EBSD Maps Research Articles

Surface Microstructure Modification in Square Extruded Al-Nb Powder Composites by Shot Peening

75%Al-25%Nb powder composites, fabricated by square shape cold extrusion, were subject to shot peening treatment with full coverage. Shot peening results in a high number of intense local deformations, with a surface roughness in our case of about l6gm. Due to the high local deformation down to nano-scale surface grain refinement and strain accumulation was generated. Previous texture characterization was performed by neutron diffraction and laboratory X-rays (Cu Kα radiation). The first method took advantage of the high penetration power and averaging capabilities and the second method was further used taking advantage of the low penetration to characterize surface microstructure modification. Peak broadening, before and after shot peening, was analyzed by MAUD software and domain sizes and microstrains were calculated for both phases. Simultaneous EBSD and EDS scans, on 30 nm step sizes, were performed on a FESEM Quanta 200 + TSL-EDAX, showing the highly heterogeneous microstructure developed because of shot peening. Protrusions, due to particle impacts, are clearly seen on EBSD maps. Results mainly revealed that, for Al phase, domain sizes decrease, while microstrains and dislocation densities consistently increase after the materials have been subjected to SP. For Nb phase the visible effect of SP is an increment of microstrains, and related dislocation densities, but keeping the domain sizes almost constant.

A method to correct coordinate distortion in EBSD maps

Drift during electron backscatter diffraction mapping leads to coordinate distortions in resulting orientation maps, which affects, in some cases significantly, the accuracy of analysis. A method, thin plate spline, is introduced and tested to correct such coordinate distortions in the maps after the electron backscatter diffraction measurements. The accuracy of the correction as well as theoretical and practical aspects of using the thin plate spline method is discussed in detail. By comparing with other correction methods, it is shown that the thin plate spline method is most efficient to correct different local distortions in the electron backscatter diffraction maps.

In-situ EBSD Phase Transformation and Recrystallisation

With the advent of in-situ heating stages that can fit into SEM's and the combination with EBSD, it is now possible to directly observe phenomenon such as phase transformations and recrystallisation at high spatial resolution and to link these processes to microstructural parameters.This presentation will report some results from preliminary in-situ EBSD heating experiments conducted in an SEM on the transformation of ausenite to ferrite in a plain carbon steel and recrystallisation in bronze alloy strip cast on a steel substrate. The microstructural changes observed during these experiments will be reported in terms of EBSD maps, grains size and crystallographic texture that evolves during a) a heating cycle from ferrite to austenite and cooling to ferrite and b) the recrystallisation microstructure for bronze and steel during isochronal heating.

Deformation Microstructure and Mechanisms in a Metastable β Titanium Alloy Exhibiting TWIP and TRIP Effects

Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

Numerical modeling of subgrain growth of hot extruded Al–4.5Zn–1Mg alloy in the presence of nanosized dispersoids

A numerical framework has been developed for predicting the sizes of subgrains in hot-extruded Al–4.5Zn–1Mg alloy. A combined numerical and analytical approach was taken to correlate subgrain growth rate with the deformation history of the extrudate and with the conditions of cooling after extrusion. Subgrain sizes in the as-deformed condition were calculated from strain, strain rate and temperature values determined from finite-element simulations. Subgrain growth during subsequent cooling was calculated from the mobility of low angle grain boundaries and the net driving force for growth by means of a physical model. The driving force was treated as the difference between stored energy, being inversely proportional to the average subgrain size, and the Zener drag pressure. With this combined approach, the correlations of the average subgrain size at the end of hot extrusion with strain, strain rate and temperature and its further evolution with time were established. Subgrain sizes after hot extrusion at different ram speeds and at different depths from the surface of the extrudate were measured by means of EBSD mapping and the results were compared with model predictions. Good agreement between the experimentally measured average subgrain sizes at the periphery of the extrudates and those predicted from the model was obtained.

Crystallographic control on the substructure of nacre tablets

Nacre tablets of mollusks develop two kinds of features when either the calcium carbonate or the organic portions are removed: (1) parallel lineations (vermiculations) formed by elongated carbonate rods, and (2) hourglass patterns, which appear in high relief when etched or in low relief if bleached. In untreated tablets, SEM and AFM data show that vermiculations correspond to aligned and fused aragonite nanogloblules, which are partly surrounded by thin organic pellicles. EBSD mapping of the surfaces of tablets indicates that the vermiculations are invariably parallel to the crystallographic a-axis of aragonite and that the triangles are aligned with the b-axis and correspond to the advance of the {010} faces during the growth of the tablet. According to our interpretation, the vermiculations appear because organic molecules during growth are expelled from the a-axis, where the Ca-CO3 bonds are the shortest. In this way, the subunits forming nacre merge uninterruptedly, forming chains parallel to the a-axis, whereas the organic molecules are expelled to the sides of these chains. Hourglass patterns would be produced by preferential adsorption of organic molecules along the {010}, as compared to the {100} faces. A model is presented for the nanostructure of nacre tablets. SEM and EBSD data also show the existence within the tablets of nanocrystalline units, which are twinned on {110} with the rest of the tablet. Our study shows that the growth dynamics of nacre tablets (and bioaragonite in general) results from the interaction at two different and mutually related levels: tablets and nanogranules.

