Formation Of Low-angle Grain Boundaries Research Articles

Triple junction disclinations in severely deformed Cu–0.4%Mg alloys

Stress fields arising from triple junction disclinations (TJDs) play a significant role in the microstructure evolution during the plastic deformation of metals. The calculation of TJD strengths from grain orientation data was developed by Bollmann more than 50 years ago, but so far applied only to collections of a few grains. Developed here is a new methodology for calculating TJD strengths and the associated stress fields in large polycrystalline assemblies using experimental electron back-scattered diffraction (EBSD) maps. The methodology combines Bollmann’s approach with a representation of materials as cell complexes. It is computationally efficient and allows for obtaining the spatial distribution of TJD strengths from EBSD images containing thousands of grains. Analysed are the fraction, distribution, and strengths of TJDs within statistically representative microstructures of Cu–0.4%Mg alloy subjected to severe plastic deformation (SPD) by equal channel angular pressing. It is shown that the formation of low-angle grain boundaries (dislocation walls) during SPD leads to an increasing number of TJDs, whose spatial distribution is progressively more uniform and whose strength distribution remains nearly constant. This result suggests that the SPD reduces the internal stresses associated with disclinations in large regions of the material, as closely situated disclinations screen each other’s fields. Regions with high local stresses can be expected between sparsely distributed TJDs with the highest strengths. The average distance between such TJDs could be considered as a natural length scale in a material.

Room temperature creep mechanisms of Ti–6Al–4V ELI alloy with equiaxed microstructure under different applied stresses

Titanium alloys are often used to make deep-sea pressure hulls and the creep strain produced during service is nonnegligible. The variation of dislocation structures with room temperature creep stresses in equiaxed α grains of Ti–6Al–4V ELI alloy was examined using transmission electron microscope. The plastic deformation mechanism varied with the applied stress, mainly manifested by the change of dislocation patterns and the sequential activation of slip systems. At the stress below the creep threshold, even though almost no macroscopic creep was generated, a small number of immobile dislocations and dislocation networks occurred in the sample. The networks were composed of dislocations with different Burgers vectors, and part of them would evolve into low angle grain boundaries (LAGBs). At the stresses above the threshold, the density of dislocations in α grains increased with the stress level, and the basal <a>, pyramidal <a> and <c+a> slips were activated successively which also contributed to creep and participated in the formation of LAGBs. Prismatic <a> slip was always the predominant creep deformation mechanism for the studied stress range. The activation of difficult slip systems was attributed to the stress concentration and stress redistribution between soft and hard phases/grains. In general, the amount of LAGBs increased with the applied creep stress, but the LAGBs formed by dislocation networks decreased.

Role of prismaticdislocations in sub-zero temperature deformation of Mg-Gd-Y-Zr alloy

This work investigates the effect of activating and accumulating prismatic<a>dislocations on ductility of a Mg-7.37Gd-3.1Y-0.27Zr (GW83K) alloy at sub-zero temperatures. It is found that the prismatic <a> dislocations were more easily activated with decreasing temperature due to the lowering of CRSS prismatic<a>/CRSS basal<a> ratio in this alloy. Increasing the proportion of prismatic<a>dislocations shows limited effect on improving the ductility of GW83K alloy in the early-stage deformation at sub-zero temperatures, because the prismatic<a>dislocations cannot accommodate c-axis deformation. However, with the formation of low-angle grain boundaries rotating along [0001] axis induced by the accumulation of prismatic<a>dislocations, the GW83K alloy can perform a much better ductility in the late-stage deformation. Such a finding provides a new perspective on understanding the contribution of prismatic <a>dislocations to ductility in Mg alloys.

Improvement of severe plastic deformation realized by several passes rotary swaging in the microstructure and properties of Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy

The industrial applications of Mg alloys are greatly limited by their trade-off of high strength and ductility. In this work, conventional extrusion and rotary swaging (RS) were applied to Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca (wt.%) alloy to obtain high strength-ductility synergy Mg alloys. The microstructure evolution and mechanical properties of as-extruded and as-swaged alloys at different (2, 6 and 10) RS passes were investigated. The results show that large quantity of dislocations occur in twin lamellae and original dynamic recrystallized (DRXed) grains after RS. Those dislocations promote the formation of low-angle grain boundaries (LAGBs) and subgrains. As the RS process continue, these subgrains transform into fine grains with high-angle grain boundaries (HAGBs), which result in grain refinement. Compared to the as-extruded alloys, the strength of as-swaged alloys increases significantly. The edge region of the as-swaged Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy after 10 passes exhibits comprehensive excellent mechanical performance with a yield strength (YS) of 403 MPa, an ultimate tensile strength (UTS) of 427 MPa and a fracture elongation (FE) of 9.1%. The high strength of as-swaged alloys is the results of the combination of dislocation strengthening, fine grain strengthening and dispersion strengthening. The deformed grains deflected to ED and the tensile twins produced during RS enhance the basal texture and reduce the plasticity of the alloy.

