Relaxation Of Compressive Strain Research Articles

Multiscale residual stress and mechanical behavior analysis in machining of Ti-6Al-4V alloy with advanced microscopic characterization

Machining of polycrystalline metal materials results in metallurgical modifications in the machined surface, and it can introduce multiscale residual stresses (macroscale and microscale residual stress), which may be detrimental to the mechanical properties thereby interfering with the service performance of machined parts. However, most of the current studies only focus on the macroscale residual stress, the microscale residual stresses have always been neglected since the limitations of the commonly used techniques for residual stress measurement in machined surfaces. In this study, the recently developed Focused Ion Beam-digital image correlation (FIB-DIC) ring core method was employed to evaluate the time-resolved strain relaxation and multiscale residual stress of the machined surface under different cutting conditions. The quantitative evaluation of residual stress and microstructural effects on hardness of the machined surface was conducted by combining with FIB incremental milling and nanoindentation technique. It has been observed that although the global tensile strain relaxation was captured in the ring-core region, local microscale compressive strain relaxation also occurred. The hardness of the machined surface was determined by the combined effects of microstructure and residual stress induced by machining. Specifically, the grain refinement and compressive residual stress contributed to hardening, while tensile residual stress resulted in softening of the machined surface. Furthermore, it was also observed the hardening/softening effects were enhanced with increasing intensity of tension and compression.

Microscale spectroscopic mapping of defect evolution and filling in large-area growth of monolayer MoS2

Defects in monolayer transition metal dichalcogenides are known to be problematic in that they usually deteriorate optical and electronic properties, thus acting as a constraint in nanodevice applications. In this regard, understanding defect-related strain and charge distributions is intriguing to identify the inhomogeneous energy landscape of two-dimensional materials. Herein, simultaneous Raman and photoluminescence results unveil how strain and charge carriers evolve over the large-area growth of monolayer MoS2. Unexpectedly, spatially-resolved correlation analysis between the in-plane and out-of-plane phonon frequencies of MoS2 reveals that both compressive strain and charge carriers increase as the morphology evolves from a triangular flake to a partially coalesced film and, finally, to a fully covered film, indicating that the number of sulfur vacancies increases with surface coverage. Theoretically calculated formation energies of sulfur vacancies support the evolution scenarios with respect to morphological variations. Photoluminescence mappings of excitons and trions provide further quantitative information on electron concentrations that are consistent with Raman correlation data. Notably, chemical treatment of defective MoS2 with a p-type dopant leads to a relaxation of compressive strain and a decrease in excess electrons, where in particular, the p-doping effects are more prominent in the fully covered film region than in the partially coalesced region.

Effects of Al doping on dislocation inclinations and strain of GaN films on Si substrates

We present how the interaction between Al dopants and threading dislocations affects dislocation inclinations and then plays an important role in controlling residual strain in GaN-on-Si epitaxial films. When the Al concentration in the GaN epitaxial film is increased to 0.85%, the dislocations extend almost in the growth direction, contributing to a strain-free epitaxial film. We suggest that the Al atoms could substitute for Ga vacancies at the dislocation cores on the growth surface and then inhibit the dislocation inclinations. The suppressed dislocation inclinations lead to a reduced relaxation of compressive strain. The results pave a new way to control dislocation movements and strain in GaN epitaxial films on Si substrates.

Observation of threading dislocations and misfit dislocation half-loops in GaN/AlGaN heterostructures grown on Si using electron channeling contrast imaging

We implemented invisibility criterion and black–white contrast orientation analysis into low-tilt electron channeling contrast imaging (ECCI) for dislocation-type discrimination in GaN and AlGaN layers grown on a Si(111) substrate. Our ECCI and x-ray diffraction (XRD) analysis attained consistent threading dislocation densities for GaN and AlGaN grown on Si, but demonstrated drastic discrepancy in the percentage of edge-type dislocations, potentially due to the lack of appropriate consideration of mixed-type (a→+c→) dislocations in XRD. Further ECCI analysis of GaN/AlGaN heterointerface revealed mixed-type (a→+c→) dislocation half-loops and dislocation bending due to compressive strain relaxation, validating that not all the dislocations originated from the mosaic or columnar structure. As a result, XRD analysis based on the mosaic block model does not give reliable edge-to-screw dislocation ratio. The observation of classic van der Merwe–Matthews-type dislocation half-loop nucleation and dislocation gliding could be associated with potential GaN/AlGaN optoelectronic device degradation issues.

