High resolution electron back-scatter diffraction analysis of thermally and mechanically induced strains near carbide inclusions in a superalloy

Cross-correlation-based analysis of electron back-scatter diffraction (EBSD) patterns has been used to obtain high angular resolution maps of lattice rotations and elastic strains near carbides in a directionally solidified superalloy MAR-M-002. Lattice curvatures were determined from the EBSD measurements and used to estimate the distribution of geometrically necessary dislocations (GNDs) induced by the deformation. Significant strains were induced by thermal treatment due to the lower thermal expansion coefficient of the carbide inclusions compared to that of the matrix. In addition to elastic strains the mismatch was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 × 10 13 m -2 in regions around the carbide. Three-point bending was then used to impose strain levels within the range ±12% across the height of the bend bar. EBSD lattice curvature measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbides particles. The higher GND densities near the carbides (order of 10 14 m -2) are generated by the large strain gradients produced around the plastically rigid inclusion during mechanical deformation with some minor contribution from the pre-existing residual deformation caused by the thermal mismatch between carbide and nickel matrix. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Probing deformation and revealing microstructural mechanisms with cross-correlation-based, high-resolution electron backscatter diffraction

High-resolution, cross-correlation-based, electron backscatter diffraction (HR-EBSD) is an emerging technique capable of measuring elastic strains, lattice rotations, and defect populations in crystalline materials. Here we briefly review development of the technique and the fundamental method. Application of HR-EBSD to metallic samples is illustrated with three examples: nickel-superalloy matrix/carbide interactions during cyclic deformation; interaction of a slip band and grain boundary; and patterning of stress and dislocation storage in deformed copper. These three examples highlight the ability of HR-EBSD to deliver new science by revealing new insights into the fundamental nature of deformation, as well as validating existing models. Application of the technique is now commonplace, and emergence of the technique is opening it up to the wider materials science community to tackle grand challenges. © 2013 TMS

A review of advances and challenges in EBSD strain mapping

High sensitivity cross-correlation based analysis of EBSD patterns was introduced by Wilkinson, Meaden and Dingley in 2006 [1, 2]. This paper will describe the basis of the method and the various modifications and improvements that have been made to it over the past few years. Strain sensitivity of ∼ 10-4 is readily obtained and sensitivity to lower strains is achievable if signal to noise ratio in the patterns is improved by simple integration. The method allows maps of local stress and dislocation density distributions to be generated in parallel with information concerning grain orientation, grain boundary misorientation and the presence of other microstructural features. The method is illustrated by example maps from a Ni-based superalloy and deformed Cu

Elastic strain tensor measurement using electron backscatter diffraction in the SEM.

The established electron backscatter diffraction (EBSD) technique for obtaining crystallographic information in the SEM has been adapted to permit elastic strain measurement. Basically, the displacement of crystallographic features in an EBSD pattern, such as zone axes, which result from strain in a crystal, is determined by comparing those same features as they appear in a pattern from an unstrained region of the crystal. The comparison is made by cross-correlation of selected regions in the two patterns. Tests show that the sensitivity to displacement measurement is 1 part in 10 000, which translates to a strain sensitivity of 2 parts in 10 000. Eight components of the strain tensor are determined directly and the ninth is calculated using the fact that the free surface of the sample is traction-free. Examples discussed are taken from studies of a lenticular fracture in germanium, the strain distribution surrounding a carbide precipitate in a nickel base alloy and grain boundary studies in another nickel base alloy

High-resolution electron backscatter diffraction: an emerging tool for studying local deformation

Electron backscatter diffraction (EBSD) is a widely available and relatively easy-to-use scanning-electron-microscopy-based diffraction technique. Recently, Wilkinson, Meaden, and Dingley presented two papers on a new cross-correlation-based analysis of EBSD patterns which allow variations in the elastic strain and lattice rotation tensors to be measured at a sensitivity of about 10 -4 at high spatial resolution. This paper briefly describes the basis of the technique and how the resulting lattice curvatures can be used to estimate the geometrically necessary dislocation (GND) content in a sample. To illustrate the utility of the method for microscale deformation studies the following examples are described: first, nanoindentation near a grain boundary in α-Ti; second, transformation-induced GNDs in a dual-phase steel; third, thermally-induced and mechanically-induced deformation near carbides in a superalloy; fourth, GND accumulation during fatigue of a polycrystalline Ti-6Al-4V alloy

A review of advances and challenges in EBSD strain mapping

High sensitivity cross-correlation based analysis of EBSD patterns was introduced by Wilkinson, Meaden and Dingley in 2006 [1, 2]. This paper will describe the basis of the method and the various modifications and improvements that have been made to it over the past few years. Strain sensitivity of ∼ 10-4 is readily obtained and sensitivity to lower strains is achievable if signal to noise ratio in the patterns is improved by simple integration. The method allows maps of local stress and dislocation density distributions to be generated in parallel with information concerning grain orientation, grain boundary misorientation and the presence of other microstructural features. The method is illustrated by example maps from a Ni-based superalloy and deformed Cu

