Dynamic Restoration Mechanism Research Articles

Dynamic recrystallization under hot deformation of additively manufactured 316 L stainless steel

The high-temperature deformation behavior of additively manufactured 316 L stainless steels prepared via the laser powder bed fusion (LPBF) technology was investigated using Gleeble compression testing in a wide temperature range of 473–1273 K under a constant strain rate of 0.001 s−1, perpendicular to the building direction (BD). The softening flow property upon continuous concurrent isothermal heating deformation displayed an excellent thermal stability performance up to the temperature of 873 K, preserving the material strength higher than 600 MPa compared to the room-temperature property of the as-built material. By the gradual increase of testing temperature, the compressive strength of the material was continuously decreased to even <50 MPa at a temperature of 1273 K. After hot compression; all tested specimens were characterized across different regions in terms of deformation substructure evolution and crystallographic texture using electron backscattering diffraction (EBSD) and constitutive analyses. Accordingly, the estimated activation energies based on established constitutive modeling proposed the operation of dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX) mechanisms for the hot deformation of additively manufactured 316 L stainless steel below (~9.77kJmol) and higher (~553.23kJmol) than the critical temperature of 873 K, respectively. The competition between strain accumulation and thermal heating induced two different Brass deformation and Goss recrystallization textures below and above this critical temperature by altering the operative dominant dynamic restoration mechanism, with various texture severity depending on the testing temperature.

Workability and dynamic restoration mechanisms of Al-6.2Zn-2.1Mg-0.15Zr alloy during medium temperature compression

Medium temperature compressions were implemented in the range of 433–593 K and 0.001–5 s−1 to reveal the workability and dynamic restoration mechanisms of quenched Al-6.2Zn-2.1Mg-0.15Zr alloy. The activation energy evolution with processing parameters was inspected through the modified kinetic equation that derived by integrally considering the strain and deformation parameters, and the results suggested activation energies decreased with the increasing strain rate and deformation temperature while less affected by the increase of strain at a specific strain rate condition. The 3D processing map was constructed to optimize the processing regions, in which two unstable domains yielded by distinct microscopic instabilities were marked whilst the safe working regions could be identified at 533–593 K, 0.001–0.1 s−1 with dissipation efficiency >20%. The microstructural evolution of the compressed specimens was examined, and the results confirmed different types of dynamic recovery (DRV) involving dislocation migration and rearrangement were dominate restoration process during the compression. Also, the microstructures of the specimens deformed at unstable domains were characterized, the results revealed fluctuating flow localization, micro cracking and particles debonding as well as dynamic strain aging were responsible for compression failure.

Workability and Dynamic Restoration Mechanisms of Al-6.2zn-2.1mg-0.15zr Alloy During Medium Temperature Compression

Workability and Dynamic Restoration Mechanisms of Al-6.2zn-2.1mg-0.15zr Alloy During Medium Temperature Compression

Hot-deformation characteristics of Al-alloyed δ-ferritic and martensitic dual-phase steel

The hot working behavior of Al-alloyed δ-ferritic/martensitic dual-phase steel was investigated using a Gleeble-3800 thermomechanical simulator and compression tests at deformation temperatures and strain rates ranging from 800 to 1100 °C and 0.1 to 3 s−1, respectively. The shape of the flow stress curve of the experimental steel resembled that of traditional dynamic recovery (DRV) flow stress curves produced by work hardening, ferritic DRV, ferritic dynamic recrystallization (DRX), and austenitic dynamic phase transformation (DPT). Additionally, constitutive equations were established to predict the steady-state stress. The apparent deformation activation energy of the steel was approximately 264.7 kJ/mol. Moreover, the microstructure following deformation was investigated using electron backscatter diffraction. The primary dynamic restoration mechanisms of δ-ferrite comprised DRV and DRX at temperatures below and above 900 °C, respectively. However, with increasing deformation temperature and strain rate, the primary dynamic restoration mechanism of δ-ferrite shifted from continuous DRX (CDRX) to discontinuous DRX (DDRX). Additionally, the processing maps of the steel were established using the dynamic materials model. Further, based on the microstructure results, it was found that the power dissipation efficiency of ferritic CDRX, ferritic DDRX, ferritic DRV, and austenitic DPT gradually decreased. Additionally, only one domain of flow instability was identified, and it was located at 1025–1075 °C/0.6–3 s−1. Thus, to avoid unstable deformation regions, a two-stage design of the optimal process parameters for industrial processing is highly essential.

