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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The interface character distribution of cold-rolled and annealed duplex stainless steel

The interface character distributions (ICDs) of cold-rolled and annealed duplex stainless steel specimens, previously solid-solution-heated (SHT) at 1573 K and 1323 K, are investigated using electron backscatter diffraction (EBSD) and five-parameter analysis (FPA). For the δ-ferrite phase, high concentrations of low angle grain boundaries (LAGBs) are developed, and the boundary planes are predominantly oriented on (111). High angle grain boundaries (HAGBs) with misorientations ranging from 50{sup o} to 60° are mostly pure tilt boundaries, and the boundary planes are primarily located on (112). For the austenite phase, very high density of coherent twin boundaries are introduced, implying that the boundary planes are exactly oriented on (111). The phase boundary character distribution (PBPD) appears to be connected with the K-S and N-W orientation relationships (ORs) terminating on (110) {sub F}‖(111) {sub A} and (110){sub F}‖(hkl){sub A}, respectively. - Highlights: •Five-parameter analysis was used to determine interface character distribution. •ICD results of statistical meaning were achieved. •Initial microstructure has a significant effect on ICD. •Low and high angle GBs in δ phase prefer in (111) and (112) planes, respectively. •K-S phase boundaries terminate on (110){sub F}‖(111){sub A} while N-W ones on (110){sub F}‖(hkl){sub A}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Grain boundary inter-connections in polycrystalline aluminum with random orientation

Highlights: • The GBICs in a pure aluminum are not random, showing remarkable preference on the planes of low Miller index. • {1 1 1}/{1 1 1} is the most frequent GBIC in pure aluminum. • {1 1 1}/{1 1 1} GBIC is mainly coming from the GBs formed by rotations around , and axes. • {1 1 1}/{1 1 1} GBIC possesses higher PCSD and definite dislocation structures compared to the general GBs. • To pursue {1 1 1}/{1 1 1} GBIC is a pint-cut for the GBE research in high stacking fault energy FCC materials. - Abstract: Grain boundary inter-connection (GBIC) is the matching of two crystallographic planes from two abutting grains ending on the grain boundary (GB) position in polycrystalline materials, which can be expressed as {h_1k_1l_1}/{h_2k_2l_2}. GBIC is the critical parameter that intrinsically defines the character and properties of a GB. In current work, a zone-melted polycrystalline aluminum bar with a purity higher than 99.99% (mass fraction) was subjected to a multi-directional forging (MDF) with a true strain of 4 followed by a recrystallization annealing at 360 °C. Such process repeated at least 4 times until an equiaxed-grain microstructure with random orientation and averaged grain size approximate 30 μm was achieved. Then the GBICs were determined by electron backscatter diffraction (EBSD) measurement and stereology-based five-parameter analysis (FPA) coupled with crystallographic analysis after the grain boundaries (GBs) were filtered according to their misorientations (e.g. angle/axis pair). The results revealed that the GBICs for any group of GBs with a given misorientation are not random, showing remarkable preference on the planes of low Miller index forming mixed and twist GBs. The work also demonstrated that among the high angle boundaries (HABs), {1 1 1}/{1 1 1}, including coherent ∑3 boundaries, is the most frequent GBIC, mainly due to the GBs formed by rotations around <1 1 1>, <1 2 2> and <1 1 2> axes. Near coincidence site (NCS) and O-lattice theory analyses indicate that the {1 1 1}/{1 1 1} GBICs usually possess higher planar coincidence site density (PCSD) and definite dislocation structures compared to the general GBs, implying their more structural stability when only crystallography is taken into account. This result agrees very well with the recent results obtained by molecular dynamic simulations. It is significant to the grain boundary engineering (GBE) in the high stacking fault energy (SFE) face-centered cubic (FCC) materials such as aluminum and its alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Study on grain boundary character and strain distribution of intergranular cracking in the CGHAZ of T23 steel

