Elongated Grains Research Articles

Grain Refinement and Phase Transformation of Pure Titanium Foils During Asymmetrical Rolling

Ultrafine‐grained microstructure and face center cubic (FCC) phase are produced in the pure titanium foils through asymmetrical rolling (ASR) to explore the mechanical properties of pure titanium foils with different thicknesses. Herein, the grain refinement process and phase transformation behavior are systematically investigated. The grain refinement process can be described as: 1) coarse equiaxed grains are elongated along RD due to the compressive strain and shear strain caused by the ASR process. 2) elongated grains are split into thin laths and subgrains by dislocation walls under greater strain. 3) thin laths and subgrains are further refined and ultrafine‐grained microstructures are obtained. In addition, hexagonal close‐packed (HCP)‐FCC phase transformation is also discovered during the ASR process. The orientation relationship between the HCP and FCC phases is determined as [0001]HCP//[001]FCC, (100)HCP//(20)FCC and the phase transformation is caused by Shockley partial dislocation with Burgers vector 1/6[110] gliding on each (100) plane. Uniaxial tensile tests on heat‐treated pure titanium foils of different thicknesses show a size effect on tensile properties. As the thickness decreases, tensile strength and elongation are reduced, with fewer/single‐layer equiaxed grains at the section of the specimen.

Wire electron beam additive manufacturing of copper

The potential of additive manufacturing of copper components is not yet sufficiently investigated despite the existence of a large market for copper products, especially in the energy, mobility, and aerospace sectors. Wire arc additive manufacturing (WAAM) and laser directed energy deposition (DED) with conventional lasers, the latter due to the high light reflection in copper, have difficulties in the additive manufacturing of this material. The electron beam, on the other hand, achieves a high energy input even with copper and thus the desired efficiency. Wire electron beam additive manufacturing (WEBAM) is capable to produce pore free copper components with high deposition rates in the range of 2 kg/h. The microstructure of pure copper WEBAM parts exhibits large, elongated grains growing epitaxially and additional structures related to the layered approach specific for additive manufacturing. The company pro-beam additive GmbH investigates several applications required by different markets. As an example, for the aerospace industry a model for a rocket motor (650 mm height, 295 mm maximum diameter, 22 kg weight) was developed. For the energy sector, the capability of WEBAM to generate multi-material components was applied to create copper structures on steel base components with good adherence.

Effect of Tempforming on Strength and Toughness of Medium-Carbon Low-Alloy Steel.

The effect of tempforming on the strength and fracture toughness of 0.4%C-2%Si-1%Cr-1%Mo-VNb steel was examined. Plate rolling followed by tempering at the same temperature of 600 °C increases yield stress by 25% and the Charpy V-notch impact energy by a factor of ~10. Increasing rolling reduction leads to the reorientation and elongation of grains toward the rolling direction (RD) and the development of a strong {001} <110> (rotated cube) texture component that highly enhances fracture toughness. A lamellar structure with a spacing of 72 nm between boundaries and a lattice dislocation density of ~1015 m-2 evolves after tempforming at 600 °C with a total strain of 1.4. Two types of delamination were found, attributed to crack branching and the propagation of secondary cracks along the rolling plane perpendicular to the propagation direction of the primary crack. Delamination toughness is associated with the nucleation of secondary cracks in RD and their propagation over a large distance. The critical condition for delamination toughness is the propagation of primary cracks by the ductile fracture mechanism and the propagation of secondary cracks by the brittle quasi-cleavage mechanism.

Evidence for two-stage hardening in an Al-Zn-Mg-Cu alloy processed by high-pressure torsion

Experiments were conducted to assess the effect of torsional straining on the microstructural evolution of an Al-5.7Zn-1.8Mg-2.1Cu alloy containing dispersed second phase particles prior to deformation. The results show that the material exhibits two distinct and consecutive hardening stages when processed through 1/8, 1, 5, 30, 50 and 100 turns in high-pressure torsion (HPT). There is an initial increase in the Vickers hardness from ∼120 Hv in the unprocessed state to ∼240 Hv after 1 turn. In this first stage, hardening occurs primarily through the accumulation and rearrangement of dislocations into fine cells/subgrains. A quasi-stationary condition is achieved with further straining where the microhardness remains nearly constant and the grain size is ∼140 nm up to 30 revolutions. Thereafter, the microstructure consists of ultrafine and slightly elongated grains but the second phases are continuously fragmented. This fragmentation produces a new hardening stage and the development of a uniform array of highly elongated grains with an average size of ∼93 nm after 100 HPT turns. It is shown that this two-stage hardening is consistent with conventional Hall-Petch strengthening and the development of the second stage is attributed to a reduction in grain boundary mobility due to the segregation of solutes.

