Fractional Softening Research Articles

Flow softening and recrystallization accompanied flow in AZ61 magnesium alloy during thermomechanical processing

The present work deals with characterizing the recrystallization accompanied flow and high temperature softening behavior of an extruded AZ61 magnesium alloy. This was supported by conducting a set of hot compression tests at temperatures in the range of 250–450 °C under the strain rate ranging from 0.001 to 0.1s−1. The flow curves at all thermomechanical conditions indicated high fractional softening representing the domination of dynamic recrystallization mechanism. Through a new quantitative approach, “Arrhenius type model”, “modified Avrami equations” and Poliak and Jonas method were simultaneously employed to investigate the kinetic of dynamic recrystallization. It was revealed that the strain required for the same amount of recrystallization fraction increased with decreasing deformation temperature, and at a specified temperature, the required strain increases with increasing strain rate. Interestingly, an anomaly was found at 400 °C under the strain rate of 0.001s−1, where the recrystallization kinetic was faster than that of what was recorded at 450 °C. This anomaly was discussed relying on the nanoprecipitation of γ-phase at the prior boundaries and sub-boundaries which prohibited the grain boundary migration and also the rotation and coalescence of adjacent sub-grains.

Effect of nitrogen content on the static recrystallization and precipitation behaviors of vanadium–titanium microalloyed steels

Abstract Double compression tests were performed on vanadium–titanium microalloyed steels with different nitrogen contents by using a Gleeble-3800 thermo-mechanical simulator to study the softening behaviors of the deformed austenite during different time intervals between the two passes. The static recrystallization fractions were calculated by the stress offset method and static recrystallization diagrams for the tested steels were obtained. The effects of deformation temperature and interval time on the softening behaviors were analyzed. Especially, the effect of nitrogen on the softening behaviors of the tested steels is discussed in detail. The results showed that the softening behaviors of the tested steels with various nitrogen contents are different. As far as the steel with low nitrogen content is concerned, the softening fraction increases monotonically with increasing time interval, and higher temperature can promote the static recrystallization. However, with more nitrogen added into vanadium–titanium microalloyed steel, precipitated particles of vanadium titanium carbonitride can be observed in the tested steel at the temperature of 850 °C or 800 °C, which leads to the formation of plateaus on the softening curves. An increase in nitrogen content in the steel is favorable for vanadium titanium carbonitride precipitation, which leads to a stronger prohibition of static recrystallization and a longer plateau on the softening curves. Moreover, the precipitated particles in the tested steel will not play an inhibition role in static recrystallization until the nitrogen content in the steel reaches a critical value.

Role of Cerium on the Intricacies of Deformed State and Softening Mechanisms of Low‐Carbon Steels

The present study investigates the softening kinetics of two cerium (Ce)‐modified steels after 60% cold rolling and annealing at 600 °C for 2–16 h. Cold rolling accumulates substantial strain in the ferritic matrix of low Ce (LCe) steel (0.03 wt% Ce) compared to high Ce (HCe) steel (0.6 wt% Ce). The acicular ferrite and Fe3C partition the imposed strain preferentially inside the ferrite matrix of LCe sample. Contrarily, a homogenous strain distribution in HCe sample is promoted by soft Ce2O3 particles embedded in ferrite. Both the steels achieve partial recovery and recrystallization even after 16 h of annealing. LCe steel experiences a softening fraction of ≈28 vol% after 16 h, inferior to HCe steel (≈34 vol%). During initial stage of annealing, nucleation of strain‐free grains are observed in LCe sample due to availability of grain and interphase boundaries. Subsequently, recrystallization kinetics get delayed because of the pinning effect exerted by fine CeO2 and Fe3C particles. In HCe samples, the early stage of ferrite nucleation is hindered by the segregation of Ce at grain boundaries. However, at a later stage, the recrystallization kinetics are accelerated owing to the ineffective pinning of dislocations and boundaries by coarse Ce2Fe17 and Ce2C3 particles.

