Formation Of High-angle Boundaries Research Articles

Enhancement and underlying fatigue mechanisms of laser powder bed fusion additive-manufactured 316L stainless steel

In this study, the enhancement of additively manufactured (AM) 316L, by annealing, to the fully reversed tension-compression fatigue performance, in terms of fatigue life and fatigue damage, were investigated under two conditions: as-built (AB) and heat-treated (HT) at 900 °C. The underlying fatigue mechanisms were comprehensively characterised through intensive microstructural observations of cyclic-strained microstructures and fracture surfaces using laser confocal scanning microscopy (LCSM) and secondary electron imaging using scanning electron microscopy (SEM). The experimental results showed that the fatigue resistance of HT 316L was significantly enhanced by 100% as the fatigue limit was increased from 75 to 150 MPa for AB and HT 316L, respectively. The fatigue cracking mechanism in AB 316L is mainly related to two imperfections of the AM-induced microstructural components: residual stresses, which cause highly localised deformation, and dendritic cellular structures, which possess a weak link in their grain boundaries against crack propagation. Upon heat treatment at 900 °C, the residual stresses and dendritic structure were effectively reduced. Consequently, the fatigue life of AM 316L was significantly enhanced by promoting the formation of high-angle boundaries. More precisely, the cyclic deformation processes in fatigued HT 316L involve persistent slip bands and strain hardening.

Characterization of fragmented structure developed during necking of iron tensile specimen

The microstructure fragmentation during necking of the iron tensile specimen was investigated by the electron backscatter diffraction technique. The material under study was the commercially pure iron containing particles of manganese sulfides and oxides. The undeformed mean grain size was ≈54 μm. The aim of the research was the characterization of the structural state preceding ductile rupture. The deformation microstructure was examined on the longitudinal section of the specimen in the locations corresponding to various true strains from ≈1 up to 1.8, which allowed studying the structure evolution in a single specimen. Special attention was paid to the deformation-induced high-angle boundaries, which are known to be preferable sites of microcracks nucleation. It was shown that a considerable number of high-angle boundaries appear already at a strain of ≈1, and their fraction rises with further straining. Large-scale fragmentation peculiarities of different kinds associated with the formation of high angle boundaries were found and discussed in terms of the effect of the neighborhood of grains on their fragmentation. Besides, multiple shear microbands of submicron width inclined to the tensile direction by angles ≈35° and larger were observed in the longitudinal section, which intersect the previously formed deformation substructure. The boundaries of these microbands were shown to make significant contribution to the accumulation of high angle boundaries.

Bonding and microstructure evolution in electromagnetic pulse welding of hardenable Al alloys

Electromagnetic pulse welding (EMPW) is a promising solid-state joining process, offering fast and strong bonding with no heat affected zone. Despite the growing interest in this process, there is little understanding of the dynamic phenomena that lead to bonding and microstructural changes during EMPW of key engineering materials such as age-hardenable aluminium alloys. This study combines experiments with numerical modelling of plastic deformation to provide an insight to these phenomena in joining of a high-strength aluminium alloy in the T4 and T6 temper conditions. Initially, bonding criteria are postulated in view of the calculated plastic strain at the interface of the T4 sample. These criteria are then used for the prediction of the extent of bonded interfaces for different sets of materials and process parameters. The predictions are shown to be in quantitative agreement with the experimental results for the T6 sample. The corresponding microstructural studies show that bonding is associated with remarkable microstructural changes in the samples, including dissolution of precipitates, formation of high-angle boundaries, and recrystallisation, especially near the bonded interfaces. Moreover, the results of post-weld heat treatments and mechanical testing demonstrate that the impact-induced deformation in EMPW can also influence subsequent precipitations, hence result in improved properties of the entire sample, in a way not achievable by conventional age hardening treatments.

