Misorientation Distribution Research Articles

Probabilistic Reconstruction of Austenite Microstructure from Electron Backscatter Diffraction Observations of Martensite.

The fine microstructure resulting from the martensitic transformation drives many of the desired mechanical properties for quench-and-temper steels. The scale of martensite features correlates with austenite grain size, such that finer austenite grains produce finer martensitic structures. Measuring prior austenite grain (PAG) size from observations of the hierarchical martensitic structure using etch-based techniques remains challenging, especially for fine-grained specimens with low C and P content. This has driven an interest in our field in reconstructing prior austenite microstructure from electron backscatter diffraction (EBSD) data collected on tempered martensite. However, reliably reconstructing the data is not straightforward because (1) up to 24 variants can theoretically be observed from a single PAG; (2) the shear-induced plasticity that arises from martensite formation produces significant scatter in the pole figure; (3) there is inherent noise in Hough-indexed EBSD data; (4) annealing twins exist within many PAGs, producing martensite variants of similar orientations when compared with the parent grains; and (5) variation in austenite orientation can occur across a single PAG. To the author's knowledge, the majority of reconstruction algorithms published to date utilize either point-to-point or flood-fill approaches, which can produce artifacts due to the relatively high probability of adjacent EBSD observations falling within any reasonable tolerance angle with 24 variants in a parent and four annealing twin orientations. Therefore, we propose a probabilistic austenite reconstruction technique based on a clustering algorithm known as graph cutting. We demonstrate its application to steels and a binary ferrous alloy, then validate the results against chemical etching and retained austenite orientations. The graph cutting technique utilizes the misorientation distribution of martensitic variants and the potential austenite orientations for each observation, as estimated via Bayesian inference of the experimental orientation relationship and its scatter. It is found that this technique produces accurate reconstructions with identification of annealing twins in the PAGs, even in the presence of a large fraction of poorly indexed data points.

Geometry-considered 3D pseudorandom grain-scale modelling for crystalline material miniature parts

Grain-scale modelling and simulation is of vital importance during the full lifecycle of crystalline material miniature parts, whose mechanical responses depend on the actual material microstructure, part geometry and their correlation. Unfortunately, current studies fail to comprehensively consider indispensable microstructure information, and lack the consideration regarding part geometry. Therefore, this paper proposes and implements a geometry-considered 3D pseudorandom grain-scale modelling method, which takes into account the necessary actual microstructure information, part geometry and their mutual relations to generate accurate and efficient microstructure-dependent crystal plasticity finite element (M−CPFE) models. Firstly, the 3D actual grain-scale microstructure was acquired efficiently by combining 2D actual microstructures. Then the geometry-considered grain-scale microstructure was created pseudorandomly based on the actual 3D grain size, orientation and misorientation distributions. On this basis, the M−CPFE model was constructed accurately. The proposed method has been proved to have higher accuracy and robustness than the Voronoi random method on macroscopic response prediction, microstructure generation and microstructure evolution prediction. Furthermore, the applicability in micro-manufacturing and service has been demonstrated by micro sheet bending and micro impeller rotation simulations. There is a great potential for the full lifecycle tracking of crystalline material miniature parts with the help of the proposed method.

Effect of pre-strain temperature on the subsequent dynamic recrystallization of parent β phase and transformed α-variant selection in a novel near α titanium alloy

The microstructure and micro-orientation evolution of a novel high-temperature titanium alloy were characterized during a two-step deformation process. In the first step, the specimens were deformed at temperatures of 800, 850, 900 and 950 °C, and subsequently deformed in the same β-phase region of 1050 °C. The specimens pre-strained at 800 °C and 950 °C were fully recrystallized during the β-phase deformation process. By contrast, the specimens pre-strained at 850 and 900 °C were not fully recrystallized during the β-phase deformation process. This indicates that the extent of dynamic recrystallization did not increased monotonically with the pre-strain temperature. The extent of dynamic recrystallization was related to the stored energy accumulated in the α and β-phase during the pre-strain deformation. The change in stored energy was based on the difference in microstructure morphology produced by different pre-deformation temperatures,and stored energy was quantitatively estimated based on the local misorientation distribution. Meanwhile, the generation of dynamically recrystallized grains was beneficial to decreasing the generation possibility of the macro-zone. This decrease was attributed to the refinement of β-grains and randomly distributed orientation, which was also related to the decrease in the typically distributed “two parallel wings” orientation between the α and β phase induced by uncrystallized deformation. Additionally, the two-step deformation process with the pre-strain at the (α + β) middle regime was an effective strategy to promote the dynamic recrystallization and decrease the generation probability of the macro-zone.

