Crystallographic Orientation Mapping Research Articles

Laser powder bed fusion spatters of Zr-Cu-Al-Nb metallic glass

Understanding the morphology, composition and structural changes experienced by metallic glass powder particles upon laser powder bed fusion (LPBF) is the first step towards evaluating the impact of spatter generation on part quality and on feedstock degradation. This study presents the characterization of Zr-Cu-Al-Nb spatter particles by means of scanning and transmission electron microscopy with automated crystallographic orientation mapping and energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The powder exposed to the LPBF environment exhibits an average surface composition similar to the original feedstock, but with a rougher morphology. The typical spatter encountered in the remaining lack-of-fusion is a vapour-entrained particle, which probably underwent partial melting leading to the formation of α-Zr(O) dendrites, and “big-cube” Cu2Zr4O nanocrystals in its heat affected zone. In the present work, spatter characteristics are discussed considering glass properties and their connection to lack-of-fusion defects is addressed.

Uncovering the white etching area and crack formation mechanism in bearing steel

The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.

The Origin of Lattice Rotation during Dendritic Crystallization of Clinopyroxene

AbstractUnderstanding dendritic crystallization is key to obtaining petrological information about rapid crystallization events. Clinopyroxene dendrites from a basaltic rock fulgurite from Nagpur, India, exhibit curved branches with corresponding lattice rotation that exceeds 180° for some branches. This paper combines crystallographic orientation mapping with microstructural observations and compositional information to determine the dendrites’ 3-D morphology and their bending mechanism. Dendrites exhibit a network of branches in the (010) plane, following either {001}* (normal to {001} planes, strong lattice curvature) or < 10–1 > (weak lattice curvature). Three or more orders of branches are observed in the (010) plane, alternating between {001}* and < 10–1>. Side branches with weak lattice curvature extend sub-perpendicular to the (010) plane, following either {021}* (sprouting from {001}* branches) or < 12–1 > (from <10–1 > branches) and defining curved ‘ribbons’ containing their respective central branch. All branches rotate about [010], with a consistent rotation sense regardless of elongation direction in sample or crystal coordinates. Bending must therefore be caused by local asymmetric thermal and compositional fields in the melt, generated by dendritic growth itself, not by sample-scale compositional, thermal or mechanical gradients. The most likely cause of bending is asymmetric distribution of melt supersaturation around branch tips, related to unequal growth rates perpendicular to different facets. Lattice rotation is inferred to occur via preferential incorporation of high densities of [001] (100) edge dislocations of one sign. High inferred dislocation densities imply that the preservation of bent dendrites requires rapid quenching. Higher inferred degree of undercooling (based on microstructural observations) correlates with greater lattice curvature. Bent dendrites can thus potentially be used to deliver information about spatial variations in degree of undercooling and place limits on the history of a sample after dendritic crystallization. Finally, finding lattice rotation exclusively about [010] is a new criterion to identify cryptic dendritic growth stages in euhedral crystals.

Evidence that the liquid structure affects the nucleation of the primary metastable L12-Al3Zr in additive manufacturing

The strategy aiming at promoting the precipitation of the primary metastable L12-Al3X phase is the most widely used to refine grain size in Al alloys designed for additive manufacturing. It is generally admitted that Al grains nucleate onto the primary L12-Al3X precipitates by epitaxy resulting in a cube-cube orientation relationship. Here we report TEM observations showing clusters of precipitates of this primary L12-Al3X phase. Using Automated Crystallographic Orientation Mapping in the TEM, we show that such clusters consist of various orientations. Analysis of those clusters reveals that some orientations share common 5-fold rotational symmetry axes suggesting that they have nucleated from an icosahedral template. We conclude that Icosahedral Short Range Order (ISRO) in the liquid can play a role in the nucleation of the primary metastable L12-Al3Zr phase through the ISRO-mediated nucleation mechanism.

