Annealing Twin Research Articles

Interactions between twins and dislocations during dynamic microstructure evolution for hot shear-compression deformation of Fe-38Mn austenitic steel

The interactions between twins and dislocations were investigated at different stage of dynamic microstructure evolution during hot shear-compression deformation of the Fe-38Mn austenitic steel. It was proved that the step-like structure located at the boundaries of pre-existing annealing twins was induced by the interactions of twins and dislocations in the case of low strain level. These steps provided favorable nucleating positions for the new grains. Benefited from the large strain and high strain rate during the hot shear-compression deformation, the deformation twins in different scales appeared with further increasing the strain. Moreover, the interactions between the deformation twins and dislocations were frequently found. As a result, the nano-grain was induced by the migration of incoherent twin boundaries in the case of large strain. The above works indicate a dynamic mechanism of grain refinement in term of the interactions between twins and dislocations during hot deformation with large strain and high strain rate.

Microstructure development in a high-nickel austenitic stainless steel using EBSD during in situ tensile deformation

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with 〈111〉 or 〈001〉 parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only.

EBSD study of grain growth behavior and annealing twin evolution after full recrystallization in a nickel-based superalloy

The grain growth behaviors of a nickel-based superalloy after full recrystallization are investigated by hot compression test plus annealing treatment. Electron backscattered diffraction technology (EBSD) is utilized to analyze the grain boundary features. It is found that the annealing parameters greatly affect the average grain size, and annealing twin boundaries, including ∑3, ∑9, and ∑27 boundaries. The fully-recrystallized grains can rapidly grow up at relatively high annealing temperatures with the increase of annealing time. Meanwhile, large fractions of annealing twin boundaries form at the initial stage of annealing treatment. However, with the further increase of annealing time, the content of annealing twins slightly drops due to the migration of grain boundaries or twin boundaries. When the annealing temperature is increased, the shape of annealing twin is apparently lengthened while the number of annealing twin sharply decreases. In addition, the kinetics model for grain growth after full recrystallization is established and validated by the experimental observations.

An investigation of effect of annealing at different temperatures on microstructures and bulk textures development in deformed Zircaloy-4

In the present work, the effect of various annealing temperatures on the microstructural and textural evolution in deformed Zircaloy-4 has been investigated. Zircaloy-4 was subjected to a rolling reduction of 90% corresponding to a true strain of 2.3 along the ND - ED and ND - TD planes of the as-received Zircaloy-4bar at room (298K) and cryogenic (77K) temperatures. Following this, the various deformed conditions of the samples were subjected to annealing treatment at various temperatures such as 400°C, 450°C and 500°C. The deformed samples were characterized by a high dislocation density and nano scale subgrains, which after annealing at various temperatures were marked with annealing twins of nano - size along with some other dislocation configuration. A higher dislocation density and hardness (258.54±2.91HV) values were observed after annealing the cryorolled alloy along the ND - TD plane at 450°C. This high cold work energy provided by cryorolling along the ND - TD plane, after annealing at 450°C produced an optimum twin growth, which impeded the dislocation motion and slightly increased the hardness along this plane. Finally, the textures evolved after annealing indicated the role of annealing twins combined with <a> prismatic, <c+a> pyramidal & basal glide activity.

Effect of annealing on microstructure and tensile properties of cold-rolled Cu-2.7Be sheets

The cold rolled Cu-2.7Be sheets were annealed at different temperatures. The evolution of microstructure, texture and tensile properties was investigated using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and tensile tests, respectively. Results showed that preponderant ∑3 annealing twins were produced during recrystallization. The ∑3 fraction and average grain size increased with increasing annealing temperature. Besides, it turned out that the extension and depletion of twin boundaries occurred simultaneously during grain growth process, affecting the ∑3 fraction. Recrystallization texture became relative isotropic because of twinning mechanism which involved the formation of new orientations from the parent grain. It was noteworthy that the weak recrystallization cube texture was observed, which was attributed to appearance of preponderant twin boundaries. Tensile tests results showed that twin boundaries should be considered as effective obstacles to dislocations motion, similar to regular grain boundaries.

