Electron Channeling Contrast Imaging Research Articles

The Effect of the Garnet Content on Deformation Mechanisms and Weakening of Eclogite: Insights From Deformation Experiments and Numerical Simulations

AbstractWe performed deformation experiments on omphacite‐garnet aggregates at a temperature of 1000°C, a confining pressure of 2.5 GPa, and a strain rate of 3 × 10−6 s−1 and complemented them by numerical simulations to gain insight into the role of garnet fraction for the deformation behavior of dry eclogite, with a focus on strain weakening mechanisms. We determined the spatial and temporal evolution of strain and strain rate by basing numerical simulations on experimentally derived microstructures, and thereby identified characteristic deformation mechanisms. Pure omphacite and garnet aggregates deform by two different mechanisms. Internally strained clasts and low‐angle grain boundaries indicate crystal plasticity for omphacitite; the fracture dominated fabric of garnetite documents brittle deformation. Electron channeling contrast imaging, however, revealed low‐angle grain boundaries and free dislocations in garnet crystals, suggesting that minor crystal plasticity accompanies the brittle failure. Eclogitic aggregates show varying deformation behavior between the two end‐members shifting from crystal plastic toward brittle deformation with increasing garnet content. All samples exhibit strain weakening. The intensity of weakening shows a positive correlation with the garnet content. Our combined experimental, numerical, and microstructural investigations suggest that the majority of strain weakening is associated with crystal plastic processes in omphacite. Numerical simulations and experiments show that a garnet content above 25% enhances the activity of crystal plastic processes in omphacite and results in strain localization, which subsequently weakens the eclogite.

Superior resistance to high–temperature creep in an additively manufactured precipitation–hardened CrMnFeCoNi high–entropy alloy nanocomposite

A carbon–containing CrMnFeCoNi high–entropy alloy (C–HEA) nanocomposite was additively manufactured via a laser powder bed fusion (LPBF) process combined with subsequent heat treatment. Exposure of the LPBF C–HEA to this additional heat–treatment step not only resulted in heterogeneous–structured grains of which the substructure is decorated with a dislocation network, but also uniformly distributed nanosized carbide precipitates at the grain and substructure boundaries. A multistep creep test conducted on the LPBF C–HEA at 873 K enabled the stress exponent, n, to be obtained with values of 3.03 in the low stress region (175–250 MPa) and 6.99 in the high stress region (250–325 MPa). The LPBF C–HEA alloy exhibited superior high–temperature creep resistance (i.e., the creep rate and threshold stress were minimized) compared to other CrMnFeCoNi HEAs. The mechanism underlying the creep resistance and unique creep deformation behavior of the LPBF C–HEA alloy is discussed on the basis of electron channeling contrast imaging and electron backscatter diffraction analyses.

Nanomechanical response of tungsten carbide single crystals in extreme conditions: Temperature and strain rate dependence

The mechanical response of tungsten carbide (WC) single crystals under extreme conditions was investigated by means of pendulum-based nano-impact testing, from ambient to high temperatures. Dynamic hardness measurements and damage accumulation observations were conducted by single and multiple impact tests, respectively, at high strain rates (∼ 103-104 s-1). Large WC single crystals with different orientations, namely, (0001) basal and (101¯0) prismatic crystals, were studied. The results obtained from impact testing were compared to results acquired by high temperature nanoindentation in the quasi-static regime (10−2 s−1), at a similar length scale, up to 600 °C. Whilst basal WC crystals were shown to exhibit a higher dynamic hardness at high strain rates across the entire range of temperatures, the hardness of prismatic WC crystals at high strain rates were only found higher at temperatures below 200 °C and comparable at elevated temperatures. Post-mortem topographical scans revealed the generation of substantial plastic deformation during impact and large pile up formation in the vicinity of the impact imprints in prismatic crystals. Electron Channelling Contrast Imaging (ECCI) of the repetitive impacts suggested preferential planes for the accommodation of stress through plastic flow at high strain rates, as previously reported in the quasi-static regime. The results shed light on the mechanical behaviour of WC in extreme conditions, evidencing anisotropic mechanical behaviours.