Interplay between solidification microsegregation and complex precipitation in a γ/γ’ cobalt-based superalloy elaborated by directed energy deposition

This study examines the segregations and the precipitation appearing during the solidification path of a Co-Ni-Al-W-Ta-Ti-Cr γ/γ’ cobalt-based superalloy processed by directed energy deposition (DED). Observations reveal characteristic elements partitioning in the liquid during additive manufacturing. Due to this microsegregation, complex multiphase precipitation occurs and the various precipitates are identified and characterized in a cobalt-based superalloy fabricated by DED. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to investigate the spatial distribution and nature of the various phases detected in the as-fabricated microstructure. Energy dispersive X-ray spectrometry (EDS), wavelength dispersive X-ray spectrometry (WDS), and electron energy loss spectroscopy (EELS) are coupled with a fine analysis of the diffraction patterns to identify the different phases decorating the interdendritic regions. These characterizations allow the identification of different submicronic precipitates: Al 2 O 3 , (Ta,Ti)(N,C), HfO 2 , Cr 3 B 2 and (Ti,Zr,Hf) 2 SC. The solidification sequence is discussed at light of the experimental results. This work provides a first understanding of the interplay between solidification segregations and the formation of second phase precipitation in a cobalt-based superalloy processed by DED. • A cobalt-based superalloy was processed by Direct Energy Deposition. • Solidification segregations lead to the precipitation of interdendritic phases. • Automated crystallographic orientation mapping and chemical analyses were used. • Various interdendritic phases have been identified: oxides (Al 2 O 3 and HfO 2 ), carbonitrides (Ta, Ti)(N, C), sulphocarbides (Ti, Zr, Hf) 2 SC, and borides Cr 3 B 2 . • The solidification path can be established based on the experimental observations.

Towards an alloy design strategy by tuning liquid local ordering: What solidification of an Al-alloy designed for laser powder bed fusion teaches us

High-strength aluminium alloys from the 2XXX, 6XXX, and 7XXX series suffer from severe hot cracking issues during 3D printing. Thus, there is a need to design new alloys with an improved hot cracking resistance while taking advantage of the non-equilibrium processing conditions of laser powder bed fusion to achieve optimized properties. Here, we report the successful fabrication of a new Al-Mn-Ni-Cu-Zr alloy designed for L-PBF. The as-built microstructure was characterized at different scales using synchrotron X-ray nano-tomography and microscopy with a special focus on automated crystallographic orientation mapping (ACOM) in transmission electron microscopy. Based on this multiscale microstructural investigation, we identify various mechanisms involving the presence of a locally ordered liquid during solidification under conditions typical of additive manufacturing. This local liquid ordering is often defined as Icosahedral Short Range Order (ISRO). The multiple consequences of ISRO in the liquid on the hierarchical microstructure inherited from additive manufacturing are discussed. ISRO enables refinement of the grain size thus improving the processability via an increase of the hot cracking resistance, producing equiaxed grains and twinned dendrites contributing to randomizing the crystallographic texture, and possibly extending supersaturated solid solutions through a cage effect. It is concluded that promoting ISRO should be considered a promising pathway to design alloys for additive manufacturing with good processability and optimized properties.

The complex structural and chemical nature of monolithic U-10Mo fuel and Zr barrier layer

Nanoscale microstructural characterization by advanced transmission electron microscopy techniques on a U-10Mo/Zr barrier layer monolithic fuel plate was performed to evaluate the microstructural evaluation after high burn-up. Gas bubble superlattice evolution, grain restructuring, and evolution of the Zr interaction layer is investigated through detailed electron microscopy characterization. The use of automated crystallographic orientation mapping to irradiated U-10Mo fuel highlights that the restructured ultra-fine grains are separated by high angle grain boundaries at a burn up of 4.42 × 1021 fissions/cm3. Additionally, advanced chemical analysis and multi-variable statistical analysis shows spatial clustering of solid fission product precipitates. Finally, characterization of a newly observed porous nanocrystalline Zr region in the barrier layer is studied. This work provides insights into the grain subdivision and restructuring process while using advanced microscopy techniques to analyze fission products in neutron-irradiated U-10Mo fuel.

A novel laser powder bed fusion Al-Fe-Zr alloy for superior strength-conductivity trade-off

New aluminium alloy design strategies for laser powder bed fusion (LPBF) are needed to target high strength and conductivity applications and substitute the traditional 6xxx series which suffers from hot cracking during LPBF processing. This study presents the route followed to design a novel Al-Fe-Zr alloy offering good processability, superior yield strength (310 MPa) and thermal conductivity (180 W/m·K) after direct ageing (400°C/4h). The multi-scale characterization, combining scanning and transmission electron microscopy with automated crystallographic orientation mapping and energy dispersive spectroscopy, reveals the precipitation of plate-like Al13Fe4 and coherent Al3Zr-L12 nanoprecipitates upon ageing, enhancing the material's strength. In turn, the associated solid solution depletion results in a significant conductivity increase. The strategy to select low vapour pressure elements and slow diffusers in Al, as well as the low solubility of Fe in Al demonstrates the ability through alloy design for additive manufacturing to fill gaps in the material property space.

