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International audience; Determination of strain path across martensitic transformation in materials with two transformation orientation relationships (ORs) is challenging due to the temporal and spatial limitation of the modern techniques. In this work, an analyzing strategy, i.e., the determination of transformation path via the variant organization feature of martensite, was suggested and applied for the Ni-Mn-based alloys as an example, based on the consideration that the different crystallographic symmetries of transformation systems will result in distinct organizations of martensite variants. For the selected Ni-Mn-based alloys, the orientation examination revealed that both the K−S and the Pitsch OR are respected. Further analyses in terms of the strain and stress compatibility condition and the minimum energy criteria showed that, theoretically, the K−S path produces 2 variants as a self-accommodated variant group, whereas the Pitsch path produces 4 variants as a self-accommodated variant group. Compared with the experimental results, the Pitsch path is the energetically favorable transformation path and actually occurs in stress-free austenite of the Ni−Mn based alloys. The significance of this work is multi-fold. It first resolves the long puzzling issue of transformation strain path of Ni-Mn-based alloys. Moreover, the analyzing scheme can be generalized to determine the transformation characters for martensitic transformation in other systems with multiple possible transformation orientation relationships.

Polycrystalline alloys are used pervasively across structural applications contingent upon extensive experimental testing. A statistically representative number of tests are necessary to expose the variability in the material's performance, as a result of non-uniform microstructures and associated micromechanical fields. In a more direct means of capturing this pertinent information, multi-modal experimental techniques are presented to measure and track the complete micromechanical state, evolving during loading, of each and every grain within the regions of interest. Specifically, a combination of high-energy X-ray diffraction microscopy and digital image correlation coupled with electron backscatter diffraction are conducted on a specimen for each of the alloys, Haynes 282 and Ti7Al. The results of the multi-modal analysis definitively demonstrate that the degree of heterogeneity increases with deformation level and is used to assess the number of grains necessary for a representative volume element description of the stress state for each of these materials. Moreover, higher resolution imaging is used for identification of the slip system activity and subsequently used to study slip transmission events. An accurate knowledge of the resolved shear stress in adjacent grains (grain interactions) is demonstrated to be a key descriptor of the slip transmission events.

High angular resolution electron backscatter diffraction (HR-EBSD) was coupled with focused ion beam (FIB) slicing to characterize the shape of the plastic zone in terms of geometrically necessary dislocations (GNDs) in W single crystal in 3 dimensions. Cantilevers of similar size with a notch were fabricated by FIB and were deformed inside a scanning electron microscope at different temperatures (21$^{\circ}$C, 100$^{\circ}$C and 200$^{\circ}$C) just above the micro-scale brittle-to-ductile transition (BDT). J-integral testing was performed to analyse crack growth and determine the fracture toughness. At all three temperatures the plastic zone was found to be larger close to the free surface than inside the specimen, similar to macro-scale tension tests. However, at higher temperature, the 3D shape of the plastic zone changes from being localized in front of the crack tip to a butterfly-like distribution, shielding more efficiently the crack tip and inhibiting crack propagation. A comparison was made between two identically deformed samples, which were FIB-sliced from two different directions, to evaluate the reliability of the GND density estimation by HR-EBSD. The analysis of the distribution of the Nye tensor components was used to differentiate between the types of GNDs nucleated in the sample. The role of different types of dislocations in the plastic zone is discussed and we confirm earlier findings that the micro-scale BDT of W is mainly controlled by the nucleation of screw dislocations in front of the crack tip., Comment: accepted (Acta Materialia)

Several theoretical studies have reported that the geometry and structure of grain boundaries in polycrystalline materials could impose a significant effect on the Hall-Petch slope. However, experimental observations are primarily limited by the ability of the techniques to accurately quantify the grain boundary strength and validate these theoretical models. Using high-resolution electron backscatter diffraction (HR-EBSD), the local stress tensor ahead of a slip band blocked by a grain boundary was quantified and coupled with a continuum dislocation pile-up model to assess the barrier strength of specific grain boundaries to specific slip systems, referred to as micro-Hall-Petch coefficient. For basal slip system in a deformed Mg-4Al alloy, the micro-Hall-Petch coefficient ( k μ b a s a l ) varied significantly, from 0.054 to 0.184 MPa − m 1 / 2 for nine different grain boundaries. These results were correlated with geometric descriptors of the respective grain boundaries, with three-dimensional GB profile additionally measured via focused ion beam milling. It was found that the angle between the two slip plane traces on the grain boundary plane was the most sensitive parameter affecting k μ b a s a l , followed by the angle between the slip directions. A functional form for calculation of k μ b a s a l depending on these two angles is proposed to augment crystal plasticity constitutive models with slip resistance dependent on some measure of the grain size. The method allows a new pathway to calibrate grain size strengthening parameters in crystal plasticity models, allowing further computational investigations of the interrelationship between texture, grain morphology, and the Hall Petch effect.