Microstructure evolution in laser surface remelting of Ni–33wt.%Sn alloy

Abstract Laser surface remelting experiments have been performed on a Ni–33 wt.%Sn hypereutectic alloy. The microstructure characteristic, grain orientation and microhardness were investigated. The lamellar eutectic spacing decreased with increasing the scanning velocity, and the lamellar eutectic spacing is reduced significantly after laser rapid solidification. The anomalous eutectic was obtained in the bottom of the molten pool when the sample was melted thoroughly with the low scanning velocity of 0.1 mm/s. It is found that the free nucleation and rapid growth of two eutectic phases should be a necessary condition for the formation of anomalous eutectic. The EBSD maps and pole figures indicated that the grain orientations of anomalous eutectic obtained by laser remelting are analogous to that obtained by high undercooling solidification. The microhardness of anomalous eutectic is lower than that of regular lamellar eutectic.

Microhardness and EBSD microstructure mapping in partially-pressed al and cu through 90º ECAP die

Equal-channel angular pressing (ECAP) is widely recognized as an effective method for processing ultrafine-grained and even nanostructured materials. Important details on processes occurring in the die intersections can be obtained by mapping the microhardness and EBSD microstructures in partially-pressed aluminum and copper through the 90o die of ECAP. Precise measurements were made using grids of partially-pressed Al and Cu and detailed color maps were plotted and compared with EBSD maps. A narrow region along the bisector of two channels reflects altering in microhardness level of the material subjected to simple shear. The microstructural evolution suggests significant refining of the grain structure but there is noticeable inhomogeneity in the microhardness and microstructure for both materials. Factors contributing to the inhomogeneous hardness distribution include the coarse initial grain size, and inhomogeneous deformation across the plane of the die channel intersection due to die wall friction and the formation of a dead zone at the outer corner.

Modeling of Surface Dynamic Recrystallisation During the Finish Turning of the 15-5PH Steel

Modelling surface microstructural changes is a key issue when considering the manufacture of critical metallic parts. During machining, extreme conditions of temperature and deformation appear in the cutting zone. Consequently, surface microstructure is strongly affected with a direct impact on its integrity and the fatigue resistance of the part. Thus the machining effect on the 15-5PH steel surface has been underlined. To explain and predict the formation of this modified surface layer, the dynamic recrystallization (DRX) has been considered.Firstly, a DRX model has been identified using dynamic compressive tests. The calibrated DRX model is composed of two parts: a criterion of initiation and an evolution law. EBSD maps obtained from compressed samples show recrystallized microstructures comparable with micrographys of machined surface.Then, the DRX model has been implemented in an ALE orthogonal cutting model. Numerical simulation results predict the recrystallized layer at the machined surface. The DRX proportion rapidly decreases from 100% at the surface to 0% at few micrometers depth. So, experimental observations are successfully reproduced.

Accelerated Potts model for grain growth – Application to an IF steel

A modification of the Potts model for grain growth is presented. We add to the site energy a grain size factor that favors growth of large grains and whose magnitude has been optimized with respect to various aspects. The motivation for such a modification as well as the consequences in term of calculation speed are presented. The capabilities of this modified model are checked by comparing several characteristics of the microstructures obtained after various grain growth simulations with experimental data obtained for an IF steel. One large EBSD map, made at the end of the primary recrystallization, is used directly as the initial image for the calculations. A second EBSD map, obtained after grain growth (with a mean diameter of the grains about twice the initial value) was used for comparisons between the simulations and the experiment. The grain size distribution, the grain boundary character distribution and the crystallographic texture are reproduced rather well with this modified model using a moderate grain size factor effect. Also the speedup of the calculation is quite significant.

