Small-angle Grain Research Articles

Research on the Re-Deformation Characteristics of Hot Stamping of Boron Steel Parts with Tailored Properties

Traditional hot-stamping products have super-high strength, but their plasticity is usually low and their integrated mechanical properties are not excellent. Functionally graded property structures, a relatively novel configuration with a higher material utilization rate, have increasingly captured the attention of researchers. Hot stamping parts with tailored properties display the characteristics of local high strength and high plasticity, which can make up for the limitations of conventional hot stamping and optimize the crash safety performance of vehicles. This new idea provides a means of personalized control in the hot-stamping process. In this paper, a new strategy of local induction heating and press hardening was used for the hot stamping of boron steel parts with tailored properties, of which the microstructure from the hard zone to the soft zone shows a gradient distribution consisting of a martensite phase, multiphase and initial phase, with the hardness ranging from 550 HV to 180 HV. The re-deformation characteristics of hot stamping parts with tailored properties have been studied through the uniaxial tensile test, in cooperation with digital image correlation (DIC) and electron backscattered diffraction (EBSD) techniques. The experiments show that there are easily observable strain distribution characteristics in the re-deformation of hot stamping parts with tailored properties. In the process of tensile deformation, the initial phase zone takes the role of deformation and energy absorption, with the maximum strain before necking reaching 0.32. The local misorientation of this zone was high, and a large number of low angle grain boundaries were formed, while the proportion of small angle grain boundaries increased from 13.5% to 63.3%, and the average grain size decreased from 8.15 μm to 3.43 μm. Meanwhile, the martensite zone takes on the role of anti-collision protection, with a maximum strain of only 0.006, and its local misorientation is mostly unchanged. The re-deformation experimental results show that the hot stamping of boron steel parts with tailored properties meets the functional requirements of a hard zone for anti-collision and a soft zone for energy absorption, suitable for automobile safety parts.

Direct Imaging of Correlated Defect Nanodomains in a Metal-Organic Framework.

Defect engineering can enhance key properties of metal–organic frameworks (MOFs). Tailoring the distribution of defects, for example in correlated nanodomains, requires characterization across length scales. However, a critical nanoscale characterization gap has emerged between the bulk diffraction techniques used to detect defect nanodomains and the subnanometer imaging used to observe individual defects. Here, we demonstrate that the emerging technique of scanning electron diffraction (SED) can bridge this gap uniquely enabling both nanoscale crystallographic analysis and the low-dose formation of multiple diffraction contrast images for defect analysis in MOFs. We directly image defect nanodomains in the MOF UiO-66(Hf) over an area of ca. 1000 nm and with a spatial resolution ca. 5 nm to reveal domain morphology and distribution. Based on these observations, we suggest possible crystal growth processes underpinning synthetic control of defect nanodomains. We also identify likely dislocations and small angle grain boundaries, illustrating that SED could be a key technique in developing the potential for engineering the distribution of defects, or “microstructure”, in functional MOF design.

Study on the morphology and microstructure of 5A06 alloy by high-pulse-frequency pulsed laser micro polishing

The surface quality of aluminum alloy seriously affects its service life and following processing quality. The innovative high-pulse-frequency pulsed laser micro-polished 5A06 alloy provides defect-free high-precision micro-domain modified surfaces. The effects of surface morphology and microstructure under processing speed and spot distance were investigated. As the processing speed and spot distance decreased, the macroscopic uniformity increased. The minimum surface roughness was 0.53 μm. The processing speed causes the difference in line energy to affect the spread of the molten pool. Spot distance distinguishes the size of the remelted area. Both affect the surface roughness. After laser micro polishing, the remelting layer and HAZ were observed in the cross-section. The EBSD results show the remelting layer is a single grain but has more small angle grain boundaries, i.e. higher dislocation density. The second phase of HAZ was significantly refined, and the grain size was reduced by 11.36% compared to base material. Mg evaporation in the remelting layer was researched using saturated vapor pressure.

