Strain-induced Grain Research Articles

A quasi-in-situ EBSD study on abnormal grain growth in 2219 aluminum alloy friction stir welded joints during post-weld heat treatment

Abnormal grain growth occurs in the friction stir welds of 2219 aluminum alloy during the solution treatment. To further investigate abnormal grain growth, the quasi-in-situ electron backscatter diffraction (EBSD) experiment, including the EBSD testing and heating by thermal simulation testing machine, was proposed to observe the grain growth process in the weld. The results show that the abnormal grain growth behavior in the stir zone and the thermo-mechanically affected zone is different, and different microstructures at different positions in the weld promote the growth of abnormal grains. In the stir zone, the modified Humphreys’ model is used to analyze the abnormal grain growth behavior. The grains with an advantage size and low strain are more likely to grow abnormally. The non-uniform pinning caused by the dissolution of second-phase particles further promotes abnormal grain growth. In the thermo-mechanically affected zone, the abnormal grains are formed by unstrained equiaxed grains near the stir zone or recrystallized sub-grains. The growth of abnormal grains in the thermo-mechanically affected zone is a strain-induced grain boundary migration process. The research is helpful to understand the abnormal grain growth in friction-stir welds during post-weld heat treatment.

Investigation of Precipitation Behavior of a Novel Ni-Fe-Based Superalloy during High-Temperature Aging Treatment.

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ'-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ' remained spherical, and the size kept increasing with aging time according to the Lifshitz-Slyozov-Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ'; as γ' gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ' increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ' achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ' precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Enhancing mechanical property and corrosion resistance of Al0.3CoCrFeNi1.5 high entropy alloy via grain boundary engineering

In the present study, to improve the performances of Al0.3CoCrFeNi1.5 high entropy alloys (HEAs), grain boundary character distribution (GBCD) of Al0.3CoCrFeNi1.5 HEA has been optimized by an appropriate thermo-mechanical processing. The experiment results showed that the fraction of low-Σ coincidence site lattice (CSL) boundaries could reach approximately 80 % through cold rolling with deformation of 8 % and subsequent annealing at 1050 °C for 5 min. The reason for GBCD optimization could be attributed to sufficient strain-induced boundary migration (SIBM) or grain growth after recrystallization. While recrystallization is not favorable for optimizing GBCD. The mechanical properties and corrosion resistance have been enhanced, with a more pronounced improvement observed in the corrosion resistance. The corrosion current density icorr of the GBEM specimen stands at 0.23 μA∙cm−2, representing a reduction of 66 % in comparison to the BM specimen (0.68 μA∙cm−2). The improvement of corrosion resistance of Al0.3CoCrFeNi1.5 HEA resulted from the discontinuous random grain boundaries (RGBs) broken by the high fraction of low-ΣCSL boundaries, especially Σ3 boundaries suppressed the propagation of corrosion crack.

Microstructure-thermal transport property correlation after cold/cryo rolling deformation of alloy D9

ABSTRACT Plastic deformation alters the microstructure of a material due to the formation of twins, high density of dislocations and strain-induced phases and grain refinement. Such microstructural modifications can affect thermal transport properties of a material. In this work, variation in thermal diffusivity (TD) was investigated in cold worked (CW) and cryo rolled (CR) alloy D9 (ASTM designation A771/UNS S38660), using laser flash analyser after 20–80% thickness reduction. Deformed samples were characterised using XRD and microscopy techniques for structure and microstructure modifications besides correlating with the change in hardness. Temperature dependence of thermal diffusivity was measured from room temperature up to 1273 K. Microstructural features confirmed the formation of deformation-induced martensite in CR specimen. This was accompanied by up to 172% increase in hardness in 80% CR compared to annealed specimen. The dislocation density estimated from XRD structural profile showed an increase with the percentage of deformation. For all specimens, thermal diffusivity showed an increasing trend with temperature but decreased with deformation. The reduction in thermal diffusivity was non-negligible beyond 40% thickness reduction for all deformed specimen and was substantial specifically for CR alloy D9.