A new EBSD based methodology for the quantitative characterisation of microstructures formed by displacive fcc–bcc transformations

This work is concerned with a new methodology that can be used to quantify the degree to which grains in the microstructure are aligned in the form of packets. The methodology is based on a crystallographic definition of the term packet which is used to deduce the theoretically ideal misorientations of intra-packet grain boundaries. A misorientation distribution obtained from extensive EBSD mapping can thus be split into intra- and inter-packet misorientations and the corresponding fractions can be determined by integration. The theoretical framework of the methodology is explained and a step-by-step description of the procedure is given. Results from a trace analysis are provided to justify the assumptions made regarding habit plane and examples are included showing how the grain boundary network can be split into two separate parts, one for lath boundaries and the other for packet boundaries. Moreover, example weld metal microstructures along with the corresponding misorientation distributions as well as quantitative values of the microstructures are presented.

In situ observation of crack growth in a WC-Co hardmetal and characterisation of crack growth morphologies by EBSD

Controlled crack growth in a WC-Co hardmetal has been observed in-situ in the SEM using an ultra-stiff test rig to allow controlled application and removal of loads. Cracks were observed to propagate in jumps of between 20μm and 70μm with each jump occurring in less than 200ms. High resolution SEM images and EBSD mapping have been used to characterise the effects of microstructure and WC grain orientation on the crack path and the degree of plastic deformation in both binder and carbide phases. The fracture in the hardmetal grade examined is dominated by transgranular fracture of the carbide phase with increased plastic deformation in grains fracturing on {10–10} planes observed by EBSD. At variance with some published research, plastic deformation in the Co binder phase ahead of the crack tip has been observed over distances of several times the Co mean free path.

Plastic deformation and development of antigorite crystal preferred orientation in high-pressure serpentinites

We have inferred the deformation mechanisms of antigorite by high-resolution EBSD mapping of samples from Cerro del Almirez ultramafic massif (Betic Cordillera, SE Spain). Textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600–630°C and 1.6–1.9GPa. Deformation was followed by static annealing of antigorite at ca. 680°C. The Crystal Preferred Orientation (CPO) of antigorite is characterised by a strong alignment of (001) poles normal to the foliation plane and a weaker, but clear, parallelism between [100] axes and the macroscopic lineation defined by the elongation of magnetite aggregates. Analysis of misorientations across subgrains shows predominance of [010] rotation axis, consistent with activation of the [100](001) slip system. However, tilt subgrain boundaries subparallel to (100), probably formed by edge dislocations of this system, are subsidiary. Most subgrain boundaries are subparallel to (001) planes. They are interpreted as (001) twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins. Modelling of the evolution of the antigorite CPO using a lower bound approach and considering different deformation regimes and sets of basal and non-basal slip systems has shown that intensities of [100] and [010] maxima reflect the relative strength of the antigorite [100](001) and [010](001) systems. Activation of other basal or non-basal slip systems does not change significantly the CPO patterns, but results in less concentrated CPO. 3D transpression models better reproduce the CPO of natural antigorite serpentinites. We propose that the widespread occurrence of strong CPO in high-pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on [hk0](001), together with the activation of twinning, implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge.

Quantitative Microstructural Characterization of Thick Aluminum Plates Heavily Deformed Using Equal Channel Angular Extrusion

A detailed quantitative analysis of the microstructure has been performed in three orthogonal planes of 15-mm-thick aluminum plates heavily deformed via two equal channel angular extrusion (ECAE) routes. One route was a conventional route A with no rotation between passes. Another route involved sequential 90 deg rotations about the normal direction (ND) between passes. The microstructure in the center of these plates, and especially the extent of microstructural heterogeneity, has been characterized quantitatively and compared with that in bar samples extruded via either route A or route Bc with 90 deg rotations about the longitudinal axis. Statistically robust data were obtained in this work using gallium enhanced microscopy and EBSD mapping of large sample areas. For the plate processed using route A, the fraction of high-angle boundaries was found to strongly depend on the inspection plane, being smallest in the plane perpendicular to the ND (plane Z), where the largest subgrain size and most profound microstructural heterogeneities were also revealed. In comparison, the plate extruded with 90 deg rotations about the ND was less heterogeneous and contained smaller subgrains in plane Z. Comparing the plate and bar samples, the most refined and least heterogeneous microstructure was observed in the route Bc bar sample. The differences in the microstructure are reflected in the hardness data; the hardness is lowest after ECAE via route A and greatest in the bar sample processed using route Bc.

