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DOE OSTI2017

Correlation Between Irradiation Defects and Transition Dimension for TEM In Situ Mechanical Testing

The objective of this study is to establish a relationship between specimen size effect transition dimension and defect number density. Transmission electron microscopic (TEM) in situ mechanical testing enables one to directly observe fundamental plastic phenomena simultaneous to quantitative measurement of applied load and displacement, in specimens having dimensions on the order of nanometers to micrometers. Thus, these techniques hold great promise for enhancing our scientific understanding of deformation mechanisms in volume-limited materials, such as shallow ion irradiated layers. The 'specimen size effect' is a major detractor, however, of any miniaturized mechanical testing technique. This effect is the direct result of low-volume specimens containing too few dislocations, such that plasticity is governed by the stress required to introduce more dislocations to the specimen. The consequence is an inflated yield stress measurement relative to the 'bulk' yield stress. As the specimen size increases, so too does the number of dislocations present, and plasticity becomes controlled by the stress needed to move these dislocations. Yield stress measurements decrease to their bulk values. The minimum specimen dimension for which plastic yield is controlled by dislocation motion rather than dislocation introduction, is what we herein refer to as the 'transition dimension'. However, the overall yield stress is governed by the superposition of dislocations, grain boundaries, and dispersed obstacles of varying morphologies. It is theorized that the higher the number density of these dispersed obstacles, the lower the transition dimension. Since it is well known that irradiation introduces a high density of defects, we hypothesize that the transition dimension is sufficiently low in irradiated materials so as to enable meaningful quantitative values of yield stress through TEM in situ mechanical tests.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Revisiting the Al/Al 2 O 3 Interface: Coherent Interfaces and Misfit Accommodation

We report the coherent and semi-coherent Al/α-Al 2 O 3 interfaces using molecular dynamics simulations with a mixed, metallic-ionic atomistic model. For the coherent interfaces, both Al-terminated and O-terminated nonstoichiometric interfaces have been studied and their relative stability has been established. To understand the misfit accommodation at the semi-coherent interface, a 1-dimensional (1D) misfit dislocation model and a 2-dimensional (2D) dislocation network model have been studied. For the latter case, our analysis reveals an interface dislocation structure with a network of three sets of parallel dislocations, each with pure-edge character, giving rise to a pattern of coherent and stacking-fault-like regions at the interface. Structural relaxation at elevated temperatures leads to a further change of the dislocation pattern, which can be understood in terms of a competition between the stacking fault energy and the dislocation interaction energy at the interface. In conclusion, our results are expected to serve as an input for the subsequent dislocation dynamics models to understand and predict the macroscopic mechanical behavior of Al/α-Al 2 O 3 composite heterostructures.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Multiscale Modeling of Inclusions and Precipitation Hardening in Metal Matrix Composites: Application to Advanced High-Strength Steels

In this study, the strengthening effect of inclusions and precipitates in metals is investigated within a multiscale approach that utilizes models at various length scales, namely, Molecular Mechanics (MM), discrete Dislocation Dynamics (DD), and an Eigenstrain Inclusion Method (EIM). Particularly, precipitates are modeled as hardsoft particles whose stress fields interact with dislocations. The stress field resulting from the elastic mismatch between the particles and the matrix is accounted for through the EIM. While the MM method is employed for the purpose of developing rules for DD for short range interaction between a single dislocation and an inclusion, the DD method is used to predict the strength of the composite resulting from the interaction between ensembles of dislocations and particles. As an application to this method, the mechanical behavior of Advanced High Strength Steel (AHSS) is investigated and the results are then compared to the experimental data. The results show that the finely dispersive precipitates can strengthen the material by pinning the dislocations up to a certain shear stress and retarding the recovery, as well as annihilation of dislocations. The DD results show that strengthening due to nano sized particles is a function of the density and size of the precipitates. This size effect is then explained using a mechanistic model developed based on dislocation-particle interaction.