Microstructural Evolution and Strengthening of Dual-Phase Stainless Steel S32750 during Heavily Cold Drawing

S32750 dual-phase stainless steel (DSS) wires were prepared by cold drawing with a strain of ε = 0~3.6. The mechanical behavior and microstructural evolution of these DSS wires at different strains were investigated. Specifically, the yield strength and ultimate tensile strength of a S32750 DSS wire at a strain of ε = 3.6 reached 1771 MPa and 1952 MPa, respectively. The microstructure of the wire was transformed into a heterogeneous microstructure, which consisted of ferrite fiber grains and a nanofibrous grain structure consisting of austenite and strain-induced martensite nanofiber grains. A sub-grain structure was observed inside the ferrite fiber. The austenitic phase followed the evolutionary steps of stacking faults, twinning, ε-martensite, α-martensite, and, finally, austenite, before transitioning into a nanofibrous grain structure. This nanofibrous grain structure significantly contributed to the strength compared with the relatively coarse ferrite phase.

Constitutive Model and Microstructure Evolution of Ti65 Titanium Alloy.

The hot deformation behavior and mechanism of Ti65 alloy with a bimodal microstructure were investigated by isothermal compression experiments conducted on the Thermecmastor-Z simulator equipment at temperatures ranging from 950 to 1110 °C and strain rates ranging from 0.01 to 10.0 s-1. The Arrhenius constitutive model, based on strain compensation, and Grey Wolf optimization-neural network with back propagation model (GWO-BP), were both established. The differences between the experimental and predicted value of flow stress were compared and analyzed using the two models. The results show that the prediction accuracy of GWO-BP in the two-phase region is higher than that of Arrhenius model. In the single-phase region, both methods demonstrated high prediction accuracy. Compared to the single-phase region, the flow stress of Ti65 alloy shows a higher degree of softening in the two-phase region. During deformation in the two-phase region, the initial lamellar α phase transformed from a kinked and elongated morphology to a globularized topography as the strain rate decreased. Boundary-splitting was the primary mechanism leading to the spheroidization process. The degree of recrystallization increased with the increase in strain rate during the deformation in the single-phase region, while dynamic recovery and strain-induced grain boundary migration were the main deformation mechanisms at a lower strain rate. Discontinuous dynamic recrystallization may be the dominant recrystallization mechanism under a high strain rate of 10 s-1.

Preparation of ultra-high ductility and high strength Mg-Sn-Zn-Zr alloy by differential thermal ECAP (DT-ECAP) induced heterogeneous structure

Developing high-ductility magnesium (Mg) alloys has become an imminent issue for their wide application. In this work, a new Mg-Sn-Zn-Zr alloy with ultra-high ductility (elongation, El. over 40 %) and high ultimate tensile strength (UTS, ∼309–354 MPa) was prepared by a novel differential thermal equal-channel angular pressing (DT-ECAP). Heterogeneous structures, including bimodal grain structures and inhomogeneous distribution of second phases composed of banded structure and particle free zone (PFZ), were induced by DT-ECAP process. Based on the results of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED), the bimodal grain structure originated from incomplete dynamic recrystallization (DRX) dominated by Zener pinning, strain-induced grain boundary migration (SIBM) and the limitation of polycrystallization due to lower dislocation density. Meanwhile, the bimodal distribution of second phases was highly associated with the defect density and initial structure. More importantly, the enhanced strength of DT-ECAPed alloys can be primarily attributed to hetero-deformation induced (HDI) strengthening, grain boundary strengthening, and precipitation strengthening. Moreover, HDI hardening, texture weakening or randomizing activation of non-basal slip, high density of dislocations in sub-structures, and twining induced superior work-hardening effect, which was highly responsible for the ultra-high ductility in sixth pass (6P) alloy. The current work provides a novel DT-ECAP process for inducing heterogeneous structure and offers beneficial insight into the development of ultra-high ductility and high strength for rare-earth-free Mg alloys via a combination of HDI strengthening and hardening and other vital mechanisms.

Strain-induced grain evolution of pure nickel under warm power torsional rolling process

Strain-induced grain evolution of pure nickel under warm power torsional rolling process

Microstructure, texture and magnetic properties of the semi-processed strip-cast non-oriented electrical steel

{100}<0vw> texture is the favorable texture in non-oriented electrical steels (NOES). However, the conventional processes are difficult to produce NOES with strong {100}<0vw> texture. In this study, the 0.35 mm thick 3.1% Si NOES with strong {100}<0vw> texture was produced by the top-side pouring twin-roll casting, cold rolling with or without intermediate annealing, and final annealing process. The microstructure and texture were characterized by optical microscopy, X-ray diffraction and electron backscatter diffraction. Compared to the one-stage cold-rolled strip-cast NOES, the semi-processed (two-stage cold-rolled) strip-cast NOES with intermediate annealing at 650 °C and 700 °C have stronger {100}<0vw> texture and better magnetic properties. The reason is that the semi-processing method can avoid the encroachment on {100}<0vw> grains by other oriented grains, which can encroach on the {100}<0vw> grains by strain-induced grain boundary migration mechanism. Furthermore, the effect of intermediate annealing temperature on the microstructure, texture, and magnetic properties of the semi-processed strip-cast NOES were studied.

