Crystallographic Misorientation Research Articles

A combined crystal plasticity and graph-based vertex model of dynamic recrystallization at large deformations

A mesoscale model of microstructure evolution is formulated in the present work by combining a crystal plasticity model with a graph-based vertex algorithm. This provides a versatile formulation capable of capturing finite-strain deformations, development of texture and microstructure evolution through recrystallization. The crystal plasticity model is employed in a finite element setting and allows tracing of stored energy build-up in the polycrystal microstructure and concurrent reorientation of the crystal lattices in the grains. This influences the progression of recrystallization as nucleation occurs at sites with sufficient stored energy and since the grain boundary mobility and energy is allowed to vary with crystallographic misorientation across the boundaries. The proposed graph-based vertex model describes the topological changes to the grain microstructure and keeps track of the grain inter-connectivity. Through homogenization, the macroscopic material response is also obtained. By the proposed modeling approach, grain structure evolution at large deformations as well as texture development are captured. This is in contrast to most other models of recrystallization which are usually limited by assumptions of one or the other of these factors. In simulation examples, the model is in the present study shown to capture the salient features of dynamic recrystallization, including the effects of varying initial grain size and strain rate on the transitions between single-peak and multiple-peak oscillating flow stress behavior. Also the development of recrystallization texture and the influence of different assumptions on orientation of recrystallization nuclei are investigated. Further, recrystallization kinetics are discussed and compared to classical JMAK theory. To promote computational efficiency, the polycrystal plasticity algorithm is parallelized through a GPU implementation that was recently proposed by the authors.

On topotaxy and compaction during antigorite and chlorite dehydration: an experimental and natural study

Dehydration reactions result in minerals’ replacement and a transient fluid-filled porosity. These reactions involve interface-coupled dissolution–precipitation and might therefore lead to fixed crystallographic orientation relations between reactant (protolith) and product phases (i.e. topotaxy). We investigate these two phenomena in the dehydration of a foliated antigorite (atg) serpentinite by comparing the crystallographic preferred orientation (CPO) developed by olivine (ol), orthopyroxene (opx) and chlorite (chl) during high-pressure antigorite and chlorite dehydration in piston-cylinder experiments and in natural samples recording the dehydration of antigorite (Cerro del Almirez, Betic Cordillera, Spain). Experiments were performed under undrained conditions resulting in fluid-filled porosity and in strong CPO of the prograde minerals, controlled by the pre-existing antigorite CPO in the reactant foliated serpentinite. The orientation of a ol,opx and $$c_{\text{chl}}^{ * }$$ is parallel to $$c_{\text{atg}}^{ * }$$ from the protolith. The Cerro del Almirez samples show similar, locally well-developed topotactic relations between orthopyroxene, chlorite and antigorite, but the product CPOs are weaker and more complex at the thin section scale. In contrast to the experiments, olivine from natural samples shows a weak correlation between b ol and the former $$c_{\text{atg}}^{ * }$$ . We relate the strengthening of local topotactic relations and the weakening of the inherited CPO at a larger scale in natural samples to compaction and associated fluid migration. Microstructural features that might be related to compaction in the natural samples include: (1) smooth bending of the former foliation, (2) gradual crystallographic misorientation (up to 16°) of prismatic orthopyroxene due to buckling by dislocation creep, (3) inversion of enstatite to low clinoenstatite (P21/c) along lamellae and (4) brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite. The coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction. An lower bound for the strain rates in the order of 10−12 to 10−13 s−1 and shear stresses of 60–70 MPa are estimated based on creep data. Lower shear stresses (20–40 MPa) are retrieved using a theoretical approach. These data point to slow compaction (and fluid extraction) in nature if the system is not perturbed by external forces, with rates only marginally higher than the viscoplastic deformation of the solid matrix.

Effect of directional solidification on texture and magnetic-field-induced strain in Ni–Mn–Ga foams with coarse grains

Ferromagnetic Ni–Mn–Ga shape memory alloys with large magnetic-field-induced strains are promising candidates for actuators. Here, we cast replicated Ni–Mn–Ga foams with 57vol.% of 355–500μm open pores, with and without directional solidification. The 10M martensitic phase was determined in all foam samples. Directionally solidified foam had a fiber texture, with <100> closely aligned with the solidification direction. In contrast, foams without directional solidification were more randomly textured. One directionally solidified foam showed a maximum magnetic-field-induced strain of 0.65%, which was twice the value displayed by other foams without directional solidification. This improvement is consistent with a reduction in incompatibility stresses between neighboring grains deforming by twinning, generated by a reduction in crystallographic misorientation in textured foam.