Effect of corrosion exposure on the mechanical performance of aeronautical aluminum alloy 2024 riveted sheets

The effect of corrosion exposure on the tensile mechanical properties of aeronautical aluminum alloy 2024-T3 with the presence of holes for their subsequent connection with rivets as well as the corrosion mechanism is investigated. Three different experimental series were performed for the simulation of corrosion attack on aircraft structures. The first experimental series included the investigation of corrosion mechanism for the specimens exposed for different times to EXCO solution on all their surfaces, including the interior of the hole. The second experimental series included the specimens corroded only within the hole, while the third experimental series consisted of the specimens that were exposed only on the surfaces outside the hole. The separation in the above-mentioned experimental series was carried out to simulate the case of damage on the aircraft’s anti-corrosion coating at points of joining of their structural parts. According to the results of the present work, it was found that corrosion affects the different surfaces of the material in a different way, with the specimens corroded inside the hole exhibiting greater corrosion susceptibility (or less corrosion resistance) and the interior of the hole (through thickness area) to play a key contributing role in the total reduction of elongation at fracture Αf, due to the ease of corrosion penetration between the highly elongated grains in this area.

Effect of grain growth in bimodal microstructure on cutting performance of self-toughening Dy-α-SiAlON ceramic tool inserts

Although significant increase in mechanical properties has been achieved in ?-SiAlON ceramics by introducing self-toughening bimodal microstructure, the bimodal microstructure may not always result in better cutting performance of ?-SiAlON tool inserts. Herein Dy-?-SiAlON ceramics were prepared by hot-pressing at 1900?C under 30MPa. Diameter and aspect ratio of the large elongated ?-SiAlON grains pivotal to formation of bimodal microstructures were regulated by using 0-5wt.% Dy2O3 as a sintering additive. Mechanical properties and cutting performance of ?-SiAlON tool inserts were evaluated. The results showed good Vickers hardness (19.0-19.7GPa) and fracture toughness (3.61-4.18MPa?m1/2) increasing with crack propagation length. However, transverse section of the large elongated grains was revealed as critical crack to cause premature fracture of the ?-SiAlON ceramics. Fracture rather than attrition wear was recognized as dominant mechanism for tool failure, indicating that for better cutting performance of the ?-SiAlON tool inserts a selftoughening microstructure consisting of thinner grains with large aspect ratio is required.

Aluminum 4.5 wt. % Copper Alloy Powder Sintering Temperature Effect on Microstructure and Mechanical Properties of Spark Plasma Sintered Compact

The aluminum 4.5 wt. % copper alloy was melted in an electrical resistance furnace and atomized to produce the powder of 350 µm to 500 µm size of different shapes and sizes. The atomized powder was graded using standard set of sieves. The powder is compacted with the help of spark plasma sintering by applying the required pressure and temperature simultaneously. The sintering pressure of 25 MPa and temperatures of 480 °C and 540 °C was employed to produce sintered compacts at two different temperatures. The density of the sintered compact was measured using Archimedes principle. The surface topography of both the compacts revealed the cellular, elongated grains and dendritic microstructure. The average micro Vickers hardness of compact sintered at 480 °C and 540 °C were found to be 55.9 HV1 and 65.4 HV1 respectively. The compression strength of compact sintered at 480 °C is 185 MPa and the compact sintered at 540 °C is reported as compacts was reported as 250 MPa.

Band Gap Reduction and Improved Ferromagnetic Ordering via Bound Magnetic Polarons in Zn(Al, Ce)O Nanoparticles

ABSTRACT Polycrystalline nanoparticles of Al-doped and Ce-co-doped ZnO were processed through sol-gel co-precipitation. Crystallite sizes determined from XRD were in the range of 10.90 to 48.77 nm. FTIR spectra indicated a stretching mode of Zn-O bond. The insertion of Al and Ce at Zn-O site created Al-O and Ce-O bonds. UV-vis spectra favoured the formation of impurity levels by doping and co-doping. Their overlapping with the conduction band edge led to the reduction of band-gap. Blue-green emission which arises from radiative recombination of a photogenerated hole with an electron occupying oxygen vacancy was observed in photoluminescence spectra. FESEM suggested granular growth in undoped ZnO which changed to diverse structures such as nano-bar and cluster of elongated grains. Significant improvement in room temperature ferromagnetism (RTFM) was noticed by co-doping of Ce in Al-doped ZnO. Formation of bound magnetic polarons (Ce3+-Vo-Ce3+) was the primary mechanism responsible for the improvement in RTFM.