Strain-induced grain boundary migration and grain rotation in polycrystalline metals: Atomic-and meso-scale phase field simulations

The study reveals that in phase field (PF) simulations with over 300 grains, grain boundary (GB) migration exhibits anisotropic behavior and higher velocities under applied external loading. However, these simulations fail to provide detailed atomic-scale information about the evolution of grain orientation. Therefore, the researchers utilize phase field crystal (PFC) simulations to track the evolution of dislocation configurations and the annihilation of GB dislocations. The PFC simulations confirm a newly proposed plasticity process in nanocrystalline metals, where grain rotation is primarily dominated by dislocation climb and dislocation absorption at triple-junction points on GB, rather than GB sliding or diffusional creep as previously suggested. Furthermore, the PFC simulations demonstrate that the analysis of misorientation angles and GB dislocation evolution between neighboring grains can be quantitatively described using the Frank-Bilby equation. Thus, a hybrid approach of PF and PFC successfully describes in-situ transmission electron microscopy observations of GB migration and grain rotation for the first time. This multi-scale simulation method provides a deeper understanding of plasticity formation and development in nanostructured materials.

Investigation of the Abnormally Large Grains of a Super304H Heat-Resistant Steel Tube over Long-Term, High-Temperature Service

Abstract Abnormally large grains (ALGs) were detected in the outer walls of Super304H superheater steel tubes of an ultra-supercritical unit in long-term service. In this study, the microstructure and mechanical properties of steel tube samples were investigated to reveal the formation mechanism of the ALGs. The results showed that the room-temperature and high-temperature strength and plasticity of the coarse-grain zone were much lower than those of the fine-grain zone. Inhomogeneous residual strain distribution in the outer wall led to high distortion energy differences in the as-supplied steel tube. The low-distortion austenite grains annex the neighboring high-distortion grains via the strain-induced grain boundary migration mechanism. Moreover, the two-dimensional step mechanism of the faceted grain boundaries also causes the ALGs to form.

Strategy for suppressing abnormal grain growth of ZK60 Mg alloy during solution by pre-compression: A quasi-in-situ study

Abnormal grain growth (AGG) easily takes place in Mg alloys during high-temperature solutions, resulting in deterioration of mechanical properties. Hence, the compression prior to solution (pre-compression) was conducted to suppress AGG, and the microstructure evolution as well as suppressing mechanisms was investigated based on quasi-in-situ analysis. After compression along the transverse direction, <11–20>//ED grains preferentially nucleated and rapidly grew up, and the initial <10–10>//ED texture was weakened. Two grain growth modes of heat-induced and strain-induced grain boundary migrations were found. The former was attributed to the high interfacial energy of grain boundaries with large curvature. The latter consumed the adjacent grains with high storage energy, forming abnormal grains with irregular shapes. The compression with a reduction > 6% could obviously suppress AGG. The suppressing effects were mainly attributed to weakening the size advantage of <11–20>//ED grains, increasing nucleation, reducing grain boundary character distribution, and redistributing storage energy distribution. After 12% compression along the transverse direction, 30° misorientation of <11–20>//ED grains and high energy grain boundaries were reduced. The {10–12} tensile twins and {10–15} high index twins induced by compression increased the nucleation of static recrystallization. Beside, compression introduces high-density dislocations, which also contributed to suppressing AGG behavior during solution.