Microstructure, deformation behavior, and dynamic recrystallization kinetics of FeNiMnCu-based high entropy alloys prepared by mechanical alloying and spark plasma sintering

In the present research, FeNiMnCuX (X: Co, Cr, Mo, Ti, W) high entropy alloys (HEAs) were successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). SPS transformed the base and Co-containing alloys into FCC structures, Cr- containing alloys changed into a mix of FCC and BCC structures, while other alloys formed a combination of FCC/BCC structure and an intermetallic compound. The base alloy and alloys with Co, and Cr additions exhibited plastic deformation in cold compression tests, whereas other alloys exhibited brittle behavior. During hot compression tests, dynamic recrystallization occurred with a peak in flow stress followed by a gradual decrease to a steady state stress in alloys with Co, Mo, Ti and W additions. The highest peak stresses were observed at 800 °C for Ti- and W- containing alloys. The dynamic recrystallization and fractional softening kinetics are explained by the Avrami kinetic model. An Avrami exponent around 1 was obtained based on the kinetics of dynamic recrystallization for the alloys doped with Co, Mo, Ti, and W.

Dynamic recrystallization behavior of selective laser melted 316L stainless steel

The present work deals with the hot deformation behavior of selective laser melted 316L stainless steel with an emphasize on the involved recrystallization mechanisms. Toward this end, a series of hot compression test have been conducted in the temperature range of 700–1000 °C under the various strain rates of 0.001, 0.01 and 0.1s−1. The flow pattern analysis through Jonas-Poliak method verifies the occurrence of dynamic recrystallization in whole investigated temperature range. The intensified substructure development and continuous recrystallization is identified as the main softening mechanism in the course of thermomechanical processing. This causes appearance of a broad peak and low softening fraction in corresponding flow curves. The cellular substructure in the as-printed state facilitates low-angle into high-angle boundaries conversion, which provide a proper condition for the occurrence of continuous mechanisms and increases the rate of recrystallization. However, detailed investigation of the sub-boundary maps indicates the contribution of discontinuous and geometric dynamic recrystallization mechanisms. Dynamic recrystallization causes substantial refinement; and the percentage of the grain holding size lower than 10 μm increases from 2 % in printed microstructure to 16.8 % and 24 % in the microstructure deformed at 700 and 1000 °C.

Evolution of austenite microstructure and microalloy precipitation state during double-twist torsion testing on Nb-Ti-bearing steels

Double-twist torsion testing was applied to two Nb-Ti-bearing microalloyed steels to develop the relationship between the mechanical response and the resulting microstructure. The double-twist torsion test applies pairs of isothermal deformation passes to a single specimen at progressively lower temperatures. The test captures the effect of multiple deformation and recrystallization events and allows changes in recrystallization behavior to be assessed with fractional softening calculations. Conventional methods for determining fractional softening were applied to the torsional data to infer changes in recrystallization behavior and estimate the temperature regime where partial recrystallization occurs. The non-recrystallization temperature (Tnr) was determined with multi-step hot torsion testing and compared to the partial softening regime from the double-twist torsion test. The traditional concept of Tnr as a single temperature was thereby assessed with respect to the fractional softening calculations and corresponding microstructural changes at the micro- and nano- scales. Tnr was within the partial softening regime assessed with double-twist torsion testing, suggesting partial recrystallization near Tnr. Measurements of grain aspect ratio matched the trends observed in the fractional softening data and confirmed that strain accumulation occurred throughout the partial softening regime. Higher Nb contents corresponded to greater strain accumulation at lower temperatures within the partial softening regime. The precipitation of Nb(C,N) occurred at high temperatures in the form of caps precipitating on pre-existing Ti-rich cores, while fine-scale precipitation occurred at lower temperatures. Increasing the Nb-content increased both the extent of cap formation and the extent of fine-scale precipitation.

