Microstructure evolution of CuZr polycrystals processed by high-pressure torsion

The microstructure evolution of extruded Cu–0.18 wt% Zr polycrystals processed by high-pressure torsion (HPT) at room temperature at the pressure of 4 GPa and the different number of the HPT revolutions (i.e. different strain) was investigated using the combination of the electron back-scatter diffraction, microhardness measurements and the X-ray diffraction. A significant transition from the inhomogeneous microstructure after few HPT revolutions into the homogeneous equiaxed microstructure with increasing number of HPT rotations was observed. HPT straining leads to the grain size refinement by a factor more than 100 after the 25 HPT revolutions. Moreover, the EBSD revealed an increase in the fraction of high-angle grain boundaries (HAGBs) with increasing HPT straining reaching the value of 70% after 25 revolutions. Additionally, a slight increase of the twin-related CSL Σ3 grain boundaries occurred during the microstructure refinement. The microhardness measurements confirmed the billet radial inhomogeneity at early stages of the HPT straining, whereas with increasing number of the HPT rotations, causing the specimen fragmentation and homogenization, the microhardness values increased. The average crystallite size and the average dislocation density in individual specimens determined by the XRD diffraction were in the range of approximately 100–200 nm and 2 × 10¹⁵ m⁻², respectively. Moreover, XRD measurements confirmed the absence of residual stresses in all specimens.     

Strengthening mechanisms of pure tungsten subjected to high pressure torsion: Deviation from classic Hall-Petch relation

In this work, the ultrafine grained (UFG) tungsten has been fabricated via high pressure torsion (HPT) of various turning numbers at a temperature of 823 K and a pressure of 1.5 GPa. The microstructure characteristics and mechanical properties of initial and high pressure torsion processed tungsten have been comparatively investigated by means of x-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and microhardness tests. It is shown that high pressure torsion leads to microstructure refinement with an average grain size of ∼0.92 μm and the fraction of high angle grain boundaries (HAGB) increasing to 62.9 %. Moreover, the dislocation density increases from initial 1.17×10¹⁴ m⁻² to 3.89×10¹⁴ m⁻². The microhardness tests revealed that hardness increased gradually and its distribution became more homogeneous with torsion strain increasing. The strengthening model was established considering the grain boundary strengthening and dislocation strengthening mechanisms. Resultantly, deviation of Hall-Petch slope was found from the classic values, which is attributed to the easy movement of the extrinsic dislocations in the high angle grain boundaries with high distortion energy and high density of defects.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.     

Modelling the hot plane strain compression test Part 1 – Effect of specimen geometry,strain rate,and friction on deformation

Abstract

Thermomechanically coupled finite element analysis of the hot plane strain compression test has been carried out to investigate the effect of various test parameters on the measured response and deformation of specimens. The results are presented in a series of papers. In this paper (Part 1), the results of two-dimensional simulations are discussed, evaluating the effects of material type, specimen geometry, strain rate, and friction on the overall deformation behaviour. The effects of spread and friction are detailed in Part 2, and the effects of asymmetry during the test are detailed in Part 3. The present results show that the local deformation behaviour is independent of the type of material and strain rate, at least up to 50 s^-1. The behaviour, however, depends strongly on friction and initial specimen geometry, with deformation becoming more uniform with decreasing initial specimen thickness, i.e. with increasing tool width w to specimen thickness h ratio. The deformation is constrained within the predicted slip line fields, and is controlled by the instantaneous geometry and not by the strain history. At low values of w/h the strain rate gradients are very high, and the raw data must be corrected for local strain rates and strains. Master curves have been produced to account for these gradients. The present results can be treated as generic, since the deformation is independent of material and the strain rate of deformation.     