Microstructural evolution and mechanical properties of functionally graded austenitic–low-carbon steel produced via directed energy deposition

In this study, the additive manufacturing of a functionally graded material (FGM) via directed energy deposition was investigated as an alternative to joining dissimilar metals. The metal powder composition of the FGM was gradually changed from fully low-carbon steel to austenite steel along the building direction. A convolutional neural network model was employed to classify the austenite, martensite, and ferrite phases in the FGM. The volume fraction of the phases was calculated using X-ray diffraction Rietveld refinement and compared with that predicted by the thermodynamic model and that determined from electron-backscattered-diffraction maps. The volume fraction of the bcc phase gradually increased, and the grain size decreased from top to bottom. Nanostructural investigations confirmed the absence of carbide and twin structures due to the relatively low carbon concentration in the upper layers and the presence of a hexagonal ω-Fe phase with twin structures in the interlayers. Furthermore, electron channeling contrast images and kernel average misorientation maps revealed the activation of the deformation twinning and strain-induced transformation of the retained austenite to martensite, which increased the strain-hardening rate. This study can guide the selection of a tailored manufacturing strategy and process parameters to obtain the required material distribution.

High-Temperature Tensile Properties of a Cobalt-Based Co-20Cr-15W-10Ni Superalloy with a Bimodal Grain Structure

Cobalt-based superalloys are common materials for the manufacturing of various components used in aerospace applications. Conventional cobalt-based superalloys with a unimodal grain structure generally exhibit low strength and ductility at high temperatures. A bimodal grain structure of a cobalt-based superalloy, Co–20Cr–15W–10Ni (CCWN), was designed to achieve both high strength and ductility at high temperatures. The deformation behavior and tensile properties of a CCWN alloy with unimodal fine-grain (FG), coarse-grain (CG), and bimodal (FG/CG) structures were investigated at 900 °C. The microstructures and substructures after high-temperature deformation were examined via electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to determine the deformation mechanisms. The microstructural observation showed that the bimodal grain structure consisted of FG and CG domains. During high-temperature deformation at 900 °C, the FG structure was mainly deformed by dynamic recrystallization (DRX), maintaining a similar FG structure. The CG structure was mainly deformed by DRV, resulting in a small amount of DRX grains and a large amount of dynamic recovery (DRV) grains. However, the bimodal grain structures were mainly softened via DRX and transformed into a new bimodal structure, ultrafine grain (UFG) and FG. The FG domains tended to deform via dislocations, and the CG domains via twinning. The high-temperature tensile tests revealed that the bimodal-structured alloy exhibited both higher strength and ductility than those of the alloy samples with unimodal FG or CG structure. This is associated with the newly developed UFG/FG structures in the bimodal grain-structured samples during high-temperature deformation. This work may provide new insight into the development of high-temperature alloys with bimodal grain structures.

GaAs/Si Tandem Solar Cells with an Optically Transparent InAlAs/GaAs Strained Layer Superlattices Dislocation Filter Layer

Epitaxial growth of III–V materials on Si is a promising approach for large-scale, relatively low-cost, and high-efficiency Si-based multi-junction solar cells. Several micron-thick III–V compositionally graded buffers are typically grown to reduce the high threading dislocation density that arises due to the lattice mismatch between III–V and Si. Here, we show that optically transparent n-In0.1Al0.9As/n-GaAs strained layer superlattice dislocation filter layers can be used to reduce the threading dislocation density in the GaAs buffer on Si while maintaining the GaAs buffer thickness below 2 μm. Electron channeling contrast imaging measurements on the 2 μm n-GaAs/Si template revealed a threading dislocation density of 6 × 107 cm−2 owing to the effective n-In0.1Al0.9As/n-GaAs superlattice filter layers. Our GaAs/Si tandem cell showed an open-circuit voltage of 1.28 V, Si bottom cell limited short-circuit current of 7.2 mA/cm2, and an efficiency of 7.5%. This result paves the way toward monolithically integrated triple-junction solar cells on Si substrates.