Investigation of the influence of misorientation-dependent anisotropy on the microstructure evolution under magnetic field: A multi-phase-field study

The influence of magnetic field-driven grain growth in a polycrystalline microstructure is studied via the multi-phase-field approach with the assumptions of isotropic and anisotropic grain boundary (GB) energy. Simulations are carried out using a two-dimensional representative volume element of titanium polycrystalline with the hexagonal crystal structure. The GB energy is a function of the misorientation angle by Read-Shockley equation and is independent of the GB inclination. The simulation results illustrate that the grain growth kinetics and final texture influentially depend on the initial texture when considering the misorientation-dependent anisotropy of GB energy. If the Euler angle φ 2 is randomly distributed for all of the grains, the simulation results are similar for isotropic and anisotropic GB energies. Whereas the simulation results for anisotropic GB energy are significantly different from the isotropic case by considering φ 2 = 0 for all grains. The elongation of grains, the aggregation of grains with analogous orientations, and deceleration of texture evolution and grain growth kinetics are its most important features. Also, the impact of magnetic field intensity is examined and, it is illustrated that the difference in simulation results for isotropic and anisotropic GB energies decreases with increasing the magnetic field intensity.

A facile method to produce rodlike β‐Si3N4 seed crystallites for bimodal structure controlling

Abstractβ‐Si3N4 rodlike seed crystallites were successfully produced by single‐step heat treatment of commercial α‐Si3N4 powder at 1900°C for 20 h under an N2 gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α‐ ⇀ β‐Si3N4 phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α‐Si3N4 dissociation temperature (1933°C) under N2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α‐Si3N4 crystal lattice was suggested as the driving force for the present oxide additive‐free α‐ ⇀ β‐Si3N4 phase transformation. β‐Si3N4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd2O3. Starting α‐Si3N4 powder with 10 vol% rodlike β‐Si3N4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β‐Si3N4 ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m−1/2 and 109.3 ± 0.4 W m−1 K−1, respectively, retaining a high fracture strength of about 1 GPa.

An OpenMP GPU-offload implementation of a non-equilibrium solidification cellular automata model for additive manufacturing

In this paper, performance strategies on GPU-based HPC platforms of a cellular automata (CA) simulation code for non-equilibrium solidification, including nucleation, grain growth, solute partitioning and transport for the metal additive manufacturing (AM) process are investigated using OpenMP 4.5. To accurately report the speed-up for multicore CPUs and GPUs, a rigorous performance analysis employed optimizations appropriate for both CPU-only code (baseline) and GPU offload codes for an isothermal test problem. The performance results on Summit at the Oak Ridge Leadership Computing Facility indicate that using a precomputed list of interface cells significantly decreased the wall-clock time on GPUs. The speedup due to GPU acceleration was evaluated for a full Summit node and measured to be 1.8X when comparing a 6 MPI tasks run with 6 GPUs versus 36 MPI tasks on the CPU only. That speed-up was found to be 7.9X when comparing 6 MPI tasks with 6 GPUs versus the 6 MPI tasks running on the CPU only. Performance measurements showed that system total time is almost constant for runs with more than 96 MPI tasks (or GPUs), indicating that the GPU-accelerated code showed an excellent weak scaling performance.Finally, a rapid directional solidification problem was considered to demonstrate the CA code capability on Summit. It was found that a mesh size of at least 0.05 μm is recommended for the AM-like simulations in order to obtain accurate elongated grain microstructure and elongated subgrain features, which are in qualitative good agreement with experimental data. The results presented in this study indicate that the performance strategies on GPU-based HPC platforms for the CA code are appropriate for novel HPC exascale platforms.

The Effect of Preheating Temperature on the Forming Limit Diagram of AA1050/AA7050 Al Multilayered Sheets Produced by Accumulative Roll Bonding (ARB)

Multilayered sheets of AA1050 and AA7050 Al alloys produced by hot accumulative roll bonding (ARB) are tested to estimate their forming limit diagrams (FLDs). Sheets processed with preheating at 450 and 500 °C for up to six ARB cycles are submitted to Nakazima tests with an in situ digital image correlation system and tensile tests to analyze the mechanical behavior. X‐ray measurements and electron backscatter diffraction are performed to obtain the crystallographic texture and the mesotexture, respectively, and thus characterize the heterogeneous microstructure. A bimodal grain size distribution is observed in these sheets since AA7050 layers present elongated and fine grain size with nanometric precipitates and AA1050 layers present coarser and equiaxed grains. In addition, texture analysis indicates both rolling and shear components in the sheets. The FLDs show that the forming properties did not follow the expected rule of mixtures when compared with monolithic alloy sheets. Overall, the multilayered AA1050/AA7050 processed by means of ARB at 500 °C presents better formability than that at 450 °C, due to the combination of lower anisotropy measured by the Lankford coefficients values close to 1 and low normal anisotropy Δr values.