Representation of electron backscatter diffraction data

Abstract

Current methods of data representation for electron backscatter diffraction (EBSD) measurements are reviewed. Obtaining diffraction data from microstructures using EBSD has become a relatively straightforward procedure, and EBSD software packages are used to represent these data as qualitative statistics in the form of ideal orientations, pole figures, inverse pole figures, Euler space, and Rodrigues–Frank space. Quantitative statistics in the form of secondary computations allow full microtextural analysis. Additionally, the power of EBSD is demonstrated through positional information representation. Through experimental examples, the conversion of EBSD data to statistical information to facilitate interpretation of results is demonstrated.

MST/3678     

Effects of strain rate on grain boundary evolution in Ti-55531 alloy

Ti-55531 alloy with an acicular α initial microstructure was hot-compressed at 998?K using strain rates of 0.001 and 1?s^?1. The effects of strain rate on the fragmentation of acicular α were investigated using electron backscatter diffraction (EBSD) from the standpoint of grain boundary evolution, e.g. low/high angle boundaries (LABs/HABs), the evolution of microtexture and the destruction of the Burgers orientation relationship (BOR) at α/β boundaries. Changes in the LAB fraction and average misorientation up to equilibrium indicate that the formation of LABs and a subsequent transition to HABs are prerequisites for the fragmentation of acicular α. The weakening of microtexture results from the formation of LABs/HABs in acicular α and the destruction of the BOR at α/β boundaries.     

Effect of hot compression and annealing on microstructure evolution of ZK60 magnesium alloys

Microstructural evolution of ZK60 magnesium alloys, during twin roll cast (TRC) and hot compression (HC) with a strain rate of 0.1 s⁻¹ at 350 °C and subsequent annealing at temperatures of 250–400 °C for 10²−5 × 10⁵ s, has been observed by optical microscopy (OM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The distribution of average grain size and recrystallized grain size at different annealing conditions were calculated. Activation energy and recrystallized volume fractions during annealing were discussed using analysis of static recrystallization (SRX) kinetics. Based on examination of microstructure evolution during annealing, it was found that several SRX mechanisms were co-activated. Subgrains with high misorientation angles to surrounding grains were formed by dislocation rearrangement, and they seemed to evolve into newly recrystallized grains.     

Dynamic recrystallization behavior of GH4169G alloy during hot compressive deformation

The microstructure evolutions and nucleation mechanisms of GH4169 G alloy were studied by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The hot compression tests were performed different imposed reductions in the range of true strain from 0.12 to 1.2 at the temperatures of 930 ℃-1050 ℃ with strain rates of 0.01 s⁻¹-1 s⁻¹. It is found that cumulative and local misorientation increase firstly and then decrease when the strain is increased due to the progress of dynamic recrystallization (DRX). The low angle boundaries (LAGBs) rapidly develop to high angle boundaries (HAGBs) at relatively high deformation temperature or the low strain rate. There are three DRX mechanisms observed for GH4169 G alloy during hot deformation. Discontinuous dynamic recrystallization (DDRX) as the dominant mechanism for GH4169 G alloy is characterized by typical necklace structures and bulged-original boundaries. Besides, different deformation bands with dislocation cells formed in deformed matrix at low temperature and large strain, which indicates that continuous dynamic recrystallization (CDRX) contributed to the DRX process. The twin boundaries lost their coherent characteristics and provide sites for nucleation, which also accelerates the nucleation of DRX.     

A procedure for indirect and automatic measurement of prior austenite grain size in bainite/martensite microstructures

An alternative procedure for indirect and automatic measurement of the prior austenite grain size (PAGS) in bainite/martensite is proposed in this work. It consists in the determination of an effective grain size by means of statistical post-processing of electron backscatter diffraction (EBSD) data. The algorithm developed for that purpose, which is available on-line, has been applied to simulated EBSD maps as well as to both nanocrystalline bainitic steel and commercial hot-rolled air-cooled steel with a granular bainitic microstructure. The new proposed method has been proven to be robust, and results are in good agreement with conventional PAGS measurements. The added value of the procedure comes from its simplicity, as no parent reconstruction is involved during the process, and its suitability for low-magnification EBSD maps, thus allowing a large step size and coverage of a substantially broader area of the sample than the previous methods reported.     

