Showing papers on "Electron backscatter diffraction published in 2012"

Twinning and martensite in a 304 austenitic stainless steel

Abstract: The microstructure characteristics and deformation behavior of 304L stainless steel during tensile deformation at two different strain rates have been investigated by means of interrupted tensile tests, electron-backscatter-diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The volume fractions of transformed martensite and deformation twins at different stages of the deformation process were measured using X-ray diffraction method and TEM observations. It is found that the volume fraction of martensite monotonically increases with increasing strain but decreases with increasing strain rate. On the other hand, the volume fraction of twins increases with increasing strain for strain level less than 57%. Beyond that, the volume fraction of twins decreases with increasing strain. Careful TEM observations show that stacking faults (SFs) and twins preferentially occur before the nucleation of martensite. Meanwhile, both ɛ-martensite and α′-martensite are observed in the deformation microstructures, indicating the co-existence of stress-induced-transformation and strain-induced-transformation. We also discussed the effects of twinning and martensite transformation on work-hardening as well as the relationship between stacking faults, twinning and martensite transformation.

Transmission EBSD from 10 nm domains in a scanning electron microscope

Abstract: Summary The spatial resolution of electron diffraction within the scanning electron microscope (SEM) has progressed from channelling methods capable of measuring crystallographic characteristics from 10 μm regions to electron backscatter diffraction (EBSD) methods capable of measuring 120 nm particles Here, we report a new form of low-energy transmission Kikuchi diffraction, performed in the SEM Transmission-EBSD (t-EBSD) makes use of an EBSD detector and software to capture and analyse the angular intensity variation in large-angle forward scattering of electrons in transmission, without postspecimen coils We collected t-EBSD patterns from Fe–Co nanoparticles of diameter 10 nm and from 40 nm-thick Ni films with in-plane grain size 15 nm The patterns exhibited contrast similar to that seen in EBSD, but are formed in transmission Monte Carlo scattering simulations showed that in addition to the order of magnitude improvement in spatial resolution from isolated particles, the energy width of the scattered electrons in t-EBSD is nearly two orders of magnitude narrower than that of conventional EBSD This new low-energy transmission diffraction approach builds upon recent progress in achieving unprecedented levels of imaging resolution for materials characterization in the SEM by adding high-spatial-resolution analytical capabilities

Role of Solute in the Texture Modification During Hot Deformation of Mg-Rare Earth Alloys

Abstract: Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing, the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer alloys (≥0.17 at. pct) show the weakening effect, whereas the most dilute alloy (≤0.03 at. pct) does not. Electron backscattered diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation (GND) content in dilute, hot-rolled alloys contain primarily basal 〈a〉 dislocations. At higher concentrations, the dislocations are predominantly prismatic 〈a〉 type. This change in the GND content suggests a change in dynamic recrystallization (DRX) mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering. Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening phenomenon in Mg rare earth alloys.

EBSD study of a hot deformed austenitic stainless steel

Abstract: The microstructural evolution of a 304 H austenitic stainless steel subjected to hot compression was studied by the electron backscattered diffraction (EBSD) technique. Detailed data about the boundaries, coincidence site lattice (CSL) relationships and grain size were acquired from the orientation imaging microscopy (OIM) maps. It was found that twins play an important role in the nucleation and growth of dynamic recrystallization (DRX) during hot deformation. Moreover, the conventional discontinuous DRX (DDRX) was found to be in charge of grain refinement reached under the testing conditions studied. Furthermore, the recrystallized fraction (X) was determined from the grain average misorientation (GAM) distribution based on the threshold value of 1.55°. The frequency of high angle boundaries showed a direct relationship with X. A time exponent of 1.11 was determined from Avrami analysis, which was related to the observed single-peak behavior in the stress–strain flow curves.

A study of the heterogeneity of plastic deformation in IF steel by EBSD

Abstract: The objective of this experimental study is to recognize the roles of several quantities like grain size and orientation distributions on the development of plastic heterogeneities. The measurements are performed on an interstitial free (IF) steel by Electron Back Scattered Diffraction (EBSD) at different states of deformation (from 0% to 17% tensile deformation). For each level of deformation, EBSD maps are performed before and after the deformation on exactly the same area. Several parameters as the Grain Orientation Spread (GOS), the Grain Orientation Spread over the grain Diameter (GOS/D) and the Geometrically Necessary Dislocation (GND) densities can thus be determined for different subpopulations of grains ranked as a function of individual grains sizes to follow the evolution of the deformed-induced microstructure. It appears that none of these grain scale measures are deciding and that grain neighborhood interactions play an important role.

Orientation informed nanoindentation of α-titanium: Indentation pileup in hexagonal metals deforming by prismatic slip

Abstract: This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516–3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal 〈a〉-glide and pyramidal〈c + a〉-glide, respectively.

Fracture Toughness of Polycrystalline Tungsten Alloys

Abstract: Tungsten and tungsten alloys show the typical change in fracture behavior frombrittle at low temperatures to ductile at high temperatures. In order to improve theunderstanding of the effect of microstructure the fracture toughness of pure tungsten,potassium doped tungsten, tungsten with 1wt% La2O3 and tungsten rheniumalloys were investigated by means of 3-point bending -, double cantilever beam -and compact tension specimens. All these materials show the expected increase infracture toughness with increasing temperature. The experiments demonstrate thatthe grain size, texture, chemical composition, grain boundary segregation and dislocationdensity seem to have a large effect on fracture toughness below the DBTT.These influences can be seen in the fracture behavior and morphology, where twokinds of fracture occur: on one hand the transgranular and on the other hand the intergranularfracture. Therefor techniques like electron backscatter diffraction, augerelectron spectroscopy and x-ray line profile analysis were used to improve the understandingof the parameters influencing fracture toughness.