Grain Boundary Assemblies in Dynamically-Recrystallized Austenitic Stainless Steel

The grain boundary misorientation distributions associated with the development of dynamic recrystallization were studied in a high-nitrogen austenitic stainless steel subjected to hot working. Under conditions of discontinuous dynamic recrystallization, the relationships between the grain or subgrain sizes and flow stresses can be expressed by power law functions with different grain/subgrain size exponents of about −0.76 (for grain size) or −1.0 (for subgrain size). Therefore, the mean grain size being much larger than the subgrain size under conditions of low flow stress gradually approaches the size of the subgrains with an increase in the flow stress. These dependencies lead to the fraction of high-angle boundaries being a function of the flow stress. Namely, the fraction of ordinary high-angle boundaries in dynamically-recrystallized structures decreases with a decrease in the flow stress. On the other hand, the fraction of special boundaries, which are associated with annealing twins, progressively increases with a decrease of the flow stress.

On recrystallization texture and magnetic property of Cu-Ni alloys

The crystallographic texture, micro-hardness and magnetic property have been investigated in Cu-57at.% Ni, Cu-53at.% Ni, and Cu-49at.% Ni alloy used as substrates for coated conductors. In this work, a similar rolling texture was observed in the cold rolled condition for the three alloys. For each material, annealing at higher temperature results in a much stronger cube texture but the fraction of annealing twin boundary was quiet low. When annealing at 700°C and 800°C, it was presumed that the increase of Cu element increased grain boundary mobility, which then lead to the enhancement of twin boundary and weakened the cube texture. In addition, magnetization measurements indicate that the increase of Cu content can effectively reduce the saturation magnetization in Cu-Ni alloys. The comparatively low ferromagnetism for both Cu-53at.% Ni and Cu-49at.% Ni alloys suggests that substrates produced using these materials will benefit from decreasing hysteresis losses for alternating current applications.

Effect of preferential orientation on the annealing twins during the low temperature treatment in nickel

The influence of preferential orientation (namely cube texture) on annealing twins in a cold rolled (95% thickness reduction) pure nickel has been investigated during isothermal annealing at low temperature. The results reveal that the cube oriented grains in cube related twinning (CT) regions have larger average boundary migration rate in comparison with grains in non-cube related twinning (NCT) regions. However, the twin density is always lower in CT regions than that in NCT regions. This is contrary to the growth accident model that suggests that twin density was proportional to the grain boundary velocity and grain boundary distance. It can be interpreted by the fact of the selective nucleation and growth mechanism for cube oriented grains which are less dependent on annealing twinning during recrystallization. Furthermore, during grain growth the average length of annealing twin boundary segments has a significant increment, which is probably attributed to lengthening of low energy twin boundaries along with the migration of other neighboring boundaries.

Development of microstructure and mechanical properties during annealing of a cold-swaged Co–Cr–Mo alloy rod

In this study, we investigated the evolution of the microstructure and mechanical properties during annealing of a cold-swaged Ni-free Co–Cr–Mo alloy for biomedical applications. A Co–28Cr–6Mo–0.14N–0.05C (mass%) alloy rod was processed by cold swaging, with a reduction in area of 27.7%, and then annealed at 1173–1423K for various periods up to 6h. The duplex microstructure of the cold-swaged rod consisted of a face-centered cubic γ-matrix and hexagonal closed-packed ε-martensite developed during cold swaging. This structure transformed nearly completely to the γ-phase after annealing and many annealing twin boundaries were observed as a result of the heat treatment. A small amount of the ε-phase was identified in specimens annealed at 1173K. Growth of the γ-grains occurred with increasing annealing time at temperatures ≥1273K. Interestingly, the grain sizes remained almost unchanged at 1173K and a very fine grain size of approximately 8μm was obtained. The precipitation that occurred during annealing was attributed to the limited grain coarsening during heat treatment. Consequently, the specimens treated at this temperature showed the highest tensile strength and lowest ductility among the specimens prepared. An elongation-to-failure value larger than 30% is sufficient for the proposed applications. The other specimens treated at higher temperatures possessed similar tensile properties and did not show any significant variations with different annealing times. Optimization of the present rod manufacturing process, including cold swaging and interval annealing heat treatment, is discussed.