Flow softening and microstructural evolution of near β titanium alloy Ti-55531 during hot compression deformation in the α + β region

In order to investigate the microstructural evolution and dynamic softening mechanisms of Ti-55531 alloy during hot compression in the α + β phase region, hot compression tests were performed at temperatures of 760–840 °C and strain rates of 0.001–1 s−1, and the microstructure of the material under each deformation condition was analyzed by scanning electron microscope and electron backscatter diffraction techniques. The results show that the flow softening phenomenon is influenced by a combination of mechanisms. The deformation heat causes significant flow softening of the material at a strain rate of 1 s−1, while dynamic recrystallization contributes little to flow softening. Due to the shift in αp phase volume fraction, the flow softening fraction f∆Vα reduces as deformation temperature and strain rate increase, from 16.6% at 760 °C and 0.001 s−1 to 1.4% at 840 °C and 1 s−1. The flow softening fraction fΔσSF is 3.95% at 790 °C and 5.34% at 820 °C as a result of the texture evolution.

Recrystallisation Characteristics of a Cu Bearing HSLA Steel Assessed Through High Temperature Compressive Deformation

Dynamic (DRX) operative duringdeformation and static recrystallization (SRX) operative after deformation are considered responsible for the changes in microstructure and texture of deformed materials. Especially in the case of advanced steels that are required in the form of plates of various thicknesses, hot rolling is the main manufacturing process during which the steel undergoes DRX under the rolls and SRX between rolling passes/strands. Knowledge on DRX and SRX characteristics of such steels is crucial for optimisation of hot rolling parameters,achieving the desired microstructure and consequently the targeted mechanical properties.In this study, certain key aspects of both dynamic (DRX) and static recrystallisation (SRX) behaviour of aCu-bearing HSLA steel, which was developed at DMRL, have been explored through high temperature deformation studies using Gleeble thermo-mechanical simulator. Through uniaxial compression testing in the austenitic regime, domains of continuous and discontinuous DRX prevalent in the steel were identified and critical parameters for initiation of dynamic recrystallisation viz., critical strain (ec), critical stress (sc), peak stress (sp) and peak strain (ep) were determined as a function of temperature and strain rate. SRX characteristics were assessed through uniaxial double hit compression tests with fixed strain rate and strain per hit but at different temperatures and with different imposed intermediate static recrystallisation (ISRT) times. From the fractional softening data, parameters such as time for 50% recrystallisation, t0.5, temperature for 50% recrystallisation, T0.5, and activation energy, QSRX have been estimated.Although the steel exhibited good plastic deformation characteristics, the results suggest that the role of copper in retarding recrystallisation is significant.

Unveiling the influence of dendrite characteristics on the slip/twinning activity and the strain hardening capacity of Mg-Sn-Li-Zn cast alloys

This work explores the correlation between the characteristics of the cast structure (dendrite growth pattern, dendrite morphology and macro-texture) and strain hardening capacity during high temperature deformation of Mg-5Sn-0.3Li-0 and 3Zn multi-component alloys. The three dimensional (3D) morphology of the dendrite structure demonstrates the transition of the growth directions from<11 2 ¯ 3>,<11 2 ¯ 0> and <11 2 ¯ 2> to< 11 2 ¯ 3> and <11 2 ¯ 0> due to the addition of Zn. The simultaneous effects of growing tendency and the decrement of dendrite coarsening rate at the solidification interval lead to dendrite morphology transition from the globular-like to the hyper-branch structure. This morphology transition results in the variation of the solidification macro-texture, which has effectively influenced the dominant deformation mechanisms (slip/twin activity). The higher activity of the slip systems increases the tendency of the dendrite arms for bending along the deformation direction and fragmentation. Apart from this, the dendrite holding hyper-branch structure with an average thickness below 20 µm are more favorable for fragmentation. The dendrite fragmentation leads to considerable softening fractions, and as an effective strain compensation mechanism increases the workability of dendritic structure.

