) and temperature (1020–1140 °C) of pre-deformed samples, the microstructure was comprised of large, equiaxed grains with straight GBs and precipitates (20 nm) formed by cooling during super-solvus treatment (super-SHT).

Model for characterizing static softening behaviour of GH4500 superalloy during two-pass thermal compression

In this work, the static softening behaviour of GH4500 superalloy during the two-pass thermal deformation was investigated via thermal-simulation compression experiments at the temperature range of 1293 K to 1373 K, strain rate range of 0.01 s−1 to 1 s−1 and interval time range of 0 s to 180 s. The metallographic structure acquired from optical microscope (OM) indicated that both static recovery (SRV) and post dynamic recrystallization (PDRX) would occur during the holding stage. Meanwhile, the influence of interval time, deformation temperatures and strain rates on static softening behaviour were revealed. The results showed that the softening effects principally induced by PDRX strengthened with the increase of the interval time, deformation temperatures and strain rates. To quantitatively describe the PDRX kinetic process of GH4500 superalloy, a model on grounds of Avrami kinetics was established to predict the PDRX softening fraction via eliminating the effect of static recovery. The predicted data of the PDRX softening fraction were well consistent with the experimental data, manifesting that the model could precisely evaluate the PDRX softening behaviour in the stage of inter-pass holding.

Simulation of Continuous Dynamic Recrystallization Using a Level-Set Method

Dynamic recrystallization is one of the main phenomena responsible for microstructure evolution during hot forming. Consequently, obtaining a better understanding of dynamic recrystallization mechanisms and being able to predict them is crucial. This paper proposes a full-field numerical framework to predict the evolution of subgrain structures upon grain growth, continuous dynamic recrystallization, and post-dynamic recrystallization. To be able to consider a subgrain structure, two strategies are proposed. One relies on a two-step tessellation algorithm to generate a fully substructured microstructure. The second strategy enables for the simulation of the formation of new subgrains during hot deformation. Using these tools, the grain growth of a fully substructured microstructure is modeled. The influence of microstructure topology, subgrain parameters, and some remaining stored energy due to plastic deformation is discussed. The results highlight that the selective growth of a limited number of subgrains is observed only when mobility is a sigmoidal function of disorientation. The recrystallization kinetics predicted with different criteria for discrimination of recrystallized grains are quantitatively compared. Finally, the ability of the framework to model continuous dynamic and post-dynamic recrystallization is assessed upon a case study representative of the hot extrusion of a zircaloy-4 billet (T=650 °C;ε˙=1.0s-1;εf=1.35). The influence of grain boundary properties and nucleation rules are quantified to evaluate the model sensitivity and suitability. Application of these numerical tools to other thermomechanical conditions and microstructures will be presented in an upcoming article.

Effect of inter-pass annealing on the deformation microstructure of Ti-48Al-2Cr-2Nb alloy

TiAl alloy is a potential candidate structural material for lightweight and high temperature in aerospace, and multi-pass thermo-mechanical treatment, where dynamic recrystallization (DRX), static recrystallization (SRX), and post-dynamic recrystallization (PDRX) have significant effects on microstructure evolution, is commonly applied for microstructure control due to its poor plasticity. Accordingly, the complex recrystallization behaviors of Ti-48Al-2Cr-2Nb alloy were investigated by hot simulation compression experiments. The results of metallographic statistics showed that microstructure inhomogeneity of uniaxial compression could not be improved by changing the temperature and strain rate to adjust the DRX degree. Additionally, discontinuous dynamic recrystallization (DDRX) characterized by the formation of twining boundaries was a dominant DRX mechanism and generated at grain boundaries. Meanwhile, continuous dynamic recrystallization (CDRX) was formed within the grains and at grain boundaries, accompanied by the relatively high distortion at the initial stage. Residual deformed microstructure owing to the incomplete DRX could be eliminated by SRX and PDRX during the inter-pass annealing. A method based on the grain size difference induced by various recrystallization mechanisms was proposed to reveal that grain growth resulting from PDRX played a primary role and SRX was impaired with the increasing strain. The interaction effect of DRX, SRX, and PDRX during the multi-pass deformation could achieve the uniform microstructure, which is consistent with a near-isothermal multi-pass multidirectional canned forging process designed in the paper. Eventually, room-temperature tensile tests demonstrated that the mechanical properties of wrought billet were distinctly enhanced.