Crystallographic orientation mapping of lizardite serpentinite by Raman spectroscopy

Abstract. The serpentine mineral lizardite displays strong Raman anisotropy in the OH-stretching region, resulting in significant wavenumber shifts (up to ca. 14.5 cm−1) that depend on the orientation of the impinging excitation laser relative to the crystallographic axes. We quantified the relationship between crystallographic orientation and Raman wavenumber using well-characterised samples of Monte Fico lizardite by applying Raman spectroscopy and electron backscatter diffraction (EBSD) mapping on thin sections of polycrystalline samples and grain mounts of selected single crystals, as well as by a spindle stage Raman study of an oriented cylinder drilled from a single crystal. We demonstrate that the main band in the OH-stretching region undergoes a systematic shift that depends on the inclination of the c-axis of the lizardite crystal. The data are used to derive an empirical relationship between the position of this main band and the c-axis inclination of a measured lizardite crystal: y=14.5cos 4 (0.013x+0.02)+(3670±1), where y is the inclination of the c-axis with respect to the normal vector (in degrees), and x is the main band position (wavenumber in cm −1) in the OH-stretching region. This new method provides a simple and cost-effective technique for measuring and quantifying the crystallographic orientation of lizardite-bearing serpentinite fault rocks, which can be difficult to achieve using EBSD alone. In addition to the samples used to determine the above empirical relationship, we demonstrate the applicability of the technique by mapping the orientations of lizardite in a more complex sample of deformed serpentinite from Elba Island, Italy.

Reconstructing dual-phase nanometer scale grains within a pearlitic steel tip in 3D through 4D-scanning precession electron diffraction tomography and automated crystal orientation mapping

The properties of polycrystalline materials are related to their microstructures and hence a complete description, including size, shape, and orientation of the grains, is necessary to understand the behavior of materials. Here, we use Scanning Precession Electron Diffraction (SPED) in the Transmission Electron Microscope (TEM) combined with a tilt series to reconstruct individual grains in 3D within a polycrystalline dual-phase cold wire-drawn pearlitic steel sample. Nanoscale ferrite grains and intragranular cementite particles were indexed using an Automated Crystallographic Orientation Mapping (ACOM) tool for each tilt dataset. The grain orientations were tracked through the tilt datasets and projections of the individual grains were reconstructed from the diffraction data using an orientation-specific Virtual Dark Field (VDF) approach for tomographic reconstruction. The algorithms used to process and reconstruct such datasets are presented. These algorithms represent an extension to the ACOM approach that may be straightforwardly applied to other multi-phase polycrystalline materials to enable 3D spatial and orientation reconstructions.

Crystallographic preferred orientation, seismic velocity and anisotropy in roofing slates

Mica-rich rocks (shales, slates, schists) are a major and significant component of the continental crust. They are often characterized by a strong seismic anisotropy, a key factor in the seismic interpretation of the architecture of the continental crust. Roofing slates are a group of natural rocks used in construction that must possess a continuous and undeformed slaty cleavage. This makes roofing slates an exceptional benchmark to study the relation between the rock microstructure and the development of seismic anisotropy in single-foliated low-grade slates. Direct measurements of seismic velocities using an ultrasonic wavemeter at room conditions were done on eight cubic samples from active quarries in the NW Iberian Peninsula. Seismic velocities were also calculated using the Hill averaging method from crystallographic preferred orientation (CPO) maps of slate-forming minerals on the same sample blocks using electron backscatter diffraction (EBSD), achieving unprecedented coverages of up to 87%. The seismic velocities estimated by both methods are similar parallel to the slaty cleavage but much slower using direct measures in the axial direction, producing anisotropies for Vp and Vs1 up to 7 and 5 times higher than the calculated velocities. The results are indicative of the highly anisotropic contribution that cracks, pores and fractures have in single-foliated slates, given the strong shape fabric that the rocks present. The samples selected also show that not all slates exhibit near-transverse isotropy, as it has been commonly assumed.

