Dynamic Recrystallization Rate Research Articles

Effect of temperature on the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloy during the large height-to-diameter ratio upsetting and extrusion

In this paper, the Mg-9Gd-4Y-2Zn-Zr (wt. %) alloy was used as the research object. The effect of deformation temperature on the microstructure and mechanical properties of the alloy after one pass of large height-to-diameter ratio (5.5) upsetting and extrusion was studied. The microstructure evolution of one pass large height-to-diameter ratio upsetting and extrusion process was studied, which provided a theoretical basis for further study of large height-to-diameter ratio reciprocating upsetting and extrusion. The results show that with the increase of deformation temperature, the grain size decreases, the dynamic recrystallization rate increases, the granular β phase increases, the texture of the alloy tends to the typical extrusion texture, and the texture intensity increases gradually. With the increase in temperature, the mechanical properties of the alloy after upsetting and extrusion with a large height-to-diameter ratio increase. The ultimate tensile strength (UTS) of the alloy deformed at 460 °C increased to 355 ± 6.7 MPa, the yield strength (YS) increased to 139 ± 3.1 MPa, and the elongation (EL) was 17 ± 0.7%. In the process of upsetting extrusion with the large height-to-diameter ratio, the initial discontinuous dynamic recrystallization (DDRX) changes to continuous dynamic recrystallization (CDRX).

Martensite decomposition under thermal-mechanical coupling conditions to fabricate an ultrafine-grained Ti6Al4Mo4Zr1W0.2Si alloy

The fabrication of ultrafine-grained microstructures (grain size below 1 μm) in titanium alloys is beneficial for improving their mechanical properties at room temperature and medium temperatures (400–550 °C). However, a long-standing challenge involves the low-cost manufacturing of bulk ultrafine-grained titanium alloys. In this work, we developed a facile strategy through martensite decomposition at thermal-mechanical coupling conditions, to fabricate an equiaxed microstructure in a Ti6Al4Mo4Zr1W0.2Si model alloy with an average α grain size of 315 ± 62 nm. The formation of the ultrafine-grained microstructure was because the lattice strain stored in the martensitic initial microstructure enhanced the nucleation rate of dynamic recrystallization, meanwhile, the pinning role of martensite decomposition products β and (Ti, Zr)5Si3 phases suppressed grain coarsening at high temperatures. Compared to conventional (α+β) alloys, the tensile strength of this alloy improved by 20%–30% at both room temperature and 550 °C, without decreasing its ductility. In situ SEM observations revealed that the ultrafine-grained microstructure would not only suppress dislocation motions but also contribute to the homogenous deformation in the matrix of the material, therefore, it resulted in higher mechanical performance. The research results may be of great significance to the development of next-generation aviation titanium alloys.

Simultaneous improvement of strength and damping capacity of Mg-Mn alloy by tailoring bimodal grain structure

A high damping capacity and a high strength is paradox in magnesium alloys needed to be addressed immediately. In this work, adding Mn into Mg matrix, and combined with low extruded temperature, alloys with bimodal grain structure that is made up of fine recrystallized grains and coarse unrecrystallized grains were obtained. The fine recrystallized grains are conducive to the activation of grain boundary slip, leading to alloy exhibits good plasticity. A large number of parallel dislocations exist in unrecrystallized grains, due to the dislocation recovery and dynamic recrystallization rate are evidently suppressed at low extruded temperature and the Mn precipitates impede the movement of dislocations, which cause a high work hardening rate in bimodal grain structure. This phenomenon not only brings about a high yield strength, but also improves damping capacity of Mg-Mn alloys at room temperature. Because the unrecrystallized grains initial to recrystallize at high temperature, the P2 internal friction peak rises and moves toward the low temperature. When the extruded temperature is 220 °C, the comprehensive properties of the Mg-1Mn alloy outperform the high-damping Mg-0.6Zr alloy, and the yield strength, plasticity and damping capacity at ε = 1 × 10−3 are 169 MPa, 32.5% and 0.042, respectively. Tailoring bimodal grain structure is an effective strategy to ensure strength, plasticity and damping capacity of the Mg alloy at the same time.