Pulsed Magnetic Treatment of Cobalt for Enhanced Microstructures and Mechanical Properties

In this study, the effects of pulsed magnetic treatment (PMT) on the microstructure and performance of pure bulk cobalt (Co) were investigated to reveal the underlying influencing mechanism of the pulsed magnetic field on ferromagnetic materials. The hardness of the material could be increased by PMT, with a maximum increase of 3.16% when 1T was adopted. The imposition of the pulsed field promoted the transformation of Co from face-centered cubic (FCC) phase to hexagonal close-packed (HCP) phase, which obeys the Shoji–Nishiyama orientation rule, with a decrease of α-Co by 8.60% and an increase of ε-Co by 0.19%. PMT also caused multiplication of defects and the formation of low-angle grain boundaries within the bulk Co. PMT could be an effective way to modify materials to suit engineering applications.

Designing a low-alloyed Mg–Al–Sn–Ca alloy with high strength and extraordinary formability by regulating fine grains and unique texture

Since the strategies used for improving formability usually result in a degradation of strength, evading the formability-strength tradeoff is a severe challenge for magnesium (Mg) alloys. Here, we developed a fine-grained (∼3.1 μm) Mg-2.0Al-0.8Sn-0.5Ca (wt.%) alloy that owns a satisfactory compatibility of formability and strength (Index Erichsen (I.E.) value: ∼7.8 mm, yield strength: ∼189 MPa), by a subtly designed processing scheme, i.e., multidirectional rolling followed by annealing treatment. The large I.E. value arises from the unique ring-characterized weak texture, which promotes the activation of dislocation slips and the formation of low-angle grain boundaries during the stretch forming. The high strength can be related to the fine grain structure and a high dispersion of nano-sized particles. Such a high-performance dilute Mg alloy sheet provides reference for broadening the application of Mg alloys in industries.

Microstructure and Mechanical Properties of Spark Plasma Sintered and Severely Deformed AA7075 Alloy

In this paper, the microstructure and mechanical properties of AA7075 with a coarse-fine-grained laminated microstructure produced by spark plasma sintering (SPS) and the cyclic extrusion severe deformation (KOBO) technique were investigated. It was found that an inhomogeneous grain microstructure was formed from coarse and fine grains by the SPS process and then was transformed into a coarse-fine-grained laminated microstructure by means of KOBO extrusion at room temperature. The grain refinement during KOBO extrusion resulted in a fine grained laminated microstructure created due to the formation of low-angle grain boundaries (LAGBs), followed by dynamic recrystallization, leading to high-angle grain boundaries (HAGBs). The EBSD analysis results reveal the formation of a deformed and partially recrystallized ultrafine grain microstructure owing to the generation and development of shear bands during KOBO extrusion. The ultimate tensile strength (UTS) of the AA7075 alloy rose after SPS-KOBO severe deformation up to 422 MPa, with high strains of about 33%. The obtained results clearly show that the SPS-KOBO extrusion technique allows a bimodal laminated fine gradient grain microstructure to be obtained due to deformation and dynamic recrystallization, which result in both high strength and good ductility. The new heterogeneous AA7075 alloys obtained by the SPS-KOBO combined techniques demonstrate that microstructural heterogeneities can assist in overcoming the strength–ductility trade-off.

Surface mechanical attrition treatment of additively manufactured 316L stainless steel yields gradient nanostructure with superior strength and ductility

