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Discovery of ferroelectricity in HfO

The deformation behavior of materials that exhibit a ductile-to-brittle transition have traditionally been characterized ex-situ and limited to bulk samples. Here, we use a unique experimental setup to perform in-situ cryogenic mechanical testing of Sn 96 solder alloy in a clamped beam configuration at − 142 °C, well below the alloy’s DBTT. Electron backscattered diffraction analysis before and after the in-situ experiments showed how deformation ahead of the crack tip is accommodated by the Sn matrix or intermetallic phase particles. Similar to behavior observed in pure Sn, within the Sn matrix of the Sn 96 alloy deformation twinning is the primary deformation mechanism below the DBTT. Similar to the twins previously observed in pure Sn, twinning behavior in the Sn matrix is consistent with the formation of the {301} and {101} twins in Sn. Within the intermetallic phase in Sn96, slip steps were observed that were not observed prior to bending.

The influence of dispersoids and grain boundary characteristics on the crack propagation behavior in AA3xxx during deep drawing has been investigated using scanning transmission electron microscopy, energy-dispersive spectroscopy, and the dictionary indexing method applied to transmission Kikuchi diffraction (TKD) patterns. Observations showed that cracks formed on the surface of the material at the earliest stages of deformation and lengthened with increasing deformation. Subsurface analysis showed that the cracks propagated from the surface and bifurcated as they extended deeper into the material, forming branches. All of the observed cracks followed intergranular paths with bifurcations correlating with triple junction locations, but no strong trends were found relating grain boundary characteristics to the crack propagation pathway. Dispersoids located at the grain boundaries were found where cracks terminated, suggesting that dispersoids arrest crack propagation and divert crack growth pathways. This paper demonstrates the first practical application of combining the dictionary indexing method with TKD analysis.

A micro-electromechanical system (MEMS) based technique is demonstrated for in situ transmission electron microscopy (TEM) measurements of stress relaxation with simultaneous observation of the underlying plastic deformation processes. True activation volumes are determined from repeated stress relaxation transients and thus provide a signature parameter of the governing mechanisms of plastic deformation. The technique is demonstrated with 100 nm-thick ultrafine-grained gold microspecimens under uniaxial tension. True activation volumes of approximately 3-5b3 (where b is the Burgers vector length) are obtained for tensile stresses ranging from 200-450 MPa. Grain boundary-dislocation interactions are observed via in situ TEM during stress relaxation measurements. The miniaturization of stress relaxation tests inside the TEM provides unique opportunities to characterize the plastic kinetics and underlying mechanisms in ultrafine-grained and nanocrystalline materials.

In this study, a multiscale electron microscopy-based approach is applied to understanding how different aspects of the microstructure in a notched AA6061-T6, including grain boundaries, triple junctions, and intermetallic particles, promote localized dislocation accumulation as a function of applied tensile strain and depth from the sample surface. Experimental measurements and crystal plasticity simulations of dislocation distributions as a function of distance from specified microstructural features both showed preferential dislocation accumulation near intermetallic particles relative to grain boundaries and triple junctions. High resolution electron backscatter diffraction and site-specific transmission electron microscopy characterization showed that high levels of dislocation accumulation near intermetallic particles led to the development of an ultrafine sub-grain microstructure, indicative of a much higher level of local plasticity than predicted from the coarser measurements and simulations. In addition, high resolution measurements in front of a crack tip suggested a compounding influence of intermetallic particles and grain boundaries in dictating crack propagation pathways.

Using electron backscatter diffraction, over 1000 grain boundary/twin interactions were investigated in pure Re after uniaxial compression. Crystallographic factors such as grain boundary disorientation angle, displacement gradient tensor accommodation, and twin plane and shear vector alignment were taken into account as well as the macroscopic Schmid factor. It was found that the crystallographic relationship between coincident twin pairs at grain boundaries fall into two categories. In the first category, the twin pairs satisfied two criteria: the twin plane alignment was maximized across the boundary and the twin variant was kept constant across the boundary. In the second category, comprising approximately 5% of the characterized interactions, twin variant selection appeared to be driven by the macroscopically applied stress state, as resolved by the Schmid factor. In comparison to published results on twin/grain boundary interactions in Mg and Zr, it was found that twins transmit across grain boundaries in Re far more readily, regardless of the grain boundary disorientation angle. This is likely to be a function of the anomalously low twin boundary energy in Re as well as the difference in magnitude of the twinning-induced deformation that must be accommodated at the boundary, with the {11 2 ¯ 1} 26 ¯ > system being dominant in Re and the {10 1 ¯ 2} 11 ¯ > being dominant in Mg and Zr.