Microstructure and microtexture of ultrafine ferrite formed in 0.1%C steel using new strip rolling process

Abstract

A new hot strip rolling process is discussed which is capable of producing ultrafine, equiaxed ferrite grains (i.e. less than 2 µm)in the surface region of steel strip. Both microstructural and texture analysis of low carbon steel strip that has been rolled using this method are used to show that the ferrite forms by strain induced transformation. Analysis by electron backscatter diffraction (EBSD) indicates that a strong ferrite microtexture exists within the individual austenite grains in which the ferrite nucleates. The results from bulk X-ray texture analysis confirm that the ferrite forms as a result of transformation from austenite that has undergone heavy shearing during rolling, with nucleation occurring on the austenite substructure. In the centre region of the strip, a bainitic microstructure forms after rolling during air cooling. In the transition region between the surface and the centre of the strip, ferrite is shown to nucleate to form closely spaced parallel ‘rafts’ of ferrite grains traversing individual austenite grains. Again, EBSD is used to show that the ferrite located within these rafts is strongly textured, which, in combination with microstructural evidence, suggests that this ferrite nucleates along intragranular shear bands that form in the austenite in this region of the strip during rolling.     

Strengthening mechanisms of Al wires processed by equal channel angular torsion drawing

ABSTRACT

This paper reports the microstructure evolution, mechanical properties and strengthening mechanisms of a commercial purity Al alloy (Al 1350) after severe plastic deformation by a novel continuous method called ‘Equal Channel Angular Torsion Drawing (ECATD)’. Electron backscatter diffraction (EBSD) results revealed an inhomogeneous grain refinement including a large fraction of low-angle grain boundaries. After four passes, the microhardness increases from the initial value of 35?HV up to 44 and 62?HV at the centre and near to wire surface, respectively. A combination of high strength and ductility can be achieved based on the results of a Taguchi design of experiment (DoE) for mechanical properties. The strengthening occurs due to increment of the dislocation density, and development of mainly new low-angle grain boundaries.     

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract

Recent year's equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ～40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ～10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.     

Analysis of cellular microstructures in Al–Fe–Si alloy

Abstract

Experiments have been carried out on a commercial aluminium alloy rolled to foils with various high rolling reductions. Attention was focused on the formation of cells during deformation. Microstructures were examined in the SEM using electron backscatter diffraction (EBSD) and oriented imaging microscopy techniques. A treatment of the results obtained with EBSD has been used to determine the discrete parameters of the individual grains in the aggregate. The local driving pressures were calculated enabling the estimation of grain growth velocities dR/dt. The evolution of texture could be predicted and successfully compared with the experimental results of an annealing treatment.     

Grain boundary and fracture surface analysis of Fe–C–P steel

Abstract

The present paper investigates the distribution of grain boundary types and fracture surface crystallography in an Fe–C–P alloy. It is shown that electron backscatter diffraction (EBSD) is an effective technique with which to conduct these investigations. The proportions of both Σ1 and particularly Σ3 (in coincidence site lattice notation)present after various heat treatments were higher than would have been expected for random generation. There was limited evidence that both higher annealing temperatures and longer annealing times promoted generation of Σ3 type boundaries. The standard EBSD technique was modified and extended to encompass both the novel ‘matched fracture’ specimen approach and direct mapping from fracture surfaces to provide crystallographic information. A correlation was noted between higher aging temperatures and proportions of cleavage fracture. Furthermore, there was a strong correlation between cleavage fracture surfaces exhibiting river markings and an {001} surface orientation.     

Characterisation of high temperature oxidation using electron backscatter diffraction

Abstract

Electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) has been used to characterise the oxide scales formed on a low alloy steel. The technique provides a powerful combination of local phase information and orientational relationships both within and between phase layers. It has revealed that hematite grain growth occurs almost exclusively along the 0001 direction for the entire range of samples examined. Wüstite and magnetite grains were also found to grow preferentially along orientations close to the 001 direction.