Assessment of the post-dynamic recrystallization effects on the overall dynamic recrystallization kinetics in a Ni-base superalloy

Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.

Towards Multi-Scale Modeling of Zirconium Alloys Recrystallization and Application to Thermo-Mechanical Processes Optimization

Zirconium alloys are used in the nuclear industry due to their low neutron capture cross-section and resistance to corrosion, irradiation and creep. The microstructure of the nuclear fuel components evolves during the manufacturing route and can impact the subsequent processes or the final properties. Thus, numerical modeling of thermo-mechanical manufacturing processes is of interest to understand and master these microstructure evolutions.Numerical modeling of thermo-mechanical manufacturing processes with FORGE® NxT software is applied. These models provide the thermo-mechanical history of the material at each integration point of the finite element (FE) mesh, which can be used to assess locally the continuous dynamic and post-dynamic recrystallization during hot extrusion.Mean-field models were developed in Python and integrated into FORGE® NxT software, to quantify the microstructure evolution at the macro-scale of the component. Full-field models (DIGIMU® software1) were also developed for considering microstructural heterogeneities and the influence of initial microstructure at the mesoscopic scale while improving the mean-field equations by homogenization.After validation based on experimental results, these two recrystallization models provide complementary information to optimize the process parameters at the macro-scale and to better understand mesoscopic scale phenomena, such as:• At the macro-scale: influence of hot extrusion parameters on the continuous dynamic and post-dynamic recrystallization of Zircaloy-4.• At the meso-scale: influence of the initial microstructure on the recrystallization phenomena with improved precision. Indeed, the topology of the microstructure is predicted and not only the mean values/distributions of the state variables.

Perspectives of Microstructure Refinement of Aluminum and Its Alloys by the Reciprocating Extrusion (Cyclic Extrusion Compression-CEC).

This paper presents a study on the perspectives of structure refinement of aluminum and its alloys by reciprocating extrusion (cyclic extrusion compression—CEC). The study included Al99.5 and Al99.992 aluminum and AlMg5 and AlCu4Zr alloy. Aluminum and alloys were deformed by reciprocating extrusion (CEC) in the strain range ϕ = 0.42 (1 CEC cycle) to ϕ = 59.8 (67 CEC cycles). After deformation, the structure of the specimens was investigated by optical microscopy (OM) and transmission electron microscopy (TEM), which revealed that the primary mechanism of hardening, over the range of applied strains, was the result of the propagation of shear bands throughout the specimens. The intersection of shear bands was found to divide the volume of the specimens into nano and microvolumes with dimensions limited by the width of the microbands. Due to structure renewal processes such as polygonization and dynamic geometric recrystallization, the formed micro and nano volumes were transformed into nano and micrograins with large misorientation angles. In terms of the occurrence of grain microstructure, a sustained uniform level of hardening was found, which was defined as steady-state flow. The research has shown that the steady state of flow is a result of the competitive interaction between the processes of hardening and structure renewal. The higher the metal purity, the higher the intensity of the structure renewal processes was. The formation of new grains and their growth under dynamic and post-dynamic recrystallization was observed in Al99.992 aluminum, in which high purity of the metal and high strain accumulation caused the growth of new grains at room temperature.