Intergranular reheat cracking in the coarse-grained heat-affected zone of T23 steel was produced by strain to fracture tests on a Gleeble 3500 thermal–mechanical simulator. Then the grain boundary character, as well as the strain distribution after reheat crack propagation, was studied by electron backscatter diffraction technique. The results showed that incoherent Σ3 boundaries were seldom found on the prior austenite grain boundaries. Therefore, only the type of random high-angle boundaries played a crucial role in the intergranular cracking. Microstructurally cavities and small cracks were preferentially initiated from high-angle grain boundaries. Low-angle grain boundaries and high-angle ones with misorientation angles less than 15° were more resistant to the cracking. More importantly, the fraction of high-angle grain boundaries increased with the plastic strain induced by both temperature gradient and stress in the coarse-grained heat-affected zone, which contributed to the crack initiation and propagation. Furthermore, the strain distributions in the vicinity of cavities and cracks revealed the accommodation processes of plastic deformation during stress relaxation. It also reflected the strength differences between grain interior and grain boundary at different heat-treated temperatures, which had a large influence on the cracking mechanism. - Highlights: • The coincidence site lattice boundaries play little role in the reheat cracking. • Cavity and crack occur at high-angle grain boundaries rather than low-angle ones. • The strain leads low-angle grain boundaries to transform to high-angle ones. • Strain distribution differs for cavity and crack zones at different temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Using defects as a ‘fossil record’ to help interpret complex processes during additive manufacturing: as applied to raster-scanned electron beam powder bed additively manufactured Ti–6Al–4V

Abstract Defects in parts produced by additive manufacturing, instead of simply being perceived as deleterious, can act as important sources of information associated with the complex physical processes that occur during materials deposition and subsequent thermal cycles. Indeed, they act as materials-state ‘fossil’ records of the dynamic AM process. The approach of using defects as epoch-like records of prior history has been developed while studying additively manufactured Ti–6Al–4V and has given new insights into processes that may otherwise remain either obscured or unquantified. Analogous to ‘epochs,’ the evolution of these defects often is characterized by physics that span across a temporal length scale. To demonstrate this approach, a broad range of analyses including optical and electron microscopy, X-ray computed tomography, energy-dispersive spectroscopy, and electron backscatter diffraction have been used to characterize a raster-scanned electron beam Ti–6Al–4V sample. These analysis techniques provide key characteristics of defects such as their morphology, location within the part, complex compositional fields interacting with the defects, and structures on the free surfaces of defects. Observed defects have been classified as banding, spherical porosity, and lack of fusion. Banding is directly related to preferential evaporation of Al, which has an influence on mechanical properties. Lack-of-fusion defects can be used to understand columnar grain growth, fluid flow of melt pools, humping, and spattering events. Graphical abstract

36 MATERIALS SCIENCE↗
DOE OSTI2021

Interrogating the Effects of Hydrogen on the Behavior of Planar Deformation Bands in Austenitic Stainless Steel

The effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε -martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α '-martensite was observed at the intersection of ε -martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α '-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α '-martensite is a secondary effect of increased ε -martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Two-step deformation-induced martensitic transformation in additively manufactured High-Si stainless steel

Deformation-induced phase transformation in the additively manufactured (AM) Fe–18Cr–8Ni-5.5Si (in wt.%) alloy was analyzed using the in-situ electron backscattered diffraction (EBSD) record during uniaxial tensile testing in a scanning electron microscope (SEM). Analysis of EBSD data was focused to investigate phase transformation behavior during deformation with consideration of mechanical interactions among phases and grains in the ultrafine AM microstructure. The initial EBSD phase map illustrated that the as-built microstructure was a duplex structure consisting of major austenite (γ) and minor ferrite phases, some of which showed non-traditional orientation relationship since the rapid solidification in the AM process has produced non-equilibrium state in phase formation. In the deformed as-built AM alloy, the two-step martensitic transformation (γ→ε→α´) was identified as the main deformation-induced transformation mechanism. Martensitic transformation steps were found to be associated with glides of Shockley partial dislocations on closed packed (111) planes under the influence of atomic disturbance in the γ and ε phases, respectively. Using the closed-packed plane pole figures and unit cell orientations, their evolutions and mechanisms accompanying with the transformation were analyzed. Finally, the results indicate that one variant of ε-martensite forms from their parent γ phase, while two variants of α´ form from their parent ε phase.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-Entropy Alloy R&D for Accelerator Beam Window Applications