Experimental and Numerical Evaluations of Localized Stress Relaxation for Vulcanized Rubber.

Vulcanized rubbers are commonly used to provide the energy absorption under compressive deformation from other engineering components. However, if a constant compressive deformation is maintained on rubber, the load response is not constant but decreases with time; i.e., the stress relaxation. A decrease in force response with time of rubber can be experimentally evaluated by the stress relaxation test. In the present work, the localized stress of vulcanized rubber during a compressive stress relaxation test (i.e., ASTM D6147) was evaluated. Hyperelastic behavior was assumed during rapid application of strain, while the viscoelastic behavior was assumed during stress relaxation. Hyperelastic and viscoelastic parameters were experimentally evaluated using a standard specimen. Finite element analysis (FEA) models were applied for the predictions of stress relaxations of rubbers with various geometries and applied strains. FEA results were in good agreement with results of the stress relaxation tests. Localized stresses in rubber during rapid application of compressive strain and stress relaxation were successfully evaluated. The findings can give the localized phenomena of vulcanized rubber during a stress relaxation test, which can be used as a guideline for the design, usage, and improvement of rubber and viscoelastic polymeric components.

Enhanced GeSn Microdisk Lasers Directly Released on Si

AbstractGeSn alloys are promising candidates for complementary metal‐oxide‐semiconductor‐compatible, tunable lasers. Relaxation of residual compressive strain in epitaxial GeSn has recently shown promise in improving the lasing performance. However, the suspended device configuration that is thus far introduced to relax the strain is destined to limit heat dissipation, thus hindering the device performance. Herein is demonstrated that strain‐free GeSn microdisk laser devices fully released on Si outperform the canonical suspended devices. This approach allows to simultaneously relax the limiting compressive strain while offering excellent thermal conduction. Optical simulations confirm that, despite a relatively small refractive index contrast between GeSn and Si, optical confinement in strain‐free GeSn optical cavities on Si is superior to that in conventional strain‐free GeSn cavities suspended in the air. Moreover, thermal simulations indicate a negligible temperature increase in the device. Conversely, the temperature in the suspended devices increases substantially reaching, for instance, 120 K at a base temperature of 75 K under the employed optical pumping conditions. Such improvements enable increasing the operation temperature by ≈40 K and reducing the lasing threshold by 30%. This approach lays the groundwork to implement new designs in the quest for room temperature GeSn lasers on Si.

Strain-relaxed GeSn-on-insulator (GeSnOI) microdisks

GeSn alloys offer a promising route towards a CMOS compatible light source and the realization of electronic-photonic integrated circuits. One tactic to improve the lasing performance of GeSn lasers is to use a high Sn content, which improves the directness. Another popular approach is to use a low to moderate Sn content with either compressive strain relaxation or tensile strain engineering, but these strain engineering techniques generally require optical cavities to be suspended in air, which leads to poor thermal management. In this work, we develop a novel dual insulator GeSn-on-insulator (GeSnOI) material platform that is used to produce strain-relaxed GeSn microdisks stuck on a substrate. By undercutting only one insulating layer (i.e., Al2O3), we fabricate microdisks sitting on SiO2, which attain three key properties for a high-performance GeSn laser: removal of harmful compressive strain, decent thermal management, and excellent optical confinement. We believe that an increase in the Sn content of GeSn layers on our platform can allow us to achieve improved lasing performance.

