Progressive Subgrain Rotation Research Articles

BPANN modified constitutive descriptions for flow behavior and softening mechanism in the α+β phase region of Ti-55511 alloy with equiaxed microstructure

The deformation behavior of Ti-55511 alloy was investigated by hot compression tests conducted at deformation temperatures of 993∼1113 K and strain rates of 0.001–1 s−1. To eliminate the effects of friction and adiabatic temperature rise on the flow stresses, the flow curves were corrected. Subsequently, a strain compensated Arrhenius (SCA) model was developed and further modified using the back propagation artificial neural network (BPANN) method. The modified model exhibits significant improvements compared to the SCA model. The average absolute relative error (AARE) decreases from 4.895 % to 0.756 %, while the relative error (δ) range decreases from −14.75 %–17.99 % to −4.78 %–8.01 %. Moreover, cross-validation also confirms the modified model has higher generalization ability and prediction accuracy than the SCA model. Microstructural characterization reveals dynamic recrystallization (DRX) of the α phase and dynamic recovery (DRV) of the β phase are the main mechanisms responsible for softening, and their volume fractions increase with decreasing strain rate and increasing deformation temperature. The equiaxed α phase undergoes elongation first, followed by gradual migration and transformation of grain boundaries. Finally, the β phase wedges into the α phase until the α phase separates. The continuous dynamic recrystallization (CDRX) of β phase accompanies its DRV, which is mainly characterised by progressive sub-grain rotation. The α grains aggregated by dislocations promote CDRX of the β phase.

A comparative analysis of a unique DRX process in achieving a homogeneous distribution of β grains in the single phase region of as-cast Ti644321 alloy

This study aims to investigate the distinctive characteristics of a specialized dynamic recrystallization (DRX) process, which results in a fine and uniform distribution of β grains within the single phase region of as-cast Ti644321 alloy. The formation of deformation bands (DB) and flow localization (FL) is a result of the instability region. On the other hand, the stability region exhibits significant dynamic recrystallization (DRX). The phenomenon of DRX encompasses continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX), and the coupling mechanism of DDRX and CDRX, which was discovered in this study. The occurrence of DDRX is attributed to grain boundary bulging, while CDRX is a result of the progressive rotation of subgrains. A strain rate of 0.1 s−1 was found to form a fine and uniform distribution of β grains. The formation of this microstructure is facilitated by a coupling mechanism that combines CDRX and DDRX, which involves the grain boundary strain concentration effect that triggers DDRX and the subgrain rotational nucleation process of CDRX. The deformation mechanism of the alloy during hot deformation varies depending on the η value. In the high η region, the primary deformation mechanism is mainly DDRX+CDRX, while in the medium η region, it transitions to CDRX+DRV. Ultimately, in the low η region, the primary deformation mechanism is DB + FL. The presence of a high proportion of DBs hinders DRX. This study provides novel insights into the mechanisms of DRX in β titanium alloys and emphasizes the significance of considering different forms of DRX.

Study on the microstructure evolution and dynamic recrystallization mechanism of selective laser melted Inconel 718 alloy during hot deformation

In this work, isothermal compression testing, electron back scattered diffraction (EBSD) technology and transmission electron microscope (TEM) were used to investigate the hot deformation behavior, microstructure evolution, and dynamic recrystallization (DRX) behavior of selective laser melted (SLM) 718 alloy. The results show that the deformation condition has significant impact on the flow behavior and DRX grain. With the increase of the deformation temperature or the decrease of the strain rate, the volume fraction and average size of DRX grains show an increasing trend. The DRX mechanism of SLM 718 alloy includes discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), in which DDRX is characterized by initial grain boundary and regenerated grain boundary bulging. CDRX occurs inside the grain, characterized by progressive subgrain rotation and homogeneous misorientation increase.

Hot Deformation Behavior and Microstructure Evolution of a Novel Al-Zn-Mg-Li-Cu Alloy.