Surface severe plastic deformation (S2PD) of additively manufactured/three-dimensional (3D) printed metallic parts is gaining increased attention as a post-manufacturing operation to enhance the material performance in a wide variety of applications. Surface mechanical attrition treatment (SMAT) is an S2PD technique that can yield a nanostructured surface layer induced by compressive stresses and work hardening. In the present study, SMAT was performed on 316L (austenitic) stainless steel (SS) processed by selective laser melting (SLM), and the consequent effects on mechanical response were investigated. The underlying mechanisms of microstructural evolution leading to the formation of nanocrystalline grains resulting from SMAT in SLM 316L SS are elucidated. The interactions between twins and deformation bands act as potential sites for impeding the movement of dislocations, which in turn leads to the formation of stacking faults, twinning, and occasionally transform to a different crystal structure. Twin-twin and/or twin-deformation band intersections sub-divide the matrix grains into smaller cells or low-angle disoriented blocks, which result in the formation of low-angle grain boundaries and finally in nanocrystallization at the surface. The size of nanocrystalline grains increases progressively with depth from the surface to micrometer size grains in bulk. The gradient nanostructure in the additively manufactured alloy after SMAT imparts an unusual combination of strength and ductility that markedly exceeds that of conventional, bulk nanostructured, or even high-performance 316L SS (containing nanoscale deformation twins embedded in submicron-sized austenitic grains obtained by dynamic plastic deformation processes). Analytical models revealed that strengthening results from a combination of grain boundaries and dislocations. The results of the present investigation pave the way for engineering high-performance SS for a variety of engineering applications.

Microstructural evolution in the cross section of Ni-based superalloy induced by high power laser shock processing

Microstructural evolution in the cross-section of Ni-based superalloy GH202 treated by high power laser shock processing (LSP) were investigated. The microstructures on surfaces and cross-sections of alloy specimens were characterized by electron backscatter diffraction and transmission electron microscopy. The results showed that dislocation slip and twinning were generated simultaneously during plastic deformation induced by high power LSP, with many low-angle grain boundaries and twins produced in the surface. The dynamic recrystallization occurred at the spot overlap after LSP due to the crystal migration and subgrain rotation. The dislocation multiplication promoted dislocations to pile up and form dislocation stacking and tangling in the subsurface. The multisystem slippage occurred in the plastic deformation induced by LSP from 40 to 130 μm depth and the motion of dislocation lines in two or more slip systems formed dislocation arrays and subgrains. In areas over 200 μm depth from the surface, dislocation density significantly decreased and a small amount of subgrains and dislocation cells remained. Therefore, LSP had a positive effect on grain refinement, formation of low-angle grain boundaries, and mechanical property improvements.

Texture analyses and microstructural evolution in monolithic U-Mo nuclear fuel

This work describes the microstructural evolution of prototypical monolithic U-Mo fuel plates analyzed via scanning electron microscopy and electron backscattered diffraction (EBSD). The understanding of the microstructural and textural evolution of nuclear fuel from as-fabricated to post-irradiation is important in assessing changes in material properties during irradiation. In our work it was observed that the typical fabrication techniques applied in U-10Mo monolithic fuel plates develop features associated with a cold-rolled body-centered cubic (bcc) texture and development of α and γ fiber (parallel to the rolling and normal direction). After irradiation, a loss of the fabrication-induced preferred orientation was observed with an increased spread of grain-boundary misorientation as burnup increases. Grain subdivision was observed in the irradiated samples with the formation of submicron grains (200–500 nm). Evidence for polygonization as the mechanism leading to grain subdivision was detected. This has been observed for the first time for U-Mo monolithic fuel via EBSD and has been associated to formation of low-angle grain boundaries (<15 degrees) at the site of the submicron grains. Such analyses of the microstructural and textural evolution of fuel (from fabrication to after irradiation) have the potential to help develop and validate microstructural physics-based models and provide a key feedback loop to further understand the interplay between fabrication processes and fuel performance.

Dissimilar friction stir welding of AISI 430 ferritic and AISI 304L austenitic stainless steels

Dissimilar joints of AISI 430 ferritic and AISI 304L austenitic stainless steels were produced by friction stir welding process. A sound and defect-free joint was obtained at 1 mm tool offset towards the ferritic sample located in the advancing side, and at rotational and welding speeds of 560 rpm and 50 mm/min, respectively. The XRD measurements revealed the presence of approximately equal volume fractions of ferrite (51%) and austenite (49%) phases in the stir zone (SZ). The formation of low-angle grain boundaries through the occurrence of dynamic recovery along with the presence of shear texture components in both constituent phases of ferrite and austenite in the SZ approved the occurrence of continuous dynamic recrystallization throughout the evolved microstructure. Moreover, microstructural observations showed the formation of necklace structure through the microstructure of ferrite in the SZ. Taylor map approved the strain localization in the ferrite phase. Micro-hardness measurement indicated that the hardness value is increased in the SZ. The result of tensile test showed that fracture occurred from less ductile ferritic base metal.

