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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Experimental investigation of grain boundaries misorientations and nano twinning induced strengthening on addition of silicon carbide in pulse electrodeposited nickel tungsten composite coating

Nanoindentation was performed on silicon carbide (SiC) reinforced pulse electrodeposited nickel-tungsten (Ni-W) composite coating. Addition of 5 vol.% of SiC in Ni-W coating increased the hardness from 10.31 ± 0.65 GPa to 14.32 ± 0.63 GPa and elastic modulus from 119.74 ± 3.15 GPa to 139.26 ± 2.09 GPa. Increased hardness and elastic modulus directly translates to the improved strengthening in the coating. An experimental investigation of strengthening mechanism was carried out in Ni-W-5 vol.% SiC alloy. Two simultaneous phenomena viz. grain refinement and increased internal strain was observed, which increased the dislocation density from 5.51 × 10{sup 18} m{sup −2} to 1.346 × 10{sup 19} m{sup −2} on reinforcement of 5 vol.% of SiC in Ni-W coating. Increased dislocation density promoted the formation of grain boundary misorientations and nano twinning. Low angle grain boundary, high angle grain boundary and nano twinning were identified using high resolution transmission electron microscope (HR-TEM) image and their role in strengthening mechanism was discussed in details. - Highlights: • SiC reinforced pulse electrodeposition Ni-W coating was deposited on steel. • Nanoindentation showed the increased mechanical properties on addition of SiC. • Grain refinement and increased internal strain was observed in Ni-W-SiC coating. • Dislocation density increased on reinforcement of SiC in Ni-W coating. • Increased dislocation density triggered grain boundary misorientation and twinning.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructural evolution and mechanical properties on an ARB processed IF steel studied by X-ray diffraction and EBSD

Accumulative Roll Bonding (ARB) is one of the so-called severe plastic deformation (SPD) processes, allowing the production of metals and alloys with ultrafine (micro-nano) structures. Materials with ultrafine grains present attractive properties like the simultaneous increase in strength and ductility. Our interest in these materials is focused on their microstructural evolution during ARB processing, eventually responsible for the enhancement of those mechanical properties. In the current work we follow the evolution of the microstructure in an interstitial-free (IF) steel deformed by ARB after consecutive processing cycles, by means of Electron BackScatter Diffraction (EBSD) and X-ray diffraction (XRD). Particularly, we present results related to texture, grain (GS) and domain sizes, grain boundary character, density of Geometrically Necessary Dislocations (GND), Grain Orientation Spread (GOS), lattice parameters, microstrain, dislocation density and their spatial arrangement. After 5 ARB cycles the system shows a microstructure constituted mainly by submicrometric grains with high angle boundaries and low presence of dislocations inside the grains. - Highlights: •The evolution of microstructure is followed simultaneously by using GAM, GOS and GND (EBSD) and XRD. •LAGBs and subgrains disappear after few cycles SSDs and HAGBs persist at the end. •Dynamic recrystallization counterbalances dislocation arrays and diminishes hardening rate. •Grain size stabilization is revealed as a mechanism for increasing ductility after few ARB cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure evolution and mechanical performance of copper processed by equal channel angular rolling

Highlights: • Copper was processed by equal channel angular rolling (ECAR) up to 33 passes. • A very fine grained microstructure with high dislocation density was developed. • A decrease of dislocation density was observed for high numbers of passes. • The proof stress saturated at a value of about 400 MPa. • ECAR is a possible way of commercialization of ultrafine-grained materials. - Abstract: Ultrafine-grained (UFG) oxygen free high conductivity (OFHC) Cu samples were processed by severe plastic deformation (SPD) using the method of equal channel angular rolling (ECAR) up to 33 passes at room temperature. It was found that the grain size gradually decreased from ~ 40 μm to ~ 250 nm with increasing the number of passes. A maximum dislocation density of ~ 21 × 10{sup 14} m{sup − 2} was achieved after 13 passes of ECAR. For large numbers of passes (between 23 and 33), the dislocation density decreased to ~ 14 × 10{sup 14} m{sup − 2}. The proof stress was saturated at the value of about 400 MPa. The stored energy was measured by calorimetry and compared with the values calculated from the parameters of the microstructure. The reduction of the released heat after 13 and 33 passes suggested structural relaxation of the UFG microstructure. For these numbers of passes, the reduction of area in tensile testing was improved without decreasing the proof stress. The correlation between the microstructure and the mechanical behavior was discussed in detail. It was found that ECAR is capable for the mass-production of UFG metallic materials with high strength, therefore this method is a possible way of commercialization of SPD-processed materials.