Effects of Low Temperature on Hydrogen-Assisted Crack Growth in Forged 304L Austenitic Stainless Steel

The objective of this study was to evaluate effects of low temperature on hydrogen-assisted crack propagation in forged 304L austenitic stainless steel. Fracture initiation toughness and crack-growth resistance curves were measured using fracture mechanics specimens that were thermally precharged with 140 wppm hydrogen and tested at 293 K or 223 K (20 °C or −50 °C). Fracture initiation toughness for hydrogen-precharged forgings decreased by at least 50 to 80 pct relative to non-charged forgings. With hydrogen, low-temperature fracture initiation toughness decreased by 35 to 50 pct relative to room-temperature toughness. Crack growth without hydrogen at both temperatures was microstructure-independent and indistinguishable from blunting, while with hydrogen microcracks formed by growth and coalescence of microvoids. Initiation of microvoids in the presence of hydrogen occurred where localized deformation bands intersected grain boundaries and other deformation bands. Low temperature additionally promoted fracture initiation at annealing twin boundaries in the presence of hydrogen, which competed with deformation band intersections and grain boundaries as sites of microvoid formation and fracture initiation. A common ingredient for fracture initiation was stress concentration that arose from the intersection of deformation bands with these microstructural obstacles. The localized deformation responsible for producing stress concentrations at obstacles was intensified by low temperature and hydrogen. Crack orientation and forging strength were found to have a minor effect on fracture initiation toughness of hydrogen-supersaturated 304L forgings.

Effect of stacking fault energy on mechanical properties and annealing behavior of brasses

The effect of stacking fault energy (SFE) on mechanical properties and annealing behavior of brasses was studied in Cu–20Zn alloy (SFE ∼18 mJ/m2) and Cu–20Zn–1.2Si alloy (SFE ∼9 mJ/m2) as well as Cu–20Zn–1.9Si alloy (SFE ∼6 mJ/m2) alloy. These brasses have been rolled at room temperature up to different thickness reductions. The significant improvement of strength in Cu–20Zn–1.9Si alloy is attributed to the formation of fine grains and high densities of dislocations and deformation twins by decreasing the SFE. Thermal stability is enhanced due to the reductions of dislocation mobility and grain boundaries migration during annealing by a decrease in SFE and addition of Si. High fractions of the {236} 〈385〉 Brass–R and (55; 30; 0) in Cu–20Zn–1.9Si alloy with lower SFE should be ascribed to the substantial Brass texture in deformed microstructures. Fine grains, deformation twins and abundant annealing twins were introduced into Cu–20Zn–1.9Si alloy by decreasing the SFE, resulting in superior strength–ductility combination.

Nucleation mechanisms of dynamic recrystallization for G3 alloy during hot compression

Dynamic recrystallization (DRX) mechanisms of a nickel-based corrosion-resistant alloy, G3, were investigated by hot compression tests with temperatures from 1050 to 1200 °C and strain rates from 0.1 to 5.0 s−1. Deformation microstructure was observed at the strain from 0.05 to 0.75 by electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). Work hardening rate curves were calculated to analyze the effect of deformation parameters on the nucleation process. Results indicate that strain-induced grain boundary migration is the principal mechanism of DRX. Large annealing twins promote nucleation by accumulating dislocations and fragmenting into cell blocks. Continuous dynamic recrystallization is also detected to be an effective supplement mechanism, especially at low temperature and high strain rate.