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined analytical approach based on scanning tunneling spectroscopy (STS), scanning electron microscopy, and electron back scatter diffraction (EBSD), where we perform these experiments on identical surface areas as small as 2 × 2 µm2 with a lateral resolution well below 50 nm. The topography of the absorber surfaces indicates two main morphological features: micro-faceted, long basalt-like columns and their short nano-faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typical for the presence of pronounced oxidation-induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano-faceted terminations of the basalt-like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt-like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non-polar planes. Ab-initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O-induced defects, in comparison to non-polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p-n junction formation in chalcopyrite thin-film solar cells.

Increasing magnetically favorable //ND texture components is a key challenge in the preparation of high-efficiency non-oriented electrical steels. In this study, an Fe-1.3 wt% Si steel with strong Cube ({100} ) and Goss ({110} ) texture was successfully produced by novel twin-roll strip casting, cold rolling and annealing process. The microstructure and texture of the material was characterized by optical microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The origin and formation mechanism of texture are described from the perspective of deformation and recrystallization behavior of specifically oriented grains. It was observed that initial Cube rotated toward {013} -{110} besides rotation toward {001} -{001} during cold rolling. In addition, new Cube deformation bands were developed in the deformed {115} -{115} grains. Cube components were partly retained as large block, small band structure and crystallite after heavy cold rolling. The Cube deformation structures served as nucleation sites of new Cube grains. The shear band within {114} , {112} and {111} matrix also provided some Cube nuclei. Morphology change from near bar-shaped to equiaxed occurred during the growth of Cube and Goss grains. The formation of recrystallization texture is attributed to the oriented nucleation mechanism, and the orientation pinning and size effects that impacted the intensity of texture component. The low thickness of strip and coarse solidification microstructure with strong {100} texture are the decisive factors to obtain strong Cube and Goss texture in strip-cast non-oriented electrical steel.

To further understand the internal residual stress that is microscopically generated via martensitic transformation in steels, the origin of internal strain attributed to Bain correspondence between face-centered cubic (fcc) and body-centered cubic (bcc) was evaluated from macro- and micro-viewpoints and its effect on hardness was investigated in an interstitial free Fe-16%Ni martensite. Neutron diffractometry and electron backscatter diffraction analysis showed that body-centered cubic (bcc) crystal structure of as-quenched martensite contained elastic distortions leading to small tetragonality, even in martensitic steels without solute carbon, and that the extended [001]bcc of martensite tended to be parallel to fcc of prior fcc austenite. In addition, the combination of micro-scale focused ion beam (FIB) and high-precision digital image correlation techniques revealed that a micropillar fabricated by FIB processing within a martensite block was anisotropically deformed by the release of the residual strain distributed in as-quenched martensite in correspondence with the tetragonal distortions of the bcc crystal structure. These results prove that a small part of the Bain strain remained as an internal elastic residual strain and was microscopically distributed among Bain groups in lath martensite after martensitic transformation. Furthermore, the residual strain generated a hydrostatic internal stress, and therefore, the nanohardness decreased considerably by the micropillar fabrication accompanied by the release of the internal stress. This means that the internal residual stress in martensite among Bain groups influences the mechanical properties of martensitic steel.