Environmental Molecular Sciences Laboratory↗

DOE OSTI2021

Sculpting crystals one Burgers vector at a time: Toward colloidal lattice robot swarms

Plastic deformation of crystalline materials with isotropic particle attractions proceeds by the creation and migration of dislocations under the influence of external forces. If dislocations are produced and migrated under the action of local forces, then material shape change can occur without the application of surface forces. We investigate how particles with variable diameters can be embedded in colloidal monolayers to produce dislocations on demand. Additionally, we find in simulation that when embedded clusters of variable diameter particles are taken through multiple cycles of swelling and shrinking, large cumulative plastic slip is produced by the creation and biased motion of dislocation pairs in the solid for embedded clusters of particular geometries. In this way, dislocations emitted by these clusters (biased “dislocation emitters”) can be used to reshape colloidal matter. Our results are also applicable to larger-scale swarms of robotic particles that organize into dense ordered two-dimensional (2D) arrangements. We conclude with a discussion of how dislocations fulfill for colloids the role sought by “metamodules” in lattice robotics research and show how successive applications of shear as a unit operation can produce shape change through slicing and swirling.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Nonlinear ultrasonic technique for the characterization of microstructure in additive materials

This study employs nonlinear ultrasonic techniques to track microstructural changes in additively manufactured metals. The second harmonic generation technique based on the transmission of Rayleigh surface waves is used to measure the acoustic nonlinearity parameter, β. Stainless steel specimens are made through three procedures: traditional wrought manufacturing, laser-powder bed fusion, and laser engineered net shaping. The β parameter is measured through successive steps of an annealing heat treatment intended to decrease dislocation density. Dislocation density is known to be sensitive to manufacturing variables. In agreement with fundamental material models for the dislocation-acoustic nonlinearity relationship in the second harmonic generation, β drops in each specimen throughout the heat treatment before recrystallization. Geometrically necessary dislocations (GNDs) are measured from electron back-scatter diffraction as a quantitative indicator of dislocations; average GND density and β are found to have a statistical correlation coefficient of 0.852 showing the sensitivity of β to dislocations in additively manufactured metals. Moreover, β shows an excellent correlation with hardness, which is a measure of the macroscopic effect of dislocations.

42 ENGINEERING↗

DOE OSTI2024

DiscoFluxM version 1.x

DiscoFluxM is software for modeling the deformation behavior of crystalline materials within the third-party finite-element based software, MOOSE. The software enables full-field crystal plasticity finite element simulations of deformation processes of metals and other crystalline materials. DiscoFluxM is an implementation of the DiscoFlux theoretical and algorithmic framework for coupling single-crystal plasticity with dislocation transport. The continuum mesoscale modeling framework is founded upon a nonlocal dislocation-density based crystal plasticity theory. The nonlocal theory couples continuum dislocation transport with an otherwise spatially-local single-crystal model employing nonlinear thermoelasticity and crystallographic plasticity. Dislocation transport is modeled by enforcing dislocation conservation at a slip-system level through the solution of advection-diffusion equations. The configuration of geometrically necessary dislocation density gives rise to a back-stress that inhibits or accentuates the flow of dislocations.

Luscher, Darby↗

DOE OSTI2019

In situ transmission electron microscopy investigation on [c + a] slip in Mg

Recent molecular dynamics simulations revealed that dislocations in Mg were prone to dissociation on the basal plane, thus becoming sessile. Basal dissociation of dislocations is significant because it is a major factor in the limited ductility and high work-hardening in Mg. We report an in situ transmission electron microscopy study of the deformation process using an H-bar-shaped thin foil of Mg single crystal designed to facilitate slip, observe dislocation activity, and establish the validity of the largely immobile dislocations caused by the predicted basal dissociation. In addition, through detailed observations on the fine movement of some dislocations, it was revealed that limited bowing out movement for some non-basal portions of dislocations was possible; under certain circumstances, i.e., through attraction and reaction between two dislocations on the same pyramidal plane, at least portions of the sessile configuration were observed to be reversed into a glissile one.

in-situ TEM, slip, deformation mechanism, magnesiu↗

DOE OSTI2019

Nanoindentation of Micrograins in Polycrystals under Multiaxial Stress: Control of Abrupt & Stochastic Plastic Events

