Hot Deformation Mechanism Research Articles

Hot tensile deformation mechanism and microstructure evolution of Mg 2Nd alloy with heterostructure

The deformation mechanism and microstructure evolution of the extruded Mg2Nd alloy were investigated in the hot tensile deformation process at strain rate of 2.2 × 10−4 s−1 and deformation temperatures of 150 °C–300 °C. the results showed that the peak stress declines and the elongation to fracture increases with the increasing of deformation temperature. By observation of the microstructure, the fibrous extrusion zones (FEZs) and non-fibrous extrusion zones (non-FEZs) coexist in the alloy, and the microstructure deformation of the FEZs is more obvious than that of the non-FEZs. In the initial deformation stage, the grain deformation first occurs in the FEZs. In the later deformation stage, recrystallization occurs in both FEZs and non-FEZs. Moreover, the deformation mechanism of the FEZs is different from that of the non-FEZs. The deformation mechanism of the FEZs is dominated by intragranular slip, supplemented by grain boundary slip (GBS). The recrystallization mechanism in FEZs is continuous dynamic recrystallization (CDRX). On the contrary, the deformation of the non-FEZs is dominated by GBS, while the intragranular slip plays a coordinated role. The recrystallization mechanism in non-FEZs is discontinuous dynamic recrystallization (DDRX). Therefore, the dominated hot tensile deformation mechanisms of the alloys are alternated among intragranular slip, GBS and dynamic recrystallization (DRX)

High Temperature Mechanical Properties and Microstructure Evolution of Ti-6Al-4V Alloy Linear Friction Welding Joints

The combination of linear friction welding (LFW) and hot forming processes is limited due to a lack of research on the high-temperature flow behavior and microstructure evolution of welded joints. In this paper, an electric-assisted high-temperature uniaxial tensile test platform based on digital image correlation (DIC) is built, and uniaxial tensile tests of Ti-6Al-4V alloy with LFW joints are performed at different temperatures (923–1023 K) and different strain rates (0.001 s−1–0.01 s−1). Then the microstructure of the LFW joints under different hot deformation conditions have been analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that the high-temperature flow behavior of LFW joints shows an obvious correlation between temperature and strain rate: the yield stress decreases from 203 MPa at 923 K to 105 MPa at 1023 K, and increases from 85 MPa to 130 MPa when the strain rate increases from 0.001 s−1 to 0.01 s−1 at 973 K. The hot deformation mechanisms with deformation conditions have been characterized, which changes from the mechanism of dislocation creep to the mechanism of self-diffusion as the deformation temperature increases from 923 to 1023 K. Especially, the fraction of high angle boundaries (HABs) rapidly rise from 49.2% to 64.1% with the increasing temperatures, the discontinuous dynamic recrystallization (DDRX) become the primary mechanism of nucleation during high-temperature deformation of LFW joints.

Hot deformation properties of liquid forging AZ80 alloy

ABSTRACT The hot deformation mechanism of the actual hot extrusion products was unknown, and the deformation properties of AZ80 were investigated. The results show that the hot deformation mechanism of AZ80 was dynamic recrystallisation. There is a corresponding relationship between peak stress and peak strain. The room temperature mechanical properties after hot deformation depend on the recrystallisation degree, grain size and sub-structure. The recrystallisation grain was observed to be equiaxed. The AZ80 has better hot extrusion forming performance.

Hot deformation characteristics of Ti-6Al-4V

Abstract The flow behavior and hot deformation mechanisms of Ti-6Al-4Vin both β and (α + β) phase regions have been studied by compression tests in the temperature range 750 – 1100 °C and strain rate range 0.05 –15 s –1. The flow curves displayed discontinuous yielding at temperatures above 850 °C and high strain rates, typically 5 and 15 s–1. Flow oscillations were also observed under such conditions. The relative discontinuous flow stress drop increased with increasing temperature in the (α + β) phase region and reached a steady state in the β phase region. The discontinuous yielding is attributed to the sudden increase in the mobile dislocation density occurring due to the activation of β grain boundary sources during hot deformation. The apparent activation energy estimated using the standard kinetic analysis is about 246 kJ/mol in the β region and 621 kJ/ mol in the (α + β) region while the strain rate sensitivity values are 0.34 and 0.18, respectively. The mechanism in the β region has been suggested to involve grain boundary sliding and dynamic recovery of the β phase while in the (α + β) region a transient mechanism, involving dynamic recovery followed by post deformation recrystallization, has been proposed.