High temperature mechanical properties and microstructure of die forged Al−5.87Zn−2.07Mg−2.42Cu alloy

Abstract The high temperature mechanical properties (250 °C) and microstructure of a die-forged Al−5.87Zn− 2.07Mg−2.42Cu alloy after T6 heat treatment were investigated. High temperature tensile tests show that as the temperature increases from room temperature to 250 °C, the ultimate tensile strength of the alloy decreases from 638 to 304 MPa, and the elongation rises from 13.6% to 20.4%. Transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) were applied for microstructure characterization, which indicates that the increase of tensile temperature can lead to the coarsening of precipitates, drop of dislocation density, and increase of dynamic recovery. After tensile testing at 250 °C, a sub-grain structure composed of a high fraction of small-angle grain boundary is formed.

Nucleation mechanism of {10[formula omitted]2} twin with low Schmid factor in hexagonal close-packed metals

{101¯2} twin is one of the most common twins in the plastic deformation of hexagonal close-packed metals. It has been found that some {101¯2} twinning behaviors do not comply with the Schmid law, but the mechanism is not well understood. In the present study, high resolution transmission electron microscopy and electron backscattered diffraction technique were used to comprehensively study the nucleation and growth of {101¯2} twins with low Schmid factor at small angle grain boundaries in pure Titanium. The experimental results revealed that the basal <ɑ> dislocations slipped to the small angle grain boundaries and reacted with the preexisted misfit dislocations, resulting in twinning dislocations. It was identified to be a crucial process for twin nucleation with low Schmid factor. A novel dislocation reaction mechanism for the low SF twin nucleation was proposed.

Scanning Moiré Fringe Method: A Superior Approach to Perceive Defects, Interfaces, and Distortion in 2D Materials.

Scanning moiré fringe (SMF) is a widely utilized technique for the precise measurement of the strain field in semiconductor transistors and heterointerfaces. With the growing challenges of traditional chip scaling, two-dimensional (2D) materials turn out to be ideal candidates for incorporation into semiconductor devices. Therefore, a method to efficiently locate defects and grain boundaries in 2D materials is highly essential. Here, we present a demonstration of using the SMF method to locate the domain boundaries at the nearly coherent interfaces with sub-angstrom spatial resolution under submicron fields of views. The strain field of small angle grain boundary and lateral heterojunction are instantaneously found and precisely determined by a quick SMF method without any atomic resolution images.

Ultrafine lamellar structure AM60B magnesium alloy sheet prepared by high strain rate rolling

The rolled sheet was successfully prepared for the as-cast AM60B magnesium alloy by using the high strain rate rolling (HSRR) in the temperature range of 310 °C – 370 °C. An ultrafine lamellar structure consisting of small angle grain boundaries was developed in the HSRRed sheet by the detwinning of high-density twins. The ultrafine lamellar structure was characterized by means of the high-resolution transmission electron microscopy technique, and the effect of temperature on the detwinning was analyzed. Furthermore, the morphological difference of the ultrafine lamellar structure and its effect on the mechanical properties of the rolled sheet were investigated. The results show that the dislocation motion, cross twinning, dynamic recovery and dynamic recrystallization have multiple interactions with the detwinning behavior in different modes. And the degree of detwinning and the post-rolling microstructure are greatly affected by the interaction. Dynamic precipitation of the second phase is induced by the HSRR process at 370 °C. Nano particles precipitate along twin grain boundaries and form the ‘precipitation band’. However, the detwinning is not affected by the ‘precipitation band’. In addition, it is also revealed from the results that a uniform and fine fully-lamellar structure is developed as the HSRR process is conducted at a medium temperature of 340 °C. Meanwhile, the alloy rolled at 340 °C has relatively optimal mechanical properties, and the strength and ductility are well balanced.

Effect of repair welding on microstructure and mechanical properties of 7N01 aluminum alloy MIG welded joint

The effect of repair welding on the microstructure and mechanical properties of the welded joint was investigated. The results show that the difference of microstructure in different locations of the original welded joint (OWJ) is mainly related to the production process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The grain boundary orientation difference in different positions of repair welded joint (RWJ) is various from that of OWJ, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. The tensile strength of the OWJ and RWJ are 283 MPa and 280 MPa, respectively, and the corresponding welded joint coefficients are 78.6 % and 77.8 %, respectively, which can fulfill the demands of practical engineering applications. Due to the existence of pores and inclusions, the crack of OWJ originates in the junction of HAZ I near the 7N01-T4 side, then expands along the welded zone, while the RWJ fails in the junction of the original weld and the weld passes.