Deforming objects in the world of the small has proven a much more complex task than just bending a common-sized object like a spoon. Over the last decade, it has become clear that the application of stress on micron-sized objects, such as micropillars, leads to abrupt and stochastic plastic strain jumps that do not satisfy Weibull statistics (as expected for uncorrelated defect yielding) but instead seem to follow highly wide, power-law distributions, pointing towards cooperative dislocation plasticity. Are power-law distributed plastic events the rule or the exception in crystals? If the rule, can we design protocols to control and suppress them in order to achieve deterministic and smooth microforming processes? We propose to combine experiments and theory to unravel the mysteries of abrupt microplastic deformation in a natural dislocation environment: micron-sized grains (\micrograins") of polycrystals. We propose a novel experimental design that utilizes nanoindentation and 4-point bending techniques on the substrate of polycrystalline thin films of two typical FCC metals (Al and Cu), and make concrete theoretical conjectures, based on intuition built from previous theoretical and experimental efforts in micropillar compression and nanoindentation. Based on a wide set of observations over the last decade, we put forward two distinct hypotheses that we propose to investigate in both theory and experiments: Our first conjecture is that crystal plasticity in the central region of micron-sized grains has micropillar-like stochastic features: power-law plastic strain jumps at stresses above the macroscale yield strength, displaying strong size and rate effects. We will demonstrate this similarity in spherical nanoindentation experiments in the central-region of multi-orientation grains of Al and Cu thin films. Al and Cu are selected for their disparity in stacking fault energy and consequently, their different propensity for cross-slip relaxation; however, our protocol may be applied to any crystalline material of technological interest. We will test our hypothesis by performing two-dimensional extended discrete dislocation plasticity (2.5D-DDP) simulations, inspired by comparable successful studies of micropillar plasticity, and extended to model nanoindentation with two slip systems and on a surface of a bicrystal with microhard grain-boundaries (GB), ie. not allowing dislocation absorption (116). Our model will include effective three dimensional dislocation mechanisms such as dislocation multiplication and double-cross-slip assisted glide. Second, we hypothesize that the application of uniform in-plane stress on our thin films below the yield point can smoothen the micrograin plastic response during nanoindentation. On a technological level, we propose that indentation-induced localization (sink-in) may be diminished by the application of concurrent in-plane stress that activates slow, viscoplastic relaxations near GBs. We aim to demonstrate our hypothesis through a 4-point bending apparatus that introduces a homogeneous in-plane stress state before indentation. We will perform simulations of a coarse-grained mesoscale theory of plasticity in a system with microhard GBs, benchmarked through 2.5D-DDP simulations, which will capture the effects of slow viscoplastic relaxations. Our nanoindentation abrupt event statistics studies, either in the micrograin interior or near GBs, will be correlated to direct observations of induced surface dislocation microstructures. For we will use TEM-based methods (PACOM/TKD) in combination with EBSD. Indents placed in the center of grains will be analyzed based on the orientation of the grain, while those near to GBs will be analyzed based on the orientation of each grain, the distance from the GB and the 5 degrees of freedom of the GB.

36 MATERIALS SCIENCE↗

NASA NTRS1989

The effect of hydrogen on the deformation behavior of a single crystal nickel-base superalloy

