Electron Backscatter Diffraction Investigations Research Articles

Laser powder bed fusion of a β titanium alloy: Microstructural development, post-processing, and mechanical behaviour

β-Titanium alloys demonstrate unique properties, especially their low elastic modulus, good balance of strength and ductility, and strong heat treatment response. In this study, pre-alloyed powder of a β-Ti-alloy Ti–34Nb–13Ta–5Zr-0.3O (TNTZO) was processed using Laser Powder Bed Fusion (LPBF). Dense builds that demonstrate a full β microstructure with limited texture were achieved following process optimisation, with an elastic modulus of 56.5 GPa, tensile strength of 756 MPa, elongation-to-failure of 20 %, and recoverable strain of 1.3 %. Due to the high oxygen content, the β→α′′ deformation induced transformation was supressed, which resulted in a typical elastic-plastic stress-strain behaviour unlike the double-yielding behaviour typically experienced in superelastic Ti-alloys. The alloy was particularly sensitive to ageing heat treatments that resulted in needle α-precipitation, with tensile strengths exceeding 950 MPa, elastic modulus of ∼80 GPa, and recoverable strain of 1.5 %, yet at the expense of a reduced elongation-to-failure of 9 %. Transmission electron microscopy and electron backscattered diffraction investigations revealed the deformation mechanism was slip-dominated with no trace of twinning or stress induced phase transformation.

Nano-scale corrosion mechanism of T91 steel in static lead-bismuth eutectic: A combined APT, EBSD, and STEM investigation

T91 steel is a candidate material for structural components in lead-bismuth-eutectic (LBE) cooled systems, for example fast reactors and solar power plants [1]. However, the corrosion mechanisms of T91 in LBE remain poorly understood. In this study, we have analysed the static corrosion of T91 in liquid LBE using a range of characterisation techniques at increasingly smaller scales. A unique pattern of liquid metal intrusion was observed that does not appear to correlate with the grain boundary network. Upon closer inspection, electron backscatter diffraction (EBSD) reveals a change in the morphology of grains at the LBE-exposed surface, suggesting a local phase transition. Energy dispersive X-ray (EDX) maps show that Cr is depleted in the T91 material near the LBE interface. Furthermore, we observed the dissolution of all Cr-enriched precipitates in this region. Although the corrosion is conducted in an oxygen deficient environment, both scanning transmission electron microscopy (STEM) and atom probe tomography (APT) reveal a thin surface oxide layer (presumably wüstite) at the LBE-steel interface. Using electron energy loss spectroscopy (EELS) in the STEM, as well as APT, the atomic scale elemental redistribution and 3D morphology of the corrosion interface is investigated. By combining results from these different techniques, several types of oxide phases and structures can be identified. Based on this detailed nano-scale information, we propose potential mechanisms of T91 corrosion in LBE.

Electron Backscatter Diffraction Investigation on Microstructure Evolution of TiB2(p)/Al-Cu Composite after Single-Pass Equal Channel Angular Pressing for Formability Assessment

Electron Backscatter Diffraction Investigation on Microstructure Evolution of TiB2(p)/Al-Cu Composite after Single-Pass Equal Channel Angular Pressing for Formability Assessment

Fine-Tuning Melt Pools and Microstructures: Taming Cracks in Powder Bed Fusion—Laser Beam of a non-weldable Ni-base Superalloy

Powder Bed Fusion – Laser Beam (PBF-LB) of high γ’ strengthened Ni-base superalloys, such as CM247LC, is of great interest for high temperature applications in gas turbines. However, PBF-LB of CM247LC is challenging due to the high cracking susceptibility during PBF-LB processing (solidification cracking) and heat treatment (strain age cracking, mostly caused by residual stresses). This study focuses on understanding the impact of process parameters on microstructure, residual stresses and solidification cracking. Laser power (P), speed (v) and hatch spacing (h) were varied while the layer thickness (t) was fixed. The melt pool size and shape were found to be key factors in minimizing solidification cracking. Narrower and shallower melt pools, achieved using a low line energy density (LED = P/v ≤ 0.1 J/mm), gave low crack densities (0.7 mm/mm2). A tight hatch spacing (h = 0.03 mm) resulted in reduced lack of fusion porosity. Electron backscatter diffraction investigations revealed that parameters giving finer microstructure with 〈100〉 crystallographic texture had low crack densities provided they were processed with a low LED. Atom probe tomography elucidated early stages of spinodal decomposition in the as-built condition, where Cr and Al cluster separately. The extent of spinodal decomposition was found to be affected by the LED and the hatch spacing. Samples with low LED and small hatch spacing showed higher degrees of spinodal decomposition. X-ray diffraction residual stress investigations revealed that the residual stress is proportional to the volumetric energy density (VED = P/(v. h. t)). Although low residual stresses can be achieved by using low VED, there is a high risk of lack of fusion. Hence, other parameters such as modified scan strategy, build plate pre-heating and pulsed laser mode, must be further explored to minimize the residual stresses to reduce the strain age cracking susceptibility.