Strain-rate effects on the recrystallization of molybdenum-based MZ17 alloy

Warm compression tests were performed for the Mo-based alloy MZ17 (Mo-1.7% wt.-ZrO2). Macroscopic deformations to about 67% reduction in height were imposed to cylindrical specimens over a wide range of strain rates (from 10−1 to 10 s−1) at 1000 and 1100 °C to investigate the effects of both, the strain rate and deformation temperature on the recrystallization of the alloy. The compression tests were performed under high vacuum (< 5.0 × 10−4 mbar) using a deformation dilatometer followed by fast cooling. Stress-strain curves with very similar shapes can be noticed for all testing conditions. As-deformed microstructures were imaged at the center of the samples where plastic flow is more uniformly distributed using electron backscatter diffraction (EBSD) to distinguish the recovered domains from the recrystallized grains. Electron channeling contrast imaging (ECCI) was used to visualize the dislocation structures. At all tested temperatures and strain rates, only partial recrystallization was detected. The recrystallized volume fraction increases with increasing the strain rate. There is microstructural evidence of three restoration mechanisms acting during warm deformation; i.e., dynamic recovery, dynamic recrystallization and particle stimulated nucleation (PSN), the latter to a minor extent.

Microstructures and wear resistance of Zr-4 and N36 alloys subjected to pulsed laser surface remelting

Two typical Zr alloys (Zr-4 and N36) for nuclear applications were surface-treated by laser surface remelting (LSR). Electron channeling contrast imaging and electron backscatter diffraction were used to characterize their main microstructure characteristics before and after the LSR, which were also correlated with their hardness and wear resistance. The results show that two surface modification zones with distinct microstructure characteristics appear in both the LSRed Zr alloys: (i) remelting zone (RZ) composed of fine α laths with dense internal nano-twins, and (ii) heat affected zone (HAZ) consisting of blocky α, lath α and second phase particles. The average hardness values of the RZs of Zr-4 and N36 alloys are 254.9 ± 8.5 HV and 269.5 ± 7.9 HV, respectively, ∼30% higher than their substrates. The wear rates of the LSRed Zr-4 and N36 alloys are found to be about 30% and 18% lower than their substrates, respectively, indicating significantly improved wear resistance. Comprehensive analyses reveal that refined grains, solid solution and internal nano-twins generated by ultrafast heating and cooling during the laser treatment have jointly led to the enhanced surface hardness of the LSRed Zr alloys, accounting for their improved wear resistance as well. Compared to the Zr-4 alloy, the refinement of the surface microstructure of the N36 alloy after the LSR treatment is more remarkable, contributing to higher hardness and wear resistance, which is mainly related to the addition of 1 wt.% Nb.

Study of dislocation dynamics of copper single crystals under cycling loading through acoustic emission and microstructural characterizations

During plastic deformation of crystalline materials, a soft plasticity, made of numerous uncorrelated dislocation movements coexists with a wilder plasticity, in the form of collaborative movements: dislocation avalanches. The coexistence of the two plasticities depends on the establishment of a dislocation structure, which is supposed to hinder the propagation of avalanches. We propose to study the correlation between the microstructural evolution of the dislocation arrangements and the nature of the dislocation dynamics, during cyclic loading of pure copper single crystals. Different fatigue tests with imposed stress amplitude are performed to study the influence of (i) the loading path, (ii) the loading ratio and (iii) the crystallographic orientation on the plasticity phenomena. The acoustic emission (AE) technique is used to study both types of plasticity : the continuous AE is associated with soft plasticity and the discrete AE with wild plasticity. The dislocation microstructures are studied using EBSD (crystal disorientation) and ECCI (Electron Channeling Contrast Imaging) techniques at the end of each fatigue stage. The AE-ECCI coupling gives valuable information about the dislocation dynamics of dislocations. This study shows a certain universality, with (i) the existence of dislocation avalanches, produced during a more or less deep reorganization of the structures, (ii) a progressive evolution, during the cycles, of the dislocation mean free path, associated with the emergence of a dislocation structure and (iii) an inhibition of avalanches during the emergence of a dislocation arrangement. EWGAE 35, Ljubljana, Slovenia, 13th – 16th Sep. www.ewg

