Subgrain Growth Research Articles

Large-Scale Multi-Phase-Field Simulation of 2D Subgrain Growth

The characteristics of subgrains in a deformed state after the high-temperature deformation of aluminum alloys control the subsequent recrystallization process and corresponding mechanical properties. In this study, systematic 2D phase-field simulations have been conducted to determine the role of deformed state parameters such as subgrain size and disorientation distributions on subgrain growth in an individual grain representing a single crystallographic orientation. The initial subgrain size and disorientation distributions have been varied by ±50%. To have a statistically relevant number of subgrains, large-scale simulations have been conducted using an in-house-developed phase-field code that takes advantage of distributed computing. The results of these simulations indicate that the growth of subgrains reaches a self-similar regime regardless of the initial subgrain structure. A narrower initial subgrain size distribution leads to faster growth rates, but it is the initial disorientation distribution that has a larger impact on the growth of subgrains. The results are discussed in terms of the evolution of the average diameter of subgrains and the average disorientation in the microstructure.

Significant reduction in creep life of P91 steam pipe elbow caused by an aberrant microstructure after short-term service

P91 steel is an important steam pipe for ultra-supercritical power plants due to its excellent creep strength, which generally has a design life of 100,000 h. Here, we found a significant aberrant decrease in the creep rupture life of a main steam pipe elbow after only 20,000 h of service. The microstructure in the aberrant piece exhibited a decomposition of martensitic lath into blocky ferrite due to recrystallization and accumulation of M23C6 as well as formation of the Laves phase along the prior austenitic grain boundaries, resulting in the decrease of hardness that no long meet ASME standard requirement. The creep testing of the P91 piece at 550–600 °C and 85–140 MPa shows that the influence of temperature on the cavity formation and cracking is greater than that of the applied stress. The rupture life is nearly two orders of magnitude shorter than the normal P91, attributing to the creep damage of the subgrain growth, M23C6 and Laves phase coarsening (aggregation approaching 3.4 μm). The residual life of the aberrant piece was evaluated to be 53,353 h based on the Larson–Miller parameter, which is much shorter than the design life, suggesting the safety operation of the elbow area should be paid more attention during the afterward service periods. P91 steel, main steam pipe elbow, aberrant microstructure, service degradation, creep life prediction

On the Metastable Ferrite Hot Rolling of Low‐Grade Electrical Steels: Its Microstructure, Texture and Low‐Temperature Annealed Structure

The hot‐rolled sheets of low‐grade electrical steels undergo complete austenite‐ferrite phase transformation, resulting in fine equiaxed grains in the sheets. These fine initial grains eventually lead to a strong detrimental {111} texture in the final product after cold‐rolling and annealing process. This work, based on the phase transformation delay of columnar grain structure in cast slabs, proposes a novel approach called metastable ferrite hot rolling, namely, a two‐stage heating at 900 °C and then at 1100 °C shortly and hot rolling, to avoid complete transformation and to retain original {100} oriented regions. In addition, low‐temperature annealing is employed to increase grain sizes of hot‐rolled sheets thereby improving the final texture and magnetic properties after cold‐rolling and annealing. The results indicate that metastable ferritic columnar grains can be maintained during the two‐stage heating process at 1100 °C and primarily undergoes subgrain growth. The transformation from ferrite to austenite at 1100 °C is very slow. During the hot rolling with a high 86% reduction by two passes, the notable grain refinement is mainly caused by dynamic recrystallization rather than dynamic phase transformation as evidenced by the observation from the rolling plane.

The investigation of Sn corrosion on Mo meshes under the irradiation of high-flux hydrogen plasma