Resistive switching characteristics of epitaxial NiO thin films affected by lattice strains and external forces

We investigated the resistive switching characteristics of NiO films grown epitaxially on two single-crystal Rh and Ni substrates. Two epitaxial NiO films were relatively small compressive strained on the Rh substrate, and confirmed by the x-ray diffraction experiments that the strain was relatively large compressive on the Ni substrate. Comparing the forming, SET, and RESET voltages of NiO resistive switching random access memory capacitors, NiO films grown on the Ni substrates showed relatively higher voltages than Rh substrates. We propose that this compressive strain has an effect on raising the forming, SET, and RESET voltages. Furthermore, we observed that the forming voltage decreases while reducing the compressive strain by applying pressure to the NiO thin film grown on the Ni substrate with an atomic force microscope tip. In addition, we showed that relaxation of compressive strain by external force by first-principles calculations can reduce the oxygen deficiency energy and lower the forming voltage and working voltages, which supports the experimental results well.

Formation mechanism of Ruddlesden-Popper faults in compressive-strained ABO3 perovskite superlattices.

Ruddlesden-Popper (RP) faults have emerged as a promising candidate for defect engineering in epitaxial ABO3 perovskites. Functionalities could be fine-tuned by incorporating RP faults into ABO3 thin films and superlattices. However, due to the lattice expansion at AO-AO interfaces, it is generally believed that RP faults are only energetically favorable under tensile strain. Contrary to this common cognition, here we present that compressive strain must be regarded as an alternative driving force for creating RP faults. Unlike the conventional perovskite-to-rock-salt transition, the RP faults originated from Shockley partial dislocations bounded by stacking faults on the basal plane. The edge-type partials gave rise to strain relaxation, facilitating the formation of RP faults under compressive strain. We envisage that our results will give new insights into the rational design and defect engineering in epitaxial-strained ABO3 perovskites.

Carrier localization and defect-insensitive optical behaviors of ultraviolet multiple quantum wells grown on patterned AlN nucleation layer

In this work, we report a significantly boosted defect-insensitive ultraviolet emission from InGaN/GaN multiple quantum wells (MQWs) grown on patterned AlN nucleation layer (NL) compared to samples grown on uniform NL. Carrier localization is clearly illustrated for MQWs grown on patterned AlN NL as evidenced by an S-shape profile from temperature-dependent photoluminescence characterization. The underlying mechanism for the carrier localization is demonstrated to correlate with partial relaxation of compressive strains inside epitaxial thin films. This work illustrates that carrier localization can be achieved in MQWs with very low indium content by the adoption of patterned NL during growth, and provides a promising route towards the realization of high-efficiency ultraviolet emitter.

High-crystalline-quality AlN grown on SiC substrates by controlling growth mode

We investigate the relation between growth mode and crystalline quality of AlN directly grown on an on-axis 6H-SiC(0001) substrate under various supersaturation conditions by metal-organic chemical vapor deposition. Although 500-nm-thick AlN fabricated by a two-dimensional (2D) growth mode had smooth surfaces, high threading dislocation density (TDD) and cracks were observed. In contrast, a three-dimensional (3D) growth mode produced spiral growth and crack-free AlN with low TDD. We analyze behaviors of surface morphology, TDD, and strain relaxation of AlN by the change in growth mode, revealing that TDD is correlated with crack generation due to relaxation of compressive strain during AlN growth. To obtain atomically flat AlN on 6H-SiC(0001) to suppress crack generation by the decreased TDD, we performed multi-step 3D and 2D AlN growth. We describe successful fabrication of 600-nm-thick AlN with no crack generation, low TDD, and a 500-nm-wide terrace on an on-axis 6H-SiC(0001) substrate.

Mechanism of Improved Luminescence Intensity of Ultraviolet Light Emitting Diodes (UV-LEDs) Under Thermal and Chemical Treatments

In this work, the influences of thermal annealing and chemical passivation on the optical and electrical properties of ultraviolet light-emitting-diode (UV-LED) were investigated. The electroluminescence (EL) intensities of the LEDs under KOH treatment and thermal annealing increased by 48% and 81%, respectively compared to as-fabricated LED under current level of 10 mA. Cathodoluminescence (CL) mapping of UV-LEDs confirmed no variation of the density of the non-radiative recombination centers after surface treatments, and no obvious change in surface morphology was identified due to lacking of energy for surface atom migration. However, Raman spectroscopy indicates a relaxation of compressive strains inside the thin film after both thermal and chemical treatments, and conductive atomic force microscopy (c-AFM) also illustrated reduced leakage current after KOH passivation, which are responsible for the improved luminescence properties of UV-LEDs.