36 MATERIALS SCIENCE↗
DOE OSTI2018

TEM investigations on lath martensite substructure in quenched Fe-0.2C alloys

Highlights: • Twin can be lath martensite substructure in low-carbon steels. • Detwinning behavior occurs in an auto-tempering process during quenching. • Detwinning may result in the formation of twinned laths. • Detwinning may result in the laths with dislocation-like contrast as substructure. - Abstract: Lath martensite in low-carbon steels is the main microstructural feature in the quenched state, and dislocations have been generally recognized as the substructure of lath martensite. However, the effects of quenching conditions on the quenched substructure have not yet been fully understood. In the present work, a Fe-0.2C (wt%) alloy was selected for detailed investigations on the microstructural evolution at various austenitizing temperatures. Microstructural characterization was carried out by using a conventional transmission electron microscope (TEM). The substructure of lath martensite was found to vary with austenitizing temperature. In the 1200 °C-water-quenched (WQ) samples, twinned laths (two specific variants having a {112}<111>-type twin relationship) and a twinned region (laths with the twin structure as their substructure) can be observed. Almost all lath martensite with a high density of fine twins as its substructure can be obtained in the 1300 °C-WQ samples. The twins gradually disappear or detwinning-occurs during TEM observations carried out with in situ heating. A further increase in temperature results in the formation of twinned laths and then lath with dislocation-like contrasts or lines appears as the substructure. These results suggested that a completely twin structure can be the initial substructure of lath martensite, and lath with dislocation-like contrasts or lines as its substructure is most likely the auto-tempering result of twinning.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hydrogen induced mechanical degradation in tungsten alloyed steels

Highlights: • The effect of H charging on tungsten alloyed steels is assessed by tensile tests. • The HE resistance increased for the Q&T condition compared to the as-Q state. • H trapping and absorption are determined by H characterization methods. • H diffusivity was higher in Q&T condition due to dislocation density decrease. • A correlation between the degree of HE and the amount of mobile H was established. - Abstract: The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W{sub 2}C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A → B → C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior of stainless steel weld overlay cladding exposed to a long-term thermal aging

Highlights: • Silicide phase formation with G-phase was first time to be observed. • Precipitation behavior discrepancy existed at different locations of ferrite phase. • G-phase and silicide phase formed along dislocations and within ferrite matrix. • Heterogeneous distribution of elements induced silicide phase formation. - Abstract: Thermal aging behavior of stainless steel weld overlay cladding aged at 400 °C for 10,000 h was investigated. The results showed that the precipitates along the dislocation and within ferrite matrix contained the face-centered cubic structural G-phase and silicide phase. Silicide phase, supposed to be (Fe, Mn){sub 3}Si phase, preferred to form with G-phase along the dislocations and within ferrite matrix. Silicide phase showed a cube-on-cube relationship with ferrite phase and G-phase. Heterogeneous distribution of elements within Ni-Mn-Si clusters was responsible for silicide phase formation during thermal aging process, and the corresponding precipitation behavior discrepancy at different forming sites (dislocations, ferrite matrix and phase boundary) was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characteristics of kinetic sprayed Ta in terms of the deposition behavior, microstructural evolution and mechanical properties: Effect of strain-rate-dependent response of Ta at high temperature