Stress relaxation induced spheroidization of the lamellar α phase in Ti-7333 alloy

In this paper, stress relaxation tests were performed on β-forged Ti-7333 alloy at the temperature of 750 °C. Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction (EBSD) were adopted to study the microstructure evolution and deformation mechanism. The characterization of microstructure indicates that the residual α lamellae after hot deformation could be broken down during stress relaxation so that a nearly homogeneous globular microstructure can be obtained. The stress exponent and TEM observation show that dislocation creep is the main deformation mechanism, leading to the formation of high angle boundaries across the α plates and the breaking up of α lamellae. Dynamic recovery is the main mechanism in the β phase during stress relaxation.

Deformation characteristics and microstructural evolution in friction stir welding of thick 5083 aluminum alloy

The microstructure development in the weld key-hole along the plate thickness was investigated through scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) techniques. A fundamental understanding of the contact condition at the tool/workpiece interface in a TriflatTM designed friction stir welding (FSW) tool is presented. The development of grain structure for “stop-action” weld reveals that the plasticized material undergoes complex interacting flows due to the presence of threads and flats. The localized high material velocities induce a significant increase in the strain rate within thread space and thereby tune the Zener-Hollomon parameter. An interesting analogy is drawn between the evolution of the secondary shear zone (SSZ) in a typical metal cutting and the steady state FSW processes. The presence of different morphology of the material filling the thread space is linked to the local variation of the contact state variables along weld thickness. It was found that the contact state is governed by the intrinsic interface characteristics and the mechanical property of the material. It was also shown that the presence of larger particles (> 0.5 μm) is responsible for the formation of high angle boundaries (HAGBs) and random recrystallization texture within the stir zone, reflecting random grain orientation due to particle-stimulated nucleation (PSN).

Influence of heat treatment on the formation of ultrafine-grained structure of Al–Li alloys processed by SPD

In this study, binary Al–2.3wt%Li alloy, ternary Al–2.2wt%Li–0.1wt%Zr alloy and quaternary Al–2.2wt%Li–0.1wt%Zr–1.2wt%Cu alloy in the solution treated condition and additionally in aging condition were severely plastically deformed by rolling with cyclic movement of rolls (RCMR) method to produce ultrafine grained structure. Scanning transmission electron microscopy (STEM), scanning electron microscopy with EBSD detector (SEM/EBSD) were used for microstructural characterization and hardness test for a preliminary assessment of mechanical properties. The results shows, that the combination of aging treatments with RCMR deformation can effectively increase the hardness of Al–Li alloys. Second particles hinders the annihilation of dislocations in Al matrix during deformation leading to an increase of dislocation density. Significant amount of nanometric second particles in refined structure to ultrafine scale especially in Al–2.2wt%Li–0.1wt%Zr–1.2wt%Cu alloy effectively prevents the formation of high angle boundaries.

Dynamic globularization and restoration mechanism of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy during isothermal compression

Dynamic globularization and restoration mechanism of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy during isothermal compression were investigated by employing a high-resolution electron backscatter diffraction technique (EBSD). Quantitative analysis was made in detail for further understanding the microstructure evolution. The results reveal that the dynamic globularization of primary α grains of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy is accomplished by the formation of high-angle boundaries (HABs) and the penetration of the β phase during isothermal compression, and an increase in deformation temperature leads to a more globular microstructure. The main restoration mechanism in the β phase of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy during isothermal compression is dynamic recovery (DRC) at a strain rate of 0.01s−1, while continuous dynamic recrystallization (CDRX) occurs as the strain rate increases to 1.0s−1/5.0s−1 and the α grains play an important role in recrystallization. The recrystallization in the β phase of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy during isothermal compression is promoted with the decreasing of deformation temperature and the increasing of strain rate. A strong 〈001〉 fiber texture develops where only DRC occurs and the deformation texture is weakened to a large extent after recrystallization of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy during isothermal compression.