Omnidirectional whispering-gallery-mode lasing in GaN microdisk obtained by selective area growth on sapphire substrate.

The optical properties of hexagonal GaN microdisk arrays grown on sapphire substrates by selective area growth (SAG) technique were investigated both experimentally and theoretically. Whispering-gallery-mode (WGM) lasing is observed from various directions of the GaN pyramids collected at room temperature, with the dominant lasing mode being Transverse-Electric (TE) polarized. A relaxation of compressive strain in the lateral overgrown region of the GaN microdisk is illustrated by photoluminescence (PL) mapping and Raman spectroscopy. A strong correlation between the crystalline quality and lasing behavior of the GaN microdisks was also demonstrated.

Tunable Perpendicular Magnetic Anisotropy in Off-Stoichiometric Full-Heusler Alloy Co2MnAl**Supported by the National Key Research and Development Program of China under Grant Nos 2017YFB0405701 and 2018YFB0407601, the National Natural Science Foundation of China under Grant Nos U1632264 and 11874349, and the Key Research Project of Frontier Science of the Chinese Academy of Sciences under Grant Nos QYZDY-SSW-JSC015 and XDPB12.

Off-stoichiometric full-Heusler alloy Co2MnAl thin films with different thicknesses are epitaxially grown on GaAs (001) substrates by molecular-beam epitaxy. The composition of the films, close to Co1.65Mn1.35Al (CMA), is determined by x-ray photoelectron spectroscopy and energy dispersive spectroscopy. Tunable perpendicular magnetic anisotropy (PMA) from 3.41 Merg/cm3 to 1.88 Merg/cm3 with the thickness increasing from 10 nm to 30 nm is found, attributed to the relaxation of residual compressive strain. Moreover, comparing with the ultrathin CoFeB/MgO used in the conventional perpendicular magnetic tunnel junction, the CMA electrode has a higher magnetic thermal stability with more volume involved. The PMA in CMA films is sustainable up to 300°C, compatible with semiconductor techniques. This work provides a possibility for the development of perpendicular magnetized full-Heusler compounds with high thermal stability and spin polarization.

Microstructure-mechanical property correlation in shot peened and vibro-peened Ni-based superalloy

In this experimental research, shot peening (SP) and vibro-peening (VP) processes were employed to modify the surface and sub-surface of a Ni-based superalloy Udimet®720Li. The effect of these treatments on the surface topography, microstructure evolution, and mechanical properties was investigated using instrumented tests. The compressive residual stresses and plastic strain generated through surface treatment were characterized using X-ray diffraction (XRD). The depth of plastically strained region was estimated using a quantitative analysis of grain orientation spread (GOS) data from electron back-scattered diffraction (EBSD) technique. It was observed that the magnitude of compressive residual stresses induced by VP is relatively low compared to SP, but the converse is true for depth of influence (DOI) for the same peening intensity. After thermal exposure at 650 °C (∼0.4Tm) for 5 h, the relaxation of compressive residual stresses and plastic strain was less pronounced in vibro-peening when compared to shot peening representing an improved thermal stability of the former process. Furthermore, 5.24 GPa surface micro-hardness of the untreated sample (O) was increased by ∼30% and ∼25.2% following SP and VP respectively. This resultant micro-hardness enhancement is due to grain refinement, plastic strain effects, and the introduction of low angle grain boundaries (LAGBs). Finally, a positive correlation among microstructural features and mechanical properties was fitted from the experimental data.

What Drives Metal-Surface Step Bunching in Graphene Chemical Vapor Deposition?

Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.