Highlights: • The effect of high-temperature rate-dependent response of BCC Ta on the deposition state of kinetic sprayed Ta particles with synergistic effect of powder oxidation • Abnormal increase of dislocation density above certain powder preheating temperature under the lower condition (N{sub 2} carrier gas), whereas considerable occurrence of grain refinement under the higher condition (He carrier gas). • Most significant dislocation density with the smallest volume-averaged domain size under the condition of 600 {sup o}C powder pre-heating with N{sub 2} carrier gas due to characteristic response of Ta. - Abstract: Numerous efforts have been focused on the fundamental research regarding kinetic spraying processes, but previous studies were biased to only few kinds of metallic materials. In particular, for Ta with body-centered cubic crystal structure, systematic studies are currently lacking in the literature. In this study, the characteristic deposition behaviors, microstructures, and mechanical properties of kinetic sprayed Ta under various impact conditions were systematically investigated. Generally, unusual features were usually related to strain-rate-dependent response (i.e. Peierls-Nabarro short range barriers) of Ta at high-temperature. This made thermal softening effect to be saturated above a certain degree of temperature, contributing to inferior deposition efficiency for the pre-heating at 600 °C than those of 400 and 500 °C by creating synergy with powder oxidation. In terms of microstructural evolutions, the dislocation density was abnormally increased at a certain powder preheating temperature (i.e., 600 °C), owing to the strain-rate-dependent response of Ta at high-temperature. On the other hand, grain refinement generated by rotational dynamic recrystallization was mainly observed under the higher kinetic energy condition (He carrier gas) at RT. Especially, the dislocation density and volume-average domain size evaluated via high-resolution X-ray diffraction peak profile analysis were corresponding to the results of the microstructural observations. Additionally, the nanomechanical properties (i.e., nanohardness and elastic modulus) evaluated via nanoindentation tests were in good agreement with the results from transmission electron microscopy investigations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct observations of crystal defects in polycrystalline diamond

Highlights: • Electron channeling contrast imaging of crystal defects in polycrystalline diamond • Introducing acid etching as a novel protocol for diamond surface preparation • Characterization of {111} twins, {111} stacking faults and {111} helical dislocations in diamond using ECCI and EBSD - Abstract: Crystal defects are abundant in synthetic diamond produced by chemical vapor deposition (CVD). We present the first images of crystal defects in a bulk polycrystalline CVD diamond sample using general electron channeling contrast imaging (ECCI) in a field emission scanning electron microscope (FE-SEM). For enhancement of channeling contrast of this material, we introduce a novel protocol for diamond surface preparation that involves acid etching. Using this protocol, we imaged three types of crystal defects including twins, stacking faults and dislocations. Each defect was identified based on its appearance in electron channeling contrast (ECC) micrographs. We analyzed grains containing twins and dislocations using electron backscatter diffraction (EBSD) crystal orientation mapping. We found a large population of grains that contained Σ3 type twins on {111} planes with a 60°<111> angle–axis pair of misorientation for twin boundaries. In addition, we identified {111} stacking faults and {111} helical dislocations. These observations are in agreement with reports of crystal defects in CVD diamond thin foils studied by a transmission electron microscope (TEM).

36 MATERIALS SCIENCE↗
DOE OSTI2018

The grain refinement mechanisms of various phases in shot-peened Nickel-Aluminum bronze (NAB) alloy

Highlights: • There exist two grain refinement mechanisms at the same time in the α phase. • The refinement of β' phase is attributed to the formation of dense and ordered nano twins. • The brittle strip-shape κ{sub iii} phases are fragmented after large deformation. • The κ{sub iv} and κ{sub ii} phases exhibit the twinning and dislocation deformation modes, separately. • The microhardness increases significantly owing to work hardening effect and grain refinement. - Abstract: In this study, coarse-grained Nickel-Aluminum bronze (NAB) alloy is subjected to shot-peening (SP) treatment, and the grain refinement mechanisms of various phases in shot-peened NAB alloy are investigated separately. The results show that severe deformation layers are produced on the surface. Meanwhile, refined grains with the average size below 100 nm are induced through SP treatment indicated by the results of X-ray diffraction (XRD) patterns. Various phases in NAB alloy including α, β' and three intermetallic phases (κ{sub ii}, κ{sub iii} and κ{sub iv}) display different grain refinement mechanisms. The grain refinement in the α phase involves dislocation segmentation in the precipitation-free zone and the twinning intersection in the precipitation zone. The formation of nanotwins by emitting partial dislocations at the grain boundary is found to be responsible for the grain refinement in the β' phase. As for the κ phases, the strip-shaped κ{sub iii} phases are fragmented after severe deformation, and the κ{sub iv} and κ{sub ii} phases exhibit the twinning and dislocation deformation modes, respectively. Also, the microhardness is significantly enhanced after SP treatment, which is mainly attributed to the work hardening effect and grain refinement.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formation of gradient microstructure and mechanical properties of hot-pressed W-20 wt% Cu composites after sliding friction severe deformation