Post-hot-deformation microstructure development in austenitic stainless steel

In the present work, austenitic stainless steel has been thermo-mechanically processed to study the post-deformation microstructure evolution at elevated temperatures. The samples were axially compressed (at a strain rate 0.1 s−1) and isothermally held at temperatures 900, 1000 and 1100 °C, for different time spans (2–1000 s). The electron back-scattered diffraction technique has been used to study the microstructure of the thermo-mechanically processed samples. The result shows for temperatures of 900 °C and 1000 °C, the resultant microstructures are comprised of deformed grains for samples held for 2 s. However, at 1100 °C sample isothermally held for 2 s shows a nearly complete recrystallization. The grain average misorientation approach has been used to estimate the softening fraction caused by recrystallization. The average grain size of the thermo-mechanically processed samples reveals grain refinement for the samples processed at 900 °C and 1000 °C. In addition to the grain refinement, the samples processed at 1000 °C shows a more consistent grain size. At all three temperatures, the increase in the holding time leads to a gradual decrease in the fraction of low angle boundaries and a simultaneous increase in high angle grain boundaries (HAGBs). The HAGBs are mainly contributed by Σ3 coincident site lattice boundaries. At the end, a discussion on post-deformation softening, considering strain-induced boundary migration, has been presented.

Optimization of Thermomechanical Processing under Double-Pass Hot Compression Tests of a High Nb and N-Bearing Austenitic Stainless-Steel Biomaterial Using Artificial Neural Networks

Physical simulation is a useful tool for examining the events that occur during the multiple stages of thermomechanical processing, since it requires no industrial equipment. Instead, it involves hot deformation testing in the laboratory, similar to industrial-scale processes, such as controlled hot rolling and forging, but under different conditions of friction and heat transfer. Our purpose in this work was to develop an artificial neural network (ANN) to optimize the thermomechanical behavior of stainless-steel biomaterial in a double-pass hot compression test, adapted to the Arrhenius–Avrami constitutive model. The method consists of calculating the static softening fraction (Xs) and mean recrystallized grain size (ds), implementing an ANN based on data obtained from hot compression tests, using a vacuum chamber in a DIL 805A/D quenching dilatometer at temperatures of 1000, 1050, 1100 and 1200 °C, in passes (ε1 = ε2) of 0.15 and 0.30, a strain rate of 1.0 s−1 and time between passes (tp) of 1, 10, 100, 400, 800 and 1000 s. The constitutive analysis and the experimental and ANN-simulated results were in good agreement, indicating that ASTM F-1586 austenitic stainless steel used as a biomaterial undergoes up to Xs = 40% of softening due solely to static recovery (SRV) in less than 1.0 s interval between passes (tp), followed by metadynamic recrystallization (MDRX) at strains greater than 0.30. At T &gt; 1050 °C, the behavior of the softening curves Xs vs. tp showed the formation of plateaus for long times between passes (tp), delaying the softening kinetics and modifying the profile of the curves produced by the moderate stacking fault energy, γsfe = 69 mJ/m2 and the strain-induced interaction between recrystallization and precipitation (Z-phase). Thus, the use of this ANN allows one to optimize the ideal thermomechanical parameters for distribution and refinement of grains with better mechanical properties.

Effect of Sample Geometry on Strain Uniformity and Double Hit Compression Tests for Softening Kinetics Determination

Static softening is an essential process during the hot rolling of steel to refine grain size and improve mechanical properties. The double hit test is used to measure the static softening volume fraction from the flow stress curves. Herein, the influence of different sample geometries on the determination of softening fraction from the double hit test for conditions where 0–99% softening is expected. The double hit tests are modeled in DEFORM for conventional sample geometries in both uniaxial compression and standard plane strain compression tests, and an additional simulation of a modified plane strain compression test sample geometry, designed to provide more uniform strain distributions, is also performed. A user routine is incorporated into the model to predict the localized softening volume fraction from localized strain based on known softening equations and set back the localized strain value to zero while the softening volume fraction reaches 85%. The standard plane strain compression test geometry shows a strong strain gradient resulting in inaccurate softening fraction volume predictions from flow stress, whereas the uniaxial compression test and modified plane strain compression test geometries show better predictions of softening volume fraction from the flow stress data.