Deformation microstructure and thermomechanical processing maps of homogenized AA2070 aluminum alloy

The hot deformation behavior of homogenized AA2070 Al–Li–Cu alloy was studied using isothermal compression in a temperature range of 300 °C–500 °C and a strain rate range of 0.001 s-1 to 1 s-1. Deformation microstructure was characterized using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) including weak beam dark field STEM imaging to correlate the microstructure evolution with hot deformation mechanism. TEM/STEM results show that a high density of fine T1 (Al2LiCu) phase forms in the microstructure through dynamic precipitation at compression temperatures lower than 400 °C, especially at low strain rates. Constitutive analysis shows that back stress due to dynamic precipitation of T1 phase decreases with increasing compression temperature and increasing strain rate. EBSD characterization indicates that dynamic recovery is the main softening mechanism at temperatures below 400 °C. At 400 °C and above, intermetallic phases including T1, Al20Cu2Mn3 and Al3Cu2 in the microstructure gradually dissolve, and dynamic recrystallization becomes the dominant softening mechanism, resulting in texture weakening and grain refinement. Three dynamic recrystallization mechanisms including discontinuous dynamic recrystallization (DDRX) at large angle grain boundaries, particle stimulated nucleation (PSN) and continuous dynamic recrystallization (CDRX) were observed. Based on hot work efficiency, deformation instability and microstructure evaluation, processing maps were constructed using both conventional method and Garofalo sinh equation, providing optimum processing conditions (temperature >400 °C and strain rate <0.1 s-1) for industrial forging, extrusion and rolling processes.

Hot Deformation Characteristics and Mechanism Understanding of Ti-6al-2sn-4zr-6mo Titanium Alloy

Hot Deformation Characteristics and Mechanism Understanding of Ti-6al-2sn-4zr-6mo Titanium Alloy

Revealing the Hot Workability and Deformation Mechanism of a Low-Density Fe-Mn-Al-C Duplex Steel

Revealing the Hot Workability and Deformation Mechanism of a Low-Density Fe-Mn-Al-C Duplex Steel

Effect of Ultrasonic-Assisted Casting on Hot Deformation Mechanism and Microstructure of 35CrMo Steel Ingot.

Hot compression tests were performed with strain rates (0.01–10 s−1) and temperatures (850–1150 °C). The power law relationship between the critical stress and critical strain and Zener–Hollomon parameters was determined by θ-σ curves. Microstructure was investigated by electron backscattered diffraction. The results showed that the flow behavior and microstructure of 35CrMo steel was affected by ultrasonic-assisted casting. The activation energy of non-ultrasonic and ultrasonic-assisted 35CrMo steel were 410 ± 9.9 and 386 ± 9.4 kJ/mol, respectively, and the activation energy of ultrasonic-assisted specimens was reduced by 6%. In addition, the ultrasonic-assisted treatment refines the grains to some extent and makes the softening process of ultrasonic-assisted samples progress faster, which promoted the development of dynamic recrystallization and the production of Σ3 boundaries. The discontinuous dynamic recrystallization was the main DRX nucleation mechanism of the 35CrMo steel.

Study on the Hot Deformation Characterization of Borated Stainless Steel by Hot Isostatic Pressing.

Borated stainless steel (BSS) specimens have a boron content of 1.86 wt%, and are prepared by hot isostatic pressing (HIP) conducted at different temperatures, ranging from 1000 to 1100 °C and a constant true strain rate (0.01, 0.1, 1 and 10 s−1). These tests, with observations and microstructural analysis, have achieved the hot deformation characteristics and mechanisms of BSS. In this research, the activation energy (Q) and Zener–Hollomon parameter (Z) were contrasted against the flow curves: Q = 442.35 kJ/mol. The critical conditions associated with the initiation of dynamic recrystallization (DRX) for BSS were precisely calculated based on the function between the strain hardening rate with the flow stress: at different temperatures from 1000 to 1100 °C: the critical stresses were 146.69–254.77 MPa and the critical strains were 0.022–0.044. The facts show that the boron-containing phase of BSS prevented the onset of DRX, despite the saturated boron in the austenite initiated DRX. The microstructural analysis showed that hot deformation promoted the generation of borides, which differed from the initial microstructure of HIP. The inhomogeneous distribution of elements in the boron-containing phase was caused by hot compression.

Hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy with columnar grains

In this work, hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy prepared using directional solidification (DS) were investigated by carrying out the isothermal compression tests within the temperature range of 1080–1190 °C and the strain rate range of 0.001–1 s-1 under the true strain of 0.693. The effects of temperature and strain rate on dynamic recrystallization (DRX) were analyzed, and constitutive equations were established. The activation energies for γ+γʹ dual-phase and the γ single-phase regions of Ni–Co-based superalloy with columnar grains were 1190 kJ mol-1 and 415 kJ mol-1, respectively. The results showed that the dominant deformation mechanisms were closely related to the strain rate and temperature. DRX was accelerated at low strain rate conditions, while the DRX process was apparently sluggish at strain rates higher than 1 s-1. The undissolved γʹ phase interacted with dislocations and accelerated the DRX of the matrix during γʹ sub-solvus temperature (1080–1120 °C) deformation. Continuous dynamic recrystallization (CDRX) nucleation proved to play a dominant role. During γʹ super-solvus temperature (1150–1190 °C) deformation, the dominant dynamic softening mechanism of the alloy is discontinuous dynamic recrystallization (DDRX). Local migration of grain boundaries (GBs) causes an increase of misorientation within the deformed grains and induced the formation of subgrain boundaries near the GBs, which subsequently evolved into high angle grain boundaries (HAGBs) and participated in DDRX. Furthermore, during high-temperature deformation in Ni–Co-based superalloys, the relatively rare microtwins (MTs) were detected under compression at 1100 °C/1 s-1.

Mechanical behavior and microstructure evolution of AZ31 magnesium alloy sheet in an ultrasonic vibration-assisted hot tensile test

Ultrasonic vibration-assisted tensile tests of AZ31 magnesium (Mg) alloy sheets are carried out at different temperatures (150 °C, 200 °C) and amplitudes (0–11.2 µm). The influences of the ultrasonic vibration on the stress-strain responses, microstructural evolution, fracture morphology, dynamic recrystallization (DRX), and dislocation structure of the material during hot deformation are discussed. The results show that both flow stress and elongation of the material are significantly influenced by the temperature and amplitude. The application of ultrasonic vibration delays the occurrence of DRX. However, it accelerates the continuous dynamic recrystallization process, thereby increasing the percentage of the DRX. Furthermore, an evolution of the dislocation structure occurs under the impact of ultrasonic vibration, offering valuable insights into the plasticity improvement of the material. The underlying mechanisms of ultrasonic vibration-assisted hot deformation of AZ31 Mg alloys can be described as below. Ultrasonic vibration affects the motion and distribution of dislocations, which have an effect on the dynamic recrystallization behavior, ultimately leading to changes in mechanical behavior of the material.

Hot tensile deformation behavior and microstructure evolution of Mg-1Al-6Y alloy

In this study, the hot tensile test was carried out using the extruded and annealed Mg-1Al-6Y alloy. The effect of temperature and strain rate on the hot tensile deformation behavior of the alloy was systematically studied at different temperatures (200 °C ∼ 350 °C) and different strain rates (8 × 10−5 s−1 ∼ 1.6 × 10−3 s−1). In addition, the effect of temperature on the evolution of microstructure when the strain rate is 1.6 × 10−3 s−1 was investigated. The results showed that as the temperature increased or the strain rate decreased, the peak stress decreased and the elongation increased. Hot tensile at different temperatures all increased the texture intensity, and the microstructure after deformation showed obvious characteristics of basal fiber texture ([0001]⊥ED). Correspondingly, the weaker [−15–40]//ED texture before deformation transformed into a stronger [01–10]//ED fiber texture. After deformation, the average Schmid factor (SF) of each non-basal slip was significantly increased compared with the average SF before deformation, indicating that abundant non-basal slip was activated during the deformation. When the deformation temperature was 300 °C, dynamic recrystallization (DRX) occurred significantly, and the DRXed grains accounted for 15.9%. DRX was a combination of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Furthermore, the calculated activation energy of the alloy was about 98.8 kJ mol−1. Comprehensive research showed that the hot tensile deformation mechanism mainly included intragranular slip, grain boundary slip (GBS) and DRX.