α/β phase transformation and dynamic recrystallization induced microstructure development in fine-grained Ti-6Al-4V friction stir weld

Fine-grained materials possess great industrial significance as they have exceptional strength, ductility, and superplastic formability. However, the poor thermal stability of these materials leads to an excessive grain growth and property deterioration during welding. In this regard, friction stir welding (FSW) is considered as a potential candidate for the welding of fine-grained materials as it is a solid-state welding process with a lower process temperature. In this work, the microstructure development in the stir zone of a fine-grained Ti-6Al-4V alloy during FSW was investigated. The microstructure, misorientation distribution and crystallography of the base metal and stir zone were characterized using the electron backscatter diffraction technique to explain the microstructural evolution. The microstructure of different layers of stir zone composed of either fine α + β lamellar structure or fine equiaxed α and β grains or the mixture of both. The combination of heating/cooling induced α/β phase transformations and plastic deformation induced continuous dynamic recrystallization (CDRX) was responsible for such microstructural developments in the stir zone. Homogeneous misorientation increase in low angle boundaries was found as the dominant mechanism of CDRX in the stir zone of fine-grained Ti-6Al-4V during FSW. The hardness measurement confirmed the absence any plausible softening though there was a little grain coarsening in the top layer of the stir zone. • Quenched and hot rolled fine-grained (1.21 μm) Ti-6Al-4V alloy was friction stir welded. • Stir zone composed of a mixture of α + β lamellar structure and equiaxed α and β grains. • Homogeneous misorientation increase in low angle boundaries was dominant mechanism of continuous dynamic recrystallization. • β➔α phase transformation was governed by the Burgers orientation relationship. • No softening in the stir zone though a little grain coarsening occurred.

Effect of Microstructure on Low-Temperature Impact Toughness of Multi-Pass Weld Metal of 1 GPa Class High Strength Steel

A 1 GPa class weld metal was prepared by multi-pass welding using a prototype flux-cored wire, and the correlation between microstructure and low-temperature impact toughness of the weld metal was investigated. The microstructure of the as-deposited columnar region and the equiaxed region reheated by subsequent welding passes was observed, and it was found that low-temperature transformation products such as bainite and martensite mainly developed in both regions. Since the prior austenite grains were coarse in the columnar region, the effective grain size of the final microstructure could be large; in this case, it was confirmed that the low-temperature impact toughness of the weld metal was very low. However, in the weld metal used in this study, acicular ferrite was formed inside the grains of the columnar region; the effective grain size and grain boundary misorientation distribution of the final microstructure were similar to those of the equiaxed region, and a good low-temperature impact toughness of the weld metal was obtained. Key words: High strength steel, Multi-pass weld metal, Acicular ferrite, Impact toughness

Study on the intrinsic factors determining impact toughness of TC21 alloy

Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy exhibits high specific strength and superior fracture toughness, making it attractive in the aerospace field. However, its fracture resistance under impact loading was not fully understood. Therefore, the impact toughness of TC21 alloy with various microstructures was studied using instrumented Charpy impact testing. Combining microstructure characterization, crystallographic orientation analysis, nanoindentation test, fractography observations with analysis of kernel average misorientation (KAM) distribution near crack path, the heterogeneity of deformation at small length scales, as well as crack initiation and propagation mechanisms were revealed. Analysis of load-displacement curves indicated that the crack initiation energy accounts for most of the total impact energy. The crack initiation energy is proportional to the degree of plastic deformation of the microstructures in the crack initiation region. The thick lamellar structure either in bimodal or fully lamellar structure can generate large plastic deformation, thus promote the crack initiation energy. In contrast, the KAM value in crack propagation region was much lower than that in crack initiation region, suggesting that impact energy consumed by plastic deformation during crack propagation was lower than during crack initiation. The crack propagation energy increased with the increasing degree of the tortuosity of propagation path. Especially in fully lamellar microstructure, the high angle α colony boundaries deflected the crack propagation direction and thus increased the crack propagation energy