Development of Cell Structure and Crack Initiation during Fatigue of an Fe-3 mass%Si Alloy

A cell structure development and a crack initiation during a fatigue of an Fe-3 mass%Si alloy was investigated through electron channeling contrast imaging in a scanning electron microscope and electron back-scatter diffraction analysis. The crystal rotation regions (CRRs), deformation bands (DBs), and cell bands (CBs) together formed a hierarchy in the dislocation structures. In the early stage of fatigue, deformation is constrained near grain boundaries; this impedes further dislocation propagation. This restriction is attributed the formation of CRRs with a width of several hundred micrometers. Further, DBs that were several microns wide were developed inside the CRRs, and CBs with a width of several hundred nanometers were formed inside the DBs. Meanwhile, a crack was initiated from a CRR near a grain boundary. At the crack tip, a DB penetrating the CRR was formed parallel to the crack-propagation direction. It was elucidated that the cell boundary in the DB had a high misorientation angle of approximately 10 degrees, which greatly affected to the crack initiation. In addition, the penetrating DB was composed of elongated cells and cell bands. Prior to crack initiation, the boundaries of the cell bands evolved in proportion to the increasing dislocation density during fatigue. The elongated direction of the cell boundary, which was almost parallel to the {110} plane with a tilt boundary feature, dominated the crack-propagation direction. The formation plane and the cell boundary development process can be explained by analyzing the geometrical relationship of the activated slip systems between adjacent cells.

Experimental investigation of dislocation-grain boundary interaction in coarse-grained high‑manganese steels using quasi in situ electron channelling contrast imaging

Experimental investigation of dislocation-grain boundary interaction in coarse-grained high‑manganese steels using quasi in situ electron channelling contrast imaging

Strain Relaxation of Si/SiGe Heterostructures by a Geometric Monte Carlo Approach

Solid‐state‐based quantum technologies, such as electronic spin‐qubits, constitute a leading approach to the realization of quantum computation. Electronic spin‐qubits hosted in semiconductor heterostructures demand the highest crystalline quality, specifically with respect to the structure and formation of in‐plane misfit dislocations (MDs). Here, the formation mechanisms of MD networks in such Si/SiGe heterostructures are investigated. For this purpose, strained Si layers on relaxed Si0.7Ge0.3 with thicknesses above the critical thickness are grown by molecular beam epitaxy to allow for the formation of MDs. MDs are mapped out by electron channeling contrast imaging and the experimental results are compared to MD networks calculated by Monte Carlo simulations. The simulations show that the ratio between the threading dislocation density (TDD) and the average length of MDs is a constant for a given set of parameters, here 2.9 for sufficiently large dislocation statistics and simultaneous extension of MDs. Further, the fractal dimension of the MD network determined by the box‐counting method as an alternative quantity to characterize MD networks is introduced. This allows to infer the formation mechanism of MDs in real devices and hence the quantification of resulting average relaxation within the Si‐quantum well.