Effect of Annealing Temperature on Cube Texture Formation in Ni7W/Ni12W/Ni7W Compound Substrate

This study is to fabricate a composite substrate (Ni7W/Ni12W/Ni7W) with weak magnetization, strong reinforcement, and improved cube texture via Rolling Assisted Biaxially Textured Substrates (RABiTS) route. The formation of cube texture with reduction in twin boundaries due to recrystallization annealing was investigated thoroughly. The optimized recrystallization annealing process revealed 97.4% of cube texture and 83.9% of small angle grain boundaries at an angle of (<10°). Higher W content and presence of defects (holes, dislocations, and grain boundaries) within inner layer of the substrate restricted the formation of cube texture along the normal direction (ND), as compared to rolling direction (RD). Furthermore, significant improvement in mechanical strength (σ0.2 = 205 ± 2 MPa) and saturation magnetization (2.6 emu/g) was found for Ni7W/Ni12W/Ni7W as compared to Ni7.5W substrates.

Synergistic Balance of Strength and Corrosion Resistance in Al–Mg–Er Alloys

High-strength, corrosion-resistant, and lightweight Al–Mg alloys perform an important function in harsh coastal service environments. Corrosion resistance is generally inversely correlated with strength; hence, it is difficult to simultaneously optimize both. In this study, a low-magnesium Er-containing Al-based alloy that is stronger and more corrosion resistant than Al-based alloys have been reported. The alloy contains Er, and the precipitation of Al3Er within a face-centered cubic matrix is obtained by a series of smelting–casting, heat treatment, and rolling processes. It is presumed that the strengthening phase of Al3Er pinning dislocations improves alloy strength. It also increases the recrystallization temperature of cold-rolled matrix and induces the distribution of small-angle grain boundaries, thus allowing the alloy to achieve excellent environmental corrosion resistance. As a result, strength and corrosion resistance are simultaneously improved.

Displacive phase-field crystal model

A phase-field crystal model that spontaneously undergoes displacive phase transitions is introduced by explicitly studying a two-component two-dimensional square crystal reminiscent of a perovskite. When the intercomponent free energy is a simple polynomial, the crystal undergoes displacive transitions in the $\ensuremath{\langle}10\ensuremath{\rangle}$ and $\ensuremath{\langle}11\ensuremath{\rangle}$ directions. When the interaction is a correlation function, displacements in any direction can occur. This displacive phase-field crystal (DPFC) model maps to Landau-Ginzburg-Devonshire (LGD) theories for ferroelectrics, and the DPFC and LGD models are compared in terms of phase transitions and domain walls. Dynamical simulations of quadrijunctions were also performed and found stable spiraling quadrijunctions and unstable nonspiraling quadrijunctions. Last, domain coarsening across a small-angle grain boundary demonstrates multiple forms of non-mean-curvature-driven growth.

Сегрегация легирующих элементов на малоугловые границы в ферритно-мартенситных сталях при облучении ионами

Segregation of chemical elements was studied in ferritic-martensitic steels RUSFER-EK-181 and ChS-139—promising structural materials for fast neutron reactor core. To simulate the radiation effects, Fe ions with an energy of 5.6 MeV at temperatures of 250–400°C to damage doses of ~6 dpa and at temperatures of 350–450°C to a damage dose of 30 dpa were used. RUSFER-EK-181 steel was also studied after thermal aging at 450°C for 5000 h. Z-contrast analysis of the irradiated steels revealed the segregation of alloying elements on dislocations at small-angle tilt and mixed grain boundaries. Atom probe tomography of the ion-irradiated ChS-139 steel showed that clusters enriched in Ni, Si, and Mn were formed on the dislocations, including dislocations of small-angle grain boundaries. Clusters enriched in Si were formed on dislocations in the RUSFER-EK-181 steel. In the aged state of RUSFER-EK-181 steel, segregation of Cr, V, Mn, Si, and N was shown at dislocations of the small-angle grain boundary. The calculated misorientation angles of the small-angle grain boundaries were ~1°–3°, and the twist angle of the mixed boundary was ~3°.