Lightweight structural alloys have broad application prospects in aerospace, energy, and transportation fields, and it is crucial to understand the hot deformation behavior of novel alloys for subsequent applications. The deformation behavior and microstructure evolution of a new Al-Zn-Mg-Li-Cu alloy was studied by hot compression experiments at temperatures ranging from 300 °C to 420 °C and strain rates ranging from 0.01 s−1 to 10 s−1. The as-cast Al-Zn-Mg-Li-Cu alloy is composed of an α-Al phase, an Al2Cu phase, a T phase, an η phase, and an η′ phase. The constitutive relationship between flow stress, temperature, and strain rate, represented by Zener–Hollomon parameters including Arrhenius terms, was established. Microstructure observations show that the grain size and the fraction of DRX increases with increasing deformation temperature. The grain size of DRX decreases with increasing strain rates, while the fraction of DRX first increases and then decreases. A certain amount of medium-angle grain boundaries (MAGBs) was present at both lower and higher deformation temperatures, suggesting the existence of continuous dynamic recrystallization (CDRX). The cumulative misorientation from intragranular to grain boundary proves that the CDRX mechanism of the alloy occurs through progressive subgrain rotation. This paper provides a basis for the deformation process of a new Al-Zn-Mg-Li-Cu alloy.

Using Misorientation and Weighted Burgers Vector Statistics to Understand Intragranular Boundary Development and Grain Boundary Formation at High Temperatures

AbstractDuring plastic deformation, strain weakening can be achieved, in part, via strain energy reduction associated with intragranular boundary development and grain boundary formation. Grain boundaries (in 2D) are segments between triple junctions, that connect to encircle grains; every boundary segment in the encircling loop has a high (>10°) misorientation angle. Intragranular boundaries terminate within grains or dissect grains, usually containing boundary segments with a low (<10°) misorientation angle. We analyze electron backscatter diffraction (EBSD) data from ice deformed at −30°C ( 0.9). Misorientation and weighted Burgers vector (WBV) statistics are calculated along planar intragranular boundaries. Misorientation angles change markedly along each intragranular boundary, linking low‐ (<10°) and high‐angle (10–38°) segments that exhibit distinct misorientation axes and WBV directions. We suggest that these boundaries might be produced by the growth and intersection of individual intragranular boundary segments comprising dislocations with distinct slip systems. There is a fundamental difference between misorientation axis distributions of intragranular boundaries (misorientation axes mostly confined to ice basal plane) and grain boundaries (no preferred misorientation axis). These observations suggest during progressive subgrain rotation, intragranular boundaries remain crystallographically controlled up to large misorientation angles (>>10°). In contrast, the apparent lack of crystallographic control for grain boundaries suggests misorientation axes become randomized, likely due to the activation of additional mechanisms (such as grain boundary sliding) after grain boundary formation, linking boundary segments to encircle a grain. Our findings on ice intragranular boundary development and grain boundary formation may apply more broadly to other rock‐forming minerals (e.g., olivine, quartz).

Microstructure Evolution and Deformation Mechanism of Tantalum-Tungsten Alloy Liner under Ultra-High Strain Rate by Explosive Detonation.

The microstructure evolution and plastic deformation mechanism of a Ta-2.5W liner under the ultra-high-strain-rate conditions generated by the explosive detonation were investigated in this study. For this purpose, a modular soft-recovery apparatus was designed to non-destructively recover the Ta-2.5W explosively formed projectile (EFP) in the ballistic endpoint. The electron backscattered diffraction (EBSD) method was employed to examine the microstructure of the Ta-2.5W liner before and after deformation. The microstructure of the recovered EFP exhibited significant grain refinement with preferred fiber texture. The theoretical computation results showed that the temperature of the EFP was in the range of 0.27–0.65 Tm. The deformation mechanism of the Ta-2.5W liner forming EFP driven by the detonation is the continuous dynamic recrystallization (CDRX) induced by high strain deformation, rather than the conventional dynamic recrystallization of nucleation and growth. The new grain structures evolve when the low-angle grain boundaries are transformed into the high-angle grain boundaries, and the specific grain refinement mechanism is the progressive rotation of subgrains near pre-existing grain boundaries.