The effect of hydrogen on the tensile deformation behavior of PWA 1480 is presented. Tensile tests were interrupted at different plastic strain levels to observe the development of the dislocation structure. Transmission electron microscopy (TEM) foils were cut perpendicular to the tensile axis to allow the deformation of both phases to be simultaneously observed as well as parallel to zone axes (III) to show the superdislocations on their slip planes. Similar to other nickel-base superalloys, hydrogen was detrimental to the room temperature tensile properties of PWA 1480. There was little effect on strength, however the material was severely embrittled. Even without hydrogen, the elongation-to-failure was only approximately 3 percent. The tensile fracture surface was made up primarily of ductile voids with regions of cleavage fracture. These cleavage facets are the eutectic (gamma') in the microstructure. It was shown by quantitative fractography that hydrogen embrittles the eutectic (gamma') and causes the crack path to seek out and fracture through the eutectic (gamma'). There was two to three times the amount of cleavage on the fracture surface of the hydrogen-charged samples than on the surface of the uncharged samples. The effect of hydrogen can also be seen in the dislocation structure. There is a marked tendency for dislocation trapping in the gamma matrix with and without hydrogen at all plastic strain levels. Without hydrogen there is a high dislocation density in the gamma matrix leading to strain exhaustion in this region and failure through the matrix. The dislocation structure at failure with hydrogen is slightly different. The TEM foils cut parallel to zone axes (III) showed dislocations wrapping around gamma precipitates. Zone axes (001) foils show that there is a lower dislocation density in the gamma matrix which can be linked to the effects of hydrogen on the fracture behavior. The primary activity in the gamma precipitates is in the form of superlattice intrinsic stacking faults (SISFs). These faults have also been reported in other ordered alloys and superalloys.

Walston, W. S.↗

DOE OSTI2013

TEM characterization of a simultaneously neutron-irradiated and helium-injected PM2000 ODS alloy

Transmission electron microscopy (TEM) discs of PM2000 were coated on one side with a ~4 µm thick NiAl film and irradiated in HFIR to a neutron displacement damage dose of 21.2 dpa at 500°C. The dpa are primarily produced by fast neutrons, while the in situ He injection (ISHI) to 1230 appm derives from the thermal neutron two-step 59 Ni(n,α) reaction. A uniform He injection profile extends up to about 8 µm below the NiAl coating, hence the remaining material experiences only dpa damage (21.2 dpa at 500°C). This report summarizes analytical TEM analyses to compare the two different regions, i.e., simultaneous He-injection/displacement damage versus displacement damage only, or high and very low He/dpa ratios, respectively. TEM analyses of the He implanted region reveals a high density of small (<2 nm) He bubbles predominantly associated with dislocation loops in the matrix. Because of a very large grain size, no grain or subgrain boundaries were observed in the samples prepared by focused ion beam (FIB) micromachining. The only voids formed were associated with large ODS particles, all of which were amorphous, irrespective of particle size and level of He. The ODS particles range in size from 15-30 nm, and most particles have a single faceted void attached to it that ranges in size from 5 to 15 nm. On the non-implanted side of the sample, no He bubbles could be detected, and faceted voids were found on ~10% of the ODS particles. The He-implanted side contains both <100>{100} and ½<111>{111} dislocation loops and a low density of line dislocations, whereas the un-implanted side possesses predominantly line dislocations with a lower density of dislocation loops. In both regions, nanoscale fcc precipitates exhibiting a cube-on-cube orientation relationship with the bcc matrix were observed. Chemical analysis using energy dispersive x-ray spectroscopy (EDS) indicates this phase is rich in Al and Ti. It was also noted that substantial Cr segregation occurs around the dislocation loops, concurrent with Fe depletion. The clusters of fcc precipitates that do not appear to be associated with the existing loops or line dislocations. The amorphous Y-Al-O particles contain a high Al concentration (>20 at%) and Cr segregation/Fe-depletion at the periphery.

Jung, Hee Joon↗

DOE OSTI2020

Slip band characteristics in the presence of grain boundaries in nickel-based superalloy

Shear strain profiles along slip bands in a modified Rolls-Royce nickel superalloy (RR1000) were analyzed for a tensile sample deformed by 2%. The strain increased with distance away from a grain boundary (GB), with maximum shear strain towards the center of the grain, indicating that dislocation nucleation generally occurred in the grain interior. The strain gradients in the neighborhood of the GBs were quantified and generally correlated with rotation about the active slip system line direction. This leads to an ability to determine the active slip system in these regions. The dislocation spacing and pileup stresses were inferred. The dislocation spacing closely follows an Eshelby analytical solution for a single ended pileup of dislocations under an applied stress. The distribution of pileup stress values for GBs of a given misorientation angle follows a log-normal distribution, with no correlation between the pileup stress and the GB misorientation angle. Furthermore, there is no observed correlation between various transmissivity factors and slip band pileup stress. Hence it appears that the obstacle strength of any of the observed GBs is adequate to facilitate the dislocation pileups present in the slip bands. However, slip band transmission does correlate with transmissivity factors, with the current study focusing on the Luster and Morris m’-factor. Observation of strain profiles of transmitted bands indicate dislocation nucleation locations.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Ordering effects on deformation substructures and strain hardening behavior of a CrCoNi based medium entropy alloy