The effect of SiC content in aluminum-based metal matrix composites on the microstructure and mechanical properties of welded joints

Aluminum metal matrix composites of the varying percentage of SiC reinforcement were welded using the tungsten inert gas welding technique. For all the conditions of the specimen, the welding parameters were kept identical. The welded composites were thermally aged to peak age-hardening conditions. Significant differences in microstructure exist between aged and non-age hardened composites in terms of misorientation, grain boundary fractions and stored energy. These differences in microstructure were seen through identical changes in the mechanical behavior of the welded composites. After aging, microstructure showed refinement in grain size and preferential orientation. Hardness and tensile strength increased at different rates with increase in SiC content indicating that hardness was a clear function of stored energy and strength had a dependence on the misorientation. EBSD measured suggests that due to age hardening, a rearrangement in dislocation substructures occurred with precipitation of Mg2Si or increasing SiC content followed by dynamic recovery of the weld metal region which leads to enhancement of hardness. The increase in tensile strength with decreased misorientation was a clear indication that the effect of annihilation of dislocations as a result of recovery was less pronounced and precipitation strengthening was dominant. The present study aims to address the role of SiC addition in aluminum metal matrix composites on the welded microstructure and finally relating the microstructural changes to the observed mechanical behavior of the welded joints. The study reveals that strength not only depends on the average value of misorientation but also on the distribution of misorientation in the microstructure.

Automated reconstruction of parent austenite phase based on the optimum orientation relationship

Characterization of the austenite phase at high temperatures is important for understanding the microstructural evolution during steel processing. The austenite phase structure can be reconstructed from the room-temperature microstructure employing the crystallographic orientation relationship between the parent and product phases. The actual orientation relationships in steels are often calculated on the basis of well known relations (e.g. Kurdjumov–Sachs), which may differ from the experimentally observed orientation relationships. This work introduces a new approach to improve the current state of the art in prior phase reconstruction. The proposed approach consists of two new algorithms that are sequentially applied on an electron backscatter diffraction (EBSD) measured data set of the product phase microstructure: (i) an automated identification of the optimum orientation relationship using the observed misorientation distribution of the entire EBSD scan and (ii) reconstruction of the parent phase microstructure using a random walk clustering technique. The latter identifies groups of closely related grains according to their angular deviation from the proposed orientation relationship. The results were validated by near in situ experimental observations of phase transformation in an Fe–Ni alloy whereby the experimentally measured parent phase structure could be compared point by point with the reconstructed counterpart.

Crystallographic texture influences on the thermal stability of nanocrystalline materials

The influence of crystallographic texture on the total amount of solute segregated at the grain boundaries (Xgb,tot) and consequently the weighted grain boundary energy (γw) of a nanocrystalline material is investigated. Toward this end, two different 3D nanostructures, one with random crystallographic texture and the other with {111} texture, were generated. The grain boundary misorientation distribution was obtained from both the 3D nanostructures and converted into an equivalent grain boundary energy distribution using the data from Olmstead et al. It was found that both Xgb,tot and γw are higher for the nanostructure bearing random texture vis-a-vis the {111} texture. This has implications in the development of thermally stable nanocrystalline materials. The effect of texture is also discussed through a case study on the Ni–W system by invoking the nanocrystalline stability criterion proposed by Kalidindi and Schuh.

Texture analysis and joint performance of laser-welded similar and dissimilar dual-phase and complex-phase ultra-high-strength steels

Microstructural analysis based on texture patterns and an evaluation of high-cycle fatigue performance of laser-welded similar and dissimilar ultra-high-strength dual-phase (DP) and complex-phase (CP) steel joints are presented. The microstructure, phase volume fraction, grain boundary orientation, and lattice misorientation distribution of the fusion zone (FZ), fine grain heat affected zone (FGHAZ), and inter-critical HAZ (ICHAZ) are characterized by electron back-scattered diffraction (EBSD). EBSD analysis shows that the FZ of the DP-DP joint consists of martensite and ferrite, whereas the FZs of the CP-CP and DP-CP joints are comprised of martensite, ferrite, and bainite. High-cycle fatigue performances are nearly identical for the similar and dissimilar joints.

