Microtextured Regions Research Articles

A Machine Learning Approach for Segmentation and Characterization of Microtextured Regions in a Near-α Titanium Alloy

The development of automated segmentation and quantitative characterization of microtextured regions (MTRs) from the complex heterogeneous microstructures is urgently needed, since MTRs have been proven to be the critical issue that dominates the dwell-fatigue performance of aerospace components. In addition, MTRs in Ti alloys have similarities to microstructures encountered in other materials, including minerals and biomaterials. Meanwhile, machine learning (ML) offers new opportunities. This paper addresses segmentation and quantitative characterization of MTRs, where an ML approach, the Gaussian mixture models (GMMs) coupled with density-based spatial clustering of applications with noise (DBSCAN) clustering algorithms, was employed in order to process the orientation data acquired via EBSD in the Matlab environment. Pixels with orientation information acquired through electron backscatter diffraction (EBSD) are divided and colored into several “classes” (MTRs) within the defined c-axis misorientations (i.e., 25°, 20°, 15°, 10°, and 5°), the precision and efficacy of which are verified by the morphology and pole figure of the segmented MTR. An appropriate range of c-axis misorientations for MTR segmentation was derived, i.e., 15~20°. The contribution of this innovative technique is compared with previous studies. At the same time, the MTRs were statistically characterized in the global region.

Investigation of the influence of macrozones in titanium alloys on the propagation and scattering of ultrasound

The presence of macrozones (or micro-textured regions) in Ti-6Al-4V (Ti-64) was shown to be a potential cause to the onset of cold dwell fatigue which reduces fatigue life significantly. Past research has demonstrated the potential of using ultrasonic testing for macrozone characterization, with the variation of ultrasound attenuation, backscatter and velocity in the presence of macrozones. However, due to the complexity of the microstructure, some physical phenomena that were observed are still not well understood. In this study, we propose the use of finite-element polycrystalline models to provide us with a means to systematically study the wave-macrozone interaction. Through this investigation performed using two-dimensional models, we are able to identify important correlations between macrozone characteristics (size, shape and texture) and ultrasound responses (attenuation, backscatter and velocity). The observed behaviours are then validated experimentally, and we also highlight how this understanding can potentially aid with the characterization of macrozones in Ti-64 samples.

Experimental and crystal plasticity modelling study on the crack initiation in micro-texture regions of Ti-6Al-4V during high cycle fatigue tests

Micro-textured regions (MTRs) are the presence of large regions of α grains aggregates with strong local texture, which are detrimental to the fatigue performances in titanium alloys. This work proposes an EBSD map-image based crystal plasticity model to study the fatigue crack initiation in MTRs. Crystal plasticity simulations reveal that the crack initiation may be attributed to the significantly high plastic strain accumulated on prismatic a slip systems leading to significantly high dislocation pile-ups, and high stress concentrations on the cluster of hard grains leading to the stress redistribution and cleavage fracture of the specimen.

How macrozone size and morphology influence yield in titanium alloys investigated using fast Fourier transform-based crystal plasticity simulations

Titanium alloys exhibit complex microstructures containing heterogeneities at different lengthscales. Although the size of primary α grains is typically in the micrometer range, processing of α/β titanium alloys can introduce millimeter long clusters of α particles with the same or nearly the same crystallographic orientation. These microtextured regions (MTRs), also called macrozones, are known to have a detrimental influence on the alloy performance under cyclic loadings. Recent studies evidenced an effect of the degree of microtexture on the yield strength of polycrystalline aggregates. In the present work, numerical simulations of the macroscopic response of large 3D aggregates containing MTRs with different shapes and sizes were performed using a fast Fourier transforms-based crystal plasticity model. Both macroscale response and microscale stress and strain fields were found affected by the presence of MTRs and their features. A mechanistic understanding of the relation between the mechanical behavior at different scales is discussed to obtain insights into the, still poorly understood, effect of microtextured regions on tensile properties.