Highlights: • W particles of composites were refined during sliding friction treatment (SFD). • Dislocation, dislocation tangles and boundary clearly exist in deformed W particle. • YS and UTS of SFDed sample are 308.74 MPa and 553.60 MPa owing to the refinement. • Electrical conductivity of SFDed sample decreased from 33 IACS% to 28.5 IACS%. • Fracture mode changing from inter-granular mode to trans-granular. - Abstract: W-based alloys are currently considered promising candidates for high heat flux components in future fusion reactors. In this paper, hot pressed W-20 wt%Cu composites were treated at room temperature using a sliding friction severe deformation (SFD) process, with a moving speed of 0.2 m/s and an applied load of 500 N. Microstructural evolution of composites after the SFD treatment was evaluated and compared with that of the untreated composites. Results showed that there was a gradient structure generated and an obvious refinement in tungsten particles size in the surface layer after the SFD process. The average particle size of tungsten in the SFD treated composites was 2.60 μm, whereas it was 4.5 μm for tungsten in the untreated composites. Fracture surfaces of the composites indicated that the SFD treatment destroyed the W skeleton and changed fracture mode from predominant inter-granular one to trans-granular one due to the decrease in contact area of W-W inter-particles. Yield strength and ultimate tensile strength of composites after the SFD treatment were 308 MPa and 553 MPa, respectively. The treated composites exhibited micro-hardness values with an average reading of about 308 HV. Analysis of the facture microstructures clearly suggested that the tungsten particles in the treated composites are consisted of dislocations and boundaries as well as dislocation tangles. The electrical conductivity of the composites was decreased from 33 IACS% to 28.5 IACS% after the SFD treatment, mainly due to loss or squeezing of copper into the inner surface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Low cycle fatigue behavior of AlSi10Mg(Cu) alloy at high temperature

Highlights: • The cyclic stress response of AlSi10Mg(Cu) alloy in LCF regime is temperature-dependent. • Precipitates transitions were observed after high temperature cycle loading. • Changes in the interaction mechanism between dislocations and nanoparticles lead to different cyclic stress responses. - Abstract: Low cycle fatigue (LCF) behavior of AlSi10Mg(Cu) alloy was investigated at 100 °C and 250 °C over a range of strain amplitude from 0.25% to 0.40%. Cycle stress response, hysteresis loops, fatigue life, as well as post-fatigue microstructure were studied in detail. Except 0.25%, the alloy performed cycle hardening behavior at 100 °C and softening behavior at 250 °C at any given strain amplitude, which corresponded well with the observed phenomena in hysteresis loops. At any given temperature, cyclic stress increases while fatigue life decreases with the increasing strain amplitude. The fatigue life of the studied alloy at 100 °C was higher than that at 250 °C under the same strain amplitude. Fractography indicated that most fatigue crack initiated from casting defects. The fatigue crack propagation was mainly characterized by fatigue striations and tear ridges in any conditions. Section inspection shows that the number of cracked eutectic Si increases with decreasing the temperature or increasing strain amplitude. Post-mortem transmission electron microscopy (TEM) investigations indicated that cyclic hardening behavior at 100 °C was due to the pinning effect of coherent precipitates (β″-Mg{sub 2}Si and GP zones). Most of tiny coherency precipitates and clusters transformed to large semi-coherency Q′ and β′-Mg{sub 2}Si precipitates after cyclic loading at 250 °C, leading to the sharp decrease of pinning number and dislocation density, which is a key reason for the cyclic softening. Another factor is that more dislocations can be activated and dislocation climb is easier at higher temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crystal rotation and microstructures in an aluminum single-slip system under tensile loading