Phase heritage during replacement reactions in Ti-bearing minerals

Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility. Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed phases can be inferred in ore deposits. An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following well-documented orientation relationship to the ilmenite [0001]ilmenite // rutile and < $$10{\bar{1}}0$$ > ilmenite // [001]rutile The anatase is a single crystal and shows a potential orientation relationship [0001]ilmenite = (0001)ilmenite // {211}anatase and < $$10{\bar{1}}0$$ > ilmenite // < $$0{\bar{1}}1$$ > anatase The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase production. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is important in dolomite nucleation. Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related to the ilmenite orientation by the following relationship: [0001]ilmenite // titanite and { $$10{\bar{1}}0$$ }ilmenite // (001)titanite The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite needle leading to the formation of substructure in the absence of deformation. Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from Productora tourmaline breccia complex shows misorientation distributions consistent with Rutile // cubic and Rutile // cubic Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to document the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common axis is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the aggregate is consistent with the above relationship replacing ilmenite with a { $$10{\bar{1}}2$$ } twin.

A multiscale experimentally-based finite element model to predict microstructure and damage evolution in martensitic steels

The objective of this work is to investigate the plastic deformation and associated microstructural evolution and damage in a martensitic steel at multiple length scales, using a combination of finite-element (FE) modelling and experimental measurements. A multiscale model is developed to predict damage evolution in the necked region of a uniaxial tensile test specimen. At the macroscale, a von Mises plasticity FE model in conjunction with a Gurson-Tvergaard-Needleman damage model is used to predict the global deformation and damage evolution. A physically-based crystal plasticity model, incorporating a damage variable is used to investigate the microscale plastic deformation behaviour and the changes in crystal orientation under large strains. The model predicts that slip bands form at the onset of plastic deformation and rotate to become almost parallel to the loading direction at large strain. In the necked region, the initially randomly orientated microstructure develops texture, brought about by inelastic deformation and lattice rotation towards the stable [011] orientation. The predicted crystal orientations and misorientation distribution are in good agreement with measurements obtained through electron backscatter diffraction in the centre of the necked region of the tensile test specimens. The experimental and modelling techniques developed in this work can be used to provide information on the evolution of plastic deformation and damage as well as the orientation-dependent crack initiation and microstructural evolution during large deformation of engineering materials.

Grain boundary character distributions in Si&lt;sub&gt;3&lt;/sub&gt;N&lt;sub&gt;4&lt;/sub&gt; ceramics

The structure of grain boundary has an essential influence on the physical and mechanical properties, especially the high-temperature mechanical properties of silicon nitride ceramics. In the present work, the five-parameter analysis method which is established based on electron backscatter diffraction (EBSD), and stereology and statistics are used to study the grain boundary character distributions in the two commercial silicon nitride ceramic ball samples. These two samples are both fabricated by hot isostatic pressing but made in China and abroad, respectively. The results indicate that the misorientation distributions of the two samples radically deviate from the random distribution at a rotation angle of 180°, showing that the frequency of the relevant grain boundaries is dramatically higher than that of the randomly distributed ones. These grain boundaries are mainly those possessing the misorientations of [0 1 –1 0]/180° and [–1 2 –1 0]/180°, corresponding to &lt;i&gt;Σ&lt;/i&gt;2 and &lt;i&gt;Σ&lt;/i&gt;3 boundaries, respectively. The grain boundary inter-connections (GBICs) of the &lt;i&gt;Σ&lt;/i&gt;2 boundaries are basically the same in the two samples, and they are dominated mainly by the {0 0 0 1}/{0 0 0 1} basal-to-basal inter-connection. However, the GBICs of &lt;i&gt;Σ&lt;/i&gt;3 boundaries are quite different in the two samples. they primarily the {–1 2 –1 0}/{–1 2 –1 0} prismatic-to-prismatic inter-connection for the &lt;i&gt;Σ&lt;/i&gt;3 boundaries in the sample made in China, but it is mainly the {1 0 –1 0}/{1 0 –1 0} prismatic-to-prismatic inter-connection for the &lt;i&gt;Σ&lt;/i&gt;3 boundary in the sample made abroad. Crystallographic analysis shows that the planar coincidence site density (PCSD) for the {–1 2 –1 0}/{–1 2 –1 0}, {0 0 0 1}/{0 0 0 1} and {1 0 –1 0}/{1 0 –1 0} GBIC are 2.45 /nm&lt;sup&gt;2&lt;/sup&gt;, 7.95 /nm&lt;sup&gt;2&lt;/sup&gt; and 9.10 /nm&lt;sup&gt;2&lt;/sup&gt;, respectively, implying that the degree of the structural ordering increases in turn. Further discussion emphasizes that the grain boundaries possessing {1 0 –1 0}/{1 0 –1 0} GBIC and {0 0 0 1}/{0 0 0 1} GBIC should be one type of special boundaries due to their high degrees of structural ordering.