The influence of the main alloying elements on the formation of the initial grain structure, on phase transformations and on structural changes that occur during superplastic deformation of alloys Al - 4.1 wt.% Mg - 0.5 wt.% Zr, 1420T, 1421, 1423

The article presents the results of research aimed at revealing the influence of the main alloying elements on the formation of the initial grain structure, on phase transformations and on structural changes that occur during superplastic deformation of several aluminum alloys. It was possible to form a homogeneous ultrafine-grained structure in samples of alloys Al - 4.1 wt.% Mg - 0.5 wt.% Zr and 1423 due to dynamic recrystallization during their superplastic deformation. It is revealed that the initial microstructure of the 1420T alloy samples is bimodal. The average grain size is approximately 5 μm, in some areas of the working part of the samples there are large elongated grains, the average size of which is approximately equal to 25 μm. The initial structure of alloy 1421 samples is fine-grained, and the initial structure of alloy 1423 samples is multi-grained and coarse-grained. Metallographic studies showed that the grain structure of samples of alloys 1420T, 1421 and 1423 increases slightly during superplastic deformation at high homologous temperatures. Cavitation accumulates in the samples and structural changes occur, which are probably associated with local melting at grain boundaries and at interphase boundaries. It was established that the presence of zirconium and scandium additions in the composition of the samples ensures the formation of an ultrafine-grained structure in them and counteracts the grain growth during superplastic flow. Magnesium and lithium, which are included in the samples of the studied alloys 1420T, 1421 and 1423, form several intermetallic phases with aluminum. These phases are part of mixtures of crystals of peritectic origin, which are localized in the form of layers between some grains. The occurrence of peritectic reactions at high homologous temperatures can be one of the reasons for the partial melting of samples of alloys 1420T, 1421, and 1423 during their superplastic deformation. Partial melting of samples of alloys 1420T, 1421 and 1423 can probably be carried out due to the presence of segregations of magnesium and lithium at the grain boundaries, which lower the melting temperature of the aluminum-based solid solution. Partial melting of samples of alloys 1420T, 1421, and 1423 during their superplastic deformation, which is performed at high homologous temperatures, leads to the formation of cells of a metastable liquid-solid phase at the grain boundaries, the viscous flow of which leads to the formation of fibrous structures due to the development of grain boundary sliding.

Evaluation of Surface Damage of Pd Using Cross-Sectional Electron Backscatter Diffraction Analysis

Surface damage induced by mechanical polishing of cold-rolled and annealed Pd specimens was examined by cross-sectional electron backscatter diffraction (EBSD) measurements. Fine grains with high-angle grain boundaries were detected in the outermost layer in both specimens. Less granular but layered gradation of crystallographic orientation was detected in the sub-surface layer of the annealed specimen. In the cold-rolled specimen, a lot of elongated grains were detected in the entire inner layer. The formation of the sub-surface layer seemed to be prevented in the cold-rolled specimen by pre-introduced microstructures. In the annealed specimen, the depth of the surface damage layer was dependent on the crystallographic orientation of the matrix grain. This study clearly demonstrated the application of cross-sectional EBSD analysis for evaluating surface damage in metallic materials.

Ultrahigh-temperature aging response of asymmetrically cold-rolled AA2024 alloy

In this research, the effect of ultrahigh-temperature aging time on the microstructure and mechanical behavior of asymmetrically cold-rolled AA2024 aluminum alloy was studied. The cold-rolled samples were solution treated at 500 °C and aged at 500 °C for various times of 5, 10, 30, 60, 120, and 240 min. The microstructures, mechanical properties, and fracture surfaces of samples were investigated. It was found that by increasing the aging time, the elongated grains turned into equiaxed and coarser grains as well as the size and content of precipitates increased. The average hardness value increased up to 128 HV with increasing the aging time to 240 min. Also, the maximum yield strength (199.7 MPa) and ultimate tensile strength (384.4 MPa), as well as the minimum total elongation (20.8%), were achieved for the sample after 240 min of aging. The fracture surfaces of samples showed the ductile mode. The size of dimples increased and the number of dimples reduced with increasing the time of ultrahigh-temperature aging treatment.