Effect of γ' phase and microtwins on the microstructural evolution and mechanical properties of a novel Ni–Co base superalloy

A new Ni–Co base precipitation-hardened superalloy designed using the low stacking fault energy (SFE) was used primarily in the hot components of aeroengines designed with low SFE exposed to high temperatures. Different microstructures of the new alloys were obtained by controlling the solution treatment temperature. The corresponding mechanical properties were tested at 25 °C and 750 °C. The primary γ' precipitates dissolved with increasing solution temperature, causing grain growth and an increasing microtwin fraction. Fine-grained, transient, and coarse-grained zones were formed in sequence because of the interaction between the γ′ phase and microtwins. The tensile strength decreased as the grain grew when tested at room temperature, and the strengthening mechanism was dislocation strengthening. However, when the tensile experiment temperature was increased to 750 °C, the tensile properties increased from the fine-grained to the transient zone and then decreased to the coarse-grained zone. Moreover, the fracture mechanism changed from ductile fracture to a mixed ductile and brittle fracture. The size of the secondary γ′ precipitates increased by 32 nm because the dissolved primary γ′ precipitated when the solution temperature was increased from 1090 °C to 1120 °C, which improved the tensile properties. The electron channeling contrast imaging and transmission electron microscopy results confirmed that microtwins were formed during the tensile process at 750 °C. The microtwins hindered the dislocation movement. Therefore, the transient zone of the low-SFE alloy demonstrated the best mechanical properties, which guided the optimization direction of the alloy design and service.

Room and elevated temperature sliding wear of high velocity oxy-fuel sprayed Diamalloy3001 coatings

CoMoCrSi alloys exhibit high hardness and excellent wear resistance due to the presence of intermetallic Laves phases in a metallic Co-matrix. In this study, we deposited Diamalloy3001 coatings using high velocity oxy-fuel (HVOF) spraying process and evaluated their tribology at temperatures 25ᵒC, 200ᵒC and 350ᵒC. At room temperature, formation of tribolayers composed of oxides of Co, Cr, and Mo resisted localized shear deformation and resulted in low wear rates. At elevated temperatures, detachment of the tribolayers, and formation of cavities during reciprocating sliding resulted in high wear rates. Electron channel contrast imaging (ECCI) of the worn subsurface showed that at elevated temperatures, oxidation of splat boundaries led to interparticle debonding during wear and decreased the stability of the tribolayers.

III-V material growth on electrochemically porosified Ge substrates

III-V semiconductor materials for high-efficiency multi-junction solar cells are often grown on germanium (Ge) substrates. However, apart from being considered as a rare element, Ge substrates are one of the major cost shares of a III-V multi-junction solar cell. To reduce costs and material consumption, we aim at re-usable porosified Ge substrates. Prior to the growth, the porous layers are subjected to an annealing procedure to close the wafer surface and to form a predetermined breaking area some microns below the surface. Later, the III-V epitaxial layers are mechanically lifted at the porous layer, so the substrate can be reused. Here, we demonstrate the III-V epitaxy material quality by growing Al0.5Ga0.49In0.01As/Ga0.99In0.01As double heterostructures on porous Ge substrates and characterize them in detail to understand how the porous layers affect the structural and opto-electronic properties of theIII-Vcompounds compared to a reference grown on germanium “epi-ready” substrates. We find no significant influence of the porous Ge substrate on the layer’s composition, thickness or roughness. However, cathodoluminescence measurements reveal a defect density of 4.5 × 105 cm−2 in comparison with 6.8 × 104 cm−2 for the reference case. Those defects were identified as threading dislocations by electron channeling contrast imaging. The lifetime of minority carriers measured by time resolved photoluminescence shows no difference in the low injection regime between both samples either, indicating a high quality opto-electronic material deposited on porous Ge. These first promising results indicate a path for both: reducing costs of III-V multi-junction solar cells and a reduced germanium consumption.

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained (>100 µm) aluminium alloys.

