Globular Grains Research Articles

Flow Behavior and Microstructure of Hot‐Worked TiNbTaVW Refractory High‐Entropy Alloy

The high‐temperature deformation behavior of an equiatomic TiNbTaVW refractory high entropy alloy (RHEA) was studied at temperatures of 950, 1000, and 1050 °C and a strain rate of 10−3 s−1. The flow stress, high‐temperature strength, and softening mechanisms were analyzed and compared with the IN718 nickel‐based superalloy. The results indicate that the flow stress of both TiNbTaVW and IN718 is sensitive to deformation temperature, with high temperatures resulting in reduced flow stress. Globular grains and elongated grains were observed at 950 and 1000 °C for TiNbTaVW, signifying both dynamic recovery and dynamic globularization as the softening mechanisms. Globular grains were only observed at 1050 °C for TiNbTaVW, signifying dynamic globularization as the softening mechanism. The TiNbTaVW RHEA has a higher temperature strength of 910, 870, and 658 MPa compared to the IN718 alloy with 72, 87, and 61 MPa at 950–1000 °C/10−3 s−1. By demonstrating comparable or superior performance in specific aspects, RHEAs can be considered in the near future for potential applications traditionally dominated by nickel‐based superalloys.

Relation between Tensile Strut and Compressive Foam Deformation Behaviour: Failure Mechanisms and the Influence of Dendritic Versus Globular Grain Structure in an AlSi7Mg0.3 (A356) Precision‐Cast Open‐Cell Foam

Open‐cell aluminum foams are gaining importance for the design of lightweight structures and as electrodes in lithium‐ion batteries. AlSi7Mg0.3 foams were produced by a modified investment‐casting process. By tuning the mold temperature, a change from the usual nearly monocrystalline dendritic to a polycrystalline globular grain structure was achieved. Tension and compression tests on single struts and foam specimens, respectively, were combined with digital image correlation, scanning electron microscopy, and phase‐contrast enhanced micro‐computed tomography in a synchrotron facility to correlate the mechanical properties and the failure mechanisms with the microstructure. The “globular” foams exhibited a lower strength and a less pronounced subsequent stress drop than the “dendritic” foams, and the deformation mechanism changed from shear‐band dominated failure to a layer‐by‐layer collapse, because of the lower strength and higher ductility of the “globular” struts. The “dendritic” struts have a more homogeneous microstructure, while the “globular” struts often contain silicon agglomerates in their central region. Accordingly, the latter struts exhibit a higher degree of scatter for the fracture strain. Thus, the arrangement of the silicon particles and the eutectic determines the mechanical properties on the strut level and thereby the failure behavior on the foam level.This article is protected by copyright. All rights reserved.

Effect of multi wall carbon nanotubes during semi-solid to solid phase transformation in eutectic Al-Si alloy

The study explores numerically the combined effect of the base metal pouring temperature and MWCNT (multi wall carbon nanotubes) during semi-solid to solid solidification of Al-Si-MWCNT composite material. The semi-solid to solid solidification actually differs significantly both qualitatively and quantitatively from the liquid to solid solidification. During solidification of Al-Si-MWCNT composite material, the melt temperature and the cooling rate play crucial roles in the phase transformations. During liquid to solid solidification, the MWCNT significantly affects the cooling rate, leading to multiple recalescence phenomena, and influences the formation and evolution of the columnar and globular grains. Under the same condition, the MWCNT also impacts the silicon segregation, causing fluctuations in the silicon phases and its redistribution. However, during the semi-solid to solid solidification, it is observed that by decreasing the melt temperature into a semi-solid state, the segregation rate of silicon decreases which enhances more homogeneous CNT distribution. It also increases the globular volume fraction and dominates the columnar growth. Overall, the future trend of semi-solid to solid solidification of Al-Si-MWCNT composite materials lies in optimizing their properties for specific applications, expanding their use across diverse industries, and promoting sustainable manufacturing practices.

The effect of Cu–8%P master alloy, SIMA process, thixoforming, and heat treatment on the microstructure and mechanical properties of Al–20%Mg2Si composite

In the present research, the effect of thixoforming and age-hardening heat treatment on the microstructure and mechanical properties of un-modified and modified Al–20%Mg2Si composites produced through the strain-induced melt activation (SIMA) process has been investigated. This method showed promising potential for manufacturing the ideal microstructure, achieving globular grains, and enhancing mechanical properties. Quantitative analysis has revealed that adjusting the semi-solid heat treatment parameters to a temperature of 570 °C and a holding time of 25 min optimizes the globular α-Al grains and Mg2Si particles, achieving the size of 20.62 μm and 66.71 μm and the shape factor of 0.81 and 0.83 for Mg2Si particles and α-Al grains, respectively. The yield strength of the modified Al–20%Mg2Si composite increased by 23.67% and the ultimate tensile strength increased by 21.20%, compared to the un-modified specimen. Both composites underwent the thixoforming process, illustrating the ultimate tensile strength and elongation of 93.68 MPa and 4.32% for the original composite and 138.86 MPa and 7.29% for the modified composite. The modified Al–20%Mg2Si composite was subjected to the effective combination of thixoforming and age-hardening. According to the tensile test results, the yield strength, ultimate tensile strength, elongation, and toughness raised by 138.72%, 140.92%, 267.70%, and 471.12%, respectively, compared to the as-cast specimen. Moreover, its impressive hardness of 125.1HB contributes to its overall performance.

