Initiation Of Dynamic Recrystallization Research Articles

Dislocation characteristics and dynamic recrystallization in hot deformed AM30 and AZ31 alloys

Zn addition to Mg alloys activates non-basal slip or twinning with solute softening effects. On the other hand, the effects of the Zn solute on the macroscopic dislocation behavior and dynamic recrystallization are not completely understood. Moreover, it is unclear if <c+a> slip can be affected by changes in the c/a ratio of solute atoms. This study was conducted to understand the solute strengthening of Zn addition and its effects on the dislocation characteristics and dynamic recrystallization. A hot torsion test was performed on both AM30 and AZ31 alloys up to a high strain to investigate the Zn solute effect on the dislocation characteristics and dynamic recrystallization. The dislocation components of the hot torsioned alloys were evaluated by X-ray line profile analysis and electron backscatter diffraction. The results showed that the Zn solutes slightly accelerate strain accumulation at the initial stages of hot deformation, which accelerated recrystallization at high strain. The dislocation characteristics were changed dynamically by Zn addition: fortified <c+a>-type slip, dislocation arrangement and strain anisotropy parameters. The most important point was that the dislocation characteristics were changed dramatically at the critical strain for recrystallization and high strain regions. The addition of Zn also acted greatly in these strain areas. This indicates that the rapid formation of <c+a>-type slip at the serrated grain boundaries occurs at the initiation of dynamic recrystallization and the increase in the grain triple junction because grain refinement has a great influence on the dislocation characteristics at high strain.

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.

Initiation of Dynamic Recrystallization of As-Cast N08028 Alloy

The use of high nickel content austenitic stainless steels (SASS) has significantly increased in the last decade. The corrosion and high fatigue resistance of these materials make them suitable for manufacturing oil country tubular goods (OCTG). SASS are processing by forging from casting conditions. Dynamic recovery (DRV) and recrystallization (DRX) of as-cast super austenitic stainless steel, N08028 Alloy, is investigated to study the refining effect from the as-cast grain structure to fully recrystallized austenite due to hot deformation. Both the critical stress and strain for the initiation of DRX are determined using the flow curves. To perform this analysis, hot compression tests are performed at temperatures between 900°C and 1250°C, and strain rates between 0.1 s-1 and 10 s-1, up to 0,8 final strain using a Gleeble®3800 thermomechanical simulator. Subsequently, the Johnson-Avrami-Mehl-Kolmogorow (JMAK) model is used to numerically fit the flow curves and consequently determine the critical strain. No critical points are seen for temperatures under 1100°C. Above this temperature, the JMAK model proves to be valid in all studied strain rates.

Effect of dynamic strain aging and precipitation on the hot deformation behavior of 253MA heat-resistant alloy

In this research, we performed an experimental investigation into the influence of dynamic strain aging (DSA) and precipitation behavior on the plastic deformation and microstructure evolution in alloy 253MA considering its service temperature. The serrated flow behavior, stress anomaly behavior and negative strain rate sensitivity are considered as the manifestation of DSA within a certain temperature range (TDSA), which can be classified as low (550–700 °C), intermediate (600–750 °C) and high temperatures (700–790 °C) at 0.001 s−1, 0.01 s−1 and 0.1 s−1, respectively. The activation energy of DSA was determined to be 242 kJ/mol, indicating that the diffusion of substitutional solutes, such as Cr and Ni, is mainly responsible for the DSA phenomenon. The characteristic of dislocation substructures is prone to change from linear structure to cellularization structure with a rising deformation temperature in the DSA regime. The stress anomaly behavior can be attributed to the rapid dissolution of σ phase. The initiation of dynamic recrystallization should contribute to the disappearance of DSA by suppressing the dynamical solute atom–dislocation interaction.

Microstructure Evolution of Copper by Three Roll Planetary Milling

The microstructure of commercial pure copper TP2 tube by three roll planetary mill processing was investigated. Due to work hardening and subsequent softing by dynamic recrystallization during milling process,the coarse grain structure of the copper transformed to fine grain structure. The grain refinement is achieved along the axial moving in general; in the reducing zone, the sample has a gradient structure along the radial direction; uniform equiaxed grain with size of ~2 μm could be obtained at the outlet. The initiation of dynamic recrystallization occurred in the three roll planetary mill the reduction engineering strain reach ~30%, where some equiaxed grains replaced the elongated grains because of recrystallization.