Advanced characterization of two novel Fe-rich high entropy alloys developed for laser powder bed fusion in the Al-Co-Cr-Fe-Ni-Zr system

Laser powder bed fusion (LPBF) was used to manufacture two high entropy alloys (HEAs) within the Al-Co-Cr-Fe-Ni-Zr system. The selected compositional ranges were similar to alumina forming austenitic (AFA) steels but omitting interstitials and replacing Nb with Zr as the Laves forming element. The Fe-rich austenitic HEAs were prepared with varying Al and Zr content to evaluate the influence on the presence, size, and distribution of the intermetallic (IM) precipitates. The LPBF process combined with a single (950 °C 6 h) heat treatment formed large, elongated grains with a fine dispersion of multiple different nano-sized IM phases. Synchrotron high energy x-ray diffraction (HEXRD) revealed the cubic M23Zr6 as the main Zr-rich IM phase, stabilized by the multi-element mixture (M=Co, Fe, Ni) and the high cooling rates of the LPBF process. Further HEXRD in-situ compression was performed from room temperature to 900 °C to evaluate the phase stability, thermal expansion, and the strength contribution of the austenitic matrix and the M23Zr6 and NiAl B2 IM phases. The evolution of lattice strain and full width at half maximum of the reflexes was tracked using a statistical model, enabling quantitative analysis along the deformation.

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

Uneven microstructure in ZK60 alloy as a core of AZ31/ZK60 composite rods

• ZK60-melt core was composed of equiaxed coarse and fine grains. • Large elongated grains and fine equiaxed grains were observed in ZK60-ingot. • Abnormal grain growth was revealed in ZK60-melt core during aging. • Mechanical properties of composites with ZK60-melt and ZK60-ingot cores were improved by aging. AZ31/ZK60 bimetal rods were fabricated via co-extrusion of hybrid billets with ZK60 core prepared by different styles. Uneven microstructures were revealed in ZK60 core of AZ31/ZK60 composite rods prepared by pouring melt (ZK60-melt) into AZ31 sleeve and casting to be ingots to fit AZ31 sleeve (ZK60-ingot) followed by co-extrusion. Compared with the microstructure composed of equiaxed coarse and fine grains in ZK60-melt after co-extrusion, a mixed grain structure with large elongated grains and fine equiaxed grains was observed in ZK60-ingot core in the composite rod. During aging at 180 °C for 12 h abnormal grain growth (AGG) was revealed in ZK60-melt core, but the uneven structure in ZK60-ingot remained thermally stable. Mechanical properties of AZ31/ZK60 composite rods with both ZK60-melt and ZK60-ingot cores were improved significantly by aging.

The role of the particle aspect ratio in the discharge of a narrow silo

The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 ⩽ L/d ⩽ 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range.

Heterogeneous microstructure and anisotropic mechanical properties of reduced activation ferritic/martensitic steel fabricated by wire arc additive manufacturing

• Reduced activation ferritic/martensitic steel was fabricated by wire arc additive manufacturing. • The as-deposited microstructure exhibited the heterogeneous band characteristics. • Microstructure differences caused the periodical hardness and anisotropic tensile properties. • A potentially detrimental softening zone, was found at the heat-affected zone of the as-deposited RAFM steel. Reduced activation ferritic/martensitic (RAFM) steel is an iron-based alloy as a candidate structural material in fusion reactor. This paper evaluates the compatibility of RAFM steel as a choice material for wire arc additive manufacturing (WAAM). Two specimens of RAFM steels with high and low heat input were fabricated by WAAM. The effects of the heat input on the microstructure, microhardness and tensile properties of samples were investigated. The fusion boundaries are spaced uniformly in the whole sample. Three distinctive zones were present in the periodic region, including heat-affected zone (HAZ), columnar grains zone and fine-grained zone occurred alternatively. The HAZ was affected by the heat input. The fully γ-annealed top region consisted of epitaxial elongated grains without HAZ. The periodic pattern in microhardness along the building direction was found which was related to the periodic microstructure featured. The tensile properties presented anisotropic characteristics due to the heterogeneous microstructure. Further analysis indicated that the grain coarsening in the HAZ and C precipitates distributed at the grain boundaries caused substantial softened in the HAZ, resulted in the lower localized microhardness and tensile strength. Compared to the high heat input specimen, the low heat input specimen had smaller grain sizes, higher microhardness and tensile properties.