Effect of strain-induced grain boundary migration on microstructure and creep behavior of extruded AZ31 magnesium alloy prepared by pre-compression and annealing treatment

Effect of strain-induced grain boundary migration on microstructure and creep behavior of extruded AZ31 magnesium alloy prepared by pre-compression and annealing treatment

On the formation of large grain structure after friction stir processing of ultrafine-grained aluminium alloy

ABSTRACT A tendency for formation of large grain structure after friction stir processing (FSP) of ultrafine-grained aluminium produced by severe plastic deformation was thoroughly discussed. To do so, microstructural assessments using electron backscattered diffraction (EBSD) analysis at various regions, including ahead of and beneath the FSP tool were carried out. It was found that ultrafine-grained microstructure of the base metal played an important role in dynamic recrystallisation (DRX) occurring in the stir zone. There are some enlarged grains just ahead of the FSP tool prior to being stirred by it. Partial static recrystallisation followed by grain growth as well as strain-induced grain boundary migration was responsible for the formation of such grains. The large grains could postpone grain subdivision process through continuous and geometrical DRX mechanisms. Moreover, the remained part of the stored strain provided driving force for grain growth after grain subdivision in the stir zone.

A MODEL OF MICROSTRUCTURE EVOLUTIONUNDER HIGH- STRAIN RATE DEFORMATION OF COPPER

A physical model of high-strain rate deformation of metallic materials has been developed to describe the evolution of microstructure during the process. The model is based on representations of the mechanisms of grain boundary sliding, strain-induced grain growth and dynamic fragmentation of coarse grains developed in the framework of the theory of nonequilibrium grain boundaries. These representations are modified for the description of high-speed flow and supplemented by a model of dynamic recrystallization. It is shown that during high-strain rate deformation of metallic materials, grain boundary sliding dominates in the material regions with a fine-grained submicron structure and the material deforms in the superplasticity mode. In regions of material with larger grain size, grain volume deformation dominates, accompanied by fragmentation and formation of fine-grained structure with submicron grain size. The transformation of the structure from fine-grained to coarse-grained occurs due to strain-induced grain growth. Transformation of structure from coarse grain to fine grain occurs due to dynamic fragmentation. The transition from grain boundary sliding to grain volume deformation occurs at grain size corresponding to the optimum value for the course of dynamic recrystallization. The cyclic change of deformation mechanisms (from grain boundary strain-induced to grain volume deformation) is caused by alternation of the processes of deformation-stimulated grain growth and dynamic fragmentation. The alternation of the above cycles of grain boundary and intragranular deformation in different regions of the material makes it possible to maintain a high strain rate and suppress the processes of both local hardening and local softening. The high strain rate is achieved at the expense of a high strain of the regions in which grain sliding dominates. In the regions dominated by intragranular deformation, the structure is “prepared” for superplastic flow.

Paleostress, deformation mechanisms and finite strain related to chocolate-tablet boudinage and folding of a Permian rock salt/anhydrite sequence (Morsleben site, Germany)

Polyphase chocolate-tablet boudinage of competent anhydrite layers in incompetent Permian rock salt of the Morsleben repository (Germany) developed during Triassic/Jurassic graben formation and late Cretaceous graben inversion. The competence contrast between layers and matrix is related to the deformation mechanisms of anhydrite and halite, which have been constrained in the present study using deformation microfabrics and geochemical data. Boudinage implies a finite longitudinal strain of ca. 100% in NNW-SSE direction. There is a clear impact of the layer thickness on both the deformation mechanisms and the shape of anhydrite boudins. Pervasive solution-precipitation creep is the dominant deformation mechanism in thin anhydrite layers (thickness < 15 cm), which are dominated by drawn boudins with aspect ratios ranging from 1.5 to 5.4. Deformation of thick anhydrite layers (thickness > 15 cm) was accommodated by dislocation creep succeeded by localized solution-precipitation, kinking, twinning and fracturing resulting in torn boudins with aspect ratios between 0.5 and 2.0. Halite of the incompetent rock salt matrix deformed viscously under low differential stress (2.0–3.1 MPa). The deformation mechanisms were dislocation creep in form of subgrain formation and strain-induced grain boundary migration. Moreover, close to anhydrite boudins, solution-precipitation creep of halite is indicated by decreased bromide content in halite of necks between brittle torn boudins, which should have precipitated from a fluid phase. Halite in necks of drawn boudins, on the other hand, displays a continuous decrease of bromide content, pointing to predominantly crystal plastic inflow of halite.