Fabricating Ti–22Al–25Nb intermetallic with ductility higher than 25% by advanced printing technique: Point-forging and laser-deposition

PF-LD is an advanced printing technique which introduces point-forging (PF) into the process of conventional laser-deposition (LD). Ti–22Al–25Nb intermetallic fabricated by PF-LD technique presented the excellent combination of room-temperature ultimate tensile strength (UTS: 1050.1–1169.3 MPa) and elongation after failure (EL: 20.8%–25.7%). An elaborately designed controlled trial by simultaneous use of LD and PF-LD techniques was employed to investigate the effect of PF on LD microstructural evolution. The results pointed out that the occurrence of static recrystallization (SRX) was the essential reason for the ultra-high mechanical properties. Both mechanisms of continuous-SRX (CSRX) and discontinuous-SRX (DSRX) played an important role in grain refinement. By in-situ forging and subsequent re-heating, the coarse columnar grains were first broken into fine equiaxed grains through progressive subgrain rotation. After two-pass PF-LD, the solidification texture <100>//DD disappeared while no obvious recrystallization texture was found. In subsequent PF-LD process, micron-scale CSRX grains were developed within original equiaxed grains through repeated polygonization and progressive subgrain rotation while DSRX grains appeared along grain boundaries driven by the differential dislocation density. We elucidated the excellent mechanical properties by detailed microstructural analysis. The results revealed that the finer equiaxed grains and the reasonable phase composition (O + BCC) greatly retarded the process of crack nucleation and propagation resulting in the typical micro-void accumulation ductile fracture while both high-density dislocations and finer recrystallized grains were beneficial to strength enhancement.

A Study on the Thermo-Mechanical History, Residual Stress, and Dynamic Recrystallization Mechanisms in Additively Manufactured Austenitic Stainless Steels

In this work, both numerical simulations and experimental characterization were used to obtain a broad understanding of the thermo-mechanical history, residual stress, and microstructure of the directed energy deposition (DED) process of austenitic stainless steels. To investigate the effect of process factors on residual stress, the global sensitivity analysis approach based on D-MORPH-HDMR was utilized. The results of the research reveal that the amplified effect of the influence of the three input variables (layer thickness, L; laser power, P; and scanning speed, v) on the transverse residual stress and thickness-direction residual stress is L &gt; P &gt; v; in contrast, the influence of longitudinal residual stress is P &gt; L &gt; v. We also found that general tendencies in local plastic strain accumulation are analogous to the relative distribution of geometrically necessary dislocations (GNDs). Additionally, we investigated post-solidification structures connected to residual stress, such as submicron dislocation cells and dynamic recrystallization (DRX) in austenitic stainless steels during DED. The investigation revealed that the DDRX and CDRX phenomena were caused by the bulging of initial grain boundaries and progressive sub-grain rotation (PSR). The fact that the sample bottom had more thermo-mechanical cycles than the top led to a higher dislocation density and hence more DDRX. This study presents a unique perspective on the link between residual stress and microstructure in additive manufacturing.

Microstructure, texture evolution and mechanical behavior of Ti-3.5Al-5Mo-4V titanium alloy during hot rolling in a β field

The microstructure, microtexture evolution, and corresponding mechanical behaviors of Ti-3.5Al-5Mo-4 V during hot rolling in the β field were investigated. The results showed that two distinct dynamic recrystallization (DRX) mechanisms exist during hot rolling, namely, continuous DRX (CDRX) with progressive rotation of sub-grains and discontinuous DRX (DDRX) comprising original grain boundary bulging and nucleation. Moreover, the texture evolution characterized by the orientation distribution function (ODF) suggests that the dominant textures are (001) < 110 > , (112) < 110 > , and (111) < 112 > textures owing to the occurrence of plane strain deformation and DRX. Furthermore, the expected balance between strength and plasticity was realized at a thickness reduction of 90% because of grain refinement and a reduction in dislocation density due to DRX. In addition, the increase in strength could also be attributed to the existence of (001) < 110 > and (112) < 110 > textures that were parallel to the tensile direction and perpendicular to the closely packed plane (110) of the β phase.