With recent advances in computational modeling and in situ transmission electron microscopy (TEM) technologies, there have been increased efforts to apply these approaches to understand defect-based mechanisms dictating deformation mechanics. In situ TEM experiments and atomistic simulations each have their own unique limitations, including observable length and time scales and accessibility of information, motivating approaches that combine the two approaches. In this paper, we review recent studies that combine atomistic simulations and in situ TEM experiments to understand defect mechanisms associated with deformation of metals and alloys. In addition, we discuss ongoing developments in characterization and simulation capabilities that are expected to significantly advance the field of defect mechanics and allow greater integration between atomistic simulations and in situ TEM experiments.

Plastic deformation induced pyramidal ⟨c + a⟩ dislocations in a Zr-2.5Nb alloy have been investigated by electron tomography in the transmission electron microscope. The 10 1 − 1 1st order pyramidal planes were determined to be the primary slip plane, with the 11 2 − 2 2nd order pyramidal planes acting as a secondary slip system. The slip behavior showed asymmetrical response with loading direction, characterized by restricted planar slip in ⟨c⟩ axis compressive loaded grains and frequent cross slip in ⟨c⟩ axis tensile loaded grains.

Using ex-situ transmission electron microscopy and the recently developed nanoprobe diffraction (NPD) technique, we characterize a nanoindented solution treated gum metal. Lattice rotations are resolved at a 1.2 nm length-scale and shown to be continuous within the nanoindentation pit; further, it is shown that these can be accommodated by a reasonable number of geometrically necessary dislocations at a density of ∼1015/m2. We additionally provide direct evidence that dislocations within the nanoindent, rather than secondary phase nanoparticles, can serve as potent barriers to dislocation motion. We also demonstrate that plasticity in these alloys under nanoindentation can be accommodated solely by dislocation nucleation and propagation, with no competing deformation mechanisms present. Conventional transmission electron microscopy and “g•b” analysis reveal the presence of dislocations on 〈 1 ¯ 11 〉 {110} slip systems and highly localized plastic deformation in the form of shear bands on { 1 ¯ 1 ¯ 2 } slip systems, similar to previously observed “giant faults”.

The microstructure and fatigue behavior of additive manufactured Inconel 718 (IN718) were studied using digital image correlation (DIC), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The samples were produced with selective laser melting and underwent two different heat treatments (stress relieved and fully post-processed) that produced different microstructures. The solidification process led to the formation of dendrites where misorientation between each branch was accommodated with dislocations, producing subgranular cell structures. EDS observations revealed that Ti and Nb segregate to the cell and grain boundaries, forming carbides. The stress relieved sample retained the cell boundaries, which acted as barriers for dislocation propagation, thereby distributing strain evenly throughout the microstructure. In contrast, the post-processed sample contained no cell boundaries, and strain was localized at the grain boundaries. Mechanical tests showed that the precipitates and the annealing twins in the post-processed sample were much more effective at improving the strength than small grain size and cell boundaries found in the stress relieved sample.

A rapid image segmentation and correlation-based approach to investigating the local microstructural effects on pitting corrosion susceptibility is introduced. The impact of four different microstructural features, consisting of grain orientation, proximity to grain boundaries, proximity to intermetallic particles, and local geometrically necessary dislocation (GND) density, on pitting initiation in the 5083 aluminum alloy was directly investigated using automated pit detection processing in combination with electron microscopy-based characterization. Three different thermomechanical treatments were used to vary the system microstructure and samples were immersed in 3.5 wt% NaCl aqueous solution for 24 h to induce pitting. GND density and proximity to intermetallic particle both showed positive correlations, while grain orientation and proximity to grain boundary showed no correlation with pit initiation sites.