On the Localization of Plastic Strain in Microtextured Regions of Ti-6Al-4V

The equiaxed microstructure of Ti-6Al-4V contains nominally 10µm to 15µm α-phase grains. Depending on processing, these fine grains may aggregate into large, millimeter-scale regions of similar crystallographic orientation. These so-called microtextured regions are detrimental to quasi-static and fatigue properties. Their presence may facilitate the formation of long-range strain localization—bands of plastic strain that traverse grain boundaries and terminate at the boundaries of the microtextured region which compromises strength and ductility—and can cause early fatigue crack nucleation. Furthermore, the low angle boundaries within microtextured regions offer little resistance to crack growth, and corresponding increases in crack growth rates have been observed in these regions. Despite significant research into the effects of microtextured regions on macroscopic properties, there is still a lack of rigorous definition of what constitutes a microtextured region. To this end, we present a computational study in which deformation simulations are performed to study the effect of the intensity and character of microtexture on the localization of plastic strain. Mean orientation and intensity of microtexture are parameterized in a suite of simulations. Simulations indicate that strain localization is facilitated both by the presence of some amount of orientation spread within a microtextured region, and further by the intensity of microtexture in neighboring regions. Simulations are further discussed with respect to high resolution experimental measurements of intragrain strain in a microtextured Ti-6Al-4V specimen, which exhibit similar mechanical trends.

The formation of microtextured region during thermo-mechanical processing in a near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe

Electron backscatter diffraction (EBSD) and backscattered electron imaging (BSE) were used to study the origin of microtextured region and how thermo-mechanical processing affects the macro/microtexture in near-β titanium alloys. Multi-step heat treatments were conducted on an (α+β)-warm-compressed Ti-55511 bar. It includes two-step sub β-transus annealing (830 °C, 2 h, 750 °C, 2 h) and a sub β-transus aging (600 °C, 8 h). The results show that microtextured regions, consisted of similar-shaped αp+αs lamellae and αag platelets, formed after the final aging. By analyzing the α and β texture during each thermomechanical step, the microtextured region (macrozone) formation was related to the maintained Burgers Orientation Relationship between α/β. In addition, the variant selection mechanism also leads to microtexture development.

<i>In Situ</i> Observation on the Deformation Behavior of Primary α-Ti in a Textured Ti-6Al-4V

The tensile deformation process and dislocation behavior of primary α-Ti of Ti-6Al-4V were studied by the in-situ tensile test combined with EBSD (electron backscatter diffraction). The initiation, evolution and distribution of dislocation slips at different strains were discussed. The results showed that the microtexture of the material had a significant influence on slip behavior. Typically, basal and prismatic <a> slips initiated first, but the dominant slip type was related to the local texture characteristics. Sometimes, the basal and prismatic <a> slips could still initiate when their Schmid factors were relatively low, while the pyramidal slips usually need a higher Schmid factor to initiate. With the increase of strain, the second slip system inside one grain was activated to accommodate the plastic deformation. When the deformation was localized in a specific microtextured region, basal <a> slips were dominant, but eventually the crack initiated from the <c+a> slip bands inside the grain.

Influence of hydrogen on dwell-fatigue response of near-alpha titanium alloys

The prior studies have investigated the influence of internal hydrogen on dwell-fatigue behavior of near-α titanium alloys primarily in the lamellar microstructural condition. In the current study, the effects of internal hydrogen, in the range 10–230 ppm (by weight), on the dwell-fatigue behavior of Ti-6242Si alloy were investigated. The examined alloy had a bimodal microstructure comprising approximately 70 vol% primary α grains and 30 vol% transformed β regions. The dwell-fatigue life generally increased with increasing hydrogen content. The dwell-fatigue lives were longer by a factor of as high as 6 for high (≥150 ppm) hydrogen contents than for the low (<60 ppm) hydrogen contents, which is reported for a near-α titanium alloy for the first time. The crack-initiation site was essentially faceted for all the hydrogen levels examined in current study. The crystallographic orientations of fracture facets at the crack-initiation sites were similar for the low (<60 ppm) and high (>150 ppm) hydrogen contents. Specifically, these facets were inclined at ∼8 – 17° from the basal plane. Therefore, the longer dwell-fatigue lives observed for the alloys with hydrogen contents ≥150 ppm could not be explained on the basis of any differences in crystallography of the facets at crack-initiation sites. The longer dwell-fatigue lives for higher hydrogen contents can be explained within the framework of time-dependent load shedding from the soft microtextured regions (MTRs) to the hard MTRs if the local stress redistribution at the soft MTR/hard MTR boundary due to the hold at maximum load is reduced with increasing hydrogen content.