Improved Predictability of Microstructure Evolution during Hot Deformation of Titanium Alloys.

Two different mesoscale models based on dislocation reactions are developed and applied to predict both the flow stress and the microstructure evolution during the hot deformation of titanium alloys. Three distinct populations of dislocations, named mobile, immobile, and wall dislocations, describe the microstructure, together with the crystal misorientation and the densities of boundaries. A simple model consisting of production and recovery terms for the evolution of dislocations is compared with a comprehensive model that describes the reactions between different type of dislocations. Constitutive equations connect the microstructure evolution with the flow stresses. Both models consider the formation of a high angle grain boundary by continuous dynamic recrystallization due to progressive lattice rotation. The wall dislocation density evolution is calculated as a result of the subgrain size and boundary misorientation distribution evolutions. The developed models are applied to two near-β titanium alloys, Ti-5553 and Ti-17, and validated for use in hot compression experiments. The differences in the predictability between the developed models are discussed for the flow stress, dislocation densities and microstructure evolutions. Only the comprehensive model can predict the different reactions and their contributions to the evolution of mobile and immobile dislocation densities. The comprehensive model also allows for correlating the elastic strain rate with the softening and hardening kinetics. Despite those differences, the selection of the model used has a small influence on the overall prediction of the subgrain size and the fraction of high angle grain boundaries.

Microstructure and properties of solid state joints of titanium sheets produced by ultrasonic welding

The results of the study on the microstructure and properties of solid state joints of titanium sheets obtained by ultrasonic welding (USW) with frequency 20 kHz and vibration amplitude 20 μm during 2 and 3 s under a 6 kN clamping force are presented. The thermo-mechanically affected zone is observed along the defect-free weld line (joint). It was found that the length and width of this zone increased with the welding time. In the thermo-mechanically affected zone, a significant grain growth takes place, a change in the texture is observed and maxima near 60° and 90° in the grain boundary misorientation distributions appear. The maximum lap shear failure load of the joints obtained by USW of 2 and 3 s durations are 1998±122 and 2506±226 N, respectively. There is a change in the failure mode from interface debonding after 2 s USW to nugget pull-out for the welding time of 3 s.

Microstructure and Texture Evolution in a Post-dynamic Recrystallized Titanium During Annealing, Monotonic and Cyclic Loading

The post-dynamic recrystallization behavior of ultrafine-grained (UFG: 0.44 μm) cp-Ti under annealing, room temperature (RT) monotonic and cyclic loading was investigated across a range of temperatures and deformation rates wherever appropriate. By characterizing the grain and boundary structures, it was confirmed that recrystallization and grain growth occurred due to annealing (≥ 600 °C) and R = − 1 fatigue at RT. There was a noticeable 30 deg aggregation in misorientation distribution, along with the increased grain size. However, the hypothetical correlation between 30 deg aggregation and Σ13a or the other characteristic coincidence site lattice boundaries was found to be weak. The fatigue-induced grain growth is particularly intriguing for two reasons. First, the large monotonic deformation with low strain rate cannot trigger grain growth. Second, fatigue sharpened the basal intensity around the ND and caused a weaker texture component close to TD (load axis along the LD, perpendicular to the TD–ND plane). By contrast, high-temperature annealing only strengthened the UFG processing induced basal pole but without affecting its location. Novel insights into this fatigue-induced texture evolution in UFG cp-Ti has been provided. The lattice rotation during fatigue can be attributed to the combined effect of activation of prismatic langle arangle slip parallel to LD, and basal langle arangle slip perpendicular to it. The theoretically calculated stress to activate dislocation slip by assuming a non-equilibrium grain boundary state lent support to the above assertion. Moreover, the TEM observation evidently showed the characteristics of dislocation cross-slip and multiple slip in the grain interior.Graphical