Deformation mechanisms and dynamic recrystallization of AZ31 Mg alloy with different initial textures during hot tension

Deformation mechanisms and dynamic recrystallization (DRX) of AZ31 Mg alloy during tension at a temperature of 300°C and strain rate of 5×10−4s−1 were investigated by using three tension specimens with different initial textures. It is shown that initial texture has a great influence on the twinning behavior of the AZ31. A lot of {10−12} extension twins were observed in ND specimen, and only a small amount of {10−12} extension twins were observed in ND45 specimen, while no twin was observed in RD specimen. The initial texture also affects DRX. If the c-axis is parallel to the tensile axis (TA), as seen in ND specimen, extensive DRX process was observed, and a highest volume fraction of DRX together with a smallest grain size was formed after 45% tension strain before fracture. When the c-axis is inclining 45° or perpendicular to the TA, as seen in ND45 or RD specimen, DRX occurred at a slow rate resulting with a low volume fraction of DRX compared to ND specimen. In all samples, the texture of unrecrystallized grains with the orientation of the as-received ND45 specimen rotates to that of the as-received RD specimen, while the texture of unrecrystallized grains with the orientation of the as-received RD specimen evolves into a basal fiber texture with 〈10−10〉 parallel to the TA. It is believed that the effect of initial texture on DRX of ND specimen is caused by the activation of 〈c+a〉 slip since cross-slip and climb are easier to take place than for other slip systems during tension resulting in the formation of high angle boundaries. As a consequence, the grains, which deformed by slipping, not by {10−12} extension twinning, would not survive during tension due to DRX. The evolutions of microstructures and textures of the three tensile samples were discussed based on their deformation mechanisms and DRX behavior.

Microstructural Development of Fe-20mass%Cr Alloys and Pure Copper Processed by Equal-Channel Angular Pressing

Microstructural development of ultra low C, N, Fe-Cr alloy and pure copper processed by equal-channel angular pressing (ECAP) have been examined focusing on the initial stage of the formation of ultrafine grain structure. Fe-Cr alloys were pressed at 423 K while pure copper at room temperature for 1 to 3 passes via the route Bc to compare at the equivalent homologous temperature. Microstructural evolutions were characterized by electron backscatter diffraction (EBSD) image and transmission electron microscopy (TEM). It was found that deformation structures were mostly deformation-induced subboundaries in both the materials after one pass, but the fraction of high-angle grain boundary became higher in the Fe-Cr alloys than in pure copper in subsequent passes by increasing misorientation of the boundaries. The more enhanced formation of high angle boundaries in Fe-Cr alloys was discussed in terms of the nature of crystal slip of FCC and BCC structures.

Mechanisms of grain refinement in Mg–6Al–1Zn alloy during hot deformation

Abstract Mechanisms of grain refinement were investigated in an extruded Mg–6Al–1Zn alloy during hot compression at 623 K and a true strain rate of 3 × 10−3 s−1. With increasing strain, grain fragmentation gradually took place; the dominant grain refinement mechanism, however, depended on the strain. At strains up to ɛ = 0.1, mechanical twinning contributed notably to grain refinement and to the formation of high angle boundaries. At medium to high strains, continuous dynamic recrystallization took place. Kinking contributed to grain fragmentation at all strains. The latter two mechanisms also increased the fraction of low to medium angle boundaries.

Influence of texture on the recrystallization mechanisms in an AZ31 Mg sheet alloy at dynamic rates

Abstract An AZ31 rolled sheet alloy has been tested at dynamic strain rates ( e ˙ ∼ 10 3 s − 1 ) at 250 °C up to various intermediate strains before failure in order to investigate the predominant deformation and restoration mechanisms. In particular, tests have been carried out in compression along the rolling direction (RD), in tension along the RD and in compression along the normal direction (ND). It has been found that dynamic recrystallization (DRX) takes place despite the limited diffusion taking place under the high strain rates investigated. The DRX mechanisms and kinetics depend on the operative deformation mechanisms and thus vary for different loading modes (tension, compression) as well as for different relative orientations between the loading axis and the c-axes of the grains. In particular, DRX is enhanced by the operation of 〈c + a〉 slip, since cross-slip and climb take place more readily than for other slip systems, and thus the formation of high angle boundaries is easier. DRX is also clearly promoted by twinning.