Improved prediction of strain distribution during mechanical and hydro-mechanical deep drawing processes using microstructure-based dynamic strain hardening and anisotropy

Interstitial free (IF) and drawing quality (DQ) steel sheets were subjected to conventional mechanical deep drawing and hydro-mechanical deep drawing operations. Detailed macroscopic strain measurements were made at different cup depths. These were simulated using PAM-STAMP, a commercial finite element–based software. For continuum-based finite element simulations, the required key material properties are strain hardening exponent (n) and anisotropy index ([Formula: see text]). These were kept either constant or dynamically varied during the simulation. The constant properties were taken from conventional tensile tests of the original material, while the dynamic variation in properties was extrapolated from developments in the crystallographic texture and in-grain misorientation. Considering dynamic properties during simulation provided a superior prediction of macroscopic strains. This study clearly demonstrates the need for considering evolution of critical continuum properties, such as work hardening and anisotropy index, through appropriate microstructural inputs for more realistic macroscopic predictions.

Microstructural Characterization of Mg–0.3Al–0.2Ca Alloy Using Ion Milling Surface Preparation Technique

The Vickers microindentation and spherical nanoindentation tests were carried out on a polycrystalline Mg–0.3Al–0.2Ca alloy specimen. Specimens were prepared with chemical polishing and ion milling techniques. The electron channeling contrast imaging (ECCI) and electron backscattered diffraction (EBSD) were used to study the plastic deformation inside and around the indents with a cold-field emission scanning electron microscope. The distribution of strain fields, slip lines, and twins was imaged on an indented surface. A crystallographic orientation mapping was performed to map the local crystallographic misorientation associated with channeling contrast variations. The chemical polishing technique was used to remove the material from the surface and allow the study of deformed microstructure at a maximum depth of the indent. The combination of ECCI and EBSD with chemical polishing and ion milling surface preparation techniques proved to be beneficial to study the indentation-induced plastic deformation in a polycrystalline Mg–0.3Al–0.2Ca alloy specimen.

Micro- and macro-creep damage formation for P92 under multiaxial stress related to circular notched specimen

In this study, to clarify the behaviour of micro- and macro-creep damage progression for P92 under multiaxial stress field, interrupted creep tests, analysis of multiaxial stress and detailed the cross-sectional observations were conducted on a circular notched round bar specimen which produces the multiaxial stress field due to the plastic constraint. As a result, creep voids were initiated at the early stage and they were formed up to the final fracture. These phenomena were found to be detected using direct current potential drop (DCPD) method. These results concern the development of the measurement of creep crack initiation. The distribution of high void area fraction was in good agreement with that of high hydrostatic stress and high multiaxial stress. This result indicates that multiaxial stress affects the void formation. Furthermore, the micro-creep damage of each interrupted specimen was evaluated by using the electron backscatter diffraction (EBSD) method which can analyse crystallographic misorientation caused by creep strain. The results of EBSD analyses indicated that the value of grain reference orientation deviation (GROD) closely concerns the void initiation.

The processing and properties of single grain Y–Ba–Cu–O fabricated from graded precursor powders

The preparation of single grain, Y–Ba–Cu–O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7−δ (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Electron Channeling Contrast Imaging of Plastic Deformation Induced by Indentation in Polycrystalline Nickel

Vickers microindentation and Berkovich nanoindentation tests were carried out on a polycrystalline nickel (Ni) bulk specimen. Electron channeling contrast imaging (ECCI) in conjunction with electron backscattered diffraction was used to image and characterize plastic deformation inside and around the indents using a field emission scanning electron microscope. The ECCI was performed with a 5 keV beam energy and 0° tilt specimen position. The strain field distribution, slip lines, and Taylor lattices were imaged on an indented surface. Orientation mapping was used to investigate the local crystallographic misorientation and identify specific ⟨110⟩ slip systems. An ion milling surface preparation technique was used to remove materials from the surface which permitted the study of deformed microstructure below the indent. A dislocation density of 1011 cm-2 was calculated based on the curvature of bend contours observed in the ECCI micrographs obtained from the Vickers indents. A yield strength of 500 MPa was calculated based on the size of the strain field measured from the ECCI micrographs of the nanoindents. The combination of ion milling, ECCI, and electron backscattered diffraction was shown to be beneficial to investigate the indentation-induced plastic deformation in a polycrystalline Ni bulk specimen.