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependant on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependant crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependant on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Measurements of the local tetragonality in Fe-C martensite at microstructural length-scale through pattern matching of electron backscatter diffraction patterns (EBSPs) and careful calibration of detector geometry are presented. It is found that the local tetragonality varies within the complex microstructure by several per cent at largest and that the scatter in the axial ratio is increased at higher nominal carbon content. At some analysis points the local crystal structure can be regarded as lower symmetry than simple body centred tetragonal. A linear relation between the nominal carbon content and averaged local tetragonality measured by EBSD is also obtained, although the averaged axial ratio is slightly below that obtained from more classical X-ray diffraction measurements., 33 pages, 12 figures, 2 Tables

In the present study, we combine comprehensive site-specific microstructural and mechanical characterization studies to evaluate the toughening effect induced by applying a surface SPEX milling (SSM) approach to commercially-pure Mg. Our results show that the high frequency, multi-directional deformation strain induced by SSM generated gradients in the grain size and orientation with depth from the surface. More importantly, it also resulted in a gradient in the density of twin meshes, which are defined as two or more intersecting arrays of twins, along the sample thickness direction. Tensile tests at room temperature indicate that after SSM, the samples have higher ultimate tensile strengths and two-fold increases in ductility as compared to those of the untreated samples. The effect of the initial texture and SSM parameters on the microstructural evolution and texture randomization were analyzed by electron backscattered diffraction (EBSD). Furthermore, in order to correlate the variation in microstructure with the site-specific mechanical response, TEM and in-situ micropillar compression testing in SEM were performed at various distances from the SSM-treated surface. The results show that twin meshes may be responsible for the activation of more slip systems and higher strain hardening, which result in higher uniform plastic strain.

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+ x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

FeAl-based Metallic-Intermetallic Laminate (MIL) composites exhibit enhanced strength and ductility compared to previously studied MIL composites. The deformation and fracture evolution of the FeAl-based MIL composites are investigated here via incremental compression testing. Microstructure assessment via electron backscatter diffraction suggests that deformation proceeds in a fairly homogeneous manner across gradients in the microstructure. Eventual failure is mainly induced by normal stresses, whereas other MIL composites typically fail by shear induced localizations. Geometrically necessary dislocation analysis indicates the FeAl regions deform in similar manners for the three MIL composites (Fe-FeAl-MIL, 430SS-FeAl-MIL, and 304SS-FeAl-MIL), and each fails in a similar mode. While the FeAl phase is the majority constituent of the composites, the mechanical properties are significantly influenced by the softer metal layers. The transition layer formed between the Fe-based metal layers and FeAl regions is the most critical constituent of the composites. Although the volume fraction of the transition layer is only ∼15%, a stronger transition layer can improve the work hardening behavior of the FeAl phase, increasing MIL composite strength by as much as 1 GPa. The findings can guide the design of the MIL composites to achieve even better mechanical properties.

We introduce and adopt the concept of a phenomenological parameter, microstructure entropy, S*, to understand the yield strength in alloy systems with bimodal grain size distribution obtained from large sets of experimental data. 60 groups of bimodal grain size distribution data was obtained through the application of an ingenious concept involving severe cold deformation to obtain dislocation cell-type of martensite, followed by annealing when martensite reverts to austenite by shear or diffusion mechanism, with the aim to obtain ultrafine grain size. Six factors emerged from statistical data analysis through EBSD studies in terms of grain size and grain numbers from 60 sets of data. Microstructure entropy (S*) was obtained from the bimodal structure data in a self-similar regime. Inverse of the square root of microstructure entropy ( 1 S * ) and yield strength exhibited a linear relationship. The proposed conceptual methodology has wide acceptance for any grain size distribution to obtain microstructure entropy (S*) of a specific grain structure and predict yield strength. A generic equation similar to Hall-Petch relationship, but involving microstructure entropy is proposed to predict and understand yield strength of metallic systems characterized by grain size distribution or microstructural evolution.

The effect of heat-treatment and build orientation on martensitic phase transformation in additively manufactured (AM) 304L stainless steel is studied and compared with conventionally produced wrought material. The relationships between observed martensitic transformations and material microstructures and their effects on mechanical strength are established through experimental observations. In situ high-energy X-ray powder diffraction measurements were performed to monitor the evolution of ϵ and α′ martensite during compressive loading of stainless steel. Electron backscatter diffraction (EBSD) was used to provide insight on initial grain morphology, crystallographic misorientation within grains, and crystallographic texture. Heat treatment alters the microstructure of AM samples creating different initial conditions. This difference in starting microstructure resulted in variability in martensitic transformation during compressive deformation. The rate of martensitic transformation decreased for AM samples treated with temperatures up to 1100∘C, after which the AM microstructures recrystallized, resulting in increased rate of martensitic transformation for those samples treated at higher temperatures. It was also observed that aligning the axis of compression with the AM build direction resulted in a lower rate of strain-induced martensite formation as opposed to aligning the compression axis perpendicular to it. More favorable distribution of crystal orientations in the latter loading orientation promoted martensitic transformation. These and additional experimental observations from EBSD in terms of kernel average misorientation, mean grain orientation spread, and mean crystallite size reveal strong microstructural effects on strength of additively manufactured metallic materials.