Liquid-metal divertor consisting of liquid Sn and Mo meshes would face high-flux plasma environments during service. In this work, the Sn corrosion behaviors on initial and pre-irradiated Mo meshes under hydrogen plasma irradiation were investigated to evaluate the reliability of the porous structure. According to the analysis and computation of XRD spectrum, the sub-grain size of the initial meshes would continuously grow during the corrosion process. The SEM images showed that after corrosion, the appearance and disappearance of prismatic Mo grains occurred alternately with the increase of plasma fluence. The thicknesses of corroded wires in the longitudinal and transverse directions displayed a similar repeatable change. In comparison to initial meshes, surface sub-grain growth and damage induced by the pre-irradiation treatment would result in differences in the corrosion process under hydrogen plasma irradiation. The pre-irradiated surface Mo grains with obvious grain boundaries were first dissolved to pebble-like Mo particles by liquid Sn and then stripped from the surface after corrosion. Sub-grain size was reduced from 72.5 nm to 41.1 nm at the fluence of 1.07 × 1026 ions/m2, which was inverse to the increasing phenomenon of the initial sample under same corrosion conditions. Additionally, the corrosion phenomenon, i.e., the presence and subsequent disappearance of prismatic Mo grains, occurred similarly on the surface of pre-irradiated samples. Based on the analysis of initial and pre-irradiated surface morphologies and wire thickness, Mo grains might go through an onion-peeling-like cycle of growth and fracture during the corrosion process. Besides, the wire thicknesses of all pre-irradiated Mo meshes were slightly less than that of the initial samples under the same corrosion conditions. Somehow, the comparative result reflected that the pre-irradiated samples were corroded more severely. Thus, the pre-irradiation of hydrogen plasma in this work would weaken corrosion resistance of Mo meshes in liquid Sn.

High-Temperature Compression Behaviors and Constitutive Models of a 7046-Aluminum Alloy.

High-temperature forming behaviors of a 7046-aluminum alloy were investigated by hot compression experiments. The microstructural evolution features with the changes in deformation parameters were dissected. Results indicated the formation of massive dislocation clusters/cells and subgrains through the intense DRV mechanism at low compression temperature. With an increase in deformation temperature, the annihilation of dislocations and the coarsening of subgrains/DRX grains became prominent, due to the collaborative effects of the DRV and DRX mechanisms. However, the growth of subgrains and DRX grains displayed the weakening trend at high strain rates. Moreover, two constitutive models involving a physically based (PB) model and a gate recurrent unit (GRU) model were proposed for predicting the hot compression features. By validation analysis, the predicted values of true stress perfectly fit with the experimental data, indicating that both the proposed PB model and the GRU model can accurately predict the hot compression behaviors of 7046-aluminum alloys.

Recovery of deformation microstructure in the bulk interior revealed by synchrotron X-ray micro-diffraction

A synchrotron X-ray micro-diffraction based technique, namely differential aperture X-ray microscopy (DAXM), is used to investigate the subgrain evolution in the bulk interior during ex-situ recovery annealing of a lightly deformed Al sample. This is the first 4D DAXM study of recovery. Supplementary TEM observations reveal that DAXM enables proper reconstruction of the microstructure, with the important addition that it is in 3D. The DAXM result provides a direct observation of the 3D microstructural evolution during annealing and confirms that subgrain boundary migration is an important mechanism for subgrain growth, preferentially removing small subgrains. Moreover, it gives new evidence of subgrain coalescence as an alternative mechanism for subgrain growth—some neighboring subgrains are observed to merge due to the decrease of the misorientation angle across certain subgrain boundaries. A possible interaction of the two recovery mechanisms is discussed, as well as the advantage and limitation of the DAXM technique for recovery studies.

Formation mechanism of Cube texture components ({001}) associated with dynamic recrystallization during compression in the single BCC phase region of Ti-22Al-25Nb alloy

The present study, employing EBSD as the primary method, investigates the formation mechanism of Cube texture components in Ti-22Al-25Nb alloy during uniaxial compression at high-temperature single-phase field. The research reveals that the formation of Cube components is closely related to texture evolution and dynamic recrystallization (DRX) behavior. Firstly, the formation of <001> and <111> dual fiber texture structures is a prerequisite for the development of Cube component, which mainly forms under large strains. Subsequently, continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) play distinct roles in the formation of Cube component. CDRX and DDRX occur in grains belonging to <001> and <111> fiber textures, respectively, exhibiting significant orientation dependence. <001> grains, characterized by a lower Taylor factor, undergo CDRX, causing sub-grains near the grain boundaries in <001> grains to rotate towards the Cube orientation. In contrast, <111> grains with a higher Taylor factor undergo DDRX, leading to the accumulation of high stored energy near the grain boundaries in <111> grains. When the two types of grains are adjacent, the significant orientation difference between them facilitates the rapid growth of Cube-oriented sub-grains through grain boundary migration. This results in a conspicuous Cube texture component on a macroscopic scale.