High-Entropy Alloys are a class of novel material that can offer improved resistance to beam-induced radiation damage and thermal shock. Development of these new alloys to serve as beam windows in multi-megawatt accelerator target applications is ongoing at Fermilab. Currently we are investigating AlCoCrMnTiV alloy systems of 4-6 components for service as beam windows; these compositions are predicted by CALPHAD simulation to have a low density and single-phase BCC crystal structure. The microstructures of these systems are being studied by electron microscopy techniques such as energy dispersive X-ray spectroscopy (EDS) to determine elemental homogeneity and composition, electron backscatter diffraction (EBSD) to quantify grain structure and orientation, and transmission electron microscopy (TEM) to observe defect structures and precipitate formation; nanoindentation is used to probe microstructural mechanical properties. Initial bulk property characterization utilizes differential scanning calorimetry to quantify specific heat capacity (cp), dilatometry to determine the coefficient of thermal expansion (CTE), and time-domain thermoreflectance (TDTR) to measure thermal conductivity (K). A miniature tensile testing apparatus is also being developed to test tensile properties. Post-irradiation examination is currently ongoing for several of these compositions that have been irradiated by low-energy ions to damage levels and at a temperature relevant to beam window applications at future next-generation accelerator facilities. This talk will briefly describe the alloy design and synthesis before going into more depth covering microstructural pre-characterization, and post-irradiation examination results from low-energy ion irradiated specimens. This will be followed by the plans for alloy down selection and future prototypic proton irradiations.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

EBSD characterization of the eutectic microstructure in hypoeutectic Fe-C and Fe-C-Si alloys

Highlights: • EBSD investigation of eutectic cementite and austenite in as-solidified low-alloy cast iron • Crystallographically independent growth of rapidly solidified cementite/austenite eutectic • Epitaxial growth of eutectic austenite from primary dendrites • Eutectic cementite and eutectoid ferrite frequently obey Bagaryatsky or Pitsch-Petch orientation relationship. - Abstract: Hypoeutectic Fe-C and Fe-C-Si model alloys were produced at different solidification conditions. Copper mold casting yields low cooling rates promoting the formation of a eutectic microstructure, which is characterized by two morphologies: elongated cementite plates and a rod structure growing perpendicular to the plates, i.e. austenite rods in a cementite matrix. Electron beam surface remelting generates a mainly plate-like eutectic due to rapid solidification. The microstructures were characterized by light-optical microscopy and electron backscatter diffraction (EBSD). The latter allows for a spatially resolved investigation of the growth crystallography of the eutectic phases. Thereby, a possible existence of crystallographic orientations relationships between cementite and austenite within the plate-like eutectic was assessed experimentally. The eutectic phases were found to grow largely crystallographically independently. Moreover, ferrite and eutectic cementite within the decomposed eutectic microstructure frequently comply with the Bagaryatsky or the Pitsch-Petch orientation relationship. Complementary X-ray diffraction (XRD) analysis reveals a pronounced cementite {002} texture in the microstructure produced by mold casting. Characteristic changes in the lattice parameters indicate that as-cast cementite is non-stoichiometric.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The role of titanium and vanadium based precipitates on hydrogen induced degradation of ferritic materials

Highlights: • Hydrogen induced damage is studied in Fe-C-Ti and Fe-C-V ferritic alloys. • Fe-C-Ti is less susceptible to hydrogen embrittlement than Fe-C-V. • Fe-C-V is less susceptible to blistering than Fe-C-Ti. • The role of diffusible hydrogen depends on the damage mechanism. - Abstract: The hydrogen induced damage of generic Fe-C-Ti and Fe-C-V ferritic alloys was investigated to assess the influence of precipitates on the hydrogen sensitivity of a material. The precipitates, formed during heat treatment, were evaluated by scanning transmission electron microscopy (STEM). The hydrogen/material interaction was evaluated by: 1) melt and hot extraction to determine the total and diffusible hydrogen content, respectively, 2) permeation experiments to calculate the diffusion coefficient, 3) thermal desorption spectroscopy to determine the hydrogen trapping characteristics of the materials. Furthermore, two different types of hydrogen induced damage were evaluated, i.e. hydrogen assisted cracking and blistering, resulting from electrochemical hydrogen charging with and without the application of an external load, respectively. Evaluation of the hydrogen induced damage and the role of the precipitates was performed by combining optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). An important though divertive role of diffusible hydrogen is observed in both damage mechanisms for the investigated microstructures. On the one hand, a large amount of diffusible hydrogen compared to strongly trapped hydrogen promotes hydrogen assisted cracking of materials, while on the other hand, the blistering phenomenon is delayed under such conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure characterisation of un-melted particles in a plasma sprayed CoNiCrAlY coating