(Invited) Electron Channeling Contrast Imaging for Rapid Characterization of Compound Semiconductors

As compound semiconductor devices become more advanced, their ultimate performance becomes increasingly dependent on nano to micro scale structural defects in their constituent materials. These defects, such as dislocations, stacking faults, and anti-phase boundaries often act as recombination centers in devices, limiting efficiency. Characterization of these defects – their type (such as edge vs. screw vs. mixed dislocations), their relative densities, how they interact with other defects, what growth conditions are more or less likely to propagate them, how they relate to device efficiencies and performance, etc. – is vital to mitigate their negative effects and even sometimes exploit their potential benefits depending on the desired application. In this contribution, we will discuss the use of electron channeling contrast imaging (ECCI) as an ideal characterization method for compound semiconductors and devices as well as some recent applications which include ECCI characterization in tandem with other characterization methods for a broader material understanding.Transmission electron microscopy (TEM) is a widely used technique for characterizing extended defects and local crystal orientation. However, TEM analysis usually entails thinning a sample via focused ion beam milling or chemical/mechanical polishing to around 50-100 nm, which is time consuming and only allows a relatively small area of the sample to be characterized. These drawbacks can cause bottlenecks in research progress and even render TEM impractical for certain applications. ECCI performed in a scanning electron microscope (SEM) can bypass these issues as it requires little to no sample preparation. This means ECCI can easily characterize over large areas in a fraction of the time as TEM analysis. [1-3] These benefits mean that ECCI gives more accurate analysis by not missing larger trends that can be missed with the small samples in TEM as well as by not introducing artifacts from the sample preparation.Here we highlight applications of ECCI for characterization of two very different compound semiconductors of growing importance -- Cd3As2 and GaN. Firstly, we consider Cd3As2 thin films of use as topological quantum materials, where the Cd3As2 can be either a compound semiconductor or topological Dirac semimetal, depending on the structural phase or mode of use. The ECCI of MOCVD-grown Cd3As2 shown in this work finds that the developmental thin films are not completely single crystalline. [4] Moreover, two different spatial scales of domain formation are seen. At lower magnification, we see roughly circular-shaped larger domains that are ~ 2-5 mm in diameter. Some of these domains are more prominently defined and feature single dark spots at their center, which at first suggests threading dislocations, but turns out to be occasional defect-pits formed in concert with numerous, indistinct threads. At higher magnification, we see much smaller dot-like or speck-like domains that are ~ 100 nm or less in diameter. The wide-area electron channeling patterns used to orient the sample for the ECCI imaging were visible, but rather indistinctly resolved, which is consistent with small crystallographic misorientations of the ensemble of domains composing the film. Secondly, we consider bulk GaN wafers entering use in a wide variety of functional applications, including GaN high-voltage diodes for power electronics. We will show novel “star” defects appearing in the GaN substrates when characterized by both ECCI and high-resolution electron backscattered diffraction (EBSD). [5] Through a more complete structural understanding of these star defects, we seek to improve understanding of the impact of these defects on device performance, which may in turn facilitate development of strategies for their mitigation or removal.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Viewing SiGe Heterostructure for Qubits with Dislocation Theory