New insights on the laser powder bed fusion processing of a NiTi alloy and the role of dynamic restoration mechanisms

NiTi alloys are a remarkable class of materials capable of exhibiting unique behaviors such as shape memory effect and superelasticity. Laser powder bed fusion (L-PBF), as one of the most prominent additive manufacturing processes, has been recently used by many researchers to fabricate high-quality NiTi alloys with desired properties. In this research, those findings were exploited to utilize an optimum range of volumetric energy density to produce dense parts and offer new insights into the microstructure and properties of the fabricated parts. Although the development of high residual stresses has been frequently reported, the effects of those stresses on the microstructural evolution of NiTi parts during the process have been rarely discussed. By utilizing finite element and analytical approaches, it was predicted that the driving force for the activation of restoration mechanisms, namely dynamic recovery and recrystallization, is provided at elevated temperatures, and the temperature range in which those mechanisms occur was calculated. Detailed microstructural investigations confirmed the results, revealing columnar grains along the building direction with strong substructures caused by dynamic recovery, in addition to new recrystallized grains at boundaries of the melt pools and scanned tracks. It was concluded that dynamic recovery acts as the dominant restoration mechanism during L-PBF of NiTi alloys, affecting both the microstructure and mechanical properties of the final part. Additionally, the peak temperature of the melt pool is a paramount factor that, if increased, results in further progress of the dynamic recovery mechanisms.

Effect of thermo-mechanical processing parameters on the dynamic restoration mechanism in an Mg-4Y-2Nd-1Sm-0.5Zr alloy during hot compression

Microstructure evolution and dynamic restoration mechanism of solution-treated Mg-4Y-2Nd-1Sm-0.5Zr alloy have been studied under three TMP parameters consisting of deformation temperatures (350–500 °C), strain rates (0.01–5 s−1), and strains (0.2, 0.4, and 0.8). A strong dynamic softening is observed in all stress-strain curves, even at higher strain rates (1 and 5 s−1) due to an adiabatic heating effect. Various stress-strain curves are applied to construct a processing map and develop an Arrhenius-type constitutive equation. With the prediction of the processing map, an optimal processing domain has been determined to be the temperature range 450–500 °C and strain rate range 0.01–0.1 s−1 at a strain of 0.8. The volume fraction of DRX grains is the largest in the corresponding domain of high temperature and low strain rate. For the effect of TMP parameters on the dynamic restoration, the discontinuous dynamic recrystallization (DDRX) and continuous DRX (CDRX) synergistic effect occur throughout the whole process at high temperature and high strain rate. In terms of high temperature and low strain rate, DDRX characteristics at a low strain and then the DDRX + CDRX synergistic effect is observed at a higher strain. Although the DRX process is weak at low temperature and low strain rate, deformation twins have occurred and provided nucleation sites for DRX grains.