Modeling Strain Localization in Microtextured Regions in a Titanium Alloy: Ti–6Al–4V

Large and highly textured regions, referred to as macrozones or microtextured regions, with sizes up to several orders of magnitude larger than those of the individual grains, are found in dual-phase titanium alloys as a consequence of the manufacturing process route. These macrozones have been shown to play a critical role in the failure of titanium alloys, specifically being linked to crack initiation and propagation during cyclic loading. Modeling microstructures containing macrozones using continuum-level formulations to describe the elastic–plastic deformation at the grain scale, i.e., crystal plasticity, poses computational challenges due to the large size of the macrozones, which in turn prevents the use of modeling approaches to understand their deformation behavior. In this work, a crystal plasticity-based modeling approach is implemented to model macrozones in Ti–6Al–4V. Further, to overcome the large computational expense associated with modeling microstructures containing macrozones, a modeling strategy is introduced based on a crystal plasticity description for the macrozone with a reduced-order model for the surrounding aggregate combining anisotropic elasticity and J2 plasticity, based on crystal plasticity-based training data. This modeling approach provides a grain-level description of deformation within macrozones using elastic–plastic continuum simulations, which has often been overlooked. Finally, the reduced-order model is used to investigate the strain localization within the microstructure and the effect of varying the misorientation tolerance on the localization of plastic strain within the macrozones.

A 3D analysis of the onset of slip activity in relation to the degree of micro-texture in Ti–6Al–4V

The mechanical properties of titanium alloys result from their complex multi-scale microstructural features, including micron scale precipitates and millimeter scale microtextured regions (MTRs). While previous investigations have revealed that the presence of mm-scale MTRs can degrade mechanical properties, particularly fatigue, the accompanying strain localization processes that operate at the microscale within the α grains in MTRs are not well understood. The present work is a mechanistic investigation of MTRs using crystal plasticity simulations of mm3-scale experimentally captured and synthetically generated 3D microstructure datasets. The explicit modeling of both the α grains and MTRs in Ti–6Al–4V enables assessment of the effect of microtexture and local structure variations within the MTR on overall deformation behavior and the onset of plastic slip in MTRs. The presence of MTRs with a dominant [0001] orientation results in both stress and plastic strain hotspots during the early stages of straining. Crystal plasticity predictions are compared to previous digital image correlation studies on early strain localization. The influence of MTRs on the local stress and strain fields is discussed with regard to the monotonic tension, fatigue and dwell-fatigue behavior of titanium alloys.

The Onset of Slip Activity in Relation to the Degree of Micro-Texture in Ti-6Al-4V

The mechanical properties of titanium alloys result from their complex multi-scale microstructural features such as micron scale precipitates and millimeter scale microtextured regions (MTRs). Deformation processes that operate at the scale of the α grain are of critical importance to mechanical properties, especially to fatigue performance. However, previous investigations also highlighted that the mm-scale MTRs affect the mechanical properties of titanium alloys. Specifically, MTRs promote long-range strain localization due to the low intergranular misorientation within a MTR. Furthermore, the elastic anisotropy of the alpha phase and the non-random spatial distribution of crystallographic orientations within the MTRs produce complex mechanical effects. The present work is a mechanistic investigation of MTRs using crystal plasticity simulations of mm3-scale experimentally captured and synthetically generated 3D microstructure datasets. The explicit modeling of both the α grains and MTRs in a titanium alloy is used to determine the effect of the degree of microtexture on the deformation behavior and on the onset of plastic slip. The presence of MTRs with a dominant [0001] orientation results in both stress and plastic strain hotspots during the early stages of straining. The influence of MTRs on the local stress and strain fields are analyzed and discussed with regard to the monotonic tension, fatigue and dwell-fatigue behavior of titanium alloys.

On the crack initiation and heterogeneous deformation of Ti-6Al-4V during high cycle fatigue at high R ratios

In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses were performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near parallel and perpendicular to the loading direction were identified. High-resolution digital image correlation (HR-DIC) was performed in the MTRs to study grain-level strain localization. In the other specimen, DIC was performed on a larger area and crack initiation was observed in a random-textured region. To accompany the experiments, crystal plasticity finite element simulations were performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination was associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.