Creep damage parameter extraction from ex-service 12% Cr steel using digital image correlation computed strain data

Several continuum damage mechanics (CDM) modelling approaches for predicting creep deformation of tempered ferritic steels have been developed in the literature, which have evolved from efforts to extend the operability of power plant components. Few of these models, however, focus on damage assessment of ex-service states of power plant steels through the extraction of damage parameters. Furthermore, few CDM approaches leverage the high density of creep curve data available through full-field strain measurement techniques such as digital image correlation (DIC). This work uses multiple creep curves obtained from DIC computed strain data at several stresses and temperatures from individual specimens of X20CrMoV12-1 (X20) piping steel. These curves serve as input data to a modified Oruganti continuum damage mechanics (CDM) model whereby microstructural-specific damage parameters can be extracted. Good agreement is noted between CDM-extracted parameters and microstructural, creep cavity density and hardness damage indicators. Damage parameters based on subgrain growth are particularly sensitive to the ex-service state of the X20 steel. The proposed CDM approach using DIC computed creep curves is shown to be a material efficient alternative to traditional damage assessment methods of ex-service material.

Recrystallization behavior and mechanical properties of AZ31B alloy during the hot-rolling process

In this work, microstructural characteristics, dynamic recrystallization and mechanical properties of the hot-rolled AZ31B were investigated. The results indicated that the different mechanisms of recrystallization are responsible for the microstructural evolution during the rolling deformation. It was found, as for the specimens with the rolling reduction of 30%, large number of twins are formed within the deformed matrix and the recrystallized grains are usually formed nearby the twins. This indicated that the mechanism of the twin-induced nucleation mainly dominates the recrystallization process of the specimens with the 30% reduction. As for the specimens with the rolling reduction of 75%, the recrystallized grains are nucleated on the bulged boundaries or within the deformed grains. This indicates that the mechanisms of the stress-induced grain boundary migration (SIBM) and the sub-grain growth mainly dominate the recrystallization process of the specimens with the reduction of 75%. The recrystallization process contributes to the grain refinement and to the improvement of mechanical properties of alloys. Eventually, the specimens with the rolling reduction of 75% obtain an excellent strength-ductility balance. The results of this work not only elaborate underlying knowledge on the correlation relationship between the recrystallization mechanisms and the mechanical properties, but provide an effective approach to optimize the microstructure-mechanical properties of alloys.

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.

Quantitative mechanisms behind the high strength and electrical conductivity of Cu-Te alloy manufactured by continuous extrusion

The microstructure, mechanical performance, and electrical conductivity of Cu-Te alloy fabricated by continuous extrusion were quantitatively investigated. The results demonstrate that the grain size of the Cu-Te alloy is refined significantly by incomplete dynamic recrystallization. The Cu2Te phase stimulates recrystallization and inhibits subgrain growth. After extrusion, the tensile strength increases from 217.8 ± 4.8 MPa to 242.5 ± 3.7 MPa, the yield strength increases from 65.1 ± 3.5 MPa to 104.3 ± 3.8 MPa, and the yield to tensile strength ratio is improved from 0.293 ± 0.015 to 0.43 ±.0.091, while the electrical conductivity of room temperature decreases from 95.8 ± 0.38% International Annealed Cu Standard (IACS) to 94.0% ± 0.32% IACS. The quantitative analysis shows that the increment caused by dislocation strengthening and boundary strengthening account for 84.6% of the yield strength of the extruded Cu-Te alloy and the electrical resistivity induced by grain boundaries and dislocations accounts for 1.6% of the electrical resistivity of the extruded Cu-Te alloy. Dislocations and boundaries contribute greatly to the increase of yield strength, but less to the increase of electrical resistivity.

T1 precipitate bands and particle stimulated nucleation in 2195 Al-Cu-Li alloy during hot deformation

Dynamic precipitation and recrystallization behavior during middle-temperature and appropriate strain rates provide a chance to obtain desirable structure, which significantly improve the mechanical properties of aluminum alloys. In this paper, the microstructure evolution in the typical regions of processing map of 2195 Al-Cu-Li alloy during hot compression was observed via comparative characterization. The effects of dynamic precipitation and particle stimulated nucleation (PSN) on dynamic recrystallization have been analyzed systematically. At lower temperatures and higher strain rates, microshear band leads to a low power dissipation and provides preferential nucleation sites for T 1 precipitates. With temperature increases (<500 °C) and strain rate decreases, the T 1 precipitate bands are formed gradually due to the regular nucleation process on the microshear bands, which therefore limits the further growth of subgrains. Fine recrystallized grains along the grain boundary can also be obtained at high temperature via PSN stimulated by adequate stored energy, which was provided by large deformation near coarse secondary phases. Comprehensive continuous dynamic recrystallization (CDRX) is more prone to occur with the decrease of strain rate, leading to a higher power dissipation at 500 °C / 0.01 s −1 . • The relationship between microstructure evolution and hot processing map of 2195 Al-Cu-Li alloy have been investigated. • The formation of T 1 precipitate bands was described, and its influence on dynamic recrystallization has been discussed. • The dynamic recrystallization behaviour around grain boundaries has been discussed.