Change in intragranular misorientation during stress relaxation in Al-Cu-Mg-Mn alloy subjected to continuous cyclic bending

Change in intragranular misorientation during stress relaxation in Al-Cu-Mg-Mn (2024) alloy sheet subjected to continuous cyclic bending (CCB) has been investigated by electron back scatter diffraction (EBSD) technique. EBSD analysis revealed that kernel average misorientation (KAM) value was heitened not only in the sheet worked by CCB but also during stress relaxation test. Grain orientation spread (GOS) value, another misorientation parameter, was also calculated and compared with KAM value of each sample. Both of KAM and GOS values varied with four stages about stress and stored strain: (a) unloaded before stress relaxation test (SRT), (b) loaded before SRT, (c) loaded after SRT and (d) unloaded after SRT. Change in KAM value was likely corresponding to sum of applied and residual stresses. Quantitative relationship between increase and decrease in KAM value between the stages was divided into two types according to dynamic restoration mechanisms.

Elucidation of deformation mechanisms and construction of processing maps for a Ti+Nb stabilized IF steel

Deformation characteristics of a Ti + Nb stabilized IF steel have been investigated in the present study to clarify the associated strain hardening and/or dynamic restoration mechanisms. Single-hit hot compression experiments were performed in a Gleeble thermomechanical simulator in the temperature range 650–1100 °C at various constant true strain rates in the range 10−3-10 s−1 to a true strain of 0.7. The ‘safe’ and ‘unsafe’ deterministic domains were identified through the construction of processing maps and verification of mechanisms through the examination of evolved microstructures, regardless of the phase type (austenite and/or ferrite). Among different materials models employed, the ‘safe’ workable region at austenite and ferrite phase, predicted by a modified dynamic materials model (DMM) superimposed with Poletti instability criteria has been identified to be appropriate in accord with the experimental data. Development of a fully recrystallized fine equiaxed ferrite grain structure is ascribed to the occurrence of dynamic recrystallization (DRX). The characteristics of the onset of DRX mechanism have been further analyzed in terms of critical strain and stress required. Moreover, a kinetic model has been established to evaluate the fractional transformation through DRX. In addition, industrial optimization of hot working schedule of the IF steel could be ascertained from the processing map.

Dynamic recrystallization in AlXCoCrFeNi duplex high entropy alloys

Most of the applications planned for high entropy alloys (HEAs) entail thermomechanical processing. In the current study, the hot deformation and dynamic restoration mechanisms of two duplex high entropy alloys (Al0.6CoCrFeNi and Al0.9CoCrFeNi) are revealed through a detailed electron back-scattered diffraction and transmission electron microscopy study. A standard homogenization of the cast microstructures results in face-centred cubic/body-centred cubic (FCC/BCC) duplex structures for both alloys. Al0.6CoCrFeNi has an FCC matrix containing dispersed BCC particles surrounded by interdendritic BCC. Al0.9CoCrFeNi has a BCC matrix with allotriomorphic and intragranularly dispersed FCC particles. Hot deformation at 1030 °C under the strain rate of 0.1s−1 is mainly accommodated through the matrix phase in both alloys. In Al0.6CoCrFeNi alloy, the FCC matrix undergoes discontinuous dynamic recrystallization mechanism (DDRX) while there are also some local observations of subgrain coalescence. DDRX mainly occurs at the interphase boundary mantle regions through the formation of Σ3 boundaries and the propagation of multiple twinning chains. Dislocations within the hot deformed FCC matrix have a large screw component. In the Al0.9CoCrFeNi alloy, the BCC matrix softens through continuous dynamic recrystallization (CDRX), characterised by the progressive conversion of low-misoriented subgrains into (sub)grains delineated partly by low-angle and partly by high-angle boundaries. Dislocations within the BCC matrix have a mixture of edge and screw character. The dispersed second phases in both alloys are mechanically fragmented during hot deformation and are quite evenly distributed throughout the microstructure at a strain of 1.5.