Evolution of microstructure in laser deposited Al-11.28%Si alloy

Abstract: Laser melting of Al–Si alloys has been investigated extensively, however, little work on the microstructural evolution of laser deposited Al–Si alloys has been reported to date. This paper presents a detailed microstructural investigation of laser deposited Al–11.28Si alloy. Laser aided direct metal deposition (DMD) process has been used to build up solid thin wall samples using Al 4047 prealloyed powder. The evolution of macro- and microstructures of laser deposited Al–Si samples was investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. Microstructural observation revealed that the morphology and the length scale of the microstructures are different at different locations of the sample. A periodic transition of microstructural morphology from columnar dendrite to microcellular structure was observed in each layer. The observed difference in the microstructure was correlated with the thermal history of the deposit.

Effect of friction stir processing on microstructure and mechanical properties of aluminium

Abstract: Commercially pure aluminium was subjected to friction stir processing (FSP) to study the microstructure developed and its effects on the mechanical properties. Friction stir processing refined the grain size to 3 μm in a single pass from the starting coarse grain size of 84 μm. Electron backscattered diffraction (EBSD) results showed occurrence of dynamic recrystallization and also revealed existence of different orientations within the stir zone and across the transition zone. Transmission electron microscopy (TEM) revealed fine grains with well defined boundaries. The arrangement and absorption of dislocation into the sub-grain boundaries, formed by dynamic recovery, was also revealed by TEM. The yield strength of the material was improved by a factor of 2.4 after FSP owing to grain refinement. The most important feature of the friction stir processed material was that even after this significant improvement in strength there was little loss of ductility. The hardness also improved by 34% with the peak hardness being observed towards the advancing side.

An introduction to three-dimensional X-ray diffraction microscopy

Abstract: Three-dimensional X-ray diffraction microscopy is a fast and nondestructive structural characterization technique aimed at studies of the individual crystalline elements (grains or subgrains) within millimetre-sized polycrystalline specimens. It is based on two principles: the use of highly penetrating hard X-rays from a synchrotron source and the application of `tomographic' reconstruction algorithms for the analysis of the diffraction data. In favourable cases, the position, morphology, phase and crystallographic orientation can be derived for up to 1000 elements simultaneously. For each grain its average strain tensor may also be derived, from which the type II stresses can be inferred. Furthermore, the dynamics of the individual elements can be monitored during typical processes such as deformation or annealing. A review of the field is provided, with a viewpoint from materials science.

Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel

Abstract: The microstructures of 25CrMo48V martensitic steel quenched at the different temperatures of 800–1200 °C and tempered at 650 °C were characterized by optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The tensile and impact properties were evaluated from the as-quenched and tempered specimens. The results show that the refinement of the prior austenite grains due to the decrease of quenching temperature could induce the refinement of packets and blocks, while the lath width remained stable. The yield strength and the 50% fraction appearance transition temperature (50% FATT) varied linearly with the reciprocal square root of the prior austenite grain size, packet size and block width. Their relationships followed the classical formula of Hall–Petch and FATT = A − B d − 1 / 2 , respectively. The prior austenite grain size has a remarkable effect on the strength and toughness, while the block is the minimum structure unit controlling strength and toughness.

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Abstract: Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.

Effects of Solute and Second-Phase Particles on the Texture of Nd-Containing Mg Alloys

Abstract: Hot-rolled, binary Mg-Nd alloys with compositions ≥0.095 at. pct undergo the texture weakening phenomenon that has been reported in a number of Mg–rare earth (RE) alloys. However, alloys with compositions ≤0.01 at. pct retain a strong basal texture typical of pure Mg and other Mg alloys. Measurements of intragranular misorientation axes obtained using electron backscatter diffraction (EBSD) show that more dilute alloys contain predominantly basal $$ $$ dislocations, while richer alloys contain primarily prismatic $$ $$ dislocations. It is suggested that this change in dislocation content is related to a change in the dynamic recrystallization (DRX) mechanism. Metastable second-phase Mg x Nd1–x intermetallic particles are present within the alloys, and an annealing study indicates that the alloys undergoing texture weakening have grain sizes well predicted by classical Zener drag theory. Even though the more dilute alloys also contain second-phase particles, they are not sufficient to induce pinning. The promotion of nonbasal slip and the reduction in grain boundary mobility due to Zener drag are suggested as controlling mechanisms that promote the observed texture weakening phenomena.

Asymmetric and symmetric rolling of magnesium: Evolution of microstructure, texture and mechanical properties

Abstract: In the present study, asymmetric rolling was carried out for incorporating a shear component during the rolling at different temperatures, and was compared with conventional (symmetric) rolling. The microstructures were investigated using electron back-scatter diffraction (EBSD). The strain incorporated was compared with the help of grain orientation spread (GOS). GOS was eventually used as a criterion to partition the microstructure for separating the deformed and the dynamically recrystallized (DRX) grains. The texture of the partitioned DRX grains was shifted by similar to 30 degrees along the c-axis from the deformed grains. The mechanism of dynamic recrystallization (DRX) has been identified as continuous dynamic recovery and recrystallization (CDRR). The partitioned deformed grains for the higher temperature rolled specimens exhibited a texture similar to the room temperature rolled specimen. The asymmetric rolling introduces a shear component which shifts the texture fibre by similar to 5-10 degrees from the conventional rolling texture. This led to an increase in ductility with little compromise on strength. (c) 2012 Elsevier B.V. All rights reserved.