Hot Tensile Deformation Behavior of Mg-4Li-1Al-0.5Y Alloy

The microstructure evolution and deformation mechanism of the as-extruded-annealed Mg-4Li-1Al-0.5Y alloy (denoted as LAY410) were investigated during the hot tensile deformation at the temperatures between 150°C and 300°C with strains from 8 × 10−5 s−1 to 1.6 × 10−3 s−1. The results show that when the strain rate decreases and/or the deformation temperature increases, the peak stress of the alloy gradually decreases, and the elongations to fracture gradually increases. The true stress–strain curves show typical dynamic recrystallization (DRX) softening characteristics. It is observed that the microstructure in the magnesium (Mg) alloy deformed at 150°C is mainly composed of the deformed grains and a few recrystallized grains. The microstructures in the Mg alloy deformed at 200°C consisted of substructures and a slightly increasing number of dynamic recrystallized grains. When the deformation temperature reaches 250°C, the number of recrystallized grains increases significantly, and the microstructures are dominated by recrystallized grains. Moreover, through theoretical calculation and result analysis, the activation energy was about 99.3 kJ/mol, and the hot tensile deformation mechanism was the alternate coordinated deformation mechanism among grain boundary slip (GBS), intragranular slip, and DRX.

Crystallography and microstructure of the deformation bands formed in a metastable β titanium alloy during isothermal compression

The plastic deformation of crystals accompanied by the formation of deformation bands or slip bands has been observed in previous studies; however, this phenomenon has not been examined in sufficient detail. In this study, the crystallography and microstructure of the deformation bands formed in a metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al during isothermal compression were investigated. The obtained results revealed that numerous deformation bands were formed inside the β grains and along the initial β grain boundaries. The majority of the produced bands were located in the grain interiors, and their formation was accompanied by the fragmentation and spheroidisation of the lamellar α phase. According to the trace analysis results, these bands belonged to the {110} <11¯1> slip band type and exhibited high Schmid factors. The crystallographic calculation data indicated that the rotation axis of the misorientation between the β phase in the lamellar area and that in the band area passed through the deformation bands. Furthermore, the origin of the formation of bands was caused by the relative slip of the lamellar β and α phases due to the coordinated shear under compression. Thus, the findings of this study can help achieve a better understanding of the hot deformation mechanisms of titanium alloys.

Hot deformation behavior and mechanism of a new metastable β titanium alloy Ti–6Cr–5Mo–5V–4Al in single phase region

Abstract A new metastable β titanium alloy Ti–6Cr–5Mo–5V–4Al (Ti-6554) has shown good application potential in large parts. In order to study the hot deformation behavior and mechanism of the Ti-6554 alloy in the β single phase region, hot compression tests were carried out at temperatures of 800–950 °C and strain rates of 0.001–10 s−1 on the Gleeble-3500 thermal simulation machine. Strain-compensated Arrhenius constitutive model was used to predict the flow behavior of the alloy, and the correlation coefficient between the experimental and predicted values reached 0.982. Based on the hot processing map, it was found that the instability region was mainly concentrated in the high strain rate area, and the peak efficiency of power dissipation (η) region occurs in the temperature range of 913–928 °C and the strain rate of 0.001–0.0025 s-1. There was an obvious DRX phenomenon in the stability region, while the instability region was dominated by deformation band (DB) and flow localization (FL). The continuous dynamic recrystallization (CDRX) by progressive rotation of subgrains and discontinuous dynamic recrystallization (DDRX) by grain boundaries bulging could be observed. DDRX mainly occurred in high temperature and low strain rate regions, while the CDRX process occurred in the high strain rate region. As the η decreased, the deformation mechanism changed from DDRX to CDRX and further to dynamic recovery (DRV), DB, FL.

A Comparative Study on the Activation Energy for Hot Deformation of 5CrNiMoV and S34MnV Steel

Background: The value of the activation energy is dependent on deformation conditions and sensitive to the difference in the alloying element. The research on the evolution of the activation energy with strain and alloying elements plays an important role in analyzing the mechanism of hot deformation. Objective: The purpose was to clarify the activation energy evolution with strain, to analyze strengthening or softening mechanism of each element and acquire an empirical equation for medium carbon steels to quantitatively describe the effect of each element on activation energy. Methods: Hot compression tests of the two medium carbon steel were conducted. The value of activation energy at different strain was obtained by fitting the experiment results. Through the analysis of medium carbon steels with different composition in published papers, the empirical relationship between thermal activation energy and chemical composition can be fitted. Results: It was found that the activation energy showed significant dependence on the strain and chemical composition. The activation energy’s evolution with strain was revealed. An empirical expression for medium carbon steels was proposed to characterize the influence of the chemical composition on the activation energy. It was found that the microalloying elements Nb, Ti, V and Mo, as the main solution strengthener and precipitation strengthener, have a positive effect on the activation energy. Conclusion: The evolution of the activation energy with strain was revealed and the corresponding hot deformation mechanism was analyzed through the activation energy. An empirical expression was acquied to characterize the influence of the chemical composition on the activation energy for medium carbon steels, which would be used for the research on new materials.