Tailoring the texture of IN738LC processed by selective laser melting (SLM) by specific scanning strategies

Selective laser melting (SLM) is an emerging technology of additive manufacturing, which is used to directly produce metallic parts from thin powder layers. This study aims at correlating laser scanning strategies with the resulting textures and corresponding anisotropy of the elastic behavior of bulk materials. Tensile test specimens made of the γ’-containing Ni-base superalloy IN738LC were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy-specimens). Their bulk mechanical properties were determined at room temperature and at 850°C. Specimens were investigated in the ‘as-built’ condition and after recrystallization heat treatment. SEM-based electron backscatter diffraction (EBSD) was applied to measure their crystallographic preferred orientations (texture) and to correlate the anisotropy of Young's modulus with the texture of the material. It is shown that the applied laser scanning strategies allow to tailor the crystallographic texture locally. The possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. A recrystallization heat treatment reduces the degree of crystallographic texture and thus the elastic anisotropy by abundant annealing twinning. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

Hot Ductility Characterization of Sanicro-28 Super-Austenitic Stainless Steel

The hot ductility behavior of a super-austenitic stainless steel has been studied using tensile testing method in the temperature range from 1073 K to 1373 K (800 °C to 1100 °C) under the strain rates of 0.1, 0.01, and 0.001 s−1. The hot compression tests were also performed at the same deformation condition to identify the activated restoration mechanisms. At lower temperatures [i.e., 1073 K and 1173 K (800 °C and 900 °C)], the serration of initial grain boundaries confirms the occurrence of dynamic recovery as the predominant restoration process. However, in the course of applied deformation, the initial microstructure is recrystallized at higher temperatures [i.e., 1273 K and 1373 K (1000 °C and 1100 °C)]. In this respect, annealing the twin boundaries could well stimulate the recrystallization kinetic through initiation new annealing twins on prior annealing twin boundaries. The hot tensile results show that there is a general trend of increasing ductility by temperature. However, two regions of ductility drop are recognized at 1273 K and 1373 K (1000°C)/0.1s−1 and (1100°C)/0.01s−1. The ductility variations at different conditions of temperature and strain rate are discussed in terms of simultaneous activation of grain boundary sliding and restoration processes. The observed ductility troughs are attributed to the occurrence of grain boundary sliding and the resulting R-type and W-type cracks. The occurrence of dynamic recrystallization is also considered as the main factor increasing the ductility at higher temperatures. The enhanced ductility is primarily originated from the post-uniform elongation behavior, which is directly associated with the strain rate sensitivity of the experimental material.

Evolution of the Annealing Twin Density during δ-Supersolvus Grain Growth in the Nickel-Based Superalloy Inconel™ 718

Grain growth experiments were performed on Inconel™ 718 to investigate the possible correlation of the annealing twin density with grain size and with annealing temperature. Those experiments were conducted at different temperatures in the δ supersolvus domain and under such conditions that only capillarity forces were involved in the grain boundary migration process. In the investigated range, there is a strong inverse correlation of the twin density with the average grain size. On the other hand, the twin density at a given average grain size is not sensitive to annealing temperature. Consistent with previous results for pure nickel, the twin density evolution in Inconel™ 718 is likely to be mainly controlled by the propagation of the pre-existing twins of the growing grains; i.e., the largest ones of the initial microstructure. Almost no new twin boundaries are created during the grain growth process itself. Therefore, the twin density at a given average grain size is mainly dependent on the twin density in the largest grains of the initial microstructure and independent of the temperature at which grains grow. Based on the observations, a mean field model is proposed to predict annealing twin density as a function of grain size during grain growth.

Twin boundary characters established during dynamic recrystallization in a nickel alloy

For the purpose of providing fundamentals for grain boundary engineering design for high-temperature alloys, twin boundary characters are studied quantitatively in a nickel alloy during DRX to improve the understanding of twin boundary evolution during hot deformation. The nickel alloy samples were hot compressed at temperatures of 1120–1180°C and strain rates of 0.1s−1 and 1s−1 to a fixed true strain of 1.1. Refined equiaxed grain structure is obtained with abundant twin boundaries. Most of the twin boundaries form inside the grains, being highly coherent and showing rather limited mutual interaction. An inversely proportional relationship is found between Σ3 twin boundary length density (BLDΣ3) and DRX grain size (DDRX). An analytical model is derived here, in good agreement with current experimental results, indicating that the stored strain energy difference across grain boundary is the main driving force for grain boundary migration and determines the annealing twin formation during DRX. Σ3 twin boundary length density (BLFΣ3) developed in DRX was found varying within a small range of 44%–48%, which is shown to result from the inverse proportionality between grain boundary length density (BLDGB) and DDRX as well as the inverse proportionality between BLDΣ3 and DDRX.