Study on static softening behavior and hot working performance of Fe-0.2C-7Mn steel

Static softening behavior and hot working performance of medium Mn steel (Fe-0.2C-7Mn) are studied by 2-step and interrupt hot compression test with a Gleeble-3500 thermal-mechanical simulator. The results show that static softening behavior of the steel exhibits obvious sensitivity to the deformation temperature, inter-pass time and strain rate. But deformation temperature has a more significant effect on the static recrystallization (SRX) grain size than strain rate. 3D hot working processing maps are established, that low temperature and medium strain rate (850 °C ∼ 925 °C and 0.02 s−1 ∼ 0.3 s−1) are the preferred thermal deformation conditions for the test steel. Microstructure observations indicate that both static recovery (SRV) and SRX occur within inter-pass time, and SRV is the predominant mechanism in determining softening fraction. The microstructure is composed of film-like α at room temperature, and most of the nucleation of SRX preferably take place at triple junctions of grains or prior grain boundaries. Besides, high-angle grain boundaries (HAGBs) migration produce twin boundaries during grain growth and the main nucleation mechanism of the SRX is strain-induced boundary migration. The kinetics model of static softening is established and the experimental values are consistent with predicted ones, which proves the reliability of the model.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy. The kinetics of static recovery, described based on the activation parameters (activation energy and volume of static recovery) and the static softening fraction, were mainly studied through two thermomechanical tests, namely, double-pass compression and stress relaxation tests. A new approach was proposed to measure the static softening fraction in the stress relaxation test. In general, a higher temperature or strain rate accelerates static recovery, while interestingly, the effect of pre-strain on static recovery is opposite in cases with and without external stress, owing to the enhanced static recovery by external stress during annealing. In addition, microhardness tests and electron backscatter diffraction (EBSD) characterization were also conducted to verify the accuracy of the results. The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery. It is noteworthy that the special stress hardening phenomenon occurring at 400 °C and 0.1/s is caused by strain aging, showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Effect of Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results.

Softening behavior of deformed metals during continuous cooling and its relation to isothermal reactions

Softening of work-hardened metals due to static recovery and recrystallization is an important issue during continuous cooling after hot deformation, which will affect the properties of metals. A mechanical test method called “twined double-pass mechanical tests”, has been proposed to measure the softening fraction of a deformed metal during continuous cooling, and then both non-isothermal and isothermal softening data of 304 austenitic stainless steel have been measured with the mechanical tests. An attempt has been made to build the relationship between non-isothermal softening behavior and isothermal softening kinetic data. On the assumption that the softening fraction only depends on the softening fraction X and temperature T, the softening curve during continuous cooling has been predicted from the isothermal softening kinetic data with numerical method. A good agreement was observed between the predicted curve and those values measured with twined double-pass mechanical tests. The Scheil's additivity rule was extended from the prediction of incubation period for pearlite transformation to the whole range of softening fraction of work-hardened metals. The relationship between non-isothermal and isothermal softening reactions has also been investigated with Scheil's additivity rule, and the additivity of the softening process of work-hardened metals during continuous cooling has been verified.