The reliability of electronic solder joints is strongly influenced by βSn grain orientations. Single-crystal Cobalt substrates were used to control βSn nucleation and grain orientations in solder joints. On (0001)Co, (11 2 ¯ 0)Co, (10 1 ¯ 0)Co, and (1 1 ¯ 02)Co, interfacial αCoSn3 crystals that are potent to catalyze βSn nucleation exhibited strong textures with 2–5 symmetric orientations featuring reproducible orientation relationships (ORs) with the Co. Preferred ORs and orientation selections of αCoSn3 were examined by exploring i) thermodynamic stabilities of interfaces represented by interfacial atomic matches and work of adhesion and interfacial energies calculated based on density functional theory (DFT) and ii) crystal growth kinetics indicated by the angle between (100)CoSn3 and the substrate plane. In sharp contrast to the innumerable grain orientations in joints on commonly used substrates, dramatically reduced orientations are attributed to the strongly textured αCoSn3, on which βSn grains nucleated by fixed ORs. On (11 2 ¯ 0)Co, the total number of orientations including twinned grains were reduced to 20. On (10 1 ¯ 0)Co, none of the βSn orientations exhibit the c-axis perpendicular to the substrate (i.e. parallel to the electron flow direction), which should reduce fast diffusion paths across the joint and improve the reliability of solder joints.

Mixed-mode (I/II) fracture experiments on rolled AZ31 Mg alloy are conducted using notched four point bend specimens along with in-situ imaging. Digital Image Correlation (DIC) technique is used to analyze the images and map out the deformation and strain fields. With increase in mode II component of loading, a monotonic reduction in the notched fracture toughness (Jc) is observed. Fractographs reveal that the fracture mechanism transitions first from (ductile) micro-void growth and coalescence to twin induced quasi-brittle failure and then to shear cracking as the loading changes from mode I to II. Detailed optical micrographs and EBSD maps show profuse tensile twinning in the ligament, especially near the far-edge of the specimen leading to pronounced texture changes. However, the micro void size near the notch tip as well as the twin density in the ligament reduce with higher mode II component. The pronounced drop in fracture toughness and the changes in the associated mechanism with increase in mode-mixity are rationalized from the combined effects of hydrostatic stress, plastic strain localization and tensile twinning.

This paper quantifies the plastic deformation in terms of dislocation density for a series of cold sprayed Al-Cu coatings with copper contents from 2 mass% to 5 mass%. The success of the deposition and consolidation of the feedstock powders during the cold spray process rely on the occurrence of significant plastic deformation. Inert gas atomized Al-Cu alloy powders were sprayed onto substrates made of an Al-Cu-Mg-Mn alloy (AA2024) to produce dense coatings using a low pressure cold spray system with helium as the carrier gas. X-ray diffraction patterns were obtained from the Al-Cu feedstock powder material and the cold sprayed coatings using a monochromatic X-ray source, and the dislocation density was determined via an X-ray whole profile analysis. Increasing the Cu alloy content (from 2 mass% to 5 mass%) systematically increased the dislocation density in the Al-Cu coatings from (4.3 ± 0.5) × 1014 m−2 to (7.5 ± 0.8) × 1014 m−2. The dislocation densities in the feedstock powders ranging from (0.4 ± 0.1) × 1014 m−2 to (1.8 ± 0.2) × 1014 m−2 were all lower than the dislocation densities in the corresponding coatings. The increasing deformation level in the Al-Cu coatings with Cu additions were confirmed by the classic and modified Williamson-Hall analyses of X-ray diffraction data, and peak breadth measurements from neutron diffraction data. A high density of dislocations was also observed in these coatings via electron backscatter diffraction and transmission electron microscopy.