The Weighted Burgers Vector: A Quantity for Constraining Dislocation Densities and Types Using Electron Backscatter Diffraction on 2D Sections through Crystalline Materials

The Weighted Burgers Vector (WBV) is defined as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. It can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has five advantages over other measures of local lattice distortion. 1. It is a vector and hence carries more information than any scalar measure of local misorientation. 2. It has an explicit mathematical link to the individual Burgers vectors of dislocations. 3. Since it is derived via tensor calculus, it is not dependent on the map coordinate system, in contrast to existing measures of local misorientation which are not only scalar but dependent on the coordinate system used. 4. Calculation involves no assumptions about energy minimisation. 5. The numerical differentiation involved in calculating the WBV may introduce errors, but there is a direct mathematical link to a contour integral. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations. Errors in orientation measurement will have a much smaller effect here, and dislocations can be detected which are otherwise lost in the noise of any local calculation.

Recrystallization Behaviour of Nb and Ti+Nb Added Ferritic Stainless Steels

The effect of alloying elements and rough rolling condition on the microstructure evolution of ferritic stainless steel has been investigated in order to understand the recrystallization and precipitation behaviour during hot rolling. In the present study, a series of high temperature compression tests with plane strain deformation mode were conducted for Nb added and Ti+Nb added ferritic stainless steels. Compression tests then were subjected to various conditions of deformation temperature, reduction ratio and holding time. After the tests, EBSD mapping and SEM observation were performed to analyze the recrystallization and precipitation behaviour. Nb added and Ti + Nb added steels show an increasing tendency of recrystallization with an elevation of deformation temperature, holding time and reduction ratio. An increase of holding temperature and holding time enlarges the recrystallized regime due to a decrease of activation energy for recrystallization and a growth of recrystallized grain. A higher reduction ratio also increases the recrystallized regime due to a rise of stored energy for recrystallization. Nb added steel, however, is more resistant to recrystallization because most of Nb (C, N) particles in Nb added steel are finely dispersed in the matrix.

On the Use of EBSD to Study the Heterogeneity of Plastic Deformation

This paper deals with the study of plastic heterogeneity. It aims to study the role of both grain size and orientation distributions in the development of such heterogeneity. The considered material is an IF steel. EBSD maps have been made on the same areas before and after several degrees of extension. Parameters such as GOS (Grain Orientation Spread) or GOS/D (D the diameter of the grain) or GND (Geometrically Necessary Dislocation) densities have been determined for the whole set of grains as well as for subpopulations (smallest grains, largest grains for example). It appears that the character of neighboring grains plays a more important role than any of these parameters alone.

Grain Orientation Spread Values in if Steels after Plastic Deformation and Recrystallization

From an EBSD map made on a polycrystal one can define a set of grains using a criterion of misorientation between the adjacent pixels. Once such a list of grains is obtained, various quantities can be associated to each grain such as its size, (mean) orientation, GOS (Grain Orientation Spread) etc... The GOS associated to one grain is the mean value of the misorientations between all the pixels of the grain and the mean orientation of the grain. This value is quite sensitive to the state of the material (degree of plastic deformation, degree of recrystallization for example). Therefore it can help in interpreting the evolution of a microstructure during thermomechanical treatments. It is the purpose of this presentation to provide GOS values for an IF steel after several degrees of plastic deformation as well as after several annealing treatments which lead to partially or totally recrystallized states. The sources of influence on GOS values (as grain detection limit or grain size) are analyzed. The link between the values and the state of the material will be discussed at a global stage for a given population of grains.

Automatic correlation of grains from two different EBSD maps

Assuming that two EBSD maps corresponding to the same area of a given polycrystalline material in two states are available, an automatic method is proposed to correlate the grains as detected into the two maps. The method uses a combination of several criteria (geographic localization, size, misorientation) to perform the recognition of the corresponding grains. The effectiveness of the method is illustrated in the case of an IF steel submitted to plastic deformation.

Microcracking of high zirconia refractories after t → m phase transition during cooling: An EBSD study

High zirconia refractories are composed of a zirconia skeleton surrounded by an intergranular glassy phase. In these materials, zirconia undergoes up to two successive phase transitions during the manufacturing process, c → t then t → m. This leads, after complete cooling, to the formation of microcracks. Preliminary observations have enabled to identify the mechanism mostly responsible for the observed microcracking. In particular, SEM imaging emphasizes the link between the positions of cracks and the presence of distinct crystallographic domains. Thus, our work focuses on the arrangement of the monoclinic and tetragonal domains in zirconia dendrites. The assessment by XRD of the thermal expansion coefficients of zirconia at the lattice scale and the analysis of EBSD maps show that cracking is produced by the thermal expansion mismatch between groups of crystallographic variants. The further reconstruction of both cubic and tetragonal – in the case of a presence of monoclinic zirconia at room temperature – parent grains enables to determine the impact of each transition on the final microstructure and the generated microcracking.