Pure Ti traditionally exhibits the hexagonal closed packed (HCP) crystallographic structure under ambient conditions and the body centered cubic (BCC) structure at elevated temperatures. In addition to these typical structures for Ti alloys, the presence of a face centered cubic (FCC) phase associated with thin films, interfaces, or high levels of plastic deformation has occasionally been reported. Here we show that small FCC precipitates form in freestanding thin foils during in situ transmission electron microscope (TEM) heating and we discuss the potential origins of the FCC phase in light of the in situ observations. This FCC phase was found to be stable upon cooling and under ambient conditions, which allowed us to explore its mechanical properties and stability via nanomechanical in situ TEM testing. It was found that FCC platelets within the HCP matrix phase were stable under mechanical deformation and exhibited similar mechanical deformation behavior as the parent HCP phase.

Nanocrystalline Ni thin films with different as-deposited textures were synthesized by varying the substrate temperature during pulsed-laser deposition. The influence of initial texture variations on the thermal stability and annealing behavior of the thin films was studied via in situ transmission electron microscopy annealing and transmission Kikuchi diffraction analysis. It was found that, as the substrate temperature during deposition was increased, the initial microstructure varied from a random distribution of grains to one almost entirely composed of (001) – oriented grains, with the volume fraction of (001) – oriented grains increasing with increasing substrate temperature. The (001) – oriented grains increased thermal stability and constrained grain growth during annealing, resulting in a bimodal grain size distribution in the annealed films. Despite different surface orientations after annealing, the misorientation distribution functions for the films were similar, suggesting that the boundary state is a primary factor in dictating equilibrium.

In situ straining in the transmission electron microscope and diffraction-contrast electron tomography has been applied to investigate dislocation interactions in α-Ti. Dislocation debris, in the form of small loops, was seen to form from sequential cross-slip events. Electron tomography provided direct three-dimensional visualisation of the dislocation structures, allowing accurate identification of slip planes, dislocation line directions and spatial relations between dislocations.

Domains and domain walls are critical in determining the response of ferroelectrics, and the ability to controllably create, annihilate, or move domains is essential to enable a range of next-generation devices. Whereas electric-field control has been demonstrated for ferroelectric 180° domain walls, similar control of ferroelastic domains has not been achieved. Here, using controlled composition and strain gradients, we demonstrate deterministic control of ferroelastic domains that are rendered highly mobile in a controlled and reversible manner. Through a combination of thin-film growth, transmission-electron-microscopy-based nanobeam diffraction and nanoscale band-excitation switching spectroscopy, we show that strain gradients in compositionally graded PbZr1-xTixO3 heterostructures stabilize needle-like ferroelastic domains that terminate inside the film. These needle-like domains are highly labile in the out-of-plane direction under applied electric fields, producing a locally enhanced piezoresponse. This work demonstrates the efficacy of novel modes of epitaxy in providing new modalities of domain engineering and potential for as-yet-unrealized nanoscale functional devices.

Multiscale characterization was performed on an Al–Mg alloy, Al 5754 O-temper, including in situ mechanical deformation in both the scanning electron microscope and the transmission electron microscope. Scanning electron microscopy characterization showed corresponding inhomogeneity in the dislocation and Mg distribution, with higher levels of Mg correlating with elevated levels of dislocation density. At the nanoscale, in situ transmission electron microscopy straining experiments showed that dislocation propagation through the Al matrix is characterized by frequent interactions with obstacles smaller than the imaging resolution that resulted in the formation of dislocation debris in the form of dislocation loops. Post-mortem chemical characterization and comparison to dislocation loop behaviour in an Al–Cr alloy suggests that these obstacles are small Mg clusters. Previous theoretical work and indirect experimental evidence have suggested that these Mg nanoclusters are important factors contributing to ...