Electromigration (EM) causes intermetallic and void growth that decreases the reliability and lifetime of solder joints. As a diffusion-controlled process, EM is highly dependent on the fastest diffusion pathways in a structure. In anisotropic β-tetragonal Sn those pathways are along high angle grain boundaries and along the c-axis of the Sn grains. A bicrystal sample with two major grain orientations and two major grain boundary types was EM tested for 100 h. EM was interrupted at intervals for x-ray microtomography to reveal the 3D evolution of intermetallic compounds (IMCs) and voids within the joint. Quantitative IMC growth along high angle grain boundaries and twins was captured for the first time and related to the effective Cu diffusivity of multiple grains. Using a correlative approach that combined scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) mapping, IMC growth and morphology were linked to the evolution of the Sn grain structure. Unusual void shrinkage and void splitting behavior were connected to uneven cathode interface consumption and thermal compression.

This study investigates the effects of the initial grain size and temperature (ranging from room temperature to 1100 °C) on the mechanical properties and microstructure evolution of Ti2AlC MAX phase. A Hall-Petch like relationship is observed between compressive strength and the grain size below brittle-to-plastic transition temperature (BPTT). However, the compressive strength of fine-grained MAX phase decreases more rapidly with increasing temperature resulting in inverse Hall-Petch effect above BPTT. Results from postmortem EBSD analysis reveal complex microstructural evolution in both fine- and coarse-grained microstructures during loading at different temperatures. The pronounced drop in compressive strength for fine-grained microstructures at temperatures close to BPTT is attributed to creep induced grain boundary sliding resulting in texture development with more grains oriented for easy slip. In coarse-grained microstructures, no significant texture development is observed even though grain refinement occurs at all temperatures. A mathematical model has also been formulated to predict the experimentally observed grain size and temperature dependent variation in the compressive strength of Ti2AlC over a wide range of grain sizes and test temperatures. The mathematical model accounts for the competing effects of Hall-Petch strengthening and high temperature creep induced softening mechanisms.

How soluble particles affect microstructural evolution during directional recrystallization was studied in a Ni-12Al alloy containing two different sizes of Ni3Al particles. Ni-12Al with small particles can form columnar grains, but at a higher annealing temperature compared to its particle-free Ni-3Al counterpart, whilst columnar grains were not formed in Ni-12Al with large γ’ particles for the same annealing parameters. Columnar grains only formed after the γ’ particles dissolved. γ’ particle dissolution is not a trigger for columnar grain formation but is simply a prerequisite. Whether columnar grain can form or not was determined by the texture at the time of particle dissolution. If texture pinning exists after particle dissolution, abnormal grain growth occurs, and columnar grains form; if texture pinning is absent, columnar grains cannot form and normal grain growth occurs. How particles affect the columnar grain orientations, grain boundary character, and grain length/ width during directional recrystallization was also studied.

Solid-state cooling based on the caloric effect of phase transformation materials has attracted considerable interest with the increased demand for energy-efficient and environmentally friendly cooling technologies. Here, we have systematically studied the microstructural evolution, martensitic transformation (MT) behaviors, and elastocaloric effect (eCE) of directionally solidified Ni35.5Co14.5Mn35Ti15 all-d-metal Heusler metamagnetic shape memory alloys. Electron backscatter diffraction (EBSD) analysis revealed the coexistence of Heusler-type 〈001〉-oriented dendritic crystal and Ti-poor interdendritic phase for as-solidified alloys. Upon homogenization annealing, the sample presents a columnar-like morphology with 〈105〉 textured five-layer-modulated (5 M) martensite along the growth direction. Besides, the multi-modulated martensite with 7 M and 8 M structures was identified by transmission electron microscopy (TEM). Using a unique technique of in situ digital image correlation (DIC) with the combination of infrared thermography, MT and eCE behaviors were studied. Ni35.5Co14.5Mn35Ti15 alloy yielded a large adiabatic temperature change (ΔTad) of 11.5 K with a low critical stress (σcr) of 38 MPa and a moderate stress hysteresis (Δσhy) of 54 MPa when subjected to uniaxial stress, resulting in the largest value of |ΔTad/σcr| = 0.31 K MPa−1. This improved eCE is attributed to the enhanced compatibility in the oriented polycrystalline alloy and the high mobility of the low-energy twin boundary in the multi-modulated martensite.