Highlights: • The microstructure of the CoNiCrAlY powder particles was studied. • The un-melted particles in the VPS CoNiCrAlY coating were investigated. • A semi-continuous β-phase layer was found around the un-melted particles. • Micro-cracks were formed along the un-melted particles under compression. - Abstract: Un-melted particles in thermally sprayed coatings are usually classified as impurity elements. Such impurities have a strong influence on the coating behaviour due to their different microstructural characteristics. In this work, the microstructural and mechanical characteristics of un-melted particles in a vacuum plasma sprayed CoNiCrAlY coating have been investigated. Microstructural analysis revealed the dendritic structure of the as-received powder and a spherical core inside the powder particle. The as-deposited coating comprised a dual phase structure of γ-Ni and β-NiAl, with coarse grains ranging 1–2 μm and fine grains ranging 500 nm or even less. Un-melted particles were found to exist in the coating after plasma spraying. The grain structure in the un-melted particles has not been fully developed, as characterised via electron backscattered diffraction (EBSD). It is shown that the spherical core in the initial powder was retained as un-melted particles due to insufficient melting during plasma spraying. In addition, a semi-continuous β-phase layer was found at the un-melted particle boundaries. Due to the inherently brittle nature of the β-phase layer and poor bonding at the particle boundaries, cracking along the un-melted particle boundaries occurred under compression.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Tensile and compressive creep behavior of extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy

The tensile and compressive creep behavior of an extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy was investigated at temperatures ranging from 200 °C to 300 °C and under stresses ranging from 30 MPa to 120 MPa. There existed an asymmetry in the tensile and compressive creep properties. The minimum creep rate of the alloy was slightly greater in tension than in compression. The measured values of the transient strain and initial creep rate in compression were greater than those in tension. The creep stress exponent was approximately 2.5 at low temperatures (T < 250 °C) and 3.4 at higher temperatures both in tension and in compression. The compression creep activation energy at low temperatures and high temperatures was 83.4 and 184.3 kJ/mol respectively, while one activation energy (184 kJ/mol) represented the tensile–creep behavior over the temperature range examined. Dislocation creep was suggested to be the main mechanism in tensile creep and in the high-temperature regime in compressive creep, while grain boundary sliding was suggested to dominate in the low-temperature regime in compressive creep. Precipitate free zones were observed near grain boundaries perpendicular to the loading direction in tension and parallel to the loading direction in compression. Electron backscattered diffraction analysis revealed that the texture changed slightly during creep. Non-basal slip was suggested to contribute to the deformation after basal slip was introduced. In the tensile–creep ruptured specimens, intergranular cracks were mainly observed at general high-angle boundaries. - Highlights: • Creep behavior of an extruded Mg–RE alloy was characterized by EBSD. • T5 aging treatment enhanced the tension–compression creep asymmetry. • The grains grew slightly during tensile creep, but not for compressive creep. • Precipitate free zones (PFZs) were observed at specific grain boundaries. • Intergranular fracture was dominant and cracks mainly originated at GHABs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Elucidating microstructure of spinodal copper alloy through annealing

Improvement in the characteristics of a polycrystalline material is done through engineering of microstructures. Spinodal decomposed copper alloy has been annealed at high temperature with systematic variation in time on multiple specimens. Extensive electron backscatter diffraction experiment was carried out to understand the grain boundary characteristics and morphologies of the alloy as a function of annealing time. The logical variation of annealing time strongly influences diffusion induced grain characteristics such as, shape, size, geometry, inclination, density, types, distribution, misorientations and the nature of subsequently induced twinning morphologies. - Highlights: •Spinodal copper alloy is annealed at high temperature by systematic time variation. •EBSD measurements are performed to quantify grain geometries and morphologies. •Annealing time has a strong influence on grain morphologies, twins and misorientations. •Grain morphologies and geometries are correlated with hardness data. •Elucidating microstructure development through grain boundary engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Epitaxial (111) films of Cu, Ni, and Cu{sub x}Ni{sub y} on {alpha}-Al{sub 2}O{sub 3} (0001) for graphene growth by chemical vapor deposition