Unusual substructure evolution and post-dynamic recrystallization effects on flow softening mechanism in a [formula omitted]-free Co-base superalloy

Motivation for this study was the absence of any report of stacking fault energy (SFE) estimate and post-dynamic recrystallization (PDRX) effects at temperatures >1000 °C in γ′-free Co-base superalloys. Therefore, the substructural evolution during hot compression of Co-22Cr-22Ni-14W-2Fe-0.1C superalloy (H188) at temperatures (1050–1150 °C) and at 10−1 s−1 was studied. Unusual substructural features viz. extrinsic stacking faults, dense dislocation walls, microbands, and subgrains were observed with an increase in strain (0.01–0.70), even at 1100 °C. Denoised electron backscattered diffraction data revealed an enormous increase in overall recrystallized (PDRX+DRX) fraction from 30 to 75% with increase in temperature from 1050 to 1100 °C at ε=0.7. Comprehensive microstructural analyses suggest that continuous dynamic recrystallization is the dominant flow softening mechanism in low SFE (estimated to be 14 ± 2.0 mJ/m2) alloy H188. However, the PDRX effects triggered by the presence of higher stored energy and recrystallized nuclei contributing to a massive increase in recrystallized fraction at 1100 °C.

Post-dynamic recrystallization behavior of a powder metallurgy Ni-Co-Cr based superalloy under different deformation conditions

The post-dynamic recrystallization (PDRX) behavior of a powder metallurgy (PM) Ni-Co-Cr based superalloy was researched by thermal compression tests at the temperature range of 1020–1080 °C within the strain rates of 0.01–1 s−1, followed by heat preservation for 0–45 s after deformed to 10%−50%. Electron backscatter diffraction (EBSD) and Transmission Electron Microscope (TEM) techniques were employed to investigate the effect of the deformation conditions on PDRX behavior. The results show that the deformation conditions (strain rate, temperature, degree of deformation and holding time) have great impact on the PDRX kinetics. Increasing deformation temperature, strain or holding time can result in the effective release of residual deformation stored energy, promoting the PDRX course. The irregular energy storage release of the superalloy deformed at different strain rates is mainly concerned with the various microstructure evolution during heat preservation. The evolution of twin boundary (Σ3) during PDRX exhibits the similar regularity with that of recrystallization fraction. The formation of Σ3 boundary induces the recrystallized nucleation, leading to further release of deformation storage energy. Meta-dynamic recrystallization (MDRX) and static recrystallization (SRX) are the dominant mechanisms of PDRX, and the nucleation induced by undissolved γ′ precipitates or assisted by twins also play certain roles in microstructure evolution during heat preservation of the deformed superalloy.

Investigation on transformation-related recrystallization behavior of Ti2AlNb-based alloy

To achieve grain refinement, the recrystallization behavior of B2 matrix phase in a Ti2AlNb-based alloy was investigated by performing a series of thermal compression tests at temperatures in the range of 950~1025°C with strain rates from 0.001s−1 to1s−1. Instead of water quenching immediately after deformation, samples were further held at deformation temperature for a certain time, in order to make the heat-affecting time at various strain rates consistent. Electron back scattered diffraction (EBSD) technique was employed for microstructure analysis, in combination with grain orientation spread (GOS) method to distinguish recrystallized grains from non-recrystallized grains. The results reveal that: (i) dynamic recrystallization (DRX) is not easy to occur in this alloy, and increasing strain rate leads to a decrease of DRX fraction; (ii) when evaluated on the same time scale, the sample at strain rate of ε˙=0.1s−1 has the highest recrystallized fraction among all applied strain rates, which provides sufficient nucleation rate of DRX as well as notable grain growth during post-dynamic recrystallization (PDRX); and (iii) the recrystallization evolution with temperature changes is closely linked to the phase transformation in Ti2AlNb. The presence of O and α2 phases affects recrystallization process via pinning effect and particle stimulated nucleation (PSN) mechanism. This work therefore provides a new understanding of recrystallization behavior during and post deformation of Ti2AlNb-based alloy, which is meaningful to refine grains by combining the contributions of DRX and PDRX.