Microstructure of Mo – 47% Re – 0.4% Zr alloys after rolling at room temperature. II. Peculiarities of mechanical twinning and formation of high-angle boundaries between microbands

It is shown that the Mo – 47% Re – 0.4% Zr alloy after rolling deformation (e ≈ 90%) at room temperature exhibits certain characteristic features, such as increased density of deformation defects with considerably faceted noncoherent boundaries and reorientation microbands with a specific range of high-angle disorientations around the direction of the 〈110〉 type. Using the concepts on direct-plus-reverse (along alternative systems) martensitic transformations as the mechanisms of plastic deformation and crystal-lattice reorientation, an analysis of possible mechanisms for formation of these states is made.

The effect of strain path reversal on high-angle boundary formation by grain subdivision in a model austenitic steel

The effects of large strain and strain path reversal on deformation microstructure development in austenite below recrystallization temperature were studied by hot torsion using an Fe-30 wt.% Ni model alloy. Results show that high-angle boundaries (HABs) can be generated by both dislocation accumulation and subgrain rotation. Multiple strain reversals lead to less well-developed HABs in the original grains compared to single reversal deformed to the same strain. This is attributed to subgrain rotation mechanism being less effective at small strains.

Effect of deformation and annealing temperatures on ultrafine microstructure development and yield strength of pearlitic steel through continuous recrystallization

Associated with the durability of railway wheels, microstructure development of high-carbon steel (0.67% C) during uniaxial compressive deformation and after subsequent annealing was studied using scanning electron microscopy and electron back scattering diffraction. The spheroidization of cementite is accelerated due to compressive deformation depending on the temperature. Annealing at 600 °C leads to the formation of ferrite grain boundaries due to continuous recrystallization and results in an ultrafine microstructure with a ferrite grain size of less than 1 μm. Formation of high angle boundaries due to the dislocation accumulation on low angle boundaries results in an equiaxial fine microstructure after deformation at 700 °C. Annealing at 600 °C or higher leads to the coarsening of spheroidized cementite, which reduces the pinning effect of the particles to the migration of high angle boundaries. The compressive yield stress increased approximately 25% through microstructure development, along with a decrease in the work hardening rate.

Microstructural evolution of equal-channel angular pressed interstitial-free steel

Abstract Equal-channel angular pressing (ECAP) belongs to advanced technologies for improving mechanical properties of materials. In the present work, the influence of the number of ECAP passes using route BC on both grain size and mechanical properties of interstitial-free steel was investigated by means of transmission electron microscopy, electron back-scattering diffraction, and microhardness testing. It was found that the grain size decreases with the increasing number of passes. At the same time, an increase in microhardness was observed. The evolution of microstructure with increasing strain imposed through ECAP, in particular the process of grain fragmentation and the formation of high-angle boundaries were also examined.

Texture and High-Angle Boundary Formation during Deformation Processing

Among the phenomena leading to formation of high-angle boundaries during deformation processing at low homologous temperatures is the subdivision of prior grains and formation of deformation bands. Evidence for this phenomenon during processing of AA2004, a superplastic aluminum alloy, is reviewed; fragmentation of deformation bands leads to equiaxed grains and high-angle boundaries that support superplasticity. In addition to subdivision, groups of grains undergo lattice rotation toward one or the other variant of the β orientation fibers during plane-strain deformation of pure aluminum by ambient temperature rolling. The formation of equiaxed grains from banded structures during simple shear by equal channel angular pressing (ECAP) is also considered.

Microstructural Refinement under High Plastic Strain Rates during Hydrostatic Extrusion

High strain rates have a similar influence to large deformations on the refinement of microstructure. In both cases, at large strains and high deformation rates, a strong tendency to form microbands is observed. It was found, that the width of the microbands is very sensitive to changes of the deformation parameters. It has been observed particularly, that in severely deformed materials, the width of the microbands is reduced to nanometric dimensions. Hydrostatic extrusion, which has been used in for the deformation of copper in the current work, strain rates exceeding 2 1 3.84 10 − ⋅ ε = × s were employed. In all the samples investigated, numerous microbands were found in the microstructure. The width of microbands varied from 20 to about 400 nm. Thus, the width of some of the microbands exhibited dimensions typical of nanometric materials. Additionally, a special feature was the appearance of large areas of subgrains with an average dimension of about 200 nm. These areas were identified as recrystallized dynamically, or post-dynamically. Large misorientations were found between the microbands and the surrounding “matrix’. Such misorientation facilitates the formation of high angle boundaries, which in turn contribute to the changes of microstructure and mechanical properties. The mechanism for the creation of high misorientation in the microband areas is probably different from that operating during the process of dynamic recrystalization. The results confirm the possibility of obtaining a nanometric structure at lower deformation, but at higher strain rates.