Films of (111)-textured Cu, Ni, and Cu{sub x}Ni{sub y} were evaluated as substrates for chemical vapor deposition of graphene. A metal thickness of 400 nm to 700 nm was sputtered onto a substrate of {alpha}-Al{sub 2}O{sub 3}(0001) at temperatures of 250 Degree-Sign C to 650 Degree-Sign C. The films were then annealed at 1000 Degree-Sign C in a tube furnace. X-ray and electron backscatter diffraction measurements showed all films have (111) texture but have grains with in-plane orientations differing by 60 Degree-Sign . The in-plane epitaxial relationship for all films was [110]{sub metal}||[1010]{sub Al{sub 2O{sub 3}}}. Reactive sputtering of Al in O{sub 2} before metal deposition resulted in a single in-plane orientation over 97% of the Ni film but had no significant effect on the Cu grain structure. Transmission electron microscopy showed a clean Ni/Al{sub 2}O{sub 3} interface, confirmed the epitaxial relationship, and showed that formation of the 60 Degree-Sign twin grains was associated with features on the Al{sub 2}O{sub 3} surface. Increasing total pressure and Cu vapor pressure during annealing decreased the roughness of Cu and Cu{sub x}Ni{sub y} films. Graphene grown on the Ni(111) films was more uniform than that grown on polycrystalline Ni/SiO{sub 2} films, but still showed thickness variations on a much smaller length scale than the distance between grains.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Influence of Temperature on the Texture Distribution in Two-Phase Region Hot Deformation of Grain-Oriented Silicon Steel

Hot deformation simulation test of 3% Si grain-oriented silicon (CGO) steel was carried out at different temperatures, and evolution of microstructures and recrystallized textures in two-phase region was analyzed by electron backscatter diffraction and optical microscopy. The “δ → γ” and “γ → α” phase transformation points of the two-phase region measured by differential scanning calorimetry method are 1272 and 819 °C, respectively, which are within the ranges of 1300-1350 and 800-850 °C, measured by water quenching. The hot deformation temperature has a large effect on the microstructure and texture in the steel. A higher hot deformation temperature leads to higher degree of recrystallization and larger average grain size, more γ-fiber textures and cube {100} <001> texture, but the rotated cube {100} <011> texture is stable. The recrystallized microstructure was also affected by the phase compositions. While the cube {100} <001> texture results in typical austenite recrystallization texture, {111} textures lead to typical ferrite rolling textures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Measurement of probability distributions for internal stresses in dislocated crystals

Here, we analyse residual stress distributions obtained from various crystal systems using high resolution electron backscatter diffraction (EBSD) measurements. Histograms showing stress probability distributions exhibit tails extending to very high stress levels. We demonstrate that these extreme stress values are consistent with the functional form that should be expected for dislocated crystals. Analysis initially developed by Groma and co-workers for X-ray line profile analysis and based on the so-called “restricted second moment of the probability distribution” can be used to estimate the total dislocation density. The generality of the results are illustrated by application to three quite different systems, namely, face centred cubic Cu deformed in uniaxial tension, a body centred cubic steel deformed to larger strain by cold rolling, and hexagonal InAlN layers grown on misfitting sapphire and silicon carbide substrates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Effect of small addition of Cr on stability of retained austenite in high carbon steel