Bainite Formation in a Carbon-Free Fe–Cr–N System

In order to promote the understanding of bainitic transformation in nitrogen alloyed steels, the microstructure, transformation kinetics, and diffusion/precipitation characteristics of an isothermally heat-treated carbon-free system Fe–5Cr–N were investigated. Quenching dilatometry was used to perform heat treatments at temperatures between 400 °C and 500 °C and simultaneously receive information about the transformation kinetics. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) investigations indicate a plate- to lenticular-like, fully ferritic microstructure with nanometer scale CrN. In order to analyze the diffusion and precipitation behavior of nitrogen, atom probe tomography measurements were performed. On the one hand, nitrogen segregation was found, and on the other hand, the presence of CrN precipitates and preliminary stages of CrN nitrides were validated.

Detection of fatigue degradation in austenitic stainless steel with eddy current probe and machine learning

Low cycle fatigue tests are performed on specimens of niobium stabilized austenitic steel AISI 347 (1.4550) at ambient temperature. During the test, the fatigue specimens are equipped with eddy current probes, and it can be seen here that the impedance phase shift changes significantly at very early stages of fatigue (i.e. before cracking). Electron backscattering diffraction investigations were carried out to better connect microstructure evolution with impedance phase shifts. Machine learning techniques are employed to relate the impedance shift to the fatigue degradation. This approach allows also the derivation of fatigue life curves with few specimens.

In-situ SEM and EBSD investigation of the deformation behavior of extruded Mg-6Al-1Zn-1.1Sc alloy

The present study subjects the extruded Mg-6Al-1Zn-1.1Sc (wt%) alloy to reveal the deformation during in-situ tensile testing at room temperature by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results demonstrate that the parallel slip traces are increasingly observed on the surfaces of grains with increasing strains owing to the movement of dislocations inside the grains toward the sample surface, while microcracks are frequently observed at grain boundaries. The slip trace analysis indicate that many basal slips are activated during the deformation. The twinning behaviors of the extruded alloy during tensile testing are dominated by extension twinning. The orientation of grains is demonstrated to have a profound effect on their deformation, where grains with orientations deviating greatest from 〈0001〉//TD exhibit the highest dislocation density after deformation. This can be attributed to the fact that these oriented grains are prone to activate basal slip due to the large Schmid factor (SF). Similarly, the deformation of grains is also found to be highly dependent on their size, where the dislocation density of coarser grains increases more significantly than that of finer grains during deformation because coarser grains have greater space available for accommodating dislocations than finer grains.

Continuous tempering effect induced PWHT alternative technology using wire arc additive manufacturing for application in replacing nuclear pressurized water reactor system repairing: CALPHAD, FEM simulation, and EBSD investigation