Austenite stability and M2C carbide decomposition in experimental secondary hardening ultra-high strength steels during high temperature austenitizing treatments

The present study deals with the austenite stability and M2C carbide decomposition in three secondary hardening ultra-high strength (SHUHS) steels with varying levels of Cr and Mo (2Cr-1Mo, 2Cr-3Mo and 5Cr-5Mo) investigated using Vicker's hardness, optical and electron microscopy. These steels were subjected to high temperature austenitizing treatments at 1000, 1050, 1100 and 1150 °C. It has been established that increasing both Cr and Mo to 5 wt% as well as increasing the austenitizing temperature in this class of SHUHS steels is stabilizing the austenite such that almost 100% austenite is produced upon oil quenching. Further, higher Cr and Mo is also found to influence the stability of metastable M2C carbide formed during processing of the steels. While the M2C carbide in 2Cr-3Mo steel remained untransformed, it was found to transform partially to M6C during austenitization of 5Cr-5Mo steel. Hardness measurements on these steels revealed that hardness is relatively insensitive to austenitizing temperature in 2Cr-1Mo steel, decreased in 2Cr-3Mo and decreased more drastically in 5Cr-5Mo steel with austenitizing temperature. This dependence has been correlated to the influence of composition on Ms temperature and hence on retention of austenite as well as primary carbides. The experimental results were compared against theoretical calculations using ThermoCalc, which predict the presence of only M6C in both 2Cr-3Mo and 5Cr-5Mo steels. The apparent discrepancy between theoretical and experimental observations has been correlated to kinetic factors

Effect of sample thinning on strains and lattice rotations measured from Transmission Kikuchi diffraction in the SEM

Cross correlation based high angular resolution EBSD (or HR-EBSD) has been developed for measurement of elastic strains, lattice rotations (and estimating GND density). Recent advances in Transmission Kikuchi diffraction (TKD), especially the on-axis geometry allows the possibility of acquiring patterns at higher spatial resolution. However, some controversy remains as to whether stresses/strains measured after the sample thinning process are still representative of the bulk sample. In this paper, we explore a way of applying the HR-EBSD method to study strains and lattice rotations in an initially bulk sample, that is then progressively thinned down until a similar analysis can be performed on thin (and electron transparent) samples. Thus, HR-TKD will be compared as a possible alternative to HR-EBSD, in scenarios when it is not always possible to perform EBSD on the surface of the sample. An estimate of strain relaxation in the sample as a result of sample thinning is presented

Investigation on stabilization of ladle furnace slag with different additives

Ladle furnace slag disintegrates into fine powder during cooling due to phase transformations of di-calcium silicate. This creates an adverse impact on working conditions and the environment by dust generation. In this paper, a short overview on different studies to overcome the disintegration problem is provided. An attempt was also made to study the effects of several different additives and their mixtures on disintegration of slag. Phase equilibria calculations were carried out for some additives using FactSage® to understand the phase changes in the slag. Based on the phase equilibria calculations and literature data, initial laboratory experiments were conducted at 1650 °C with different additives such as boric acid, aluminium, and fly ash. Slag samples were analyzed with X-ray fluorescence and X-ray powder diffraction for chemical and phase analysis before and after treatment. The disintegration of slag can be prevented either by addition of 0.5 wt% or more of boric acid or 9 wt% of aluminium or 6 wt% of fly ash or 4–8 wt% fly ash along with 0.125–0.25 wt% of boric acid in slag. Based on the optimized conditions, industrial trials were conducted

In situ characterisation of the strain fields of intragranular slip bands in ferrite by high-resolution electron backscatter diffraction

High angular resolution electron backscatter diffraction has been used to quantify the local elastic field at the tip of mechanically loaded intragranular slip bands observed in situ in the ferrite grains of an age-hardened duplex stainless steel (Zeron 100). The surface elastic strain field was integrated to calculate in-plane and out-of-plane surface displacements. This allowed the elastic fields to be parameterised in a finite element analysis, which used the displacement field as the boundary conditions, to obtain the potential strain energy release rate (J-integral) and three-dimensional stress intensity factors (,,). This new analysis method is demonstrated by examining the elastic fields around the tip of an incipient slip band, an array of slip bands and the loading of a slip band. Direct measurement of the stress tensor in the grain identified the active slip systems with the highest Schmid factor. The stress intensity factors ahead of the slip band, measured under load, were directly affected by the magnitude of loading and the inclination angle of the slip band to the observed surface