Microstructural evolution during recovery of deformed aluminium—Effect of deformation strain

Recovery takes place during annealing of a deformed metal, releasing some of the stored energy. Many recovery mechanisms have been proposed in the literature and these mechanisms strongly depend on the deformation microstructure. In this work, recent progress in the study of recovery in aluminium is reviewed. In lightly deformed aluminium, subgrain growth (through boundary migration and coalescence) is found to be an important recovery mechanism, whereas in heavily deformed samples, uniform coarsening through Y-junction motion is found to be the dominant recovery mechanism. The kinetics of recovery follows a universal coarsening model for aluminium deformed to both low and high strains, in which the apparent activation energy increases in the course of recovery. Furthermore, a new definition of recovery is suggested to be compatible with observations at both low and high strains.

The dynamic recrystallization mechanism of ultrasonic power on non-contact ultrasonic-assisted direct laser deposited alloy steel

Ultrasonic assistance has been used as one of the more convenient and effective means of grain refinement during alloys direct laser deposition, and the formation of fine grains is closely related to the dynamic recrystallization behavior during laser non-equilibrium metallurgy. To investigate the dynamic recrystallization behavior of the non-contact ultrasonic-assisted process in direct laser deposited alloy steel, this paper used 24CrNiMoY alloy powder and the non-contact ultrasonic-assisted direct laser deposition method to study the mechanism of ultrasonic power on the dynamic recrystallization behavior of alloy steel. The results show that the ultrasonic power had an obvious regulation on the prepared alloy steel samples’ grain size and tensile properties. The microstructure of the prepared samples was mainly composed of upper bainite, lower bainite, and granular bainite. After the non-contact ultrasonic-assisted process, the grain was obviously refined, and the smallest grain was 0.4 μm. The dynamic recrystallization behavior in direct laser deposited alloy steel at different non-contact ultrasonic powers was mainly determined by dislocation density and solidification rate. The introduction of non-contact ultrasonic caused dislocation movement in the plastic deformation zone of direct laser deposited alloy steel samples, and the low angle grain boundaries migration—subgrain growth—the transformation of low angle grain boundaries to high angle grain boundaries initiated by dislocation movement was the key process to trigger dynamic recrystallization. The optimized ultrasonic power of 360 W had the best strength-plasticity matching relationship for the prepared samples, with a tensile strength of 1108 MPa and elongation of 12%. And the strong-plastic product of sample (360 W) was increased from 6 GPa% to 13 GPa% due to the increase of recrystallization fraction. This study provides a valuable theoretical and technical reference for preparing high-performance alloy steel by ultrasonic-assisted direct laser deposition.

Sub-grain Boundary Mobility of 6XXX Alloys during Annealing

In this paper, 6061 aluminum alloy and a new type of dispersion-strengthened 6XXX aluminum alloy (named WQ) were compressed to a true strain of 1.2 at a strain rate of 0.1/s at 300/400/500°C and then annealed at different temperatures. The microstructure evolution of both heat treated alloys were characterized by EBSD. The results showed that during the annealing process of the two deformed alloys, the sub-grain size did not increase significantly. Due to the pinning effect of the dispersoids in WQ alloy, the sub-grain size of WQ was smaller than that of 6061 alloy. Under the same deformation conditions and annealing process, the deformation stored energy of WQ alloy was higher than that of 6061 alloy. The deformation stored energy of 6061 compressed alloy was more sensitive to the annealing temperature. Combining theoretical formulas and EBSD data, the sub-grain boundary migration rate curves of 6061 and WQ deformed alloys during the annealing process were obtained, which could provide a theoretical basis for the future study of the sub-grain growth process of multi-element aluminum alloys.