Progress toward combining time-resolved experiments with periodic three-dimensional analysis of the evolved microstructural state has been made recently. In situ electron microscopy is used to observe in real time the development of irradiation defects and the influence of these defects on dislocation behavior. Three-dimensional characterization provides information on the true spatial distribution of defects and clarifies effects of the free surfaces in thin films. This quasi-four dimensional analysis approach has been applied to understand the formation of channels in irradiated alloys, the depth distribution of ion damage in an electron transparent foil, and the dislocation channel interactions with grain boundaries. The new insight obtained from these experiments is highlighted and contrasted with findings from simulations.

The mechanical behavior of pure rhenium was investigated using uniaxial compression tests, transmission electron microscopy and electron backscatter diffraction characterization. The plasticity was characterized by a large amount of twin formation and propagation, including twin transmission across grain boundaries. In-depth analysis of the interactions of { 1 1 2 ¯ 1 } 〈 1 ¯ 1 ¯ 2 6 〉 twins with grain boundaries found that grain boundaries with misorientation angles below ∼25° allowed twin transmission, while grain boundaries with higher angles did not. Similar to dislocation interactions with grain boundaries, twin transmission was largely dictated by the minimization of the angle between the shear vectors of the incoming and outgoing twins.

Characterizing plasticity mechanisms below the ductile-to-brittle transition temperature is traditionally difficult to accomplish in a systematic fashion. Here, we use a new experimental setup to perform in situ cryogenic mechanical testing of pure Sn micropillars at room temperature and at −142 °C. Subsequent electron microscopy characterization of the micropillars shows a clear difference in the deformation mechanisms at room temperature and at cryogenic temperatures. At room temperature, the Sn micropillars deformed through dislocation plasticity, while at −142 °C they exhibited both higher strength and deformation twinning. Two different orientations were tested, a symmetric (1 0 0) orientation and a non-symmetric (4 5 ¯ 1) orientation. The deformation mechanisms were found to be the same for both orientations.

Recent progress in understanding dislocation interactions with grain boundaries and interfaces in metallic systems via static and in situ dynamic experimental approaches is reviewed.

The interaction of lattice dislocations with grain boundaries in proton- and ion-irradiated austenitic stainless steels is investigated. At the macroscale, interactions of dislocation channels with grain boundaries result in slip blockage, slip transmission and slip along the grain boundary with an associated in-plane displacement. At the dislocation level, slip transmission prediction is governed by the strain energy density minimization. However, the importance of the local resolved shear stress increases, in the sense that it must be of sufficient magnitude to propagate the dislocations through the damaged matrix. If this condition is not satisfied, the grain boundary will activate an alternative mechanism to relieve the strain accumulated from accommodation of lattice dislocations and the stress concentration associated with the formation of a dislocation pile-up. One observed relief mechanism involves the nucleation and propagation of a crack along the grain boundary, which is accompanied by an out-of-plane displacement of the boundary.

Recent developments in the application of electron tomography for characterizing microstructures in crystalline solids are described. The underlying principles for electron tomography are presented in the context of typical challenges in adapting the technique to crystalline systems and in using diffraction contrast imaging conditions. Methods for overcoming the limitations associated with the angular range, the number of acquired images, and uniformity of image contrast are introduced. In addition, a method for incorporating the real space coordinate system into the tomogram is presented. As the approach emphasizes development of experimental solutions to the challenges, the solutions developed and implemented are presented in the form of examples.

The dislocation microstructure beneath surface slip traces produced by fatigue loading of Haynes 230 was revealed to be a function of distance from a crack tip. The microstructure beneath these traces evolves from planar slip bands with increasing dislocation density and decreasing interband spacing as the crack tip approaches one of refined subgrains and lamellar bands at and in the vicinity of the crack tip. Similarly, beneath fatigue striations the microstructure evolves from nanosized subgrains to a banded structure with increasing distance from the fracture surface. These structures are significantly different to those predicted to develop under fatigue loading of a planar slip material. The evolved structures are considered in terms of the microstructure generated by severe plastic deformation.