Deformation Behavior of a Coarse-Grained Mg-8Al-1.5Ca-0.2Sr Magnesium Alloy at Elevated Temperatures

The compression tests were carried out on a coarse-grained Mg-8Al-1.5Ca-0.2Sr magnesium alloy samples at temperatures from 300 to 450 °C and strain rates from 0.001 to 10 s−1. The flow stress curves were analyzed using the double-differentiation method, and double minima were detected on the flow curves. The first set of minima is shown to identify the critical strain for twinning, while the second set indicates the critical strain for the initiation of dynamic recrystallization (DRX). Twin variant selection was numerically identified by comprehensive analysis of the Schmid factors for different deformation modes and the accommodation strains imposed on neighboring grains. It was found that twinning is initiated before DRX. Dynamic recrystallization volume increases with strain rate at a given deformation temperature. At high strain rate, various twin variants are activated to accommodate deformation, leading to the formation of twin intersections and high DRX volume. Fully dynamic recrystallized structure can be obtained at both high and low strain rates due to the high mobility of the grain and twin boundaries at the temperature of 400 °C.

Multi-axial forging of Fe3Al-base intermetallic alloy and its mechanical properties

Tri-axial plane-strain forging was applied to the Fe–28Al–5Cr–0.8Zr–0.04B intermetallic alloy, in order to study its grain refinement and possible improvement in mechanical properties. The forging temperature range was from 20 to 600 °C. The maximum number of forging passes was 67. The deformed microstructure was investigated using electron backscatter diffraction in a scanning electron microscope. At forging temperatures <500 °C, the alloy was very prone to brittle cracking. At the temperature range of 500–600 °C, the overall plasticity of material increased and the cracking tendency was reduced but not completely eliminated. Extensive processes of dynamic recovery/recrystallization were observed during forging at 600 °C after 10–40 passes. Recrystallized areas with an average grain size of a few micrometers were observed. However, even after severe deformation at 600 °C, resulting from 40 forging passes (ε ~ 11.3), dynamic recrystallization was incomplete and the fraction of low-angle boundaries was still high. The Vickers microhardness substantially increased from 280 HV0.1 to 450 HV0.1 at the center of the sample after both deformations at room temperature as well as after 20–40 cycles at 500–600 °C. However, a further increase of strain for a sample deformed at 600 °C after 67 passes (ε ~ 28) led to quite a significant hardness decrease at the center of the sample. This phenomenon could be associated with the initiation of dynamic recrystallization. None of the samples forged at the 500–600 °C range exhibited any ductility improvement during subsequent tensile testing at room temperature, most likely, owing to the absence of a uniform ultrafine-grained structure developed during forging.

Investigation of Microstructure in Friction Stir Welded Al-Cu-Li Alloy

It is well known that the addition of Li to aluminum alloys offers an attractive combination of low density and high modulus, which are useful for lightweight structures of aerospace vehicles. However, microstructure of Al-Li alloys are complex, which consist of a number of equilibrium and metastable phases. In addition, Al-Li alloys are weldable but the weldability is not as good as that of other aerospace alloys. This is due to the reactive property of element Li during melting and causes porosity, cracking and low joint efficiency. In friction stir welding (FSW), rotating welding tool generates frictional heat and by keeping the tool rotating and moving speed, the heat from friction causes the plate to soften without melting. Therefore, this solid state welding is adequate to Al-Li alloys. The friction stir welded joint was divided into 9 regions and each microstructure was investigated in detail to present the microstructure evolution and material flows during friction stir welding process. The recrystallized structure is observed in nugget zone and the evidence of initiation of dynamic recrystallization is found around the boundary between thermo-mechanically affected zone (TMAZ) and nugget region. This paper describes the results of a study to investigate the microstructure change of Al-Cu-Li alloy during the friction stir welding process.

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s−1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.

Dynamic recrystallization kinetics in Mg-3Gd-1Zn magnesium alloy during hot deformation

AbstractThe initiation of dynamic recrystallization during hot deformation, its kinetics according to the Johnson–Mehl–Avrami–Kolmogorov model, and the resultant grain refinement in the Mg-3Gd-1Zn magnesium alloy were studied to elucidate the mechanism of dynamic recrystallization and the opportunities it offers during high-temperature thermomechanical processing of this heat resistant light alloy. The necklace mechanism was found to be responsible for the noticeable grain refinement, during which the mean recrystallized grain size varied slightly. These results were further confirmed based on the kinetics analysis.