Constitutive modeling of flow behavior of precipitation-hardened AA7022-T6 aluminum alloy at elevated temperature

Abstract The thermomechanical behavior of precipitation-hardened aluminum alloy AA7022-T6 was studied using isothermal compression at temperatures of 623−773 K and strain rates of 0.01−1 s−1. The experimental results indicated that dynamic recrystallization (DRX) is a predominant hot deformation mechanism, especially at elevated temperatures and low strain rates. The modified Johnson−Cook (J−C) and the strain compensated Arrhenius-type models were developed to predict the hot flow behavior under different deformation conditions. The correlation coefficients of modified J−C model and the strain compensated Arrhenius-type models were 0.9914 and 0.9972, respectively, their average relative errors (ARE) were 6.074% and 4.465%, respectively, and their root mean square errors (RMSE) were 10.611 and 1.665 MPa, respectively, indicating that the strain compensated Arrhenius-type model can predict the hot flow stress of AA7022-T6 aluminum alloy with an appropriate accuracy.

The interplay relationship between phase transformation and deformation behavior during hot compression in a metastable β titanium alloy

In this work, the hot compression behavior at 600 °C of a metastable β titanium alloy is investigated to resolve the correlation between phase transformation and deformation behavior. It is demonstrated that the deformation behavior includes a near linear hardening stage, a discontinuous yielding stage, a hardening stage and a final softening stage. For the near linear hardening stage, the α precipitates are formed on the grain boundaries (αGB). The hardening behavior of the alloy in this stage balanced by boundary hardening through the formation of αGB. For the discontinuous yielding stage, the softening of deformation behavior is originated from the fragmentation of the αGB and the generation and movement of mobile dislocation. For the hardening stage follow the discontinuous yielding, the massive β to α phase transformation were occurred and mainly contributed to the hardening behavior. For the final softening behavior stage, three activities: the formation of deformation bands, the spheroidization of α phase and the dislocation slips contribute to the softening behavior. The present results provide comprehensive information on hot deformation mechanisms of metastable β titanium alloys.

Hot tensile deformation behavior of extruded LAZ532 alloy with heterostructure

Hot tensile test was performed at deformation conditions of 150–°C300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1, respectively, to systematically study the microstructure development and deformation mechanism of extruded Mg–5Li–3Al–2Zn (LAZ532) alloy with a heterostructure of both the fibrous extrusion zones (FEZs) and non-fibrous extrusion zones (non-FEZs) coexisted. The results showed that the peak stresses decreased gradually, while the fracture strains increased gradually with the decrease of strain rates or the increase of deformation temperatures, and the alloy exhibited superplastic characteristics at deformation conditions of 300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1. By microstructure observation, the alloy showed that in the initial deformation stage, the deformation of the grains in the FEZs was prior to that in the non-FEZs. In the middle deformation stage, the deformation of the grains in the FEZs was dominated by intragranular slip and accompanied with grain boundary slip (GBS), while the deformation of the grains in the non-FEZs was dominated by GBS and accompanied with intragranular slip. In the later deformation stage, the continuous dynamic recrystallization (CDRX) generated in the coarse lamellar grains in the FEZs due to dislocation pile-up, in contrast, the discontinuous dynamic recrystallization (DDRX) generated at the grain boundaries in the non-FEZs. Moreover, based on theoretical calculation and result analysis, the activation energy was about 110.0 kJ/mol, and the hot tensile deformation mechanism was each other alternating and coordinated deformation mechanism among GBS, intragranular slip and dynamic recrystallization (DRX).

Hot Deformation Treatment of Grain-Modified Mg-Li Alloy.

In this work, a systematic analysis of the hot deformation mechanism and a microstructure characterization of an as-cast single α-phase Mg–4.5 Li–1.5 Al alloy modified with 0.2% TiB addition, as a grain refiner, is presented. The optimized constitutive model and hot working terms of the Mg–Li alloy were also determined. The hot compression procedure of the Mg–4.5 Li–1.5 Al + 0.2 TiB alloy was performed using a DIL 805 A/D dilatometer at deformation temperatures from 250 °C to 400 °C and with strain rates of 0.01–1 s−1. The processing map adapted from a dynamic material model (DMM) of the as-cast alloy was developed through the superposition of the established instability map and power dissipation map. By considering the processing maps and microstructure characteristics, the processing window for the Mg–Li alloy were determined to be at the deformation temperature of 590 K–670 K and with a strain rate range of 0.01–0.02 s−1.