Heat transport behavior of bicrystal ZnO containing tilt grain boundary

Zinc oxide (ZnO), as a conventional semiconductor material, has excellent characteristics, such as piezoelectricity, photoelectricity, gas sensitivity, etc. With the improvement of nanopreparation technology, different types of nanostructrued ZnO compounds have appeared and their heat conductions have become a main research topic in nanodevices. In order to study the effects of grain boundary on the thermal properties of materials of this kind, bicrystal ZnO containing small-angle and high-angle grain boundaries are constructed by the embedded dislocation line and coincidence site lattice method. The variation of grain boundary energy with tilt angle is studied by the non-equilibrium molecular dynamics simulation. In addition, the dislocation density is calculated by using the Frank-Bilby formula. Our results show that the grain boundary energy and dislocation density increase with the increase of tilt angle in a small-angle region, and they tend to be stable in a high-angle region. The tilt angle of 36.86° is defined as the transition angle. The trend of the Kapitza resistance is the same as that of the grain boundary energy and satisfies the theoretical value from the extended Read-Shockley model. Furthermore, it is found that both the Kapitza resistance and thermal conductivity have a significant size effect. When the sample length is between 23.2 nm and 92.6 nm, the Kapitza resistance decreases sharply with the increase of the length and then tends to be stable. The thermal conductivity of the sample increases with length increasing, but is always less than that of the single crystal. At the same time, temperature is an important factor affecting the heat transport properties. The Kapitza resistance and thermal conductivity decrease with temperature increasing. At different temperatures, the Kapitza resistance of 38.94° grain boundary sample is greater than that of 5.45° grain boundary sample. In order to further explore the influence mechanism of grain boundary angle on heat conduction, the phonon state density of 5.45° and 38.94° grain boundary sample are calculated. The results indicate that the high-angle grain boundary has stronger scattering for acoustic branch phonons and the peak frequency becomes lower, whereas the optical branch ones have almost no effect on the heat conduction.

Effect of High Temperature Annealing on Crystal Structure and Electrical Properties of Multicrystalline Silicon by the Metallurgical Method

The defects of dislocations and grain boundaries (GBs) could significantly reduce the electrical properties of multicrystalline silicon (mc-Si) solar cells. In this manuscript, the influence of crystal defects on the electrical properties of mc-Si was investigated, aims to carry out the research about the influence of crystal defects on the electrical properties of mc-Si. Purified metallurgical grade N type mc-Si ingot was obtained with the method of vacuum directional solidification technique by employing industrial production ingot furnace. The variations in grain boundaries and dislocations were characterized by electron backscatter diffraction (EBSD), and their effects on the electrical properties of mc-Si before and after annealing at different heat insulating time were investigated. Experimental results showed that high temperature annealing process could significantly improve the electrical properties of mc-Si, the minority carrier lifetime was improved from 0.599 μs to 0.770 μs after 10 h insulation, with a significant increase of 28.61%. Experimental results indicated that high temperature annealing process could significantly eliminate the crystal defects of multicrystalline silicon. The crystal defects was eliminated more obviously with prolonged heat preservation time. When treated by high temperature annealing, the proportion of small angle grain boundaries in the same region can be increased by as high as 13.45%. Moreover, the whole grain sizes of partly region of silicon wafer could be increased by 20000μm2, increased by 15.89% compared with the original grain sizes. Based on the experiment phenomena, we attributed the improvement in the electrical properties of mc-Si to the elimination of dislocations, increased grain sizes and migration of GBs.

On manufacturing multilayer-like nanostructures using misorientation gradients in PVD films

Due to their applicability for manufacturing dense, hard and stable coatings, Physical Vapor Deposition (PVD) techniques, such as High Power Impulse Magnetron Sputtering (HiPIMS), are currently used to deposit transition metal nitrides for tribological applications. Cr-Al-N is one of the most promising ceramic coating systems owing to its remarkable mechanical and tribological properties along with excellent corrosion resistance and high-temperature stability. This work explores the possibility of further improving Cr-Al-N coatings by modulation of its microstructure. Multilayer-like Cr1−xAlxN single films were manufactured using the angular oscillation of the substrate surface during HiPIMS. The sputtering process was accomplished using pulse frequencies ranging from 200 to 500 Hz and the resulting films were evaluated with respect to their hardness, Young’s modulus, residual stresses, deposition rate, crystallite size, crystallographic texture, coating morphology, chemical composition, and surface roughness. The multilayer-like structure, with periodicities ranging from 250 to 550 nm, were found associated with misorientation gradients and small-angle grain boundaries along the columnar grains, rather than mesoscopic chemical modulation of the microstructure. This minute modification of microstructure along with associated compressive residual stresses are concluded to explain the increased hardness ranging from 25 to 30 GPa, which is at least 20% over that expected for a film of the same chemical composition grown by a conventional PVD processing route.