Hydrogen uptake in titanium alloys leads to the formation of hydride phases, which in-turn cause severe embrittlement upon further deformation. The embrittlement micro-mechanisms are complicated by the distinct hydrogen diffusion and solution characteristics of the α and β phases, which arise from their different crystal structures. To improve the fundamental understanding of the room temperature interplay among hydrogen, the present phases and the hydride that is formed at their interface, we investigated the hydride formation process in Ti–6Al–4V alloy by in situ scanning electron microscopy and electron backscatter diffraction analyses during hydrogen charging, and post-mortem synchrotron diffraction tests. These investigations reveal that the lattice expansion of the hydrogenated β phase introduces a localized stress field near the phase boundary, which, in turn, induces hydrogen confinement therein, eventually facilitating the hydride growth along the boundary. The volumetric expansion of the α-to-hydride transformation extends the local stress field, influencing the work-hardening and damage evolution in the alloy. We discuss these findings and the overall damage initiation and propagation taking place in the hydrogenated alloy, based on the interplay among the present phases and hydrogen.

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

The effect of aging on the accumulation of microscale plasticity, and the resulting macroscopic mechanical behavior, were examined in the magnesium alloy WE43 under uniaxial tension. Full-field strains on the length scale of the microstructure, and their relation to the underlying crystallography, were captured using a combination of electron backscatter diffraction, custom nanoparticle patterning processes for corrosion-susceptible alloys, scanning electron microscopy (SEM), in-SEM uniaxial tensile and compressive loading, and distortion-corrected digital image correlation. The as-received material exhibited an average grain size of 12 µm. The strain incurred on individual slip traces in magnesium was resolved for the first time. Insights into slip activation across the microstructure revealed that using Schmid's Law with the nominal Schmid Factor appeared to be predictive for basal and non-basal slip. The DIC results were compared with simulation using an advanced open-source crystal plasticity finite element (CPFE) code, PRISMS-Plasticity. The PRISMS-Plasticity model is a more precise determination of the local Schmid Factor and was used to simulate variations in slip and twin activity within each grain. Such simulations provide an avenue for physically interpreting the various slip traces observed in the dense DIC data and an improved understanding of the critical resolved shear stress of the various slip systems.

International audience; Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7 M and A//7 M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7 M and A//7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.

Due to the alignment of two easy 〈100〉 axes in the sheet plane, the cube orientation ({001} ) is an ideal texture for non-oriented electrical steel sheets used as core lamination for electric motors. However, this magnetically favorable texture was rarely able to be produced using conventional rolling and annealing routes in non-oriented electrical steels. In this research, inclined cold rolling − a simple rolling scheme to alter the initial texture before cold rolling − was applied to a 2.8 wt% Si non-oriented electrical steel, in order to intentionally “create” a rotated Goss ({110} ) texture before cold rolling, which was not commonly observed in hot-rolled electrical steels. Plane-strain compression (rolling) of the rotated Goss was able to produce cube crystallites within the matrix, at the grain boundaries and within the shear bands of the deformed rotated Goss grains. The cube crystallites within the shear bands had lower stored energy than their surroundings, and served as the initial seeds for nucleation. Upon annealing, the cube crystallites preferentially nucleated from the shear bands and competitively grew out of the surrounding substructure, forming a strong cube texture in the final sheet. The formation of the initial cube crystallites within the shear bands of the deformed microstructure was believed to be necessary for the development of a final cube texture in the annealed electrical steel sheet. Although inclined rolling may be difficult to be implemented in industrial production, its unique capability to produce uncommon initial texture before cold rolling provides an interesting technique for the study of texture involution during thermomechanical processing.

The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts.

Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 nm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.

Additively manufactured (AM) metallic parts exhibit substantially different microstructures compared to those that are conventionally produced. Characterization studies have revealed that the microstructure of as-built AM metallic materials is highly heterogeneous in many respects. The strongly anisotropic mechanical response under plastic deformation observed in AM metals, compared to their conventionally manufactured counterparts, lies in the aforementioned inherent microstructural disparities. In this study, we have focused on a high-manganese steel (HMnS) processed by laser powder bed fusion (LPBF), which exhibits twinning-induced plasticity (TWIP). The as-built microstructure is carefully characterized by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. To unfold the potential of metal additive manufacturing, it is essential to understand the microstructure of AM products and its connection with the mechanical properties by means of numerical modeling and simulation. The mechanical response of AM components under plastic deformation is highly complex and its simulation requires advanced modeling and numerical methods. In the present study, in order to simulate the anisotropic plasticity of the LPBF-HMnS, we used the full field method for computational polycrystal homogenization combined with physics-based crystal plasticity constitutive and statistical microstructure modeling. The impact of different process-induced microstructural heterogeneity characteristics on macroscopic strain hardening behavior of the material has been comprehensively and systematically investigated. Finally, it has been argued why the chosen AM material with the selected processing parameters and chemical composition represented an ideal candidate for a generic assessment of the anisotropic polycrystal plasticity due to microstructural heterogeneity.