Microstructural origin and control mechanism of the mixed grain structure in Ni-based superalloys

Mixed grain structure, as an incomplete recrystallization, should be homogenized and refined by subsequent processing to improve the mechanical properties of the aeroengine components. The microstructural origin and control mechanism is quite necessary for the microstructure homogenizing and grain refining through thermo-mechanical processing. In present research, microstructural origin and control mechanism of the mixed grain structure have been conducted deeply. With the consideration of temperature drop in practical industrial forging, the optimum processing parameters can be determined as 1110–1140 °C/1.0 s−1 to obtain homogeneous equiaxed grains. The microstructure evolution corresponding to the identified optimal parameters, viz., deformation temperature and strain rate, were systematically investigated. Higher dynamic recrystallization (DRX) fraction and coarser DRX grains can be observed at super-solvus temperature deformation (1140 °C). In addition, sub-solvus deformation (1110 °C) shows random orientated distribution while near< 001 > compression texture can be detected in super-solvus deformation. Particular attention was paid to the DRX mechanisms during hot deformation. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occur simultaneously, and the synergistic effect of them contributes to microstructure control. Moreover, DDRX characterized by grain boundary bulging dominants the microstructure control. Progressive subgrain rotation labeled as CDRX can be regarded as an important assistant mechanism. The findings would provide theoretical support for microstructure control of Ni-based superalloys with mixed grain structure, which is of vital importance for components to obtain excellent performance.

Microstructure evolution of 316L stainless steel during solid-state additive friction stir deposition

ABSTRACT Solid-state additive friction stir deposition was employed to three dimensionally print a 316L stainless steel. The microstructure, texture and grain boundary distribution were characterised using five-parameter boundary analysis. The bulk microstructure was exceptionally fine (∼0.7 µm) throughout the thickness of the deposited layers, consisting of equiaxed grains along with sub-grains delineated by high and low angle boundary segments. The misorientation angle distribution of the deposited layers was considerably different from the as-received microstructure, exhibiting a significant reduction in the relative areas of ∑3 annealing twin boundaries due to their distortion by severe plastic deformation. The overall texture also exhibited strong shear components associated with FCC metals, suggesting that the microstructure refinement largely took place through progressive subgrain rotation, known as continuous dynamic recrystallisation. The grain boundary plane distribution revealed significantly lower anisotropy compared with the as-received material, showing two moderate peaks at the (111) and (110) orientations. The migration of high-angle mobile boundaries, which may have been induced by thermal cycle upon subsequent deposition along with the dislocation gradients between adjacent grains, slightly enhanced the intensity of low energy (111) orientations at the expense of (110) high-energy boundaries.

Effect of pressure on the deformation of quartz aggregates in the presence of H2O

Quartzite samples of high purity with a grain size of ~200 μm have been experimentally deformed by coaxial shortening in a solid medium apparatus at 900 °C and at confining pressures ranging from 0.6 to 2 GPa. Most samples have been shortened by ~30% with 0.1 wt% added H2O. The samples deformed dominantly by crystal plasticity (dislocation creep), and there is a systematic decrease of flow stress with increasing confining pressure. Strain rate stepping tests yield stress exponents of n ≈ 1.4. The strain determined from individual grain shapes matches that determined from bulk shortening. In addition to plastic strain, mode I cracks developed in all samples, principally in the grain boundary regions. Recrystallized material, visible through cathodoluminescence colours, forms by two mechanisms: (1) progressive subgrain rotation and (2) cracking, nucleating small new grains. After high-angle boundaries have been established, grain boundary migration takes place, and a distinction of new grains nucleation origin (subgrain rotation or cracking) is impossible. At higher pressure, there is more recrystallized material forming in the deformed samples, and it is inferred that the inverse pressure dependence of flow stress is caused by enhanced grain boundary migration at higher pressure, consistent with previous studies.