Highlights: • The crystal rotation angle with the progression of ductile damage was larger in the vertical plane than that in the horizontal plane. In the measurement along 110, the relative rotation angle was the maximum at the intersection of 111 and 110. • The integrated intensity and the FWHM decreased near the notch (from 0 to 0.3 mm) as well as at the intersection of 110 and 111 in the measurement along 110. • The dislocation cell size and the dislocation density decreased at the intersection of 111 and 110 in the measurement of 110 for ε = 5%. This behavior was attributed to the strain energy being released by the rotation of the dislocation cell. We experimentally investigated the rotation angle of the (220) plane in an aluminum single-slip system in which the angles of 110 and 111 in the single crystal were 45° relative to the tensile loading direction. Crystal rotation was investigated at several spots (50 × 50 μm) along 110 and 111 using synchrotron radiation white X-rays at the BL28B2 beamline of super photon ring-8. In measurements along the 110 axis, at an applied strain of ε = 5%, the relative crystal rotation of (220) in the vertical plane increased from 0.850° to 5.438° in the range of 0.9 to 1.4 mm. We observed the microstructure using a transmission electron microscope and verified the size of the dislocation cells or subgrains at each spot along 110. The fundamental local crystallography revealed in this work should be helpful in establishing a method for evaluating ductile damage in materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Comparison of EBSD, DIC, AFM, and ECCI for active slip system identification in deformed Ti-7Al

The use of SEM-DIC, AFM, ECCI, and HR-EBSD to characterize slip-system activity was assessed on the same material volume of Ti-7Al. This study presents a robust comparison of the various methods for the first time, including an assessment of their advantages and disadvantages, and how they can be used effectively in a complementary fashion. The analysis of the different approaches was carried out in a blind, round-robin manner at three different universities. A Ti-7Al specimen was deformed in uniaxial tension to approximately 3% axial strain, and the active slip systems were independently identified using (i) trace analysis; (ii) in-SEM digital image correlation, (iii) observations of residual dislocations from ECCI, and (iv) long-range rotation gradients through HR-EBSD, with consistent trace identification in all cases. Displacement data from AFM was used to augment SEM-DIC displacement data by providing complementary out-of-plane displacement information. Furthermore, short-range dislocation gradients (measured by DIC) provided insight into the residual geometrically necessary dislocation (GND) content, and was consistent with the GND content extracted from EBSD data and ECCI images, confirming the presence of residual GNDs on the dominant slip systems resulting in visible slip bands. Furthermore, these approaches can be used in tandem to provide multi-modal information on slip band identification, strain and orientation gradients, out-of-plane displacements, and the presence of GNDs and SSDs, all of which can be used to inform and validate the development of dislocation-based crystal plasticity and strain gradient models.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Effects of Fe atoms on hardening of a nickel matrix: Nanoindentation experiments and atom-scale numerical modeling

Significant hardening effect due to Fe concentrations in Ni-based alloys with face-centered-cubic structure has been studied by using a combined experimental and atomistic-based computational approach via nanoindentation tests. The obtained experimental load–displacement data for the [0 0 1] crystal orientation reached a qualitative good agreement with molecular dynamics simulations results, leading to strong evidence that the main strengthening factors are associated to sluggish dislocation diffusion, reduced defect sizes and the nucleation of tetrahedral stacking faults. Here, interstitial type prismatic dislocation loops mainly formed by $\frac{1}{6}$$\langle$1 1 2$\rangle$ Shockley dislocations are nucleated during the loading process, where their interaction leads to the formation of pyramidal shaped stacking fault which are mainly created by $\frac{1}{3}$$\langle$1 0 0$\rangle$ Hirth dislocations lines. Observing both types of defects coexisting in the same plastic deformation zone by both approaches. Reported mechanical data, measured experimentally and interpreted numerically, are also in accordance with microstructural SEM and TEM investigations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Elucidating the microscopic origin of electrochemical corrosion and electrical conductivity by lattice response to severe plastic deformation in Al-Mn-Si alloy