Development of grain size/ texture graded microstructures through friction stir processing and subsequent cold compression of a rare earth bearing magnesium alloy

The present work deals with the effect of mechanical post-treatment on the microstructure and texture evolution of friction stir processed Mg-4.37 wt%Y-2.9 wt%RE-0.3 wt%Zr alloy. In this respect, the stir processed specimens were compressed down to engineering strain of 10 and 20%, and the resultant microstructures were characterized. Through applying subsequent compressive loading, fraction of low angle boundaries was increased, substructure was developed, the grain size was refined, and specified misorientation angles/texture components were faded or appeared. The degrees of such evolutions were different between the surface layer and stir zone, and resulted in development of graded microstructures. The fraction of strain-less and fresh grains was considerably decreased; but the fraction of coarse grains holding high grain orientation speared (GOS) was remarkably increased specially by moving away from material surface. Schmid analysis indicated the higher probability of non-basal slip within the unstable coarse grain in the course of subsequent loading. This was led to the occurrence of post-dynamic recrystallization which was traced through the sequence of substructure development and transformation of the low angle boundaries in to the high angle ones. Such gradual microstructure/texture evolution indicate the stir processing plus compression loading as capable method for fabrication of micro-scale functionally graded materials. • Mechanical post-treatment of friction stir processed WE43 magnesium alloy. • Fabricating graded microstructures regarding grain size, substructure and texture. • Contribution of continuous dynamic recrystallization during friction stir processing. • High potential of unstable coarse grains for refinement during subsequent loading. • Post-dynamic/Meta-dynamic recrystallization activates during subsequent compression.

A new front-tracking Lagrangian model for the modeling of dynamic and post-dynamic recrystallization

A new method for the simulation of evolving multi-domains problems has been introduced in previous works and further developed in parallel in the context of isotropic grain growth (GG) with no consideration for the effects of the stored energy (SE) due to dislocations. The methodology consists in a new front-tracking approach where one of the originality is that not only interfaces between grains are discretized but their bulks are also meshed and topological changes of the domains are driven by selective local remeshing operations performed on the finite element (FE) mesh. In this article, further developments and studies of the model will be presented, mainly on the development of a model taking into account grain boundary migration (GBM) by SE. Further developments for the nucleation of new grains will be presented, allowing to model dynamic recrystallization (DRX) and post-dynamic recrystallization (PDRX) phenomena. The accuracy and the performance of the numerical algorithms have been proven to be very promising in Florez et al (2020). Here the results for multiple test cases will be given in order to validate the accuracy of the model taking into account GG and SE. The computational performance will be evaluated for the DRX and PDRX mechanisms and compared to a classical FE framework using a level-set formulation.

Dynamic and Post-Dynamic Recrystallization of Haynes 282 below the Secondary Carbide Solvus

Thermomechanical processes, such as forging, are important steps during manufacturing of superalloy components. The microstructural development during processing, which controls the final component properties, is complex and depends on e.g., applied strain, strain rate and temperature. In this study, we investigate the effect of process parameters on the dynamic and post-dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. Specifically, we address the effect of deformation below the grain boundary carbide solvus temperature. During deformation, discontinuous and continuous dynamic recrystallization was observed at the grain boundaries, and particle-stimulated nucleation occurred at primary carbides. Strain rate was determined to be the governing factor controlling the recrystallization fraction for strain rates up to 0.5 s−1 above which adiabatic heating became the dominating factor. Careful examination of the temperature development during deformation showed that the response of the closed-loop temperature control system to adiabatic heating can have important effects on the interpretation of the observed behavior. During a 90 s post-deformation hold, grain growth and an increasing fraction of twin boundaries significantly changed the deformation-induced microstructure and texture. The microstructure developed during post-dynamic recrystallization was mainly controlled by the temperature and only weakly coupled to the prior deformation step.

Microstructure and Texture Evolution in a Post-dynamic Recrystallized Titanium During Annealing, Monotonic and Cyclic Loading