Recrystallization behavior of a high-manganese steel: Experiments and simulations

Abstract The thermal treatment of a 30% cold-rolled Fe–28Mn–0.28C Twinning-Induced Plasticity (TWIP) steel was investigated. The low degree of rolling served to exclude the effect of shear banding but to capture the characteristic texture evolution. A detailed microstructure and texture characterization was performed by means of SEM/EBSD, X-ray texture analysis, and hardness measurements. Specifically, the dislocation density distribution was computed using a Crystal Plasticity Finite Element Method (CP-FEM) framework and the primary recrystallization was modeled by means of a Cellular Automaton (CA) approach. The macrotexture of the recrystallized state was controlled by (i) retainment of the deformation texture components during nucleation due to oriented nucleation and (ii) the formation of new orientations by recrystallization twinning. The nucleation of new grains occurred heterogeneously at grain boundaries. The experimental results were used as input data for simulations. The results from the CP-FEM simulations in the form of orientation-resolved dislocation densities and the orientation and density of recrystallization nuclei extracted from SEM/EBSD measurements were directly transferred into a 3D CA for the simulation of primary recrystallization. The results substantiated that the consideration of realistic simulation scenarios in terms of microstructural inhomogeneity and annealing twin formation is essential for an accurate prediction of the recrystallization behavior of the investigated high-manganese steel.

Observation of annealing twin nucleation at triple lines in nickel during grain growth

Three-dimensional near-field high-energy X-ray diffraction microscopy has been used to observe the formation of new twinned grains in high purity Ni during annealing at 800°C. In the fully recrystallized microstructure annealed at 800°C, twinned grains form along triple lines. Both the grain boundary character and the grain boundary dihedral angles were measured before and after the twin formed. These measurements make it possible to show that although each new twinned grain increases the total grain boundary area, it reduces the total grain boundary energy.

Influence of Mo concentration on corrosion resistance to HF acid solution of Ni–Co–Cr–Mo alloys with and without Cu

The corrosion resistance of Ni–30Co–16Cr–6Fe–xMo and Ni–30Co–16Cr–6Fe–2Cu–xMo (x=7–15wt%) alloys was investigated by immersion tests in 5.2M HF solution at 100°C. The weight loss of the Ni–30Co–16Cr–6Fe–xMo alloys relative to Mo concentration after immersion followed a Boltzmann equation. With decreasing Mo concentration, the Ni–30Co–16Cr–6Fe–xMo alloys first lost the Mo protection of the grain boundaries, then the annealing twin boundaries, and finally the grain matrix. In contrast, decreasing Mo content did not cause a significant increase in Ni–30Co–16Cr–6Fe–2Cu–xMo alloy weight loss, but led to a slightly preferential dissolution of both grain and annealing twin boundaries.

Dynamic recrystallization behaviour at grain boundaries and triple junctions

Dynamic recrystallization (DRX) behaviour and nucleation mechanisms were investigated using copper and copper alloy bicrystals, tricrystals and polycrystals. New grains were preferentially formed along grain boundaries in the bicrystals. After grain-boundary migration and bulging, nuclei appeared behind the deeply bulged grain boundary regions. The critical strain for nucleation was about one-quarter to one-half of the peak strain. The characteristics of nucleation at a grain boundary depended sensitively on grain boundary character. In copper alloy bicrystals, nucleation was much delayed due to solute drag of migrating grain boundaries. The nucleation at triple junctions, in contrast, took place at a much lower strain. New grain formation at triple junction was stimulated by development of folds. All the new grains were twin-related (Σ3) to the matrix and were formed behind the migrating grain boundaries. Therefore, it was revealed that the DRX mechanism in copper and copper alloys was essentially controlled by annealing twin formation. Variant selection of the twinning plane depended sensitively on the direction of the grain-boundary migration and on the geometry, however, was not affected by activated slip plane or dislocation glide. The DRX nucleation mechanisms at grain boundaries and at triple junctions are discussed with respect to grainboundary migration and annealing twin formation.