Highlights: • Al-Mn-Si specimens were underwent severe plastic deformation by CGP and cold-rolling and their combined routes. • Microscopical observations depicted particles' fragmentation and non-preferential distribution under heavy straining. • XRD analysis revealed dislocations annihilation and lattice swelling (d-spacing increase) due to dynamic recrystallization. • Polarization outputs disclosed direct correlation of corrosion behavior with dislocations density. • Four-point probe test proved that electrical conductivity was a function of lattice d-spacing. - Abstract: Al-Mn-Si specimens were underwent SPD by CGP, cold-rolling and their combined paths. Acquired OM and SEM micrographs revealed that random distribution of intermetallic precipitates beside their fragmentation under heavy imposed strains deteriorated particles’ influence on the electrochemical corrosion and inhibited pitting. Analysis of XRD patterns pointed out that post-rolling of CGPed sheets resulted in dynamic annihilation of dislocations and accordingly crystal lattice swelling (d-spacing increase) due to dynamic recrystallization. From polarization results, crystallites’ refinement along with density increment of sub-grain boundaries within structure as a consequence of SPD beside dislocation density enhancement accelerated corrosion. Outputs of four-point probe electrical conductivity examination indicated that apart from the employed processing, lattice compaction and atomic layers approach would limit the electrons’ mobility space and hence reduce the conductivity of material. In this regard, combined effects of CGP and rolling as employed dual-straining route on Al-Mn-Si alloy enhanced the electrical conductivity by d-spacing growth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Strengthening mechanisms for microstructures containing unimodal and bimodal γ' precipitates in ATI 718Plus

Here, the influence of γ' precipitate size distribution on the deformation mechanisms under tensile loading in ATI 718Plus was studied. A set of aging treatments within the temperature range of 720 °C–900 °C was performed on solution-treated samples to obtain various γ' precipitate size distributions. Unimodal and bimodal γ' precipitate size distributions were achieved through single-step and two-step aging sequences, respectively, and such microstructures were tensile tested to failure to assess their yield strength, ultimate tensile strength, and elongation-to-failure. Some of the tensile samples were interrupted after achieving 3–4 % plastic strain, and the deformed microstructures were examined using transmission electron microscopy to investigate the γ' precipitate-dislocation interactions. For the unimodal γ' precipitate size distribution samples with the smaller γ' precipitates (radius ~ 7 nm), dislocations sheared through the precipitates. Both dislocation loops and paired dislocations were observed for the microstructures containing larger γ' precipitates (radius ~ 24 nm). The microstructure containing a bimodal γ' precipitate size distribution, which included average γ' precipitate radii of ~6 nm and ~28 nm, exhibited shearing as the dominant deformation mechanism, and this microstructure exhibited the highest strength values. The experimental observations were rationalized based on the theoretically-calculated critical resolved shear stress values for shearing and looping and a modified model for predicting the yield strength for bimodal microstructures was introduced.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Micro-beam bending of FCC bicrystals: A comparison between defect dynamics simulations and experiments

To understand the role of the grain boundary (GB) in plasticity at small scale, a concurrently coupled mesoscale plasticity model was developed to simulate micro-bending of bicrystalline micron-sized beams. By coupling dislocation dynamics (DD) with a finite element model (FEM), a novel defect dynamics model provides the means to investigate intricate interactions between dislocations and GBs under various loading conditions. Our simulations of micro-bending agree well with corresponding micro-bending experiments, and they show that mechanical response of bicrystals could have not only hardening but also softening depending on the characters of the GB. In addition, changing the location of the GB in the microbeams results in different mechanical responses; GBs located at the neutral plane show softening compared to single crystals, while inclined GBs located halfway along the length of the beam show little effect. In conclusion, simulation results could provide a clear picture on detailed dislocation-GB interactions, and quantitative resolved shear stress analysis supplemented by dislocation density distribution is used to analyze the mechanical response of bicrystalline samples.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tensile creep properties of a CrMnFeCoNi high-entropy alloy

Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions.

36 MATERIALS SCIENCE↗