The high temperature deformation behavior of a triplex (ferrite+ austenite+ martensite) low density steel

Abstract The present work deals with the high temperature deformation behavior of Fe-11.15Mn-5.6Al-0.07C (wt.%) triplex ferrite-based lightweight steel in the temperature range of 800–1100 °C under the strain rate of 0.001 to 0.1s−1. The compressive high temperature flow curves under the various thermomechanical conditions were accompanied by a considerable fractional softening. According to the detailed microstructural analysis, the observed flow softening was discussed relying on the occurrence of dynamic strain induced transformation and dynamic recrystallization. In this respect, a sine hyperbolic Arrhenius-type constitutive model was developed considering the three dimensional variation of the materials’ constant with strain, strain rate and temperature. This provided a proper condition for accurate assessment of the strain compensation mechanisms. The capability of the modified and un-modified constitutive models in prediction of the high temperature flow behavior of experimented low density steel were compared. According to the verified model, activation energy (Q) maps were developed and discussed in correlation with the characterized microstructure evolutions. The Q-plots were divided into three domains and a transition range was recognized at ~1100–1250 K, the extent of which decreased with increasing imposed strain. The low energy domains were attributed to the (i) activation of load transition as an effective strain compensation mechanism and the occurrence of dynamic austenite to ferrite transformation, and (ii) the high dislocation annihilation rate at high temperatures.

Dynamic and static softening mechanisms of commercial-purity Zr during double-stage hot compressive deformation

The dynamic and static softening behavior of R60702 commercial-purity Zr was studied through single-stage and double-stage uniaxial hot deformation tests in the range of 600oC–800 °C. The microstructural evolution to uniaxial compression loading for the different conditions was investigated using a combination of optical microscopy and electron back-scattering diffraction (EBSD). The results show that the static softening fraction increases with an increase in temperature as well as holding time. At 600 °C, the peak stress of the second stage was greater than that of the single-stage, and the sample did not undergo static softening. At 800 °C, the peak stress of the second stage was less than that of the single-stage, and the sample underwent rapid static softening during holding time. As the holding time increased at 700 °C, the peak stress of the second stage of the sample decreased successively. At 700 °C and 1s holding time, the sample did not undergo static softening, and dynamic recovery and recrystallization occurred in the subsequent second stage. At 700 °C and 60s holding time, the sample underwent static recovery and recrystallization; larger grain growth and deformation appeared in the subsequent second stage. This change in the microstructural morphology with the flow stress curve is discussed in the subsequent sections.

Recrystallization Behavior and Microstructure Evolution in High‐Manganese Austenitic Steel

Compression deformation experiments are conducted to investigate the recrystallization behavior during the dynamic recrystallization (DRX) and static recrystallization (SRX) of high‐manganese steel in terms of flow stress and microstructure morphology. The experimental results indicate that strain rate plays a key role on DRX style. A low strain rate promotes discontinuous dynamic recrystallization (DDRX), whereas a high strain rate encourages continuous dynamic recrystallization (CDRX). The steady‐state stress of DRX behavior decreases with increasing deformation temperature regardless of the true strain and strain rate. Deformation temperature is a vital factor in determining the softening fraction of SRX behavior. The effect of interpass time on austenite grain size is weak for SRX behavior. The SRX kinetics equation for the designed high‐manganese steel is proposed according to experimental results.

Double twist torsion testing to determine the non recrystallization temperature

A double-twist torsion testing technique has been developed using a 316 stainless steel as an exemplar material to experimentally assess recrystallization behavior and determine the non-recrystallization temperature (Tnr). This new method was compared to the traditional methods of double-hit compression and multi-step hot torsion testing. The double-twist torsion test allows Tnr to be related to the extent of austenite recrystallization through measurements of fractional softening while accommodating multiple deformation and recrystallization steps with a single specimen. The double-twist torsion test resulted in average Tnr values similar to those determined with multi-step hot torsion, and a partially recrystallized microstructure was observed in the vicinity of the calculated Tnr for all three methods. The ability of the double-twist torsion test to relate the experimental Tnr to the evolution of austenite recrystallization via fractional softening measurements while incorporating effects of multiple deformation steps offers an advantage over traditional methods for quantifying changes in austenite recrystallization during thermomechanical processing.