High carbon steels with dual phase structures of martensite and austenite have considerable potential for industrial application in high abrasion environments due to their hardness, strength and relatively low cost. To design cost effective high carbon steels with superior properties, it is crucial to identify the effect of Chromium (Cr) on the stability of retained austenite (RA) and to fully understand its effect on solid-state phase transition. This study addresses this important knowledge gap. Using standard compression tests on bulk material, quantitative X-ray diffraction analysis, nano-indentation on individual austenitic grains, transmission electron microscopy and electron backscatter diffraction–based orientation microscopy techniques, the authors investigated the effect of Cr on the microstructure, transformation behaviour and mechanical stability of retained austenite in high carbon steel, with varying Cr contents. The results revealed that increasing the Cr %, altered the morphology of the RA and increased its stability, consequently, increasing the critical pressure for martensitic transformation. This study has critically addressed the elastoplastic behaviour of retained austenite – and provides a deep understanding of the effect of small additions of Cr on the metastable austenite of high carbon steel from the macro- to nano-level. Consequently, it paves the way for new applications for high carbon low alloy steels. - Highlights: • Effect of small addition of Cr on metastable austenite of high carbon steel from the macro- to nano-level • A multi-scale study of elastoplastic behaviour of retained austenite in high carbon steel • The mechanical stability of retained austenite during plastic deformation increased with increasing Cr content • Effect of grain boundary misorientation angle on hardness of individual retained austenite grains in high carbon steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of niobium and phase transformation temperature on the microstructure and texture of a novel 0.40% C thermomechanically processed steel

Field emission scanning electron microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) have been employed to investigate the effect of niobium and phase transformation temperature on the evolution of microstructure and texture in a novel thermomechanically processed, medium‑carbon, low-alloy steel intended for slurry pipeline applications. Thermomechanical processing consisted of hot-rolling in the austenitic region with deformation both above the recrystallization limit temperature and below the recrystallization stop temperature. Immediately after rolling, specimens were directly quenched in water to two different temperatures of 560 °C and 420 °C and subsequently furnace cooled from those temperatures to simulate the cooling of coiled strip on a hot strip mill. The microstructure of samples quenched to 560 °C mostly comprised of upper bainite, whereas the samples quenched to 420 °C mainly consisted of lath-type lower bainite. The transformation texture of all samples at the mid-thickness position consisted of α, γ and ε-fibers with high intensities close to the transformed copper, transformed brass and rotated cube components. The addition of 0.013 wt% Nb refined the microstructure and sharpened the texture. The texture of the small fraction of retained austenite present in the final microstructures indicated that the main bcc texture components result from the brass and copper components in the parent austenite.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Evolution of Σ3n CSL boundaries in Ni-Cr-Mo alloy during aging treatment

Highlights: • Σ3{sup n} boundary distribution characteristics are changed during aging treatment at 600 °C. • The increase in fractions of Σ9 and Σ27 boundaries depends on interaction between Σ3{sup n} boundaries. • The consumption of Σ3{sub c} boundaries is modeled with migration of random HABs. • Generation mechanism of Σ3{sub ic} boundaries differs from traditional models. - Abstract: The evolution of Σ3{sup n} (n = 1,2,3) boundaries in a Ni-Cr-Mo alloy during an aging treatment at 600 °C was investigated based on electron backscatter diffraction (EBSD) measurements. Incoherent Σ3 (Σ3{sub ic}) and coherent Σ3 (Σ3{sub c}) boundaries were identified using the single-section trace analysis method. The results show that the Σ3{sup n} boundary distribution characteristics are changed and that a Pt{sub 2}Mo-type Ni{sub 2}(Cr,Mo) superlattice phase precipitates during the aging treatment. Through the analysis and comparison of the average grain diameters measured under the different conditions, the migration behaviors of both the Σ3{sub ic} boundaries and random high-angle boundaries (HABs) are found. The migration of the Σ3{sub ic} boundaries provides the probability of interaction between Σ3{sup n} boundaries. This interaction is responsible for some different types of triple junctions, the increase in Σ9 and Σ27 boundary fractions and interrupted connectivity of the network of random HABs. However, the Σ3{sub c} boundaries' consumption caused by the migration of random HABs and the difficulty of forming new Σ3{sub c} boundaries lead to the decreased fraction of Σ3{sub c} boundaries. The further discussion about the increase in the fraction of Σ3{sub ic} boundaries indicates that the generation mechanism of the Σ3{sub ic} boundaries during the aging treatment differs from the models applied to recrystallization and subsequent grain growth processes.

36 MATERIALS SCIENCE↗