Ti and its alloys have a variety of applications in aerospace industry and medical implants. The formation of hydride has been used in biomedical areas and can significantly influence the mechanical performance of materials. In this work, we investigate the orientation dependence of hydride precipitation in commercially pure titanium via interrupted in-situ electron backscatter diffraction (EBSD) measurements. The results reveal that hydrides during hydrogen charging at room temperature exhibit two types of orientation relationships with α-titanium, i.e., { 0001 } α / / { 1 1 ¯ 1 } δ 1 2 ¯ 10 > α / / 110 > δ with interface plane { 10 1 ¯ 3 } α / / { 1 1 ¯ 0 } δ (B-type), and {0001}α//{001}δ 1 2 ¯ 10 > α / / 110 > δ with interface plane { 10 1 ¯ 0 } α / / { 1 1 ¯ 0 } δ (P-type). Significant orientation dependence of hydride precipitation is observed, especially when { 10 1 ¯ 3 } , { 10 1 ¯ 0 } , {0001} or { 11 2 ¯ 0 } planes of the parent grains are parallel to the diffusion surface. The displacement gradient tensor based accommodation shows that the orientation dependence is attributed to the strain relaxation of hydride transition. Three types of hydride platelets are characterized: parallel hydride platelets (Type I), crossed hydride platelets (Type II) and clustered hydride platelets (Type III). The multiple morphologies of hydride platelets resulting from the hydride variant selection and interaction are dependent on the crystal orientation of the matrix.

Nanostructured ferritic alloys (NFAs) contain an ultra-high density of 2–4 nm fcc pyrochlore Y2Ti2O7 nano-oxides (NOs) embedded in a bcc Fe-14Cr ferritic matrix. Characterization of helium interactions with NOs and associated Fe-Y2Ti2O7 interfaces is important to the development of structural materials for nuclear fusion and fission applications. A benchmark 14YWT NFA was first annealed to coarsen the NOs, then insoluble helium was implanted at 700 °C to produce a high number density of bubbles. High-resolution scanning transmission electron microscopy characterization shows two dominant Fe-Y2Ti2O7 crystallographic orientation relationships (cube-on-edge and cube-on-cube). The smallest NOs (≈ 2 nm) are associated with the smaller bubbles (≈ 1.5 nm), while some of the largest NOs (> 6 nm) have larger, and sometimes multiple, bubbles. NO corner {111} facets are the preferred sites for He bubble nucleation. A refined sequence of events for He trapping and bubble formation is presented. These observations offer new insight on He management in NFAs, and provide a foundation for detailed modeling studies.

A combined experimental and simulation study of intragranular misorientation and texture development in ferritic stainless steels is presented. Cold rolling was performed on materials having different grain shapes to reveal variations of misorientations and texture with variations in microstructure. The experimental results were compared with predictions of the Visco-Plastic Self-Consistent (VPSC) model and the recently developed Grain-Fragmentation Visco-Plastic Self-Consistent (GF-VPSC) model. It is shown that the GF-VPSC model, incorporating the development of intragranular misorientations, provides a much better prediction of the texture strength compared to the standard VPSC model. The predictions of intragranular misorientation are also in good agreement with experimental measurements. Both experiments and simulations point to the importance of anisotropy of intragranular misorientation distributions in determining texture development and, importantly, texture strength.

Plastic effects during sheet metal forming can lead to undesirable distortions in formed components. Here, the three-stage work hardening and plastic strain recovery (“springback”) in a cold-rolled, α-phase commercially pure titanium is examined. Interrupted standard tensile tests with in situ x-ray diffraction and quasi-in situ electron backscatter diffraction show that twinning plays a minor role in both of these phenomena. The experiments give evidence that the observed work hardening plateau is the result of an abrupt activation and multiplication of 〈 c + a 〉 slip and a subsequent redistribution of load between grain families. The springback can be attributed to inelastic backwards motion and annihilation of dislocations, driven by backstresses from dislocation-based hardening during loading. The peak broadening behavior, observed by x-ray diffraction, suggests that the internal stress state is highest in the rolling direction, resulting in consistently higher springback magnitude along this direction.