The effects of NiO multilayer additions to pulsed-laser deposited nanocrystalline Ni on the grain growth and texture evolution during annealing have been studied using a combination of in situ annealing in the transmission electron microscope and the recently developed transmission electron backscatter diffraction technique. Grain growth in pure Ni proceeds in an abnormal manner with a small number of grains growing rapidly at the expense of the surrounding nanocrystalline matrix. The addition of NiO layers in the Ni suppressed grain growth and twin formation upon annealing, with increasing oxide content resulting in decreasing extent of grain growth. In pure Ni, the post-annealing texture was found to be composed of a strong {001} texture coupled with the four {122} twin variants from the {001}. With the addition of NiO, annealing twinning, and hence the {122} orientation, was suppressed and instead the final texture was composed of the {001} and {111} orientations.

The effects of heavy-ion irradiation on dislocation processes in stainless steels were investigated using in situ irradiation and deformation in the transmission electron microscope as well as post mortem electron tomography to retrieve information on the three-dimensional dislocation state. Irradiation-induced defects were found to pose a strong collective barrier to dislocation motion, leading to dislocation pileups forming in grain interiors and at grain boundaries. The passage of multiple dislocations along the same slip plane removes the irradiation defects and leads to the eventual formation of a defect-free channel. These channels are composed of densely tangled dislocation networks which line the channel-matrix walls as well as residual dislocation debris in the channel interiors. The structures of the dislocation tangles were found to be similar to those encountered in later stages of deformation in unirradiated materials, with the exception that they developed earlier in the deformation process and were confined to the defect free channels. Also, defect free channels were found to widen through both source widening as well as complex cross-slip mechanisms.

A combination of electron tomography, electron backscatter diffraction and traditional |g.b| analysis is used to characterize a dislocation transmission event across a Σ3 grain boundary in 304 stainless steel. The parallel {1 1 1} planes across a Σ3 boundary allow simultaneous imaging of dislocations in both grains. The interaction is explained in terms of residual strain energy density in the grain boundary due to the accommodation and emission of dislocations and the magnitude of the local shear stress.

Grain boundary migration associated with abnormal grain growth in pulsed-laser deposited Ni was studied in real time by annealing electron transparent films in situ in the transmission electron microscope. The resulting texture evolution and grain boundary types produced were evaluated by ex situ electron backscatter diffraction of interrupted anneals. The combination of these two techniques allowed for the investigation of grain growth rates, grain morphologies, and the evolution of the orientation and grain boundary distributions. Grain boundaries were found to progress in a sporadic, start/stop fashion with no evidence of a characteristic grain growth rate. The orientations of the abnormally growing grains were found to be predominately 〈1 1 1〉//ND throughout the annealing process. A high fraction of twin boundaries developed during the annealing process. The intermittent growth from different locations of the grain boundary is discussed in terms of a vacancy diffusion model for grain growth.

The mobility of defects such as dislocations controls the mechanical properties of metals. This mobility is determined both by the characteristics of the defect and the material, as well as the local stress and strain applied to the defect. Therefore, the knowledge of the stress and strain during deformation at the scale of defects is important for understanding fundamental deformation mechanisms. Here, we demonstrate a method of measuring local stresses and strains during continuous in situ deformation with a resolution of a few nanometers using nanodiffraction strain mapping. Our results demonstrate how large multidimensional data sets captured with high speed electron detectors can be analyzed in multiple ways after an in situ TEM experiment, opening the door for true multimodal analysis from a single electron scattering experiment.

In 2006, Angus Wilkinson introduced a cross-correlation-based electron backscatter diffraction (EBSD) texture analysis system capable of measuring lattice rotations and elastic strains to high resolution. A variation of the cross-correlation method is introduced using Bragg's Law-based simulated EBSD patterns as strain free reference patterns that facilitates the use of the cross-correlation method with polycrystalline materials. The lattice state is found by comparing simulated patterns to collected patterns at a number of regions on the pattern using the cross-correlation function and calculating the deformation from the measured shifts of each region. A new pattern can be simulated at the deformed state, and the process can be iterated a number of times to converge on the absolute lattice state. By analyzing an iteratively rotated single crystal silicon sample and recovering the rotation, this method is shown to have an angular resolution of approximately 0.04 degrees and an elastic strain resolution of approximately 7e-4. As an example of applications, elastic strain and curvature measurements are used to estimate the dislocation density in a single grain of a compressed polycrystalline Mg-based AZ91 alloy.