This study proposes an effective repair technology using arc additive manufacturing for pressurized water reactors (PWRs) in nuclear power plants (NPP) aimed at avoiding complete replacements and post-weld-heat treatments (PWHTs) of component parts while ensuring safety and reliability. Effective repair technology is defined as economic and process efficiency, because of maintenance costs and radiation exposure, and it is critical in related industries. The technology is designed to relieve the hardness and martensite fraction of the welding heat affected zone (HAZ) of low alloy steels (SA508) in PWRs penetration/nozzles by heat source generated in the WAAM process, thus ensuring structural integrity. In the first layer of wire arc additive manufacturing processes, 89.6% of the martensite phase was formed in the HAZ of SA508, which was significantly reduced to 45.7% due to repetitive thermal behaviors at the third layer. The resulting process dramatically reduced hardness from 450Hv in the initial layer additive manufacturing to 320Hv in 3 layers without additional heat treatments. Moreover, the study quantitatively investigated the martensite starting temperature (Ms) and bainite starting temperature (Bs) and analyzed the microstructure and mechanical behavior of the developed process using thermodynamic calculation (CALPHAD), finite element method (FEM) simulation, and microstructure quantitative analysis by electron backs-catter diffraction (EBSD). The proposed technologies and their quantitative analysis results can be a substantial alternative to the repair technology for penetration/nozzles in nuclear primary water cooling reactor applications, complying with ASME Sec.IX Qw-462.12 and ISO 15614-1.

Influence of wire rolling on Zircalloy-2: tensile behaviour and microstructural investigation

In the present work, the microstructure, tensile and texture properties of Zircalloy-2 (Zr-2) processed by the room temperature wire rolling (RTWR) have been investigated. The detail microstructural investigation was performed with the help of optical microscopy, Transmission Electron Microscopy (TEM) and Electron Back Scattered Diffraction (EBSD). Dislocation density was calculated by using X-ray diffraction (XRD) through modified Williamson Hall technique. The dislocation density increased as the true strain increased from 0.69 to 1.32, but at the same time when true strain increased from 1.32 to 2.77, dislocations density start decreasing. It was occurred due to the formation of large volume fraction of dynamic recrystallization grains (DRX) after inducing true strain of 2.77. The maximum yield strength of 750 MPa was achieved after true strain of 1.32, but as the induced true strain increased to 2.77, the yield strength decreased to 620 MPa. It was due to the formation of high volume fraction of DRX grain and grain coarsening led by the dislocations annihilation mechanism. The highest ductility was achieved after true strain of 2.77 is attributed to domination of dislocations annihilation assisted mechanism. The EBSD and tensile test investigation further confirmed the presence of Extension twinning ({101¯2}<101¯1>) after inducing a true strain of 1.32 and more, which signify the severe deformation through the RTWR. Further, deformation mechanism of Zr-2 alloy has been proposed through processing by RTWR with the help of experimental investigation.

Analysis of the dislocation activity of Mg-Zn-Y alloy using synchrotron radiation under tensile loading.

An understanding of deformation behavior and texture development is crucial for the formability improvement of Mg alloys. X-ray line profile analysis using the convolutional multiple whole profile (CMWP) fitting method allows the experimental determination of dislocation densities separately for different Burgers vectors up to a high deformation degree. A wider use of this technique still requires exploration and testing of various materials. In this regard, the reliability of the CMWP fitting method for Mg-Zn-Y alloys, in terms of the dislocation activity during tensile deformation, was verified in the present study by the combined analysis of electron backscatter diffraction (EBSD) investigation and visco-plastic self-consistent (VPSC) simulation. The predominant activity of non-basal 〈a〉 dislocation slip was revealed by CMWP analysis, and Schmid factor analysis from the EBSD results supported the higher potential of non-basal dislocation slip in comparison with basal 〈a〉 dislocation slip. Moreover, the relative slip activities obtained by the VPSC simulation also show a similar trend to those obtained from the CMWP evaluation.

Formation and Characterization of the Recast Layer Formed on Inconel 718 during Wire Electro Discharge Machining.

The present work investigates the formation and microstructural and micro-mechanical characterization of the recast layer that formed on Inconel 718 alloy in the course of the wire electro-discharge machining (WEDM). The as-machined surface contains globules, shallow cracks, and re-deposition of molten materials, together with the elements from the decomposition of wire electrode and electrolyte, which does not exceed beyond the surface of the recast layer. Under presently investigated machining parameters, the recast layer was about 6.2 ± 2.1 µm thick. There was no presence of a heat-affected zone (HAZ), as otherwise indicated for other hard-to-cut materials. The transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) investigations show that the microstructure of the recast layer is similar to that of bulk alloy. Micro-mechanical characterizations of the recast layer were investigated via in-situ micro-pillar compression on the micro-pillars fabricated on the recast layer. The strength of the superficial layer (1151.6 ± 51.1 MPa) was about 2.2 times higher than that of the base material (523.2 ± 22.1 MPa), as revealed by the in-situ micro-pillar compression.