Hot deformation behavior of a cryogenic high manganese steel based on the microstructure and texture evolution

The correlation between microstructure evolution, texture development and hot deformation behavior of high manganese steel for LNG were investigated through uniaxial hot tensile tests at different temperatures (800, 850, 900 and 980 °C). Hot tensile tests were carried out on a Gleeble 3800 thermomechanical simulator with a strain rate of 10 /s. Texture analysis was conducted by the orientation distribution functions acquired from the electron backscatter diffraction (EBSD). Detailed microstructure and dynamic recrystallization (DRX) were indicated by the grain orientation spread of EBSD and transmission electron microscopy (TEM). The results show that there are only two stages in the work hardening rate when the deformed microstructure is non-recrystallized or mostly recrystallized. The work hardening rate exhibits a wave-like fluctuation when the tensile temperature is near the critical DRX temperature, due to the continuous competition between the dislocation packing caused by deformation and the softening intrigued by DRX. The deformed microstructure contains the highly dense dislocation wall (HDDW) structure and the cell block structure, and the HDDW structure mostly occurs in the 〈001〉//td fiber while the latter corresponds to the 〈111〉//td fiber. What's more, <001>//td fiber has stronger orientation gradient and work hardening rate than <111>//td fiber. DRX preferentially nucleated on the <001>//td fiber through two nucleation methods: strain-induced grain boundary migration and twining nucleation. DRX grains only retain the weak {111} 〈112〉 F and {110} <112> A textures, and the overall texture was weakened and randomized.

Insight into microstructure, microhardness and corrosion performance of 2205 duplex stainless steel: Effect of plastic pre-strain

This study explored the microstructure evolution of 2205 duplex stainless steel (DSS) and further probed its effects on microhardness and corrosion performance. The results show that the dislocation multiplication, strain-induced martensite generation and grain refinement resulted from pre-stain increase the microhardness of DSS. Compared with the degradation of corrosion resistance at 7% pre-strain, the improved corrosion resistance at 14% and 21% pre-strain is related to the smaller surface energy difference, more strain-induced martensite and more low-angle grain boundaries. The changed pitting nucleation sites with the rise of pre-strain strongly depend on the increased dislocation density and strain-induced martensite.

High Strain Rate Quasi-Superplasticity Behavior in an Ultralight Mg-9.55Li-2.92Al-0.027Y-0.026Mn Alloy Fabricated by Multidirectional Forging and Asymmetrical Rolling.

To explore new approaches to severe plastic deformation and the ductility of a multicomponent magnesium-lithium alloy, an ultralight microduplex Mg-9.55Li-2.92Al-0.027Y-0.026Mn alloy was made by novel multidirectional forging and asymmetrical rolling, and the superplasticity behavior was investigated by optical microscope, hot tensile test, and modeling. The average grain size is 1.9 μm in this alloy after multidirectional forging and asymmetrical rolling. Remarkable grain refinement caused by such a forming, which turns the as-cast grain size of 144.68 μm into the as-rolled grain size of 1.9 μm, is achieved. The elongation to failure of 228.05% is obtained at 523 K and 1 × 10-2 s-1, which demonstrates the high strain rate quasi-superplasticity. The maximum elongation to failure of 287.12% was achieved in this alloy at 573 K and 5 × 10-4 s-1. It was found that strain-induced grain coarsening at 523 K is much weaker than the strain-induced grain coarsening at 573 K. Thus, the ductility of 228.05% is suitable for application in high strain rate superplastic forming. The stress exponent of 3 and the average activation energy for deformation of 50.06 kJ/mol indicate that the rate-controlling deformation mechanism is dislocation-glide controlled by pipe diffusion.

Strain-induced strengthening in superconducting β-Mo2C through high pressure and high temperature

High-strain-induced microstructural refinement and dislocations are critical for the strengthening of metallic materials; however, this is difficult to achieve in ceramic materials due to their unique bonding characteristics and electronic structure. Here, a series of β-Mo2C bulk ceramics are consolidated by the high pressure and high temperature (HPHT) strategy. Our results demonstrate that high strain-induced grain plastic deformation at high pressure produces a lamellar sub-grain structure with high-density dislocations, and that the dislocations at the lath-like grain boundaries eventually evolve into low-angle grain boundaries. The mechanical properties, superconducting behavior, and onset of oxidation of the specimens are investigated. It is found that superconductivity and hardness arise from the high density of states at the Fermi level, while the high intrinsic hardness is attributed to the strong hybridization between Mo-4d orbitals and C-2p orbitals. Furthermore, strain-induced defect structure (dislocations and low-angle grain boundaries) mainly mainly enhances the intrinsic structure of β-Mo2C.