Chemical and physical heterogeneity within native gold: implications for the design of gold particle studies

Studies of populations of gold particles are becoming increasingly common; however, interpretation of compositional data may not be straightforward. Natural gold is rarely homogenous. Alloy heterogeneity is present as microfabrics formed either during primary mineralization or by modification of pre-existing alloys by chemical and physical drivers during subsequent residence in either hypogene or surficial environments. In electron-probe-microanalysis (EPMA)-based studies, the combination of Cu, Hg, and Pd values and mineral inclusion suites may be diagnostic for source style of mineralization, but Ag alone is rarely sufficient. Gold characterization studies by laser-ablation-ICP mass spectrometry linked to both quadrupole and Time-of-Flight (ToF-MS) systems show that only Ag, Cu, and Hg form homogenous alloys with Au sufficiently often to act as generic discriminants. Where present, other elements are commonly distributed highly heterogeneously at the micron or submicron scale, either as mineral inclusions or in highly localized, but low concentrations. Drawing upon our own data derived from individual inspection and analyses of approximately 40,000 gold particles from 526 placer and in situ localities worldwide, we show that adequate characterization of gold from a specific locality normally requires study of a minimum of 150 particles via a two-stage approach comprising spatial characterization of compositional heterogeneity, plus crystallographic orientation mapping, that informs subsequent targeted acquisition of quantitative compositional data by EPMA and/or laser-ablation ICP-MS methods. Such data provide the platform to review current understanding of the genesis of gold particle characteristics, elevating future compositional studies from empirical descriptions to process-focused interpretations.

Towards Crystallographic Orientation and Strain Mapping of 1D & 2D Tellurium from 4D-STEM

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Fatigue life enhancement in alpha/beta Ti–6Al–4V after shot peening: An EBSD and TEM crystallographic orientation mapping study of surface layer

Shot peening is a well-established surface treatment technique to improve metallic components’ fatigue lives by introducing compressive residual stress at their surface layers. Using α/β Ti–6Al–4V alloy as an example, the fatigue life of Ti64 was significantly enhanced after a compressive residual stress was introduced to a depth of 208 µm via shot peening treatment. The top ~24 µm layer from surface was found interestingly unable to be indexed under electron backscatter diffraction (EBSD), a depth coinciding with the presence of maximum compressive residual stress as revealed by center hole drilling method. Transmission electron microscope (TEM) with crystallographic orientation mapping was therefore employed to probe the top surface layer in order to unveil the origin of such thick non-indexed region observed in EBSD after shot peening and its effect on enhancement of component's fatigue life.

Development of a heterogeneous nanostructure through abnormal recrystallization of a nanotwinned Ni superalloy

This work explores the development of a heterogeneous nanostructured material through leveraging abnormal recrystallization, which is a prominent phenomenon in coarse-grained Ni-based superalloys. Through synthesis of a sputtered Inconel 725 film with a heterogeneous distribution of stored energy and subsequent aging treatments at 730°C, a unique combination of grain sizes and morphologies was observed throughout the thickness of the material. Three distinct domains are formed in the aged microstructure, where abnormally large grains are observed in-between a nanocrystalline and a nanotwinned region. In order to investigate the transitions towards a heterogeneous structure, crystallographic orientation and elemental mapping at interval aging times up to 8 h revealed the microstructural evolution and precipitation behavior. From the experimental observations and the detailed analysis of this study, the current methodology can be utilized to further expand the design space of current heterogeneous nanostructured materials.

Optical characterization of grain orientation in crystalline materials

Characterizing crystallographic orientation is essential for assessing structure-property relationships in crystalline solids. While diffraction methods have dominated this field, low throughput and high cost limit their applicability to small, specialized samples and restrict access to well-funded research institutions. We develop a complementary method that expands applicability and broadens access. We demonstrate crystal orientation mapping over centimeter-scale surfaces using nothing more than a conventional optical microscope and commercial laptop. Our approach relies on a novel analysis method that correlates crystal lattice orientation to optical reflectance of specially etched surfaces. We successfully apply the method to metal and semiconductor surfaces. The simplicity, low cost, and enhanced sample throughput of our method promise to expand the availability of crystallographic orientation mapping significantly, making it readily available in education as well as academic research and industrial settings.

Three-dimensional cellular automaton modeling of silicon crystallization with grains in twin relationships

A three-dimensional model is proposed to simulate the grain structure in directionally solidified silicon. This includes the nucleation and growth of grains in twin relationship whose formation is very frequent during silicon solidification. Based on analyses of in-situ and real-time observations of the crystallization front, a three-dimensional cellular automaton method is developed to model the dynamic of {111} facets, groove formation at grain boundaries, nucleation and growth of grains in twin relationship. The model is applied to well-characterized experiments assuming the frozen temperature approximation. A comparison of solidification sequences, crystallographic orientation maps, and coincidence site lattice maps in both experiment and simulation results is achieved. Results demonstrate that the model could be applied to optimize crystallization processes for both polycrystalline and cast-mono silicon fabrication processes.