Simple shear extrusion versus equal channel angular pressing: A comparative study on the microstructure and mechanical properties of an Mg alloy

Two severe plastic deformation (SPD) techniques of simple shear extrusion (SSE) and equal channel angular pressing (ECAP) were employed to process an extruded Mg−6Gd−3Y−1.5Ag (wt%) alloy at 553 K for 1, 2, 4 and 6 passes. The microstructural evolutions were studied by electron back scattered diffraction (EBSD) analysis and transmission electron microscopy (TEM). The initial grain size of 7.5 μm in the extruded alloy was reduced to about 1.3 μm after 6 SPD passes. Discontinuous dynamic recrystallization was suggested to be operative in both SSE and ECAP, with also a potential contribution of continuous dynamic recrystallization at the early stages of deformation. The difference in the shear strain paths of the two SPD techniques caused different progression rate of dynamic recrystallization (DRX), so that the alloys processed by ECAP exhibited higher fractions of recrystallization and high angle grain boundaries (HAGBs). It was revealed that crystallographic texture was also significantly influenced by the difference in the strain paths of the two SPD methods, where dissimilar basal plane texture components were obtained. The compression tests, performed along extrusion direction (ED), indicated that the compressive yield stress (CYS) and ultimate compressive strength (UCS) of the alloys after both SEE and ECAP augmented continuously by increasing the number of passes. ECAP-processed alloys had lower values of CYS and UCS compared to their counterparts processed by SSE. This difference in the mechanical responses was attributed to the different configurations of basal planes with respect to the loading direction (ED) of each SPD technique.

Effect of Mg content on the dislocation characteristics and discontinuous dynamic recrystallization during the hot deformation of Al-Mg alloy

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

Role of Al addition in modifying microstructure and mechanical properties of Mg-1.0 wt% Ca based alloys

In this work, the role of Al addition in modifying the microstructure and mechanical properties of the Mg-1.0Ca-xAl (x = 0.6, 1.0 wt%; termed as XA10 and XA11, respectively) based alloys has been systematically investigated. Microstructural examination showed that, when the Al content is low, the Al and Ca atoms were inclined to segregate to the dislocation lines in the XA10 sample. As a result, the mobility of dislocation was restrained and the dynamic recovery behavior was hindered, which leads to a low degree of dynamic recrystallization. When the Al content is high, in contrast, no apparent Ca/Al segregations along dislocations have been found in XA11-0 extrusion billet during the whole stages of processing, which results in the enhanced dislocation mobility. Consequently, the recovery of dislocations and dynamic recrystallization rate can be largely enhanced at the earlier stage of extrusion. More importantly, subsequently formed high density Guinier-Preston (G.P.) zones and nano-disk Al2Ca phases in the high-Al content Mg sample can play an effective role in inhibiting the growth of subgrains and dynamically recrystallized (DRXed) grains, which enhance the thermal stability of high density low angular grain boundaries (LAGBs) in the later extrusion stage. The high density nano-disk phases, high volume fraction of DRXed grains, as well as the high density LAGBs in the un-DRXed grains, can together contribute to both high strength and good elongation of the XA11 sample. The findings lead to the controllable Mg-Ca based alloy designing strategy that can improve the strength and ductility simultaneously.

Microstructure and Corrosion Behavior of the As-Extruded Mg–4Zn–2Gd–0.5Ca Alloy

In order to study the corrosion resistance of extruded magnesium alloys, the Mg–4Zn–2Gd–0.5Ca alloy was extruded at the speed of 0.01–0.1 mm/s with the temperature of 280–360 °C in present study. Hot extrusion results show that the volume fraction of precipitates (Vpre), VDRX (the dynamic recrystallization rate) and the average size of DRXed grain (dDRX) decrease with the decrease in extrusion speed, and the corrosion rate of the alloy also shows a downward trend. On the contrary, the values of Vpre, VDRX and dDRX increase with the increase in extrusion temperature, and the corrosion resistance of Mg–4Zn–2Gd–0.5Ca alloy decreases. When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280 °C, the alloy has the best corrosion resistance. The corrosion of extruded Mg–4Zn–2Gd–0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone. With the further corrosion, the corrosion continues to spread along the phase, and the corrosion area gradually increases. Galvanic corrosion plays a leading role in the corrosion process. Moreover, there are a large number of basal plane textures in the unDRXed region, which is conducive to improving the corrosion resistance of magnesium alloys. In addition, the decrease in grain size also makes the corrosion of magnesium alloy more uniform.

Dynamic Recrystallization Behavior of High Carbon Chromium Bearing Steel

The effect of process parameters on dynamic recrystallization of GCr15 bearing steel was investigated by a single-pass compression tests using Gleeble-3500 thermal simulator. The results indicate that the flow stress decreases with the increasing of temperature and the decreasing of strain rate. The dynamic recrystallization rate increases with the increasing of deformation temperature. By regression analysis, the mean activation energy and the stress exponent are determined to be 435.1KJ/mol-1and 5.0505 respectively. Meanwhile, the kinetic mathematical model of dynamic recrystallization and constitutive equation are established.The relationship of peak stress, peak strain and Z parameters is founded by using the data of flow stress curves.

Dynamic recrystallization behavior of a typical nickel-based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980°C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener–Hollumon parameter.