Assessment of Selected Structural Properties of High-Speed Friction Welded Joints Made of Unalloyed Structural Steel.

Commonly used S235JR structural steel, generally associated with good weldability, was joined by high-speed friction welding (HSFW). The friction welding tests were performed with a rotational speed of n = 8000 rpm and four different values of the unit pressure in the friction phase (pf) in the range of 64−255 MPa. The obtained joints were subjected to metallographic observations using an optical microscope; in selected zones of friction joints the average grain size was specified in accordance with the EN ISO 643:2012 standard; the hardness of friction joints was measured using the Vickers method. The friction-welded joint with the highest pf was EBSD-investigated. The obtained friction-welded joints resembled an hourglass, and the microstructure of individual zones of the joints differed depending on the height (axis, radius) of the observations. The generated joining conditions resulted in a significant refinement of the microstructure in the friction weld—the average grain size is about 1 µm2 (for base material it was 21 µm2). The highest increase in hardness above 340 HV0.1 was recorded in the friction weld of the welded joint with the lowest used value pressure in the friction phase. Such a sharp increase in hardness can make the resulting friction-welded joint become sensitive to dynamic or fatigue loads. The electron backscatter diffraction (EBSD) investigation confirmed the strong refinement of the microstructure in the friction-welded joint and the occurrence of the phenomenon of dynamic recrystallization (DRX). The friction weld was also characterized by a large share of high-angle boundaries (HAGBs) >80%. These results may indicate that during high-speed friction welding it is possible to create conditions like those obtained during the High-Pressure Torsion (the method used to produce UFG materials) process.

{[formula omitted]} twin–twin intersection-induced lattice rotation and dynamic recrystallization in Mg–6Al–3Sn–2Zn alloy

This study investigated the formation mechanism of new grains due to twin–twin intersections in a coarse-grained Mg–6Al–3Sn–2Zn alloy during different strain rates of an isothermal compression. The results of electron backscattered diffraction investigations showed that the activated twins were primarily {101¯2} tension twins, and 60° <101¯0> boundaries formed due to twin–twin intersections under different strain rates. Isolated twin variants with 60° <101¯0> boundaries transformed into new grains through lattice rotations at a low strain rate (0.01 s−1). At a high strain rate (10 s−1), the regions surrounded by subgrain boundaries through high-density dislocation arrangement and the 60° <101¯0> boundaries transformed into new grains via dynamic recrystallization.

Effect of notch severity and crystallographic texture on local deformation and damage in commercially pure titanium

The evolution of local deformation and damage in commercially pure titanium was studied for flat un-notched, U-notch and V-notch tensile specimens with two distinct initial textures namely off-basal (BA) and prismatic-pyramidal (PP) orientations to establish the micro-mechanisms of anisotropic deformation and damage in titanium for different notch severity. BA orientation showed higher apparent yield stress, greater plastic deformation, better notch strength ratio (NSR) value than PP orientation for all the cases. Electron backscatter diffraction investigation showed an increase in the number of twins per grain and the thickness of twins with notch severity irrespective of orientation with BA samples showing higher twin activity than corresponding PP samples. In addition, extensive slip activity was observed within twins and parent grains in BA orientation unlike PP orientation that showed slip activity near grain boundaries. These microstructural observations correlated well with strain measurement from digital image correlation which also showed that the local maximum longitudinal strain for BA orientation increased with notch severity unlike PP orientation. Viscoplastic self-consistent crystal plasticity simulations at the mesoscale indicated that the activity of basal, pyramidal <c+a> slip systems and twinning increased at the expense of prismatic slip with an increase in notch severity for both the orientations. The experimental observations show extensive diffuse necking in BA orientation, while higher fraction of fluted voids with c-axis aligned along the major axis of the void are observed in the PP orientation. Prismatic slip promotes nucleation and growth of voids in the prism plane which is perpendicular to the tensile axis for PP orientation contributing to void growth along c-axis forming fluted voids, while higher twinning in BA orientation leads to fewer fluted voids, thus highlighting the significant role of crystallographic texture in controlling anisotropic deformation and damage mechanisms in titanium.