The analysis of hot deformation behavior of powder metallurgy Ti-10V-2Fe-3Al alloy using activation energy and Zener-Hollomon parameter

The hot deformation behavior of powder metallurgy Ti-10V-2Fe-3Al alloy was investigated by hot compression tests conducted in the temperature range 800 – 1000 °C and strain rate range 0.01 - 10 s-1. Flow stress data, the average deformation activation energy, and Zener-Hollomon parameter were developed in order to investigate the effects of variable deformation conditions on deformation behavior and microstructure evolution. The determined value of average activation energy for investigated alloy allows to predict the dynamic restoration mechanism taking place during hot working. The activation energy was found to be about 162 kJ/mol which is considered as relatively low value. The received results indicate that the main restoration mechanism is dynamic recovery. The microstructure after deformation was mainly composed of coarse β grains elongated towards flow direction with serrated grain boundaries. The number of recrystallized grains was not significant.

In-Situ Synchrotron X-Ray Diffraction of Ti-6Al-4V During Thermomechanical Treatment in the Beta Field

This work aims to identify the mechanisms of restoration occurring in Ti-6Al-4V during hot plastic deformation and subsequent heat treatment. The allotropic phase transformation that occurs during cooling distorts the interpretation of the restoration mechanisms taking place at high temperatures. Therefore, analysis of deformed samples by conventional microscopy have led to controversies in the interpretation of the main dynamic restoration mechanism. Additionally, static restoration of the microstructure can occur during slow cooling, modifying the microstructure. These facts were mainly the reasons why discontinuous dynamic recrystallization and/or dynamic recovery has been reported as the main dynamic restoration mechanism for Ti-6Al-4V. In this work, we use in-situ synchrotron X-ray diffraction combined with conventional microscopy to determine the dynamic and static mechanisms of restoration during and after deformation at different strain rates. The results show dynamic recovery as main mechanism of restoration during deformation in the β field, denoted by sub-grain formation and a misorientation dependency of the strain rate. After deformation, static recrystallization, grain growth, and coarsening of the β grains can be observed, especially at strain rates higher than 0.1s−1. It is also demonstrated that the nucleation of new grains can occur within the very first seconds of the isothermal heat treatment.

Evolution of equiaxed and lamellar α during hot compression in a near alpha titanium alloy with bimodal microstructure

Microstructural evolution of equiaxed α and lamellar α during hot compression in a bimodal-structure near alpha Ti-6Al-3Nb-2Zr-1Mo alloy were investigated. Focus was concentrated on their own dynamic restoration mechanism via electron backscatter diffraction (EBSD) technique. Based on analysis of grain boundary misorientation distribution and misorientation gradient within α grain, it was revealed that limited continuous dynamic recrystallization (CDRX) as well as adequate dynamic recovery (DRV) took place within equiaxed α, whereas CDRX occurred as the main restoration mechanism in lamellar α. Further, the deformation heterogeneous among equiaxed α grains was found to be relevant to the difference in slip systems. Under the same nominal strain of 75% height reduction, process of weak DRV, strong DRV and CDRX coexisted in distinct equiaxed α grains owning to small, medium and high Schmid Factor of prismatic slip. Besides, globularization heterogeneity induced by CDRX in lamellar α was considered to be closely related with its initial orientation to compression axis and the imposed strain. For flat colony α, the microstructure was relatively stable during straining showing a thinning process and a weak dynamic globularization, while the kinked colony α with large strain was globularized sufficiently.

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370 K and a strain rate of 0.01-10 s−1. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313 K and 0.01-0.1 s−1, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the α + β phase field. Flow instability behaviors occurred at 1263-1343 K/1.78-10 s−1, 1233-1283 K/0.1-3.16 s−1, and 1353-1373 K/0.56-1.78 s−1, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333 K and 0.01-0.1 s−1. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10 s−1.

Influence of CNTs decomposition during reactive friction-stir processing of an Al-Mg alloy on the correlation between microstructural characteristics and microtextural components.