Interfacial microstructure evolution and formation process of the joints prepared by diffusion bonding on DD6 nickel-based single crystal superalloy

Nickel-based single crystal superalloy DD6 was diffusion bonded in vacuum at 1170 °C under 55 MPa for 2–6 h. Two processes, direct bonding and bonding with interlayer, were designed for comparison. A pure 4 μm nickel (Ni) foil was selected for the interlayer. The joint strength was measured via shear test, whereas the joint microstructure and element distribution were analyzed via scanning electron microscope, electron probe analyzer and transmission electron microscope. The results indicated that the joint direct bonded for 6 h achieved the best joint quality at approximately 353 MPa of shear strength. In this case, the joint morphology and microstructure were characterized by microvoids and carbide precipitation. In contrast, in the joints bonded with interlayer, both carbide precipitation and microvoids disappeared. When 4 μm nickel foil was used as an interlayer, the foil/DD6 interface became less distinct as the bonding time prolonged. When the bonding time reached 6 h, the γ′ phase crystals connected each other with the misorientation distribution of the joint less than 5°, and maximum shear strength of 727 MPa was obtained. The fracture morphology has a mixed ductile-brittle feature.

The residual stress in as-built Laser Powder Bed Fusion IN718 alloy as a consequence of the scanning strategy induced microstructure

The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS.

Large-scale phase-field study of anisotropic grain growth: Effects of misorientation-dependent grain boundary energy and mobility

Three-dimensional grain growth behaviors under anisotropic (misorientation-dependent) grain boundary energy and mobility are investigated via phase-field simulations. Based on a multi-phase-field model and parallel graphics-processing unit computing on a supercomputer, very large-scale simulations with more than three million grains are achieved, enabling reliable statistical evaluation of anisotropic grain growth. The anisotropic boundary properties are introduced by the classical Read-Shockley and sigmoidal models; the threshold misorientation angle, Δθh, included in these models is used as a quantity to determine the anisotropy strength of the system. Systematic simulations are performed for different Δθh values, through which the correlations between the anisotropy strength and grain growth characteristics such as grain size and misorientation distributions are examined. The obtained results show that anisotropic grain growth reaches the steady-state regime irrespective of the Δθh value. However, the kinetics and microstructural morphology during the steady-state growth are largely dependent on Δθh. Furthermore, by comparison with the simulated results, the applicability of analytical grain growth theories to anisotropic systems are tested. The tests reveal that the steady-state microstructure in anisotropic growth cannot be well captured by the existing theories, which is likely because the basic assumptions of the theories do not hold for anisotropic systems.

Microstructural and texture evolution during thermo-hydrogen processing of Ti6Al4V alloys produced by electron beam melting

The present study was conducted to reveal the effects of building angles and post heat-treatments (2-step Thermo-Hydrogen Processing (THP) and conventional annealing treatment) on the density, microstructure and texture of Ti6Al4V alloy parts produced by Electron Beam Melting (EBM). The results showed that regardless of the building angle; the density, microstructure and crystallographic texture (defined with respect to building angle) of the as-produced samples were identical; having Widmanstätten α structure and columnar β-grains which are parallel to building direction. The main texture component for the α phase was (101¯0)//building direction, and for β phase (001)//building or heat flow direction. The first step of THP, namely, the hydrogenation step, produced a needle-like microstructure and increased the local misorientations due to lattice distortion. On the other hand, after application of the second step of THP, dehydrogenation step, microstructure was refined, particularly α-grains that were larger than 10 μm and located at grain boundaries. Moreover, THP randomized the crystallographic texture since it involves β to α phase transformation, at which one β-grain can produce 12 distinct α-variants. The grain boundary misorientation distributions also changed in accordance with the microstructural changes during the 2-step THP. On the other hand, annealing coarsened the grain boundary and Widmanstätten α phases; moreover, it changed the texture so that the basal planes (0001) rotated 30° around the building direction.