One-step growth of wafer-scale monolayer tungsten disulfide via hydrogen sulfide assisted chemical vapor deposition

Wafer-scale monolayer WS2 has been widely investigated. Here, we report a repeatable and low-cost one-step chemical vapor deposition method for the direct growth of a 4-in. monolayer WS2 film on a thermal oxide silicon wafer by using WO3 and H2S gas as precursors. H2S gas exhibits a high vulcanization ability and can effectively reduce the growth temperature of WS2 to 825 °C. The growth process follows a self-limiting growth to form a monolayer polycrystalline film, which is merged via many stable small-angle grain boundaries. The wafer-scale monolayer WS2 film shows uniform and high-quality electrical properties. This method helps promote the future production and application of wafer-scale monolayer sulfide.

Analysis on Fracture Toughness of the L360QS/N08825 Bimetallic Composite Pipe Welded Joint

The fracture toughness of the weld and heat-affected zone (HAZ) of the L360QS/N08825 composite pipe welded joint was evaluated by a crack tip opening displacement (CTOD) test. The fracture morphology, microstructure, and inclusion near fracture zones were observed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The grain size and grain orientation of the crack propagation zone in the weld were investigated by electron back-scattered diffraction (EBSD). The results revealed that the average CTOD values of the weld and HAZ samples were relatively high, and a greater dispersion of CTOD values of the HAZ samples is due to the pop-in phenomenon in the P–V curve. The fracture surfaces of the weld and HAZ samples showed the characteristics of ductile fracture to a certain extent, whereas the fracture of the CTOD sample with the pop-in phenomenon exhibited a quasicleavage feature. High-density dislocation and a large number of inclusions were observed in the near fracture zone of the weld and HAZ samples. The stress concentration, caused by hindering the dislocation slip, was the main reason for microcrack formation. The existence of large-size grains and large-scale small-angle grain boundary in the weld implies that the cracks propagate toward the weld.

Relationship and mechanism between microstructure and property of C70250 copper alloy strip prepared by temperature controlled mold continuous casting

Temperature controlled mold continuous casting (TCMCC) technology was used to fabricate the C70250 copper alloy strips. The influences of solidification microstructure on mechanical and electrical properties of C70250 copper alloys were systematically studied, and the relevant influencing mechanism was discussed. The results show that the microstructure of TCMCC C70250 copper alloy has a large number of fine dispersed Ni2Si phases, small angle grain boundaries and columnar grains with strong [001] orientation. The tensile strength, elongation and electrical conductivity of the C70250 copper alloy are 328 MPa, 40.4% and 24.3% IACS, respectively, which are obviously higher than the properties of the C70250 copper alloys prepared by conventional cold mold casting methods. The tensile strength of TCMCC C70250 copper alloy strip significantly increases due to the dislocation plugging inside the grains, which caused by strong pinning effect of Ni2Si phases during room temperature tensile. Straight small angle grain boundaries and columnar grains with strong [001] orientation significantly reduce the hindering effect of grain boundaries on dislocations. The deformation between and within grains is more uniform, and it is not easy to form high strain concentration, which leads to the increase of plastic deformation ability. The scattering effects of solute atoms and transverse grain boundaries on electrons are significantly reduced due to the precipitated Ni, Si atoms and the high orientation of columnar grain structure, which contributes to the improvement of the electrical conductivity.