Multipass friction-stir processing was employed to uniformly disperse multiwalled carbon nanotubes (MW-CNTs) within an Al-Mg alloy metal matrix. Decomposition of MW-CNTs occurs in situ as a result of solid-state chemical reactions, forming fullerene (C60) and aluminium carbide (Al4 C3 ) phases during reactive high temperature severe plastic processing. The effects of this decomposition on the microstructural features, dynamic restoration mechanisms and crystallographic microtextural developments are studied for the first time by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analysis. The formation of an equiaxed grain structure with an average size of ∼1.5 μm occurs within the stirred zone (SZ) under the influence of inclusions which hinder grain boundary migration via Zener-Smith pinning mechanisms during the discontinuous dynamic recrystallisation (DDRX). Formation of two strong Cubic and Brass microtextural components in the heat affected zone (HAZ) and thermomechanical affected zone (TMAZ) was noted as compared to the completely random and Cube components found in the base and SZ regions, respectively. The microstructural modification led to hardening and tensile strength improvement for the processed nanocomposite by ∼55% and 110%, respectively with respect to the annealed Al-Mg base alloy.

Plausible extension of Anand's model to metals exhibiting dynamic recrystallization and its experimental validation

Modeling and optimization of industrial hot-working operations by means of commercial finite element computer codes require robust constitutive descriptions, which could be easily implemented. Also, it is expected that such formulations are able to describe as accurate as possible the flow stress, work-hardening and work-softening rate of the material, as a function of microstructure and deformation conditions. The present communication describes the development of a novel constitutive description of metals which exhibit dynamic recrystallization during deformation under hot-working conditions. The formulation is founded on the extension of the model earlier developed by Anand and co-workers for the constitutive description of metals which exhibit dynamic recovery as the only dynamic restoration mechanism during hot deformation. The advanced constitutive formulation has been validated by conducting axisymmetric compression tests with samples of a R260 steel employed for the manufacture of rail tracks. The specimens were deformed in a wide range of temperatures and strain rates, both under constant and variable strain rate conditions. The latter involve either an increase or a decrease in the Zener-Hollomon parameter as plastic deformation is applied. The results obtained indicate that the proposed constitutive description is able to reproduce quite accurately the experimental flow stress data. It has also been shown that the correct computation of the recrystallization time and volume fraction recrystallized dynamically, by means of an appropriate differential or incremental formulation of the Avrami equation, constitutes an essential aspect of the model. Such an approach is particularly important for predicting correctly the changes in flow stress under variable strain rate conditions.

Dynamic restoration and deformation heterogeneity during hot deformation of a duplex-structure TC21 titanium alloy

The coexistence of equiaxed α, lamellar α and β phase in a duplex-structure α/β titanium alloy not only benefits the balance between strength and ductility, but also arouses a complicated microstructure evolution during hot deformation. The research objective herein is first to investigate the dynamic restoration mechanism, including dynamic recovery (DRC) and dynamic recrystallization (DRX), of both α and β phases during hot deformation of a duplex-structure TC21 titanium alloy. The results show that, after deformation, the geometric morphologies of lamellar α and β phase change more apparently than equiaxed α. Based on the analysis on the distribution and frequency of misorientation angle, it reveals that DRC and continuous DRX (CDRX) occur in all of equiaxed α, lamellar α and β phase, and the increasing deformation temperature restrains the occurrence of CDRX. Synthetically, the correlation between geometric morphology and dynamic restoration is well established. Meanwhile, the heterogeneous degree of DRX indicates a morphology dependent deformation heterogeneity between equiaxed and lamellar α, and between different local regions in β phase which is closely related to the distribution of equiaxed and lamellar α. The further discussion reveals that the morphology dependent deformation heterogeneity is fundamentally attributed to the different orientation relationship (OR) between β phase and equiaxed/lamellar α. This new find enriches the understanding on the deformation heterogeneity of titanium alloy, which was mainly attributed to the difference in crystallographic orientations in past decades.