EBSD is also well suited to characterising more complex scales such as those formed during hot working (e.g. millscale), and those formed on Fe–Ni alloys. In the latter complementary chemical information from energy dispersive spectroscopy (EDS), which was acquired simultaneously with the EBSD, enables the identity of crystallographically similar phases to be distinguished. EDS also shows that no nickel exists in the external scale and that it instead accumulates at the interface with the scale and at adjacent grain boundaries.     

Applications of electron backscatter diffraction to materials science: status in 2009

Over the last two decades electron backscatter diffraction (EBSD) in the scanning electron microscope has become a powerful tool for the characterisation of crystalline materials. Via an in-depth analysis of published work in 2008 compared with 2003, this article captures the present contribution that EBSD is making to materials science. From the analysis it is shown that some aspects of EBSD application have increased greatly in recent years, particularly the range of materials analysed, microtexture determination, general microstructure characterisation, application to interfaces and combinations of EBSD with other applications such as modelling or materials testing. On the other hand some other applications of EBSD are still emerging, such as macrotexture determination, true phase identification and three-dimensional EBSD.     

Electron backscatter diffraction: Strategies for reliable data acquisition and processing

In electron backscatter diffraction (EBSD) software packages there are many user choices both in data acquisition and in data processing and display. In order to extract maximum scientific value from an inquiry, it is helpful to have some guidelines for best practice in conducting an EBSD investigation. The purpose of this article therefore is to address selected topics of EBSD practice, in a tutorial manner. The topics covered are a brief summary on the principles of EBSD, specimen preparation, calibration of an EBSD system, experiment design, speed of data acquisition, data clean-up, microstructure characterisation (including grain size) and grain boundary characterisation. This list is not meant to cover exhaustively all areas where EBSD is used, but rather to provide a resource consisting of some useful strategies for novice EBSD users.     

Microstructure Evolution during Friction Stir Welding of Aluminum Alloy AA2219

The characterization of microstructure evolution in friction stir welded aluminum alloy was carried out by optical microscopy (OM) and transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The weld nugget consisted of very fine equiaxed grains and experienced dissolution of nearly half of metastable precipitates into the matrix during welding. Thermomechanically affected zone (TMAZ) also experienced dissolution of precipitates but to a lesser extent whereas coarsening of precip...     

Orientation, stress, and strain in an (001) barium titanate single crystal with 90° lamellar domains determined using electron backscatter diffraction

The orientation, stress, and strain in a single crystal of barium titanate (BaTiO₃), containing relatively large (0.5–14.5 μm), parallel, lamellar domains, have been determined and mapped using electron backscatter diffraction (EBSD). The strain distribution in the single crystal was determined using cross-correlation analysis of the EBSD patterns. Strain in the (001) single crystal was dominated by strain in the minority a-domain bands with peak values of ?0.006 determined in the surface plane, perpendicular to the intersection of the domain walls with the crystal surface, compared to +0.002 in the same direction for the majority c-domains. The out-of-plane strains were negatively correlated with the in-plane strains and were about a factor of two smaller. The experimentally determined strains peak at domain boundaries and suggest a contraction of the c-axis and an expansion of one of the a-axes as the domain wall is approached. The ratios of the in-plane and out-of-plane strains were consistent with the bulk elastic constants of BaTiO₃. Stress values determined from the strains and the elastic constants peaked at 400 MPa.     

Application of electron backscattered diffraction and crystal orientation mapping to recrystallisation studies in high strength interstitial free steels

Abstract

Electron backscattered diffraction (EBSD) has been demonstrated to be a valuable tool in gathering data to aid in the understanding of microstructural development in a variety of materials. A high strength interstitial free steel has been studied using the capabilities of a modern EBSD system, particularly by use of the crystal orientation mapping facility. Grain boundary characteristics have been determined and used to develop an understanding of the recrystallisation process. Little evidence in favour of the oriented growth theory has been found. It is proposed that the nucleation process during annealing is critical in controlling the fully annealed texture.