TEM investigation of the role of the polycrystalline-silicon film/substrate interface in high quality radio frequency silicon substrates

The microstructural characteristics of two polycrystalline silicon (poly-Si) films with different electrical properties produced by low-pressure chemical vapour deposition on top of high resistivity silicon substrates were investigated by advanced transmission electron microscopy (TEM), including high resolution aberration corrected TEM and automated crystallographic orientation mapping in TEM. The results reveal that the nature of the poly-Si film/Si substrate interface is the main factor controlling the electrical resistivity of the poly-Si films. The high resistivity and high electrical linearity of poly-Si films are strongly promoted by the ∑3 twin type character of the poly-Si/Si substrate interface, leading to the generation of a huge amount of extended defects including stacking faults, ∑3 twin boundaries as well as ∑9 grain boundaries at this interface. Furthermore, a high density of interfacial dislocations has been observed at numerous common and more exotic grain boundaries deviating from their standard crystallographic planes. In contrast, poly-Si film/Si substrate interfaces with random character do not favour the formation of such complex patterns of defects, leading to poor electrical resistivity of the poly-Si film. This finding opens windows for the development of high resistivity silicon substrates for Radio Frequency (RF) integrated circuits (ICs) applications.

Characterization of recovery onset by subgrain and grain boundary migration in experimentally deformed polycrystalline olivine

Abstract. To apprehend plate tectonics and the dynamics of the lithosphere–asthenosphere boundary, composed principally of olivine, we need to understand the mechanisms that control plastic deformation of olivine in the relevant temperature domain. After more than 50 years of laboratory studies and investigations on natural rocks, the interplay of several key parameters (e.g. temperature, pressure, vacancy concentration, dislocation densities, grain size, strain rate) controlling polycrystalline olivine plasticity remains difficult to assess. Here, we study four olivine polycrystals, which have been deformed in axial compression under a confining pressure of 300 MPa, at 1273 or 1473 K. Despite significant differences in mechanical properties (stress–strain curves), previous characterization by scanning (SEM) and transmission electron microscopy (TEM) did not reveal significant differences in dislocation microstructures which could explain these contrasted behaviours. We have undertaken automatic crystallographic orientation mapping (ACOM) analyses in TEM to increase the spatial resolution of characterization compared to previously obtained electron backscatter diffraction maps to further decipher the microstructures at nanoscale. With this novel technique applied to olivine, a noticeable difference in the onset of microstructural recovery has been identified between specimens deformed at 1273 and 1473 K. The microstructures of the olivine polycrystals deformed at 1473 K exhibit numerous curved grain and subgrain boundaries, advocating for recovery by boundary migration. In contrast, the microstructures of the olivine polycrystals deformed at 1273 K have significantly fewer subgrain boundaries and show more straight boundaries (i.e. closer to an equilibrium microstructure) than in the specimen deformed at 1473 K. Characterization by ACOM-TEM has permitted the identification of the onset of recovery, which is led by boundary migration even for very low macroscopic finite strains.

Dislocation structures and the role of grain boundaries in cyclically deformed Ni micropillars

Transmission electron microscopy and finite element-based dislocation simulations were combined to study the development of dislocation microstructures after cyclic deformation of single crystal and bicrystal Ni micropillars oriented for multi-slip. A direct correlation between large accumulation of plastic strain and the presence of dislocation cell walls in the single crystal micropillars was observed, while the presence of the grain boundary hampered the formation of wall-like structures in agreement with a smaller accumulated plastic strain. Automated crystallographic orientation and nanostrain mapping using transmission electron microscopy revealed the presence of lattice heterogeneities associated to the cell walls including long range elastic strain fields. By combining the nanostrain mapping with an inverse modelling approach, information about dislocation density, line orientation and Burgers vector direction was derived, which is not accessible otherwise in such dense dislocation structures. Simulations showed that the image forces associated with the grain boundary in this specific bicrystal configuration have only a minor influence on dislocation behavior. Thus, the reduced occurrence of “mature” cell walls in the bicrystal can be attributed to the available volume, which is too small to accommodate cell structures.