Effect of carbon content on hot deformation behaviors of vanadium microalloyed steels

The compressive deformation behaviors of a low carbon vanadium microalloyed steel and a medium carbon vanadium microalloyed steel were investigated at the temperatures from 900°C to 1100°C and strain rates from 0.005s−1 to 10s−1 on Gleeble-1500 thermo-mechanical simulator. It was found that increasing the carbon content of vanadium microalloyed steels decreased the flow stress at lower strain rates, whilst at higher strain rates carbon addition led to higher flow stress, especially at the initial stage of deformation. The flow stress constitutive equations of hot deformation were developed for the experimental steels; results showed that carbon addition has the trend to reduce the hot deformation activation energy. The dependence of the characteristic points under different deformation conditions on the Zener–Hollomon parameter and the relationship between critical strain (critical stress) and peak strain (peak stress) of the experimental steels were analyzed, and the results were in reasonable agreement with those reported before. The work hardening behavior of both steels was investigated and it was found that carbon addition can decrease the work hardening rate when strained at lower strain rates. Dynamic recrystallization analysis showed that carbon addition can accelerate the occurrence and the rate of dynamic recrystallization.

Variation of Ti‐in‐quartz in gneiss domes exposing the world's youngest ultrahigh‐pressure rocks, D'Entrecasteaux Islands, Papua New Guinea

The D'Entrecasteaux Island gneiss domes, Papua New Guinea, expose the world's youngest ultrahigh‐pressure rocks (5–8 Ma), which were exhumed nearly isothermally to the mid‐crust at rates of cm yr−1. Because quartz grains are internally distorted by deformation bands and have strongly interlobate grain boundary microstructures indicative of rapid grain‐boundary migration, we infer that Ti content in the quartz was frozen in by dynamic recrystallization that accompanied the continued deformation of the gneisses as they were exhumed through the crust toward the surface. We infer that average Ti content of an assemblage of quartz grains in a sample depends on the ratio of cooling rate to dynamic recrystallization rate. An increase in the former preserves and captures a larger fraction of relatively high‐Ti, high‐temperature grains, whereas an increase in the latter removes such relict grains and replaces them with a newer set of lower‐Ti, low‐temperature recrystallized grains. Based on this concept, we use Ti‐in‐quartz as a tool to infer domains of differing relative cooling rate across four D'Entrecasteaux Island domes. Ti concentrations in quartz grains from 89 samples of quartzofeldspathic gneiss and eclogite increase inwardly away from the dome margins (2.5–20 ppm) toward the center of the domes (20–>100 ppm). From this, we infer that the most rapidly cooled rocks were exhumed near the center of the domes. Our data reinforces structurally based arguments that the symmetric domes were emplaced vertically into the crust as diapirs, rather than laterally exhumed as a result of large‐magnitude slip on dome‐bounding detachment faults.

Effect of zirconium addition on microstructure and mechanical properties of Mg97Y2Zn1 alloy

The effects of zirconium addition on the microstructure and mechanical properties of Mg97Y2Zn1 alloy were investigated. The microstructure of as-cast Mg97Y2Zn1 alloy is refined by the addition of zirconium. During the extrusion, the initial nucleation sites of the alloy are mainly original grain boundaries and secondary phase. The addition of zirconium could stimulate the DRX process because more grain boundaries are formed, which increases the dynamic recrystallization rate. Both the strength and elongation of the alloy are increased by the addition of zirconium.

The rate of dynamic recrystallization in 17-4 PH stainless steel

The hot working behavior of 17-4 PH stainless steel (AISI 630) was studied by hot compression test at temperatures of 950–1150 °C with strain rates of 0.001–10 s −1. The progress of dynamic recrystallization (DRX) was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the reciprocal of the time corresponding to the DRX fraction of 0.5% or 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. The Zener-Hollomon parameter ( Z) was found to be inappropriate for analysis of DRX kinetics. Therefore, the dynamic recrystallization rate parameter (DRXRP) was introduced for this purpose. The DRXRP may be determined readily from the Avrami analysis and can precisely predict the rate of DRX at hot working conditions.

Effect of homogenization on the hot-deformation ability and dynamic recrystallization of Mg–9Gd–3Y–0.5Zr alloy

Two states of Mg–9Gd–3Y–0.5Zr (wt.%) (GW93) alloy, as-cast (note as F state) and homogenized (note as T4 state), were hot compressed at strain rate of 0.1 s −1 and temperatures range from 623 K to 723 K. The stress-strain curves were established and volume fractions of dynamic recrystallized (DRX) grains were determined using point counting method. The results show that flow stresses are sensitive to the compression temperature and pretreatment state. Comparing with as-cast alloy, T4 state alloy has a higher flow stress and strain hardening rate θ, more complicated deformation microstructures, a smaller dynamic recrystallization rate and Avrami exponents. The phenomena are the results of the difference of initial nucleation sites and eutectic particles stimulated nucleation (EPSN) of two states. The initial nucleation sites of as-cast alloy include the original grain boundaries and eutectic particles, but as for T4 alloy, only original coarse grain boundaries can be act as nucleation sites.