Microstructural evolution of Ti-added interstitial free steel in high strain deformation by hot torsion

The dynamically evolved microstructure under high strain deformation condition does still have many debatable aspects, particularly in the case of easy-recovery metals like bcc-iron. In this research, microstructural evolution in high strain deformation by hot torsion of Ti-added interstitial free (IF) steel was systematically investigated. Torsion specimens were deformed up to an equivalent strain of ∼ 7 at different temperatures (650 °C - 850 °C) and strain rates (0.01 s-1 - 1.0 s-1), i.e., under various values of the Zener-Hollomon (Z) parameter. Immediately after the deformation, samples were water-quenched and microstructures were investigated by electron backscattering diffraction (EBSD) measurements and electron channelling contrast imaging (ECCI). Flow stress-strain curves of the IF steel under various deformation conditions showed typical flow curves of high stacking fault energy metals at low Z values, i.e., a peak stress followed by slight softening. On the other hand, under the high-Z deformation conditions, the specimens showed a larger stress drop after a certain amount of deformation. EBSD-based quantitative analysis was used to study the microstructural transition between high and low Z values. At low Z values, the occurrence of strain induced boundary migration (SIBM) as an initiation of dynamic recrystallization (DRX) was clearly observed. On the other hand, at high Z values, grain subdivision phenomena led to very fine and elongated structures.

Effect of Nitrogen Content on Hot Deformation Behavior and Grain Growth in Nuclear Grade 316LN Stainless Steel

316LN stainless steel with 0.08%N (08N) and 0.17%N (17N) was compressed at 1073–1473 K and 0.001–10 s−1. The hot deformation behavior was investigated using stress-strain curve analysis, processing maps, and so forth. The microstructure was analyzed through electron backscatter diffraction analysis. Under most conditions, the deformation resistance of 17N was higher than that of 08N. This difference became more pronounced at lower temperatures. The strain rate sensitivity increased with increasing temperature for types of steel. In addition, the higher the N content, the higher the strain rate sensitivity. Hot deformation activation energy increased from 487 kJ/mol to 549 kJ/mol as N concentration was increased from 0.08% to 0.17%. The critical strain for initiation of dynamic recrystallization was lowered with increasing N content. In the processing maps, both power dissipation ratio and unstable region increased with increasing N concentration. In terms of microstructure evolution, N promoted dynamic recrystallization kinetic and decreased dynamic recrystallization grain size. The grain growth rate was lower in 17N than in 08N during heat treatment. Finally, it was found that N favored twin boundary formation.

A new mathematical model for predicting flow stress up to the critical strain during hot deformation

By considering the linear relationship between work-hardening rate and stress, a new mathematical model was proposed for predicting flow stress up to the critical strain during hot deformation. This model is expressed in terms of critical strain, critical stress, and saturation stress. The model shows the critical strain and thus considers the initiation of dynamic recrystallization (DRX). A new material parameter L, resulting in the model’s higher accuracy compared with that of the previous studies. Compared with the measured values, the predicted values show better accuracy, whereas the model of flow stress up to the peak shows lower levels of accuracy. Therefore, compared with the previous model, the mathematical model for predicting flow stress up to the critical strain presents better precision and better reflects the internal mechanism of the changes in the material microstructure.

The Influence of Initial Grain Size and Strain Sequence of Slab Hot Rolling on the Austenite Evolution of Peritectic Microalloyed Plate Steels

The influence of the strain sequence during slab hot rolling (also known as “roughing”) on the evolution of austenite in plain carbon, C-Mn-V and C-Mn-Nb-Ti-V steels was investigated. Reheating and roughing simulations were conducted in a Bähr deformation dilatometer using a constant austenitising temperature, constant soaking time and various heating rates and roughing strain sequences. Stress analysis was used to quantify the austenite softening behaviour and the prior austenite grain size was measured from quenched specimens. The austenite grains of the plain carbon steel were coarser than those of both microalloyed steels, with the C-Mn-Nb-Ti-V grade being the finest due to effective pinning of the grain boundaries. Pass strains greater than 0.2 were sufficient for initiation of dynamic recrystallisation (DRX) for the C-Mn and C-Mn-V steels and led to uniform austenite microstructure with austenite grain sizes less than 40µm after the roughing stage.