3-inch GaN-on-Diamond HEMTs With Device-First Transfer Technology

Based on a device-first transfer process, a 3-inch polycrystalline diamond substrate is bonded within $1.5~\mu \text{m}$ of the junction in GaN high electron mobility transistors (HEMTs) to enhance heat removal of the high-power RF devices. Highly preserved electrical performance is demonstrated by comparison exactly on the same HEMT device prior and after substrate transfer. The residual compressive strain relaxation of the whole GaN epilayer does not reduce the 2-D electron gas sheet density. The dc characteristics show weakened self-heating in the GaN-on-diamond HEMT with maximum current density increasing from 968 to 1005 mA/mm. The power density increases from 4.8 to 5.5 W/mm with the PAE slightly reducing from 50.9% to 50.5%. On-wafer infrared measurement is performed on a 1.25-mm GaN HEMT at power dissipation of 10 W/mm, and the peak juncture temperature of the device decreases from 241 °C to 191 °C after transferring to the diamond substrate.

The effect of He + irradiation on hardness and elastic modulus of Fe-Cr–40 wt.% TiB 2 composite rod designed for neutron absorbing

Abstract A composite rod, designed for the neutron absorption in nuclear research reactors or Nuclear Power Plants (NPPs), was synthesized by ball milling and subsequent Hot Isostatic Pressing (HIP-ing), similarly to the one reported in Ref. [3]. Its core consists of the oxide dispersion strengthened ferritic Fe-Cr matrix and 40 wt.% of TiB 2 reinforcement, surrounded by a Ti tubing/cladding with a functionally graded interface layer. The Specific Surface Area (SSA) for the ball-milled pre-alloyed powders increases from 0.64 to 2.92 m 2 /g and XRD shows that the crystallite/grain size of TiB 2 is ∼38 nm. Berkovich nanoindentation study after irradiation with 160 keV He + ions at a fluence up to 1 × 10 17 He + /cm 2 shows an initial hardening effect at fluences of 1 × 10 15 and 1 × 10 16 ions/cm 2 . Hardness and elastic modulus of the Fe-Cr -TiB 2 core rapidly drops when the fluence reaches 1 × 10 17 ions/cm 2 . The Ti cladding seems to be relatively impervious to increased radiation fluence since its hardness and elastic modulus change very slightly with increasing ion fluence. The observed changes in the mechanical properties are discussed in terms of vacancy/dislocation loops and He bubble formation in the irradiated microstructure. Although the He-vacancy complexes are widely regarded as being immobile, it is hypothesized here, based on the grazing incidence XRD (GI XRD), that interstitial helium diffuses outward through the boundaries of the (100) hcp-TiB 2 nanograins. As a result, the relaxation of compressive strains due to high concentration of vacancies in a nanograin crystalline lattice finally leads to the hcp-TiB 2 unit cell contraction.

Effects of Si-doping on structural, electrical, and optical properties of polar and non-polar AlGaN epi-layers

The polar (0001)-oriented c-plane and non-polar (112¯0)-oriented a-plane wurtzite AlGaN epi-layers were successfully grown on polar (0001)-oriented c-plane and semi-polar (11¯02)-oriented r-plane sapphire substrates, respectively with various Si-doping levels in a low pressure metal organic chemical vapor deposition (MOCVD) system. The morphological, structural, electrical, and optical properties of the polar and non-polar AlGaN epi-layers were studied with scanning electron microscopy (SEM), X-ray diffraction (XRD), Hall effect, and Raman spectroscopy. The characterization results show that Si dopants incorporated into the polar and non-polar AlGaN films induced a relaxation of compressive residual strain and a generation of biaxial tensile strain on the surface in consequence of the dislocation climbing. In particular, it was found that the Si-induced compressive strain relaxation in the non-polar AlGaN samples can be promoted by the structural anisotropy as compared with the polar counterparts. The gradually increased relaxation of compressive residual strain in both polar and non-polar AlGaN samples with increasing Si-doping level was attributed to the Si-induced enhancement in the opportunity for the dislocations to interact and annihilate. This implies that the crystal quality for both polar and non-polar AlGaN epi-layers can be remarkably improved by Si-doping.

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.