High angle boundary formation by grain subdivision in equal channel angular extrusion

Intensive work on transmission electron microscope (TEM) was carried out to clarify high angle boundary (HAB) formation during equal channel angular extrusion (ECAE). It was demonstrated that newly developed HABs are mainly formed by the occurrence of grain subdivision, and the occurrence of grain subdivision is strongly influenced by the original grain orientations and ECAE strain path.

Deformation bands and the formation of grain boundaries in a superplastic aluminum alloy

Superplastic aluminum alloys are often classified according to the mechanism of microstructural transformation during annealing after deformation processing. In Al-Cu-Zr materials, such as Supral 2004, the presence of fine (10 to 50 nm) second-phase particles retards dislocation rearrangement and the formation and migration of boundaries during either annealing or elevated temperature deformation after thermomechanical processing. This leads to predominance of recovery in the evolution of microstructure, although high-angle boundaries must still form in order to account for the superplastic response of such materials. The mechanisms of high-angle boundary formation in such circumstances have remained unclear. The term “continuous recrystallization” (CRX) has been used as a phenomenological description of recovery-dominated processes that take place uniformly throughout the microstructure and lead to the formation of fine grains with high-angle boundaries. Orientation imaging microscopy (OIM) methods have been employed to assess the as-processed microstructure of this alloy and its evolution during annealing at 450 °C, as well as during superplastic deformation at this temperature. Orientation images demonstrate the presence of deformation bands of alternating lattice orientations that corresponds to the symmetric variants of the brass, or B, texture component (〈112〉{110} in rolled material). During annealing, the high-angle grain boundaries (disorientation of 50 to 62.8 deg) develop from transition regions between such bands while the lower-angle boundaries (i.e., up to 20 deg) separate an evolving cell structure within the bands. Further OIM results show that the bands remain distinct features of the microstructure during either annealing alone or during deformation under superplastic conditions.

Texture, Grain Boundaries and Deformation of Superplastic Aluminum Alloys

Two distinct microstructural transformation processes have been observed to enable superplastic response in aluminum alloys, depending of composition and thermomechanical processing (TMP). Texture observations, including microtexture data acquired by newly developed electron backcsatter diffraction (EBSD) analysis methods, and mechanical property results are summarized in this work. These data reveal a clear dependence of superplastic response on the mechanism of microstructural transformation that leads to a superplastically enabled condition. Transformation by continuous recrystallization involves recovery-dominated changes that occur gradually and homogeneously throughout the deformation microstructure in the absence of high angle boundary formation and migration. Important features of microstructural evolution include deformation banding leading to grain boundaries that are interfaces between symmetric variants of the main texture component, and the presence of a fine, cellular structure within the bands. Both grain boundary sliding and dislocation creep operate in the superplastic regime and alloys such as Al-5%Ca-5%Zn and Supral 2004 respond in this manner. Alternatively, transformation via primary recrystallization processes involves the heterogeneous formation and growth of grains by the migration of high-angle grain boundaries from the deformation zones surrounding coarse precipitate particles. This leads to essentially random grain orientations and a predominance of high-angle boundaries in the microstructure. The mechanical behavior of these materials is governed by grain boundary sliding in the superplastic range and alloys such as 5083 and 7475 behave in this way. The implications of these results for aluminum alloy development and the design of deformation processing procedures for grain refinement and superplasticity will be discussed. Finally, we present a table describing the superplastic behavior of a number of aluminum alloys according to the two patterns mentioned above.