Significance of crystallographic misorientation at phase boundaries for fatigue crack initiation in a duplex stainless steel during high and very high cycle fatigue loading

The current investigations on the austenitic–ferritic duplex stainless steel X2CrNiMoN22-5-3 documents that the misorientation at a phase boundary determines its strength concerning the transition of plastic deformation from the austenite to the ferrite phase. The misorientation can be described by means of the tilt, twist and slip direction angle between two slip planes of neighboring grains. It has been found that plastic deformation can be transferred from an activated slip system in an austenite grain to a slip system in a neighboring ferrite grain with a moderate Schmid factor, even if not-activated slip systems in this ferrite grain have higher Schmid factors. For this, the tilt and twist angle between the activated austenite slip system and the ferrite slip system with the lower Schmid factor have to be smaller as compared to that of the tilt and twist angle between the activated austenite slip system and the ferrite slip system with the higher Schmid factor. The transition is very localized and mostly followed by microcrack initiation. Beside their significance concerning microcrack initiation, phase boundaries also determine significantly the short fatigue crack propagation.

The development of grain boundary orientation distribution in quartz

The fabric of high-angle boundaries in recrystallized quartz aggregates deformed under distinct strain and metamorphic conditions has been investigated in samples from iron formations recrystallized under lower greenschist to lower amphibolite facies conditions. Orientation of grain boundaries was measured using U-stage, while crystallographic fabric and misorientation between grains were measured using EBSD. In domains where grains were recrystallized under lower greenschist facies conditions, grain boundaries are sutured, composed of several small straight segments, whose traces can be parallel to traces of any crystallographic plane. In contrast, in domains of upper-greenschist–lower-amphibolite facies, grain boundaries are predominantly composed of long straight segments and belong to girdle of rhombohedral planes, while segments within girdle of basal or prism planes are rare. The longest segments (larger than 270 μm) are commonly within girdle of rhombohedral planes for the two grains that share a boundary surface, possibly representing Dauphine twin boundaries. Under higher temperature condition of the lower amphibolite facies (500–550 °C), boundary segments are longer and tend to represent coincident crystallographic orientations for the two grains that share a boundary.

Quantitative in-situ TEM study of stress-assisted grain growth

Abstract

Misorientation defects in coalesced self-catalyzed GaN nanowires

The coalescence of self-catalyzed GaN nanowires grown on Si substrates has been examined in order to identify the cause of coalescence and to characterize the defect structures resulting from it. Coalescence is found to occur due to both nanowire crystallographic misalignment and initial nanowire proximity independent of the growth conditions. Defects occasionally result from coalesced nanowires, and the root cause of defect creation is found to be crystallographic misorientation between the nanowires. Several different defect structures are identified, and the types of defects that may form are found to depend on the degree of that misorientation. In the most extreme cases “zipper”-like arrays of dislocations and stacking faults were observed.

Use of misorientation values to further understand deformation in rail steels

The high contact stresses between a railway wheel and rail result in local plastic deformation in both the rail and wheel steel. Optical metallography and hardness testing have traditionally been used to quantify the extent of deformation present; however, these methods only give limited information about the deformation mechanism and the role of microstructure. In this study, electron backscattered diffraction has been used to assess the depth and degree of deformation using the kernel average misorientation function; where the crystallographic misorientation between and within pearlite colonies has been quantified using a local average misorientation function. This technique gives invaluable crystallographic information about the deformed microstructure to aid understanding of the deformation mechanism. The application of the kernel average misorientation function has been modelled for idealised rail microstructures, including after the simulation of deformation via shear, in order to understand how the average kernel average misorientation values are developed under the high levels of deformation seen in rail steels.

Effects of local grain misorientation and β-Sn elastic anisotropy on whisker and hillock formation

Abstract

Geometrical influence for creep damage evaluation of IN738LC using electron backscatter diffraction

The Ni-based superalloy IN738LC is an outstanding heat-resistant material, but it does have the disadvantage of being brittle. Therefore creep degradation such as deformation of components or initiation of microscopic creep voids only appear at the end of its available creep life. To date, electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy has been used for damage evaluation. Some results showed that the misorientation was correlated with the magnitude of macroscopic plastic or creep strains. In this study, by using the EBSD method, the characteristics of the change and distribution tendencies of crystallographic misorientation on creep life have been investigated. In addition, the geometrical influence of stress concentrated areas for misorientation analysis has been investigated, which is important to evaluate damage in practical components like blades. As a result, the grain reference orientation deviation parameter which corresponds to the spread of creep strain has been found to be useful to estimate creep life among the misorientation parameters. Also, misorientation analysis allows the evaluation of creep damage before the appearance of obvious defects without being subjected to the geometrical influence of the stress concentrated areas.