A comprehensive study of microstructure development and its corresponding tensile properties in nano/ultrafine-grained metastable austenitic steel during accumulative roll bonding (ARB)

The evolution of microstructure together with mechanical properties change of Fe-28.5Ni steel, used in design of marine and nuclear related products, processed by different deformation amount during ARB process was elaborated by Electron Backscattered Diffraction (EBSD) method. EBSD results showed that the initial structure of the material contained retained austenite as the main phase besides martensite phase. Martensite phase was diminished after successive ARB cycles due to reduction of martensite start (Ms) temperature. EBSD microstructural results exhibited that a stabilized nano structure was attained by 6-cycle or 8-cycle of the ARB process with the latter having slightly higher grain size due to the grain growth effect of dynamic restoration mechanism resulting from extreme deformation. Tensile test data on different cycles of ARB processed specimens showed that yield and ultimate tensile strengths were substantially increased after the first cycle of the ARB coinciding with the substantial amount of dislocations in the form of low angle grain boundaries (LAGB). Strengths increased in the successive cycles, though in a much slower pace. Ultra-fine grains were formed instead after high cycles of ARB process. Although such increase in strength generally came at the expense of ductility, the material after 8-cycle had still substantial ductility.

Strain rate dependence of ferrite dynamic restoration mechanism in a duplex low-density steel

The ferrite major dynamic softening mechanisms, particularly in duplex microstructures, have long been a matter of debate among steel scientists. It has been shown in the present work that a marked increase in the strain rate at a high temperature leads to a transition in the ferrite softening mechanism in low density steels. In contrast to the current widespread view, ferrite was found to soften through continuous dynamic recrystallization at a low strain rate and via discontinuous dynamic recrystallization at a high strain rate. The latter mechanism is largely associated with the interphase mantle regions, rather than the scarce original ferrite-ferrite boundaries. In these regions, the new grains were observed to develop through the growth of highly-misoriented subgrains.

Deformation behaviour and mechanisms during hot compression at supertransus temperatures in Ti-10V-2Fe-3Al

Deformation mechanisms in Ti-10V-2Fe-3Al at above β transus temperatures were studied mainly using Electron backscattered diffraction (EBSD). The volume fraction of dynamic recrystallization is lower than 10% for all test conditions in the single β phase field, showing that the main dynamic restoration mechanism is dynamic recovery. The activation energy was calculated to be 172 kJ/mol, which also supported the dominant role of dynamic recovery. Besides dynamic recovery, continuous dynamic recrystallization (CDRX) by progressive lattice rotation was also observed. When it occurred, the concentration of high Kernel Average Misorientation (KAM) values close to grain boundaries and low KAM values at the grain center was observed. Geometric dynamic recrystallization hardly occurred, as high angle grain boundaries (HAGB) spacing was much larger than subgrain size. Discontinuous dynamic recrystallization (DDRX) was rarely observed.

Flow Behavior and Microstructural Evolution of 7A85 High-Strength Aluminum Alloy During Hot Deformation

Hot deformation behavior of 7A85 high-strength aluminum alloy was investigated at 593 K to 713 K (320 °C to 440 °C) and 0.01-10 s−1. The manifestation of flow curves was related to the strain rate. Typical single-peak curves were shown below 10 s−1, while two stress peaks appeared in the case of 10 s−1 and the second peak strain was almost three times larger than the first one. A constitutive equation considering the effect of strain was developed. Flow stress values predicted by the constitutive model demonstrated a good agreement with the experimental results over the entire range of strain rates and temperatures. Microstructure characterization revealed that dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX) which depended on the Zener–Hollomon parameter (Z) closely, co-occurred at large strain (e = 0.7). With decreasing Z-value, the dominant dynamic restoration mechanism gradually transformed from DRV to CDRX. The average subgrain size (d sub) showed a power-law relationship with Z. Recrystallization was sensitively dependent on the strain rate at above 683 K (410 °C). The fine equiaxed grains appeared at original grain boundaries and in deformed grains interior owing to CDRX. The high-curvature subgrain boundaries can also cause the nucleation of recrystallization within deformed grains.