Here, a CrCoNi based medium entropy alloy with small additions of Ti, Al and Nb (denoted as (CrCoNi) 93 Al 4 Ti 2 Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Dark field transmission electron microscopy (TEM), atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with atom probe tomography (APT) show that spatially-localized long range ordering (LRO) L1 2 domains exist in this alloy. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and STEM based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2<110>dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi) 93 Al 4 Ti 2 Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of LRO domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of LRO domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Intrinsic size dependent plasticity in BCC micro-pillars under uniaxial tension and pure torsion

The mechanical behavior of submicron body-centered cubic (BCC) micro-pillars is investigated by three-dimensional dislocation dynamics (DD) simulations to better understand the governing mechanisms for size dependent plasticity under uniaxial tension and pure torsion. A formula is developed to compute the incremental plastic twist due to dislocation motion in DD simulations. The DD simulations show that different dislocation microstructures are created depending on the loading conditions, which leads to different size dependent mechanical behavior. While in tension plasticity is mainly governed by the kinetics of dislocation motion controlled partly by the surface dislocation sources, plastic flow in torsion is controlled by dislocation pile-ups associated with strain gradients. The simulation results also reveal a Bauschinger effect and plastic recovery under cyclic twist, which have been observed in recent experiments.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Radiation resistance of oxide dispersion strengthened alloys: Perspectives from in situ observations and rate theory calculations

Here, in situ ion irradiation and rate theory calculations were employed to directly compare the radiation resistance of an oxide dispersion strengthened alloy with that of a conventional ferritic/martensitic alloy. Compared to the rapid buildup of dislocation loops, loop growth, and formation of network dislocations in the conventional ferritic/martensitic alloy, the superior radiation resistance of the oxide dispersion strengthened alloy is manifested by its stable dislocation structure under the same irradiation conditions. Thus, the results are consistent with rate theory calculations, which show that high-density nanoparticles can significantly reduce freely migrating defects and suppress the buildup of clustered defects.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Characterization of electrically-active defects in ultraviolet light-emitting diodes with laser-based failure analysis techniques

Laser-based failure analysis techniques demonstrate the ability to quickly and non-intrusively screen deep ultraviolet light-emitting diodes (LEDs) for electrically-active defects. In particular, two laser-based techniques, light-induced voltage alteration and thermally-induced voltage alteration, generate applied voltage maps (AVMs) that provide information on electrically-active defect behavior including turn-on bias, density, and spatial location. Here, multiple commercial LEDs were examined and found to have dark defect signals in the AVM indicating a site of reduced resistance or leakage through the diode. The existence of the dark defect signals in the AVM correlates strongly with an increased forward-bias leakage current. This increased leakage is not present in devices without AVM signals. Transmission electron microscopy analysis of a dark defect signal site revealed a dislocation cluster through the pn junction. The cluster included an open core dislocation. Even though LEDs with few dark AVM defect signals did not correlate strongly with power loss, direct association between increased open core dislocation densities and reduced LED device performance has been presented elsewhere [M. W. Moseley et al., J. Appl. Phys. 117, 095301 (2015)].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

An analysis of key characteristics of the Frank-Read source process in FCC metals