Spin-dependent electron transport in manganite bicrystal junctions

Magnetic bicrystal films and junctions of magnetic La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) films epitaxially grown on NdGaO3 substrates with the (110) planes of their two parts misoriented (tilted) at angles of 12A degrees, 22A degrees, 28A degrees, and 38A degrees are investigated. For comparison, bicrystal boundaries with a 90A degrees misorientation of the axes of the NdGaO3 (110) planes were fabricated. The directions of the axes and the magnetic anisotropy constants of the films on both sides of the boundary are determined by two independent techniques of magnetic resonance spectroscopy. The magnetic misorientation of the axes in the substrate plane has been found to be much smaller than the crystallographic misorientation for tilted bicrystal boundaries, while the crystallographic and magnetic misorientation angles coincide for boundaries with rotation of the axes. An increase in the magnetoresistance and characteristic resistance of bicrystal junctions with increasing misorientation angle was observed experimentally. The magnetoresistance of bicrystal junctions has been calculated by taking into account the uniaxial anisotropy, which has allowed the contributions from the tunneling and anisotropic magnetoresistances to be separated. The largest tunneling magnetoresistance was observed on LCMO bicrystal junctions, in which the characteristic resistance of the boundary is higher than that in LSMO boundaries.

Quantitative analysis of grain boundary sliding by atomic force microscopy for early detection of intergranular damage

Atomic force microscopy (AFM) was used to analyze grain boundary sliding. This method was applied to measure the out-of-plane component of grain boundary sliding on two 304H grades and one X-750 grade. The main objective was to examine the initial stage of grain boundary sliding, after short-time constant load tests. The results showed that grain boundary sliding is activated at initial loading, producing a few tens nm high steps. The influences of grain boundary trace orientation, the applied load and the crystallographic misorientation were analyzed. The latter is the most distinctive parameter on grain boundary sliding propensity. An ‘initial stage’ of grain boundary sliding was identified. The proposed method allows early detection of intergranular damage sensitivity of industrial grades after short-time tests, and can be used to refine further long-time tests on heavier experimental devices, such as creep tests or stress corrosion cracking tests.

Microstructures, crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions

Naturally deformed hematite aggregates from 15 different iron ore mines located in Quadrilátero Ferrífero region, Brazil, were analyzed in order to verify the influence of increasing temperature and deformation intensity on their microstructural and textural aspects as well as the deformation mechanisms associated with the metamorphic conditions. The electron backscattered diffraction (EBSD) technique was applied in order to get qualitative and quantitative data concerning with microstructural parameters, crystallographic preferred orientation (CPO) and misorientation between hematite grains. The microstructures of these aggregates vary from randomly oriented hematite grains with approximately equant grains, to strongly oriented and elongated grains following the increase in deformational and metamorphic polarity toward east in the region. In the low deformation domain (western region) the deformation mechanisms are typically microfracturing and dissolution precipitation creep for magnetite rich aggregates. In the high-strain domain (eastern region), the deformation is accommodated by a combination of basal intracrystalline slip (c) (<a>) and grain boundary sliding, with rotation around hematite [c] axis. No evidences for recrystallization processes in these aggregates can be supported by our results, probably due to the superposition of subsequent processes.

Garnet crystal plasticity in the continental crust, new example from south Madagascar

AbstractGarnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ∼300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (1010 m−3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (&gt;1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.

Determination of the Mechanism of Restoration in Subtransus Hot Deformation of Ti-6Al-4V

Titanium alloys are attractive for structural applications in the aerospace industry due to their high specific strength in comparison with other engineering materials. These properties are strongly related to the microstructure obtained during thermo-mechanical processes. The influence of the processing parameters on the microstructure is investigated to determine criteria for the control of the forming processes. Pre-forged specimens of alpha-beta Ti-6Al-4V alloy with elongated primary alpha grains are deformed below the beta transus temperature between 0.1 and 10/s of strain rate. Compression is carried out parallel and perpendicular to the preferential orientations of the primary alpha grains. The local strain within the compressed samples is determined by finite element methods and correlated to the microstructure observed there. The alpha content is affected by the temperature of deformation and the morphology of the alpha grains is influenced by the strain and strain rate. Specimens with previous primary alpha grains parallel to the compression axis show a rotation of the alpha grains which were oriented almost perpendicular to the load axis. EBSD measurements are used to determine the restoration mechanism involved during hot deformation. Continuous dynamic recrystallization in the alpha grains is revealed by increasing the cumulative crystallographic misorientation towards the grain boundary and the formation of new grains. This misorientation increases with increasing values of the Zener Hollomon parameter (Z). For lower values of Z restoration occurs mainly in the beta phase.