Mechanistic investigation of a low-alloy Mg–Ca-based extrusion alloy with high strength–ductility synergy

High strength–ductility synergy is difficult to achieve in Mg alloys. Although high strength has been achieved through considerable alloying addition and low-temperature extrusion, these techniques result in low ductility (2%–5%). In this work, a novel low-alloy Mg–Ca-based alloy that overcomes this strength–ductility trade-off is designed. The alloy has an excellent tensile yield strength (∼425 MPa) and exhibits a reasonably high elongation capacity (∼11%). A microstructure examination reveals that a high density of submicron grains and nano-precipitates provides the alloy high strength, and the leaner alloy additions and higher extrusion temperatures initially improve ductility. As a result, the density of residual dislocations is reduced, and the formation of low-angle grain boundaries (LAGBs) is enhanced. With fewer residue dislocations, it becomes less probable for the newly activated mobile dislocations to be impeded and transformed into an immobile type during the subsequent tensile test. The LAGBs function as potential sites to emit new dislocations, thus enhancing the dislocation–multiplication capability. More importantly, they can induce evident sub-grain refinement hardening and guarantee that the alloy achieves high strength. The findings lead to a controllable Mg alloy design strategy that can simultaneously afford high strength and ductility.

Influence of homogenization on the microstructure, mechanical properties and texture of the twin-roll cast AA8006 sheet processed by accumulative roll-bonding

Unhomogenized and homogenized AA8006 (Al-Fe-Mn-Si) alloy sheets twin-roll cast to strip 8.5 mm thick, processed by accumulative roll-bonding (ARB) on 2 mm thick sheets up to 6 cycles were studied. The microstructure was characterized by means of scanning and transmission electron microscopy and electron backscattered diffraction (EBSD), mechanical properties were monitored by hardness measurements and tensile tests. The macroscopic texture was determined by X-ray diffraction. The initial texture of the unhomogenized sheet is rotated cubic {001}&lt;110&gt; combined with recrystallization R texture {011}&lt;211&gt;, while the homogenized sheet has a cubic texture {001}&lt;100&gt; with remaining rolling component C {112}&lt;111&gt;. One ARB cycle leads to the formation of low-angle grain boundaries (LAGB) in original coarser grains and to a low angle rotation of the subgrains bounded by LAGB. After the 3rd and 6th ARB cycle unhomogenized and homogenized sheets show a common rolling texture of cold-rolled aluminium. The thermal stability of sheets processed by 6 ARB cycles was tested by isochronal annealing for 30 min up to 450°C. The homogenized sheet starts to recrystallize at 250°C, while the fine grain structure of the unhomogenized sheet is thermally stable up to 400°C.

Texture Evolution in AA6082-T6 BFSW Welds: Optical Microscopy and EBSD Characterisation.

One of the difficulties with bobbin friction stir welding (BFSW) has been the visualisation of microstructure, particularly grain boundaries, and this is especially problematic for materials with fine grain structure, such as AA6082-T6 aluminium as here. Welds of this material were examined using optical microscopy (OM) and electron backscatter diffraction (EBSD). Results show that the grain structures that form depend on a complex set of factors. The motion of the pin and shoulder features transports material around the weld, which induces shear. The shear deformation around the pin is non-uniform with a thermal and strain gradient across the weld, and hence the dynamic recrystallisation (DRX) processes are also variable, giving a range of observed polycrystalline and grain boundary structures. Partial DRX was observed at both hourglass boundaries, and full DRX at mid-stirring zone. The grain boundary mapping showed the formation of low-angle grain boundaries (LAGBs) at regions of high shear as a consequence of thermomechanical nature of the process.

Effect of alloying Au on the microstructural, mechanical and electrical properties of Ag-based alloy wires

In this study, the microstructure and performance of different Ag-based conductive wires were investigated. Ag-based wires that contain 8, 15, 20, and 28 wt% Au were produced by multiple drawing and rapid annealing processes to substitute commercial gold wires in electronic packaging industries. The cross-sectional gain structures observed using focused ion beam showed the formation of a slender granular structure at the center, and the outer portion changed from equiaxed to elongated grains with the increase in Au content. High-resolution transmission electron microscopy showed a remarkable decrease in the twin thickness and stacking defect of 9R structures dissociated from incoherent twin boundaries. Electron backscatter diffraction analysis revealed that slender grains at the center showed strong crystallographic orientation, and the equiaxed grains near the surface were in the preferred orientation. The high Au solute concentration in Ag-based wires induced strong interaction on dislocations, thereby resulting in the formation of low-angle grain boundaries (LAGBs) and nano-twins. Ternary Ag-based alloy wires exhibited high strength and hardness but low conductivity and elongation with the increase in Au content because relatively the high LAGB density and Au solute atoms caused strengthening and electron scattering.