The post-dynamic recrystallization behavior of ultrafine-grained (UFG: 0.44 μm) cp-Ti under annealing, room temperature (RT) monotonic and cyclic loading was investigated across a range of temperatures and deformation rates wherever appropriate. By characterizing the grain and boundary structures, it was confirmed that recrystallization and grain growth occurred due to annealing (≥ 600 °C) and R = − 1 fatigue at RT. There was a noticeable 30 deg aggregation in misorientation distribution, along with the increased grain size. However, the hypothetical correlation between 30 deg aggregation and Σ13a or the other characteristic coincidence site lattice boundaries was found to be weak. The fatigue-induced grain growth is particularly intriguing for two reasons. First, the large monotonic deformation with low strain rate cannot trigger grain growth. Second, fatigue sharpened the basal intensity around the ND and caused a weaker texture component close to TD (load axis along the LD, perpendicular to the TD–ND plane). By contrast, high-temperature annealing only strengthened the UFG processing induced basal pole but without affecting its location. Novel insights into this fatigue-induced texture evolution in UFG cp-Ti has been provided. The lattice rotation during fatigue can be attributed to the combined effect of activation of prismatic langle arangle slip parallel to LD, and basal langle arangle slip perpendicular to it. The theoretically calculated stress to activate dislocation slip by assuming a non-equilibrium grain boundary state lent support to the above assertion. Moreover, the TEM observation evidently showed the characteristics of dislocation cross-slip and multiple slip in the grain interior.Graphical

Mechanisms of formation of the structure of welded joints, performed by friction stir welding

The analysis of welded joints of structural materials is carried out. As a result of this, in addition to the traditionally formed zone of thermal influence (HAZ), a zone of thermal deformation influence is formed in the near-seam zone, located between the welded nugget and the HAZ. The welded nugget joints in FSW is formed in the regime of structural superplasticity (SP). The structure of the welded nugget has a sheared band structure in the form of an "onion pattern" consisting of a large number of lamellas forming shear bands under the influence of a movable welding tool. The attainment of the superplastic state (SPS) during the formation of the welded nugget is ensured by the stepwise flow of various mechanisms of plastic deformation in the regime of simple, collective, anomalous dynamic recrystallization, prepared by dynamic return processes, polygonization with transition to postdynamic recrystallization by the mechanisms of Bailey- Hirsch, Kahn-Burgers-Taylor. For the group of metals and alloys with polyformism (high-alloy steels of different structural classes, Ti- alloys), SPS is supported additionally by recrystallization by twinning (copper, austenitic steel and other structural classes, Ti and its alloys) and as a result of phase transformations of alpha-gamma or alpha-beta phases).

Joining of Titanium and its Alloys with Aluminum Alloys by Friction Stir Welding

This article is devoted to the study of the mechanism of formation of dissimilar welded joints Ti-2Al-1.5Mn, pure titanium (Ti35A) and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys obtained by friction stir welding (FSW). The investigated microstructure of the weld joint nugget (WN), zones of thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ) formed at FSW between Ti-2Al-1.5Mn, Ti35A and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys. Zones of welded joints at FSW are formed in the mode of structural superplasticity (SP) with a specific shear-band layered structure with alternating layers. The achievement of superplastic state (SPS) in the formation of WN, TMAZ, HAZ is provided by the step–by–step transformation of various mechanisms of plastic deformation in the mode of simple, collective, abnormal dynamic recrystallization, prepared by the processes of dynamic return, polygonization with the transition to post-dynamic recrystallization by the mechanisms of Bailey–Hirsch, Kahn-Burgers-Taylor. At FSW aluminum and titanium alloys with polymorphism, SPS is supported additionally due to recrystallization by twinning and as a result of phase transformations of alpha-gamma or alpha-beta phases.

Effect of Warm to Hot Rolling on Microstructure, Texture and Mechanical Properties of an Advanced Medium-Mn Steel

The deformation microstructures and mechanical properties were studied in a medium-Mn austenitic steel subjected to warm-to-hot rolling. During warm rolling at temperatures of T < 1073 K the structural changes were controlled by dynamic and static recovery leading to a pancaked work-hardened microstructure, during hot rolling at T ≥ 1073 K—by discontinuous dynamic and post-dynamic recrystallization resulting in equiaxed grains. The grain size decreased while the dislocation density increased with a decrease in rolling temperature. A decrease in rolling temperature enhanced the texture development, which consisted of relatively strong Brass, S and P components. The Brass component exhibited the strongest temperature dependence. A decrease in rolling temperature resulted in significant strengthening of the steel. The yield strength increased from 340 to 950 MPa as rolling temperature decreased from 1373 K to 773 K. Both the grain refinement and the work-hardening contributed to the strengthening.