Effect of Zener-Hollomon parameter on microstructure evolution of a HEXed PM nickel-based superalloy

The microstructure evolution of a hot extrusion (HEXed) powder metallurgy (PM) nickel-based superalloy was investigated by hot compression tests under different deformation conditions. Optical microscope (OM), electron backscatter diffraction (EBSD) technique and transmission electron microscope (TEM) were applied to investigate the effect of Zener-Hollomon (Z) parameter on the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). The results show the stress-strain curves of the experimental superalloy exhibit obvious characteristics of DRX. Microstructure observations reveal that with the decrease of the Z parameter, both the degree of DRX and the size of DRX grains for the experimental superalloy gradually increase. The nucleation mechanism of DRX of the experimental superalloy is related to the Z parameter. Under the condition of lower Z value, discontinuous dynamic recrystallization (DDRX) characterized by bulging of grain boundaries is the main mechanism of DRX, while continuous dynamic recrystallization (CDRX) is considered to be an auxiliary mechanism for DRX nucleation in hot deformation. The existence of twins and γ' phase also has a major effect on the DRX nucleation mechanism of the experimental superalloy. Under the condition of higher Z value, CDRX and DDRX occur simultaneously, while CDRX characterized by progressive subgrain rotation can also be regarded as an important nucleation mechanism of DRX.

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 continuous dynamic recrystallization constitutive model combined with grain fragmentation and subgrain rotation for aluminum alloy 2219 under hot deformation

The large temperature gradient resulted from rapid heat dissipation and a long process period in the hot deformation of aluminum alloy causes complex microstructure evolutions. Accurate constitutive models describing the evolution of the microstructures over a wide temperature range are acquired. Thus, a physically-based continuous dynamic recrystallization constitutive model combined with grain fragmentation and subgrain rotation was established in this investigation. Uniaxial hot compression tests for aluminum alloy 2219 were conducted at temperatures of 250 °C–450 °C and the evolution of microstructures were analyzed via metallographic technique. Then internal state variables were proposed to describe the migration of grain boundary, dislocation density, average grain boundary misorientation, and subgrain/grain size. The net torque on the grain is introduced to precisely describe the progressive subgrain rotation mechanism. Moreover, geometrically necessary dislocations are considered to reasonably simulate the rise in flow stress associated with grain fragmentation. The results show that continuous dynamic recrystallization (CDRX) and dynamic recovery are the main softening mechanisms. The CDRX mechanism is gradually changed from grain fragmentation to subgrain rotation due to the increase in temperature. The new constitutive model may precisely predict the flow stresses, subgrain/grain size, and average misorientation in the wide temperature range. Comparing with the existing continuous dynamic recrystallization model which only considers subgrain rotation, the overestimate of the grain size in the low-temperature range is avoided in the new constitutive model. Furthermore, the contributions of the grain fragmentation to the increase in strain hardening can reasonably be covered.

Dynamic Recrystallization Modeling and Mechanisms in Inconel 690 Alloy during Hot Compressive Deformation

The dynamic recrystallization (DRX) behavior of INCONEL 690 (IN690) alloy was studied using electron backscatter diffraction (EBSD) and transmission electron microscopy. Isothermal compression tests were performed on a Gleeble-3500 simulator under temperatures 950-1100 °C, strain rates 0.01-1 s−1, and the maximum true strain 0.8. An Avrami-type model was developed to investigate the DRX behavior of IN690; it was found to be extremely sensitive to the processing parameters of temperature, strain rate, and true strain. As temperature increased and strain rate decreased, the activation energy increased, thus accelerating movement of dislocation and migration of grain boundaries, causing DRX behavior acceleration. The DRX behavior is accompanied by dislocation rearrangement and annihilation. The DRX mechanism of IN690 is dominated by discontinuous dynamic recrystallization (DDRX) and supplemented by continuous dynamic recrystallization (CDRX). A key characteristic of DDRX is grain-boundary bulges, while CDRX features progressive sub-grain rotation, leading to migration of low-angle grain boundaries to high-angle grain boundaries. A series of constitutive models were embedded into finite-element method software to study the DRX behavior of IN690. The results show that the microstructure evolution regularity obtained by the simulation method is consistent with the experimental values, which provides a basis for computer simulations of the hot machining process.