Formation of low-angle grain boundaries under different solidification conditions in the rejoined platforms of Ni-based single crystal superalloys

Abstract

Structural defects and sp2 localization in CVD diamond

Transmission electron microscopy and energy loss near edge spectroscopy were used to characterize microstructural defects in polycrystalline CVD diamond films. Of particular interest here is the change in sp2 content catalyzed by structural defects such as twin boundaries, random grain boundaries, and dislocations. Changes in bonding configuration from sp3 to sp2 are known to have significant impact on many properties of interest in CVD diamond. I have shown here that coherent Σ = 3 twin boundaries do not lead to sp2 formation, while dislocations and random grain boundaries do. Another feature of interest here is the fivefold twining structure, which is typically found in CVD diamond. It comprises four Σ = 3 coherent twin boundaries, but topological constraint for filling the entire space induces the formation of low-angle grain boundaries and multiple stacking faults in one of the twin domains. I have shown that such defects are indeed present and primarily localized in one domain. The strain associated with these defects increases the localized sp2 content in the diamond.

Crystal orientation and morphology of α lamellae in wrought titanium alloys: On the role of microstructure evolution in β processing

To obtain high-quality aviation forgings of titanium alloys, β forging is an essential processing step which must be considered throughout a production process. In this work, the effect of β forging on the crystal orientation and morphology of lamellar α was experimentally investigated in a two-phase titanium alloy. Strong dynamic recovery during β working resulted in the formation of low-angle grain boundary (LAGBβ) inside β grains. The lamellar α can penetrate through the LAGBβ, leading to similar intra α LAGBs on subgrain boundaries. Deformation banding occurs at high strain rates, and both diffusive and sharp boundaries of deformation bands can be observed. A continuous change of the β orientation in diffusive boundaries results in the formation of fine and disordered α lamellae without intra-lamellar boundary to hold the Burgers orientation relationship (OR). On sharp boundaries, it is prone to producing continuous grain boundary α (αGB) with a highly similar orientation along the boundaries. Meanwhile, there may exist several lower-angle boundaries within the grain boundary α for a smoother orientation change on the β grain boundary.

Restructuring in high burnup UO2 studied using modern electron microscopy

Modern electron microscopy techniques were used to conduct a thorough study of an irradiated urania fuel pellet microstructure to attempt at an understanding of high burnup structure formation in this material. The fuel was irradiated at low power to high burnups in a light water reactor, proving ideal for this purpose. Examination of grain size and orientation with strict spatial selectivity across the fuel pellet radius allowed for capturing the progression of the restructuring process, from its onset to full completion. Based on this information, the polygonization mechanism was shown to be responsible for restructuring, involving formation of low-angle grain boundaries with their initiation occurring at the original high-angle grain boundaries of the as-fabricated pellet and at the gas bubble-matrix interfaces. The low-angle character of boundaries between the subdivided grains disappeared in the fully developed high burnup structure, likely due to creep deformation in the pellet.

On the role of twinning and stacking faults on the crystal plasticity and grain refinement in magnesium alloys

We have investigated the detailed microstructural mechanisms associated with the excellent crystal plasticity and ultra-fine grain refinement observed under high strain-rate deformation of Mg alloys, focusing on ZK60. Firstly, we have identified the clear formation of stacking faults in deformation-induced twinned crystal segments. Specifically, we have found that intrinsic I1 and I2 stacking faults bounded by 16<2¯ 023> and 13<101¯ 0> partial dislocations, respectively, were found to occur in very high number densities within the twins. This was due to the high Schmid factor for stacking fault shearing in twins and the critical role that twin boundaries played in emitting partial dislocations. Secondly, we have clarified the interplay between twinning and stacking faults on the enhanced crystal plasticity. Apart from the strain accommodated by the extensive twinning itself, we propose that the improved plasticity during high strain-rate deformation is mainly due to the nucleation of 13<11¯ 23>{112¯ 2} dislocation within twins, which provides enough independent slip systems to achieve a homogeneous deformation in the material. Finally, we have demonstrated the interplay between twinning and stacking fault formation on the nucleation of new grains via dynamic recrystallisation. The twin boundaries and stacking faults, especially those of the I1 type, facilitate the formation of low-angle grain boundaries that can subsequently transition into high-angle grain boundaries, and form ultra-fine dynamically recrystallised grains.