A Hierarchical Multiscale Modeling Investigation on the Behavior of Microtextured Regions in Ti-6242 α/β Processing

Ti-6242 is a near alpha titanium alloy, which has excellent high-temperature creep resistance and is widely used in jet engine compressors. This alloy is susceptible to creep fatigue failure under dwell loading below 473 K. The existence of microtextured regions (MTRs) contributes significantly to this fast crack propagation. Mechanical processing in the alpha + beta region has been employed to eliminate MTRs, but the efficiency depends significantly on the applied strain path. Previous investigations based on crystal plasticity finite element (CPFE) simulations have demonstrated the relationship between breakdown efficiency and loading direction. Therein, MTRs with regular geometry and pure initial orientation were used to isolate the effect of loading direction from initial microstructure. In this paper, the behavior of MTRs with realistic initial microstructure was investigated using a hierarchical multiscale modeling framework, and the microscale results were analyzed in detail to understand the behavior of MTRs under different loading conditions. It was shown that a hierarchical multiscale model with realistic initial microstructure at the microscale can reflect the influences from different strain paths, initial orientation distributions, and positions of the region simultaneously. The combined effect of initial orientation distribution and loading direction on the MTR breakdown efficiency is discussed in detail.

Modeling the evolution of microtextured regions during [formula omitted] processing using the crystal plasticity finite element method

Titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) is frequently used in the high-pressure compressor of aero engines. While exhibiting high strength at elevated temperatures, it is susceptible to dwell fatigue at temperatures below ∼473 K due in part to the presence of microtextured regions (MTRs), also known as macrozones. This work investigates the role of forging direction on the mesoscale mechanical response of MTRs. The crystal plasticity finite element (CPFE) method was used to simulate large strain compression of MTRs with different initial crystallographic and morphological orientation with respect to the axial direction of the extruded billet. These simulations included cases where the c-axis of neighboring MTRs were (i) both perpendicular, (ii) one at 45° and one perpendicular, and (iii) one parallel and one perpendicular, to the compression direction. The effectiveness of each processing direction on the breakdown of MTRs is inferred through the extent of lattice rotation and the development of internal misorientations within the MTRs. The calculations reveal that case (i) leads to the most effective MTR breakdown but the c-axis remains similarly aligned; the c-axis is more scattered in case (iii) but the extent is limited by the high critical resolved shear stress of the pyramidal slip systems. Under uniaxial compression, competitive slip system activity correlates with positive divergence of reorientation velocity field in Rodrigues’ space as well as efficient breakdown of MTR.

Influence of Microtexture on Early Plastic Slip Activity in Ti-6Al-4V Polycrystals

Microtextured regions are known to influence the fatigue performance of titanium alloys. Previous studies revealed that crack initiation, accounting for most of the fatigue life, is triggered by slip activity. The influence of microtextured regions on the early plastic slip activity was presently investigated by means of an in situ tensile test performed inside a scanning electron microscope on a bimodal Ti-6Al-4V polycrystalline specimen. A slip trace analysis was carried out in several regions with different crystallographic textures to highlight potentially different deformation behaviors. Significant stress heterogeneities were revealed through an early slip activation in microtextured regions with a predominant [0001] orientation. This point was shown to be related to a locally increased resolved shear stress. Consequences on behavior under cyclic loadings are finally discussed.

Mechanical study of crystalline orientation distribution in Ti-6Al-4V: An assessment of micro-texture induced load partitioning

Microtextured regions are known to have a detrimental effect on the fatigue performance of near α and α/β titanium alloys. Due to the elastic anisotropy of α titanium, stress heterogeneities are expected to result from such microstructural features. The present work is an assessment of their influence on the elastic stress field, using Fast Fourier Transform-based computations on a 2D extruded aggregate built from a real microstructure. The contribution of the microtextured regions was then isolated by comparing simulations obtained on a synthetic aggregate with similar grain morphology and texture but removed microtextured regions through a redistribution of crystalline orientations. The analysis of the computed fields revealed that the average stress experienced by α grains is only weakly affected by the neighborhood and mostly determined by the elastic anisotropy of α titanium. Nevertheless, complex mechanical interactions between microtextured regions were evidenced. Indeed, load partitioning and non-local influence on the stress field were found to result from the presence of microtextured regions. The implications on fatigue crack initiation are discussed in light of these results.