Preparation of Carbide-Free Bainitic Steels for EBSD Investigations

Abstract Carbide-free bainitic steels are composed of bainitic ferrite laths, which are separated by films and larger islands of austenite, which is stabilized by carbon enrichment. Due to their multi-phase microstructure, the preparation of such steels for a characterization by means of electron backscatter diffraction (EBSD) represents a challenge. Attention must particularly be paid to prevent the transformation of the retained austenite to martensite during the preparation. This study compares the sample preparation by vibratory polishing using different suspensions to the electrolytic preparation. During vibratory polishing, it must be ensured that very little force is applied in order to prevent the metastable austenite from transforming. No influence of the different suspensions could be found. Electropolishing with a voltage of 40 V for 10 s at 23 °C is well suited for microstructural investigations. However, the surface relief is too pronounced for EBSD analyses. OP-U polishing for 15 min subsequent to electropolishing accomplishes the best results.

Quasi-in-situ electron backscattered diffraction investigation of twin-induced recrystallization during electrical pulse treatment in AZ61 alloy

In this work, static recrystallization induced by deformations twins in cryogenic rolled AZ61 alloy during electric pulse treatment is investigated by using quasi-in-situ electron backscattered diffraction. Besides of the {10 1- 2} tensile twins, {10 1- 1} compression twins, {10 1- 2}-{10 1- 1} double twins are also observed in the as-deformed AZ61 alloy. It has been found that recrystallization occurs preferentially at the early stage of electric pulse treatment, with {10 1- 2}-{10 1- 1} double twins and {10 1- 1} compression twins being effective nucleation sites for recrystallization. In contrast, lenticular {10 1- 2} tensile twins are consumed by surrounding grains that are with growth advantage. Our results suggest a feasible way to tailor the microstructure of Mg alloys through cryogenic rolling and electrical pulse accelerated recrystallization.

An EBSD based investigation on the deformation mechanisms under HCF-creep interaction in a Ni-based superalloy (alloy 617M)

The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.

Surface patterning allowing combined EBSD and DIC investigations during in-situ deformation experiments

A simple method for surface patterning using silica nanoparticles is presented and it is shown how under appropriate patterning conditions digital image correlation (DIC) can be carried out with sub-micrometer resolution, while still allowing collection of electron backscatter diffraction (EBSD) data over the same area with negligible effect on the EBSD indexing during multi-step in-situ deformation experiments. A demonstration based on in-situ tensile deformation of aluminum reveals significant heterogeneities in several EBSD- and DIC-derived parameters, both between, and within individual grains, with only a weak correlation between these parameters on the scale of the dislocation patterning.

A comprehensive study on solidification (hot) cracking in austenitic stainless steel welds from a microstructural approach

The present work evaluates solidification cracks in stainless steel welds that have been produced on a laboratory scale by carrying out a detailed EBSD (electron backscattered diffraction) investigation. The welding speed presented a near-linear relationship with the susceptibility to cracking. A wide range of microstructures were generated by varying the welding speed. The cracks were predominantly transgranular type and mainly propagated through grains with lower Taylor factors. These cracks were accompanied by a strong presence of a localized strain field (high local misorientation), which was observed by a higher presence of low angle grain boundaries (LAGBs). A correlation was found between the local in-grain misorientation and the crack sensitivity. With increased thermal tensile strains, certain LAGBs transformed into HAGBs to accommodate the strain and that resulted in the formation of strain-free grains. The elastic stored energy estimated from EBSD also showed an increase with the local misorientation.

TEM and EBSD investigation of a Re-doped β-NiAl coating on a Ni-based single crystal superalloy

To figure out the microstructure and precipitation phases of a Re doped β-NiAl coating, the electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were both used. Re-doped β-NiAl coating was composed of outer coarser β-NiAl grains with nanometer sized γ-Re precipitates, inner tiny β-NiAl grains with submicron sized α-W phases and bottom interdiffusion zone (IDZ, consisted of strip TCP and β-NiAl phases). Moreover, these γ-Re and α-W nanoparticle phases were not only aggregated at β-NiAl grain boundary but also appeared within the grains.