(Invited) Effects of Annealing Capped (0001) Oriented GaN Samples

Changes in the topological, structural, electrical and optical properties produced by annealing (0001) oriented GaN homo- and hetero- epitaxial films that were protected by an AlN cap and annealed at a temperature, TA, ranging from 1150 to 1300 ºC for a time, t A, ranging from 1 to 8 min are discussed. The films were as-grown or Mg implanted unintentionally doped n-type, or they were Mg doped. For as-grown hetero-epitaxial films the step-flow growth patterns on the as-grown surface survive up to TA ≤ 1300 ºC and t A ≤ 2 min. Small angle grain boundaries parallel to the {1100} M-planes become more defined on the surface of the hetero-structure films as TA and/or t A increases so long as it retains its smooth step-flow growth pattern. Small hexagonal etch pits associated with threading edge dislocations decorate these boundaries suggesting that the grain boundaries are created by polygonization. These boundaries are present in the as-grown film, but they do evolve over time at different rates at different temperatures as is illustrated in the (00.2), (10.2), (30.2) x-ray rocking curves, (10.5) ω - 2θ scans, and reciprocal space maps. Similar studies of films grown on ammonothermal bulk single crystals show similar trends, but they are much more muted. All of these studies require an annealing cap to minimize the rate of preferential evaporation of nitrogen. The GaN surface must be clean and smooth, the cap must adhere well, and one must be able to preferentially etch it off after the anneal. A thin AlN film grown epitaxially by MOCVD at low temperatures combined with a thicker sputtered AlN film meets all of these requirements, but it only does so up TA = 1300 ºC for t A < 8 min. It fails at higher TA because the N vapor pressure at 1300 ºC is about equal to the yield stress of AlN. Our caps fail when t A > 8 min at TA 1300 ºC because even some N escapes with a good seal. When the cap does not seal as well, the GaN film decomposes first at the small angle grain boundaries. Even when the surface of the film is smooth, the electrical properties show that the anneal has altered the internal structure. The conductance above the voltage knee of Schotty diodes fabricated on them initially increases with TA and t A, but it begins to drop off at TA = 1200 or 1250 ºC suggesting that at lower TA some of the structural defects are annealed out thereby increasing the mobility, but at higher TA other types of defects, possibly nitrogen vacancy, NV, complexes are created. This explanation is consistent with the observation of the conductance below the voltage knee being larger at the larger TA because the tunneling current through the defects associated with the NV is larger. The reverse leakage current follows the same trend – it being larger at the larger TA. The CV characteristics of the diode suggests that this defect layer is mostly confined to the region near the surface as the net carrier concentration as measured at the end of the depletion layer does not change with TA until TA ≥ 1250 ºC when the defected area is at the end of the depletion layer at zero bias. Defects of a different nature are created by the implant process as determined by the increased breadth of the XRD curves and the creation of another peak. But much of that damage is annealed out as the peak widths return to almost their original breadth and the second peak disappears. However, they do not return to their original breadth. It is not clear whether this is due to not removing all of the implant damage or the damage created by the annealing process contributing to the increased breadth. We are now in the process of deciphering the more complicated electrical measurements made on the Mg implanted or doped samples, and expect to report on them at the meeting.

Density, distribution and nature of planar faults in silver antimony telluride for thermoelectric applications

Defects such as planar faults in thermoelectric materials improve their performance by scattering phonons with short and medium mean free paths (3–100 nm), thereby reducing the lattice thermal conductivity,κl. Understanding statistically the microscopic distribution of these extended defects within the grains and in low angle grain boundaries is necessary to tailor and develop materials with optimal thermoelectric performance for waste heat harvesting. Herein, we analyze these defects from the millimeter down to the nanometer scale in a AgSbTe2 thermoelectric material with low angle grain boundaries. The investigations were performed using electron channeling contrast imaging combined with transmission electron microscopy. The microstructure study was complemented by estimating the effect of planar faults on the phonon scattering using the Debye-Callaway model. AgSbTe2 is a promising thermoelectric material, which exhibits extremely low thermal conductivity, κ, of 0.5 Wm−1K−1 at room temperature. In contrast to conventional alloys or intermetallic materials, in the present material small angle grain boundaries are not composed of individual dislocations but of a dense arrangement of stacked planar faults with fault densities up to NPF=1.6⋅108m−1. We explain their abundance based on their low interfacial energy of about 186 mJm−2 calculated ab-initio. The current findings show, that it is possible to reach very high densities of phonon-scattering planar faults by the correct microstructure engineering in AgSbTe2 thermoelectric materials.