Critical Strain for the Initiation of Dynamic Recrystallization in a Fe-Cr-Ni Super Austenitic Stainless Steel

In constant strain rate tests, the occurrence of dynamic recrystallization (DRX) is traditionally identified from the presence of stress peaks in flow curves. However, not all materials display well-defined peaks when tested under these conditions. It is shown that the onset of DRX can also be identified from the inflection point on the strain hardening rate (θ=dσ/dε)versus flow stress (σ) curve. In this paper, the hot compression curves can be described by an equation that fits the experimental θ-σ data from zero to the peak stress. An appropriate third order equation was fitted to the strain hardening data. The results show that the critical stress at initiation σc=-B/3A where A and B are coefficients of the third order equation. It is evident that this value depends on the deformation conditions. The stress–strain curve was then normalized with respect to the peak stress, leading to a normalized value of the critical stress (uc) equal to uc=σc/σp=-B'/3A'. Here A'and B'are coefficients of the normalized third order equation. This value is constant and independent of the deformation conditions.

Determination of Initiation of Dynamic Recrystallization in AISI 4340 Steel

In this work, two approaches for determining critical stress and strain for initiation of dynamic recrystallization (DRX) of the AISI 4340 steel were presented.The first one applied a polynomial function to represent relationship between work hardening rate and flow stress. Secondly, Cingara constitutive model were employed. To investigate hot deformation behavior of the steel, compression tests were performed at different temperatures between 850 °C and 1150 °C and strain rates between 0.01s-1and 10 s-1. Obviously, both methods provided different values of critical stress and critical strain.Accuracy of the first method depended on fluctuations of the fitted strain hardening curve. On the other hand, results of the Cingara model was primarily related to the described flow curves up to their peak points. It could be noticed that the DRX occured during hot deformation of the examined steel started when the normalized critical stress and strain reached the values of 0.735 and 0.324, respectively.

Dynamic Strain - Induced Boundary Migration during Dynamic Recovery at a High Temperature Deformation with a Lower Strain Rate

Conventional hot compression deformation and water quenching experiments were applied to investigate the evolution of austenite grain structures before the initiation of dynamic recrystallization. The experimental results reveal an interesting phenomenon that dynamic strain induced boundary migration can lower dislocation density and coarsen austenite grains. The results show that dynamic recovery is not the only way to decrease dislocation density, the mechanism of which for dynamic recovery is related to dislocations climb and annihilation, resulting in the formation of sub-grains and regular sub-boundaries. However, the mechanism of decreasing dislocation density for dynamic strain induced boundary migration is different from dynamic recovery. Therefore, dynamic strain induced boundary migration should be another softening mechanism before the initiation of dynamic recrystallization.

Determination of Critical Conditions for Initiation of Dynamic Recrystallization by New Method

ABSTRACTBased on the stress versus strain relation for materials at high temperatures, a new method to determine critical conditions for initial dynamic recrystallization (DRX) was proposed by analyzing the stress versus strain curve in a double natural logarithm coordinates (Ln–Ln plot) directly. The critical conditions of initial DRX of a low carbon steel were determined by the new method and the resultant values were compared with those determined by previous methods. The results show that the normalized strains are 0.6491, 0.7026, and 0.7055, respectively, by using the corresponding curves and all of the values fall in the range of 0.6–0.85 proposed by Sellars. The normalized stress and strain determined by using the strain hardening rate versus stress curve have a good agreement with those determined by using the stress versus strain curve in an Ln–Ln plot.

Effect of Substructure Characteristics on the Dislocation Annihilation Process during Post-Deformation Annealing

Two distinct substructures were produced in a Ni-30Fe austenitic model alloy by different thermomechanical processing routes. The first substructure largely displayed organized, banded subgrain arrangements with alternating misorientations, resulting from the deformation at a strain just before the initiation of dynamic recrystallization (DRX). By contrast, the second substructure was more random in character and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, formed through DRX. During the post-deformation annealing, the latter substructure revealed a rapid disintegration of dislocation boundaries leading to the formation of dislocation-free grains within a short holding time, though the former largely preserved its characteristics till becoming replaced by growing statically recrystallized grains.

Activation Energy in Hot Forming and Recrystallization Models for Magnesium Alloy AZ31

Kinetics of the dynamic, as well as postdynamic recrystallization of the wrought magnesium alloy AZ31, was ascertained. Continuous compression tests associated with the study of dynamic recrystallization were realized at temperatures from 523 to 723 K and at the strain rates from 0.001 to 10 s−1. The activation energy in hot forming was determined as Q = 158 kJ/mol for stress-strain curves of conventional shape, or Q = 146 kJ/mol for stress-strain curves with the concave initial phase affected by twinning. If the Zener-Hollomon parameter Z > 9.1 × 1012 s−1 the deformation necessary for the initiation of dynamic recrystallization is almost independent on the forming parameters. Using the results of the stress relaxation tests, equations describing the kinetics of metadynamic recrystallization and the grain size originated in such a way were developed and the effect of individual variables was evaluated.