Microstructure-Informed Cloud Computing for Interoperability of Materials Databases and Computational Models: Microtextured Regions in Ti Alloys

With the fast global adoption of the Materials Genome Initiative (MGI), scientists and engineers are faced with the need to conduct sophisticated data analytics on large datasets to extract knowledge that can be used in modeling the behavior of materials. This raises a new problem for materials scientists: how to create and foster interoperability and share developed software tools and generated datasets. A microstructure-informed cloud-based platform (MiCloud™) has been developed that addresses this need, enabling users to easily access and insert microstructure informatics into computational tools that predict performance of engineering products by accounting for microstructural dependencies on manufacturing provenance. The platform extracts information from microstructure data by employing algorithms including signal processing, machine learning, pattern recognition, computer vision, predictive analytics, uncertainty quantification, and data visualization. The interoperability capabilities of MiCloud and its various web-based applications are demonstrated in this case study by analyzing Ti6AlV4 microstructure data via automatic identification of various features of interest and quantifying its characteristics that are used in extracting correlations and causations for the associated mechanical behavior (e.g., yield strength, cold-dwell debit, etc.). The data were recorded by two methods: (1) backscattered electron (BSE) imaging for extracting spatial and morphological information about alpha and beta phases and (2) electron backscatter diffraction (EBSD) for extracting spatial, crystallographic, and morphological information about microtextured regions (MTRs) of the alpha phase. Extracting reliable knowledge from generated information requires data analytics of a large amount of multiscale microstructure data which necessitates the development of efficient algorithms (and the associated software tools) for data recording, analysis, and visualization. The interoperability of these tools and superior effectiveness of the cloud computing approach are validated by featuring several examples of its use in alpha/beta titanium alloys and Ni-based superalloys, reflecting the anticipated computational cost and time savings via the use of web-based applications in implementations of microstructure-informed integrated computational materials engineering (ICME).

The HCF behavior and life variability of a Ti-6Al-4V alloy with transverse texture

In this study, the microtexture effect on HCF behavior of a hot-rolled Ti-64 plate was investigated. The fatigue life scatter was relatively small and the life variability exhibited a single distribution mode due to microstructural and textural effects. All samples exhibited the Stroth-type crack initiation and the number of pile-up locations between the initiation facet and surrounding propagation facets was related to the life variability. Cracks tended to initiate at the boundary of two microtextured regions (MTRs) located in sample surface area and this is likely caused by the preferred dislocation pile-up between “hard” and “soft” α grains at MTR boundary. During early facet propagation period, crack front was deflected when crossing MTR boundaries. There was no crack front deflection when striation propagation mode became dominant in the high ΔK regime. The closely orientated grains within MTRs provided an easy path for microcracks to propagate and caused the overall deterioration of fatigue properties.

A dataset for the development, verification, and validation of microstructure-sensitive process models for near-alpha titanium alloys

Near-alpha titanium alloys are used for moderate-temperature applications in the early stages of the compressor in gas turbine engines. The quasi-static and fatigue properties of these alloys depend heavily on microstructure due to the absence of hard second phases and inclusions which can nucleate voids or cracks. Moreover, these alloys are known to exhibit a significant reduction in fatigue life when subjected to high mean stress or upon the application of dwell-fatigue cycles. Previous analysis has elucidated the microstructural features that drive these properties; the most important features are the volume fraction, size, and shape of clusters of similarly oriented alpha particles or microtextured regions (MTRs). To date, there have been few efforts to elucidate in a quantitative fashion the evolution of MTRs during thermomechanical processing (TMP). To meet this need, we have performed hot-compression tests on Ti-6Al-2Sn-4Zr-2Mo-0.1Si billet material with high-aspect-ratio MTRs at 0°, 45°, and 90° to the direction of primary metal flow during manufacture (i.e., the billet axis), thoroughly characterized the initial and final microstructures, and quantified field variables via finite-element method (FEM) process simulations for each experiment. These data can be used for a variety of purposes including the development, verification, and validation of models for microstructure/texture/microtexture evolution and defect formation.

Incipient slip and long range plastic strain localization in microtextured Ti-6Al-4V titanium

High resolution scanning electron microscope digital image correlation (SEM DIC) was performed in situ during uniaxial loading on Ti-6Al-4V rolled titanium plate to determine the dependence of strain localization on microstructure and microtexture. Individual grains with preferred orientation for basal slip exhibited plastic localization along basal planes before macroscopic yielding. With additional strain, but still below macroscopic yielding, pyramidal and prismatic plastic activity was observed as slip bands transmitting across many grains and entire microtextured regions (MTRs). The localization of long range plastic strain occurred within MTRs that allowed for slip transmission across grains with low angle boundaries. The rolled titanium plate material having strong [0001] and[101¯0]texture components showed pyramidal and prismatic type slip extending across entire MTRs at strains well below macroscopic yielding. These strain localization processes occur much earlier in straining and over lengthscales much longer than observed with conventional slip offset imaging. The implications for properties are discussed.