Grain refinement and weak-textured structures based on the dynamic recrystallization of Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr alloy

We utilized electron backscatter diffraction to investigate the microstructure evolutions of a newly developed magnesium-rare earth alloy (Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr) during instantaneous hot indirect extrusion. An equiaxed fine-grained (average grain size of 3.4 ± 0.2 µm) microstructure with a weak texture was obtained. The grain refinement was mainly attributed to the discontinuous dynamic recrystallization (DDRX) and continuous DRX (CDRX) processes during the hot indirect extrusion process. The twin boundaries formed during the initial deformation stage effectively increased the number of high angle grain boundaries (HAGBs), which provided sites for new grain nuclei, and hence, resulted in an improved DDRX process. Along with DDRX, CDRX processes characterized by low angle grain boundary (LAGB) networks were also observed in the grain interior due to effective dynamic recovery (DRV) at a relatively high temperature of 773 K and high strain rates. Thereafter, LAGB networks were transformed into HAGB networks by the progressive rotation of subgrains during the CDRX process.

The role of mesoscopic deformation heterogeneities in plastic flow and recrystallization of a magnesium sheet alloy

Unconventional rolling schemes characterized by high reductions and a low number of passes were employed to deform a new magnesium sheet alloy to large strains in order to obtain a heterogeneous deformation microstructure with large local variations in defect density. Microstructure heterogeneity was present in the form of large elongated grains, twinning, and shear banding. The rolled material was further deformed in tension at room temperature along different sheet directions to study the effect of such microstructure on plastic flow and planar anisotropy. Other rolled samples were subjected to annealing treatments before tensile testing to investigate the mechanisms of recrystallization in conjunction with deformation heterogeneity. Recrystallization chains were seen to form by stages of progressive subgrain rotation and subgrain growth giving rise to new grain boundaries capable of long-range migration during annealing. Unlike the as-rolled condition where planar anisotropy of the yield strength and maximum elongation was evident due to several reasons discussed in the study, the annealed condition with its homogeneous equiaxed grain structure and weak textures demonstrated an almost isotropic plastic flow behavior.

Microstructure evolution and underlying mechanisms during the hot deformation of 718Plus superalloy

718Plus superalloy was investigated to obtain a detailed and fundamental understanding of the microstructure evolution and the underlying mechanisms. Isothermal compression tests were conducted, and different strains were utilized to trace the whole deformation process at the selected deformation conditions. The results showed that no yield drop phenomenon (YDP) occurred at all deformation conditions involved in this research except a very slight one at 1020 °C with the strain rate of 0.1 s−1 The heterogeneity of initial microstructure was responsible for the absence or weakening of YDP. Deformation conditions and the Taylor factor differences between adjacent grains were found both strongly affected the distribution of dislocation density and local misorientation, as well as the formation of low angle grain boundaries (LAGBs), which finally influenced the dynamic recrystallization (DRX) behaviors and the underlying mechanisms. Moreover, discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) behaviors as well as the underlying mechanisms were discussed in detail. Subgrain rotation would also be observed during the DDRX process, which may lead to a misjudgment of CDRX if only focused on misorientation gradient. Hence, based on the morphology examination, misorientation gradient and misorientation angle evolution, progressive subgrain rotation labeled as CDRX was confirmed to be an important assistant nucleation mechanism during the deformation of the studied 718Plus superalloy at both sub-solvus and super-solvus temperatures.

Dynamic recrystallization mechanism and improved mechanical properties of a near α high temperature titanium alloy processed by severe plastic deformation

The effect of deformation temperature on microstructure evolution and mechanical properties of a near α titanium alloy during isothermal multiple die forging (IMDF) is systematically investigated. Present study describes the grain refinement mechanism at different deformation temperature by electron back-scatter diffraction (EBSD) characterization. The results indicated that the grain refinement as a result of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). With the decrease of deformation temperature, the mechanism of DRX converts from DDRX to CDRX. The formation of new grains by CDRX mechanism is mainly attributed to the progressive rotation of sub-grains. Moreover, the fresh grains resulted from DDRX process form along the serrated grain boundaries by bulging. It indicates that the superior tensile strengths of the IMDFed titanium alloy at room temperature (RT) and 650 °C are attributed to grain boundary strengthening and dislocation strengthening. However, grain boundary strengthening no longer works above 650 °C, leading to rapid decreases in tensile strengths.