A well-known intragranular dislocation source, the Frank-Read (FR) source plays an important role in size-dependent dislocation multiplication in crystalline materials. Despite a number of studies in this topic, a systematic investigation of multiple aspects of the FR source in different materials is lacking. Here, in this paper, we employ large scale quasistatic concurrent atomistic-continuum (CAC) simulations to model an edge dislocation bowing out from an FR source in Cu, Ni, and Al. First, a number of quantities that are important for the FR source process are quantified in the coarse-grained domain. Then, two key characteristics of the FR source, including the critical shear stress and critical dislocation configuration, are investigated. In all crystalline materials, the critical stresses and the aspect ratio of the dislocation half-loop height to the FR source length scale well with respect to the FR source length. In Al, the critical stress calculated by CAC simulations for a given FR source length agrees reasonably well with a continuum model that explicitly includes the dislocation core energy. Nevertheless, the predictions of the isotropic elastic theory do not accurately capture the FR source responses in Cu and Ni, which have a relatively large stacking fault width and elastic anisotropy. Our results highlight the significance of directly simulating the FR source activities using fully 3D models and shed light on developing more accurate continuum models.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Coupled nonlinear elasticity, plastic slip, twinning, and phase transformation in single crystal titanium for plate impact loading

High purity single crystal titanium (Ti) under shockwave loading in plate impact experiment is modelled and simulated, with help of the finite element. A thermodynamically consistent system of equations is formulated in the frame of large deformation to consider material anisotropy, rate dependence, and multi-physics including nonlinear elasticity, dislocation based plastic slip, deformation twinning, and phase transformation. A novel kinematics is proposed to consider phase transformation in the α twin variants and also to consider the dislocation based slip in all of components of parent material, primary twins and the ω phase. The thermodynamically consistent driving forces for plastic slip, twinning, and phase transformation are developed based upon the second law of thermodynamics. For nonlinear elasticity, the stiffness matrix is dependent on the volume fractions of all components, and the volumetric part of Cauchy stress is obtained from the nonlinear equation of state. Dislocation based plastic slip in multi-variant and multiphase heterogeneous materials is developed and interactions of dislocations among all slip modes are taken into account. A mechanism for dislocation density evolution during twinning and phase transformation is proposed. The shock loading along the [0001] and [10$\bar{1}1$ directions of single crystal high purity Ti is investigated computationally. Very good correspondence between simulation and experiment is obtained, which includes pole figures, volume fractions of components, free surface velocity, peak pressure, and phase transformation pressure. Multiple experimental phenomena are interpreted based upon the progression of dislocation slip, deformation twinning, and phase transformation. In compression with the [10$\bar{1}1$ crystal, a higher volume fraction in the primary twins but a lower secondary twin volume fraction in the [0001] crystal in experiment was observed. Finally, the higher propensity for phase transformation occurs in the [0001] crystal is reproduced. In addition to material texture, distributions of temperature, stresses, and plastic strains dependent on the impact loading directions are revealed.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Investigation of deformation micro-mechanisms in nickel consolidated from a bimodal powder by spark plasma sintering

Bulk polycrystalline nickel compact was processed by spark plasma sintering from heterogeneous powder consisting of a mixture of nanometer and micrometer sized particles. The consolidated samples inherited the bimodal structure of the starting powder and was composed of ~ 55 vol.% coarse-grained (with the grain size larger than 1 μm) and ~ 45 vol.% ultrafine-grained (with an average grain size of ~ 550 nm) components. The deformation mechanisms were established by EBSD, X-ray line profile analysis and in-situ TEM observations. In the ultrafine-grained volume, the deformation occurred mainly through the activation of dislocation sources emitting full or partial dislocation either from grain interior or grain boundaries. Besides dislocation activity, rolling and sliding of nanograins were also observed during deformation by in-situ transmission electron microscopy, which have a considerable contribution to the observed high strain rate sensitivity of the bimodal microstructure. The cracks formed during deformation easily propagated in the nanograin regions due to the weaker particle bonding caused by the relatively high fraction of native oxide layer on the surface of the initial nanoparticles. - Highlights: • Bulk bimodal polycrystalline Ni was processed by SPS from a heterogeneous powder. • High SRS of the flow stress was observed which enhanced ductility and strength. • In-situ TEM revealed dislocation sources inside and at the boundaries of UFGs. • Twinning, partial dislocation and NG rolling were observed at crack tip vicinity. • The high SRS pertained to both dislocation activity in CG and NG rolling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