Rapidly solidified Al-Cu-Co quasicrystalline alloy: microstructure and catalytic properties

This study presents the characterisation and evaluation of the catalytic potential of an alloy with a composition referring to the quasicrystal from the Al-Cu-Co system. The material was synthesised through a melt-spinning process, providing rapid solidification conditions and a favourable material form of thin ribbons. Microstructural and chemical analyses were performed using scanning electron microscopy, X-ray diffraction and transmission electron microscopy methods. The obtained results confirmed the presence of a major phase which was a decagonal quasicrystal in the form of globular grains and additional crystalline phases occurring along the grain boundaries. The catalytic potential of the materials was explored using phenylacetylene hydrogenation. The catalyst, obtained by pulverising the ribbons, demonstrated catalytic activity with over 40% phenylacetylene conversion and selectivity exceeding 60% for styrene production under mild reaction conditions. The recovered catalyst displayed a stable phase composition, as confirmed by X-ray diffraction, and unchanged morphology, indicating the possibility of catalyst reuse for subsequent reaction cycles. The presented results provide insight into the experimental verification of the catalytic properties of the developed environmentally friendly catalysts.

Rapid Thermal Treatment of Oriented Microstructures to Generate Fine Globular Grains

Rapid Thermal Treatment of Oriented Microstructures to Generate Fine Globular Grains

Dry sliding wear behavior of AA7075 alloy produced by thixocasting

Abstract In this study, the wear behavior of AA7075 alloy produced by thixocasting was investigated. The wear behavior of the AA7075 alloy is examined for three cases: extruded with T6 heat treatment, as-thixocast, and thixocast with T6 conditions. The dry sliding wear test was conducted with a tribometer according to ASTM G-99 standard. The microstructures were characterized by optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray analysis (EDX). The tensile and hardness tests were performed to evaluate the mechanical properties. The AA7075 alloy was successfully shaped by thixocasting. The as-thixocast sample exhibited typical globular structures with multinary eutectic structures along the grain boundaries. The globular grains transform into a polygonal structure, and the grain size increases from 50 μm to 60 μm in the thixocast + T6 sample. This microstructure exhibited excellent wear resistance under dry sliding conditions in the thixocast + T6 sample. The aging treatment with prolonged solution process improved the mechanical properties two times and the wear rate three times for the thixocast AA7075 alloy. Furthermore, the thixocast + T6 sample exhibited a significant decrease in the coefficient of friction with the lowest wear rate compared to the as-thixocast sample. The dominant wear mechanisms are microdelamination, adhesion, and oxidation in all samples.

Influence of Various Alloying Element Additions on Microstructure and Magnetic and Mechanical Properties and Corrosion Behavior of Cast Fe–Ga–Z Shape Memory Alloys

Fe–Ga alloys are attractive materials where high mechanical strength, toughness, ductility, and large low-field magnetostriction combine to give unique properties. Adding alloying elements is an effective method to further enhance these properties. In order to integrate these alloys into the operating environments, e.g., micro-robots and magnetic actuators, the corrosion behavior should be addressed. This work analyzed the microstructure, magnetization, hardness, and corrosion properties of Fe81Ga19−xZx (X = 5 at.% of Ni, Mn, or Ti, and 2 at.% Al; separately) alloys. X-ray diffraction (XRD), scanning electron microscope-electron (SEM), vibrating sample magnetometer (VSM), Vickers hardness (HV), and a potentiostat were used for characterization. XRD revealed that the prominent peak belongs to the bcc disorder A2 phase and a small peak for the cubic order L12 phase. Fe–Ga–Al alloy got the maximum Ms value, while Fe–Ga–Mn alloy gained the lowest one. However, the Mr and Hc properties for Fe–Ga alloy were distinctly improved by adding Al but slightly affected by doping Mn. Addition of Ti achieved the highest hardness, followed by Ni, Mn, and Al. The microstructure of the different alloys significantly influenced their corrosion behavior. Fe–Ga–Mn alloy with the fine globular grain structure showed the lowest corrosion rate (C R = 0.03 mm/year), whereas Fe–Ga–Al alloy with the coarse longitudinal grains exhibited the highest corrosion rate (C R = 0.19 mm/year).

Influence of multi-walled carbon nanotubes on the solidification process of an Al-12 %Si alloy

This paper investigates the influence of multi-walled carbon nanotubes (MWCNT) on the solidification process of an Al-12 %Si alloy. Amulti-phase Eulerian model of solidification is deployed to numerically handle the problem. The simulation determines the cooling curve and addresses the recalescence phenomena along with the estimation of the columnar and globular solid phases;precipitating silicon (PSP) phase and MWCNT phase distributions. The findings reveal that MWCNT significantly affects the cooling rate, leading to multiple recalescence phenomena, and influences the formation and evolution of the columnar and globular grains. MWCNT also impacts silicon segregation, causing fluctuations in silicon phases and redistribution. Furthermore, the study explores the distribution of MWCNT throughout the material and identifies a critical time period for their influence. Overall, the study enhances our understanding of the complex dynamics within the solidification process and provides insights into the role of MWCNT in shaping the microstructure through a mathematical model.