SiGe/Si/SiGe heterostructure serves as one of the ideal semiconductor material system to build large scale electron spin qubits[1]. To further push the scalability and extend the coherence times of qubits built on SiGe/Si/SiGe heterostructures, there are several factors to improve, one of that is the lattice homogeneity of strained Si. One source of lattice homogeneity break down are dislocations. Due to the difficulties of high quality SiGe bulk crystal growth [2-3], relaxed SiGe is commonly grown on Si substrates by epitaxy. Viewing the SiGe/Si/SiGe heterostructure on native Si substrate, dislocations half loops form unavoidably because of the misfit between SiGe and the Si substrate. These dislocation half loops consist of threading segments extending through the whole heterostructure (figure 1 (a) and (c)) [4]. Due to the stress caused by the strained Si quantum well layer, the threading dislocations can be driven to glide, resulting in additional misfit segments in the lower interface of strained Si layer when the critical thickness is exceeded (figure 1 (b)) [5]. The extension of these misfit segments is blocked when they meet other perpendicular dislocations (figure 1 (d)) [6]. These misfit dislocations destroy strain uniformity and cause tilt inside the Si layer, resulting in a distortion of the valley splitting and ultimately, reducing the coherence time of qubits [7-8].In this work, we present the observation of dislocations in SiGe/Si/SiGe heterostructures for qubit application by plan-view TEM (Figure 1 (right)). Here, the experimental observations agree with dislocation theory that threading dislocations (some are marked by (b)) will glide and elongate to have misfit segments (some are marked by (c)) in the interface when two requirements are fulfilled: misfit exists between epitaxial layer and substrate; critical thickness is exceeded. When the misfit dislocations meet perpendicular misfit dislocation line, they are blocked since the thickness is not enough to glide over it [6]. This is the case in figure 1 right marked by (d). Due to the misfit segment generation and blocking effects, the misfit dislocation spacing is proportional to the threading dislocation density both theoretically and experimentally. These misfit dislocations bring local strain field around them [9], which disturb the desired homogeneous strain field by misfit. Therefore, misfit dislocation generation needs to be suppressed if one wants to achieve homogeneous strain in Si thin layer in large scale.According to Matthews-Blakeslee theory, one can avoid the formation of misfit dislocations in the Si interface by controlling the strained layer within the critical thickness. The critical thickness of Si on Si0.7Ge0.3 is calculated as 8.5 nm. The Electron channeling contrast images of Si with difference thickness 10 nm and 5 nm grown on high threading dislocation relaxed Si0.7Ge0.3 buffer at 700°C by reduced pressure chemical vapor deposition in Figure 2 are collaborating with theory: obviously misfit dislocation appear in 10 nm Si on relaxed Si0.7Ge0.3 (Figure 2 (left)) and no misfit dislocation are presented in 5 nm Si on Si0.7Ge0.3 (Figure 2 (right)).Our results show that dislocation generation and movement needs to be taken into consideration to further develop SiGe heterostructure based qubits.

In situ investigation on plastic deformation behaviors in austenite-ferrite heterostructured stainless steel

Heterostructured (HS) materials have attracted extensive attention due to their superior mechanical properties. However, there are still many fundamental issues to be solved, like the microstructural evolution process of the hetero-zones during deformation. Here we reported a HS 316L stainless steel generated by exploiting the chemical heterogeneity strategy via thermomechanical processing. The plastic deformation behaviors of the hetero-zones were investigated using three-point bending tests and interrupted tensile tests characterized by electron channeling contrast imaging (ECCI) combined with electron backscattered diffraction (EBSD) techniques. These in situ observations demonstrate the evolution of hetero deformation-induced (HDI) stress field near the austenite/ferrite interfaces in terms of geometrically necessary dislocation (GND) pileups, which can modulate the plastic deformation behaviors of both austenite and ferrite phases. At the early deformation regime, the imposed plastic deformation can be well maintained by the dense and homogeneous activation of slip systems in austenite phases and the stress concentration near austenite/ferrite interfaces can be alleviated by the gradual activation of plastic deformation in the ferrite phases. At the large deformation stage, a progressive twinning-induced plasticity (TWIP) effect caused by the chemical heterogeneity can provide a stable work hardening ability in a broad strain range, which can significantly enhance the ductility. At the late stage of deformation, strain-induced α′-martensite is also triggered, which provides an additional work-hardening ability. The strategy of micro-tuning deformation mechanisms by chemical heterogeneity is versatile and can be applied to many alloys.

Preparation of an overall intergranular fracture surface caused by hydrogen and identification of lattice defects present in the local area just below the surface of tempered martensitic steel

An overall intergranular (IG) fracture surface was prepared in a load reduction test and analyzed by low-temperature thermal desorption spectroscopy (L-TDS). The purpose was to clarify on the atomic scale lattice defects present in the local area just below the surface of a hydrogen-induced IG fracture in tempered martensitic steel. The tracer hydrogen peak corresponding to vacancies and vacancy clusters was present just below the IG fracture surface. Local plasticity and nano-voids were observed on and near the prior austenite grain boundaries by the combined use of electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These findings clearly indicate that local plastic deformation and associated vacancy formation play an essential role in hydrogen-induced IG fracture in tempered martensitic steel.