Hot Deformation Behavior and Microstructure Evolution of Ti-6.5Al-1.5Zr-3.5Mo-0.3Si with an Equiaxed α+β Starting Structure

The hot deformation behavior of TC11 with an equiaxed α+β pre-form microstructure was investigated by hot compression tests in the temperature range 800°C–1090°C and strain rate range 1×10-310s-1. Several approaches have been used in this investigation, which include analysis of shapes of stress–strain curves, kinetic analysis and microstructure observation. The experimented results showed that, (1) In the β phase field, the alloy exhibits dynamic re-crystallization. The apparent activation energy in this domain is estimated to be 183kJ/ mol, which is close to that for self-diffusion in β. The re-crystallized grain size may be governed by competing of diffusion and dynamic re-crystallization rate at deforming conditions. (2) In the α+β phase field, the alloy exhibits super-plastic deformation behavior at 0.001-0.01s-1 and 980°C -850°C, and the apparent activation energy estimated in this domain is about 600 kJ/mol, which is much higher than that for self-diffusion in α-titanium due to β volume fraction is not constant over the test temperature range.

An investigation on the effect of carbon and silicon on flow behavior of steel

The effect of carbon and silicon on the occurrence of dynamic recrystallization as well as on the flow stress of steel was investigated. For this purpose, three grades of steels including low carbon, medium carbon and high silicon low carbon steel were examined. Single hit experiments at temperatures of 900–1200 °C and strain rate of 0.01–2 s −1 were performed to determine the phase transformation kinetics. Different socking times were utilized to detect the effect of initial austenite grain size on the kinetics of dynamic recrystallization and flow behavior of the steels. It was found that increasing the carbon content leads to a higher rate of dynamic recrystallization at high temperatures and low strain rates, while silicon causes postponement of the occurrence of dynamic recrystallization. Moreover, increasing the initial austenite grain size postpones the onset of dynamic recrystallization and leads to a higher flow stress at equivalent deformation conditions.

Measuring relative grain boundary energies and mobilities in an aluminum foil from triple junction geometry

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

High temperature deformation of Ti–(46–48)Al–2W intermetallic compounds

The high temperature deformation behavior of Ti–46Al–2W and Ti–48Al–2W intermetallic compounds have been investigated in isothermal compressive tests, performed at temperatures between 1000°C and 1200°C for strain rates between 10 −3 and 10 −1 s −1. The stress–strain curve during high temperature deformation exhibits a peak stress which is followed by a gradual decrease into a steady state stress with increasing the strain. The flow softening behavior after the peak stress is attributed to the effects of dynamic recrystallization during deformation. The dependence of flow stress on temperature and strain rate followed a hyperbolic sine relationship using the Zener-Hollomon parameter. The activation energies, Q, were measured as 449 kJ mol −1 and 394 kJ mol −1, and the stress exponents were measured as 3.6 and 3.7 for Ti–46Al–2W and Ti–48Al–2W, respectively. The activation energy increased with decreasing Al content in TiAl-base intermetallic compounds. The coefficient between peak stress and Zener-Hollomon parameter, A, was not a constant, but was dependent on the activation energy. The peak stresses can be predicted well by using a normalized Zener-Hollomon parameter. The dynamic recrystallization rate and recrystallized grain size increased with increasing the temperature and with decreasing the strain rate.

Segregation-induced intergranular brittleness of ultrahigh-purity Fe–S alloys

AbstractThe hot ductility of ultrahigh–purity Fe–S alloys doped with 20 to 100 ppm sulphur has been studied by tensile tests at a strain rate of e = 2 × 10−3 S−1. A well defined ductility trough between 850 and 1100°C was found in alloys containing ~50 ppm sulphur. This minimum sulphur content necessary to produce an intergranular embrittling effect is much higher than in steels containing aluminium and nitrogen doped with 10 ppm sulphur. The role of sulphur in reducing hot ductility is explained in terms of the intergranular brittleness caused by the presence of sulphur on grain boundaries, which decrease their cohesion. The intergranular hot brittleness of Fe–S alloys can be attenuated by dynamic recrystallization at low sulphur contents. In this case the dynamic recrystallization rate must be higher than the void nucleation and propagation rate. The hot ductility minimum temperature shifts with increasing sulphur content and seems to change in the same manner as the sulphur solubility limit temperature...