Roles of triple and quadruple junctions on plasticity by phase-field crystal approach

Roles of non-/symmetric triple and quadruple junctions on plasticity of ploycrystals are examined using phase-filed crystal model combined with an isovolumetric deformation technique. We designed four polycrystalline samples containing the four types of junctions respectively to simulate the deformation process under a strain rate comparable to practical situations. Our results uncover relationships between the ability of resistances to external deformations and the deformation mechanisms. For the ploycrystals containing quadruple junctions, the symmetric junction is beneficial to maintain the stability of the junctions during deformations, while asymmetric ones would be disjointed and result in formation and growth of sub-grains. For the polycrystals containing triple junctions, the stability of the junction is strongly related to the misorientation angles of the associated grains. The triple junction composed with high-angle GBs exhibits excellent stability under successive deformations and the deformation is dominated by GB migrations. Low-angle GBs migrate more easily than the high-angle ones.

The evolution of coarse grains and its effects on weakened basal texture during annealing of a cold-rolled magnesium AZ31B alloy

The nucleation, grain growth of 34 coarse grains during annealing were tracked using a quasi-in-situ EBSD method. These 34 coarse grains had different orientations and most grains were non-basal orientated. No preferable grain growth or special types of grain boundaries were identified. Only 9 coarse grains nucleated from deformed grain boundaries due to initial large grain size and limited grain boundary volume fraction. The main nucleation site of 34 coarse grains was dislocation cells or subgrains in deformed grain interiors. Their recrystallisation behaviour can be illustrated by abnormal subgrain growth (AsGG) rarely reported in Mg alloys. The coarse basal grains showed no growth advantage in terms of grain size or number over other non-basal grains, leading to a weak basal texture in AZ31B alloy.

Effect of creep annealing on the dimensional stability of dispersion strengthened copper alloy

This study aims to enhance the dimensional stability of alumina dispersion strength copper alloy via creep annealing treatment. The microstructure, residual stress and micro-yield strength of alloys after different treatments were investigated. During creep annealing, the residual stress in the alloy was dramatically reduced due to the decrease of dislocation density. The nano-Al2O3 particles in the alloy did not coarsen or agglomerate during the high temperature treatment and retarded the growth of subgrains effectively, which played important roles in high micro-yield strength of creep annealed alloy. The pores were effectively reduced by the creep deformation during creep annealing, resulting in a more uniform residual stress distribution and an increase in micro-yield strength. Compared with extruded alloy, the residual stress of alloy creep annealed at 950 °C for 12 h was reduced by 88%, while the micro-yield strength increased by 9.7%, leading to a significant enhancement in dimensional stability of the alloy.

Topological aspects responsible for recrystallization evolution in an IF-steel sheet – Investigation with cellular-automaton simulations

A cellular automaton algorithm for curvature-driven coarsening is applied to a cold-rolled interstitial-free steel’s microstructure - obtained through electron backscatter diffraction (EBSD). Recrystallization nucleation occurs naturally during the simulation, due to the highly heterogeneous and hence competitive growth among pre-existing (sub) grains. The spatial inhomogeneity of the subgrain growth that takes place derives from the large local variations of subgrain sizes and misorientations that comprise the prior deformed state. The results show that capillary-driven selective growth takes place to the extent that the prior elongated and deformed grains are replaced by equiaxed grains with no interior small-angle boundaries. Additionally, during the simulation certain texture components intensify and others vanish, which indicates that preferential growth occurs in a fashion that relates to the crystal orientations’ topology. The study of the early stages of recrystallization (i.e. nucleation) shows that the pre-existing subgrains that eventually recrystallize, exhibit certain topological characteristics at the prior deformed state. Successful nucleation occurs mostly for pre-existing matrix subgrains abutting shear bands or narrow deformation bands and particularly at regions where the latter intersect.

A novel simulation of continuous dynamic recrystallization process for 2219 aluminium alloy using cellular automata technique

Based on the previous experiments, continuous dynamic recrystallization (CDRX) was determined to be the main softening mechanism of Al 2219 alloy during the thermal deformation process. The mechanism of microstructural transformation for the CDRX process was firstly discussed, and the dislocation evolution model, subgrain growth nucleation model, low angle grain boundaries (LAGBs) migration model, and topological deformation model were established respectively. Using the probabilistic cellular automata technique, a two-step microstructure formation method of initial equiaxed grains + local subgrains dispersion was proposed, which can simulate the initial microstructure at any processing state. Then, according to the proposed micro-mechanism and models of CDRX, a novel simulation approach of the CDRX process for Al 2219 alloy during thermal processing was established on the Matlab platform to predict the substructure's evolution. The CA model was applied to extensively analyze the influence of deformation parameters and the transformation law of LAGBs. The results showed that high temperature, high strain, and low strain rate can effectively promote the development of continuous dynamic recrystallization, which is consistent with the experimental results.