Thickness dependent studies on p-type K-doped ZnS nanostructured films grown via colloid-based solution route for ambipolar optoelectronic applications

Potassium-doped Zinc Sulphide (K:ZnS) nanostructured films were deposited on silica substrates using a colloid-based solution method, with varying thicknesses from 3 to 6 coats. The films exhibited p-type electrical conductivity and favorable optical properties for ambipolar and optoelectronic device fabrication. X-ray analysis confirmed a polycrystalline cubic phase in all the films, with crystallite sizes of 7–27 nm. Scanning electron micrographs revealed spherical globular grains of sizes 1.20–2.84 μm, leading to improved compactness with thicker films. UV/VIS analysis demonstrated a red-shifted band gap with increasing thickness. Photoluminescence under UV excitation revealed a peak at 477 nm attributed to K-induced trap levels. The electrical resistivity (1.34–6.00) × 102 Ω⋅cm, carrier concentration (4.63–13.33) × 1015 cm−3, and mobility (2.25–3.50) cm2/V·s varied with film thickness. The minimum resistivity of 1.34 × 102 (Ω⋅cm), maximum conductivity of 7.46 × 10−3 (Ω−1⋅cm−1), and highest mobility of 3.50 (cm2/V·s) were obtained for K:ZnS-6 film.

Microstructure evolution and grain fragmentation induced by the co-activation of slip and [formula omitted] nanotwin during cryo-forging of a Ti-6Al-4V alloy

The nanostructured titanium alloy usually exhibits superior mechanical properties than the coarse-grained titanium alloy. In the present work, the α-grain fragmentation induced by slip/nanotwin co-activation during cryo-forging was studied in a bimodal Ti-6Al-4V alloy, in which the nanosized (022‾1‾) deformation twin was revealed. The results show that the lamellar α-grains fragmented to small globular grains after 25% compression at 0.1 mm/s and 0.5 mm/s. The high strain rate and large deformation increased the dislocation density and flow stress, therefore strengthened the deformation texture and promoted the grain fragmentation. The (0001) <112‾0> basal slip system was regarded as the dominant slip mode during cryo-forging, which activated more intensely in lamellar α-grains than that in equiaxed α-grains, and was promoted as the deformation ratio and stain rate increased. In addition, the activation of basal slip system was inhomogeneous in lamellar α-grains. The partial lattice rotation caused high angle grain boundary (HAGB) formation and grain fragmentation. On the other hand, the (022‾1‾) nanosized deformation twins appeared within fragmented α-grains after 25% cryo-forging at 0.5 mm/s, in which the (011‾1) planes between matrix and twin existed a ∼27° misorientation. The (022‾1‾) deformation twins contributed to grain fragmentation by twin boundary grooving and twin-twin intersection. This work demonstrated an available route to achieve refined or gradient grained structure in Ti-6Al-4V alloy by slip/twinning co-activation.

Microstructure and Mechanical Properties of an Advanced Ag-Microalloyed Aluminum Crossover Alloy Tailored for Wire-Arc Directed Energy Deposition

The implementation of wire-arc directed energy deposition requires the development of novel, process-adapted, high-performance aluminum alloys. Conventional high-strength alloys are, however, difficult to process as they are prone to hot-cracking. Crossover alloys based on Al-Mg-Zn combine good processability with good mechanical properties following artificial aging. Here, we present an effort to further improve the mechanical properties of Al-Mg-Zn crossover alloys using Ag microalloying. No cracks and few porosities were observed in the samples. The microstructure is dominated by fine and globular grains with a grain size ≈ 26.6 µm. The grain structure is essentially free of texture and contains fine microsegregation zones with ≈ 3–5 µm thickness of segregation seams. Upon heat treatment these microsegregation zones are dissolved and T-phase precipitates are formed as clarified by diffraction experiments. This precipitation reaction results in a microhardness of ≈ 155 HV0.1, a yield strength of 391.3 MPa and 418.6 MPa, an ultimate tensile strength of 452.7 MPa and 529.4 MPa and a fracture strain of 3.4% and 4.4% in transversal and in longitudinal directions, respectively. The gained results suggest that highly loaded structures can be manufactured by wire-arc directed energy deposition using the newly developed aluminum crossover alloy.

Kinetics and crystallography of globularization in the additively manufactured Ti–6Al–4V

This work investigates the kinetics and crystallography of globularization in the additively manufactured Ti–6Al–4V by optical microscopy, transmission electron microscopy and electron back scatter diffraction. It is found that it follows the LSW (Lifshitz–Slyozov–Wagner) model with boundary splitting when the laser-melted Ti alloy suffers from post annealing thermal treatment, where coarsening efficient 0.124. Additionally, boundary splitting does not change the orientation relationship and the orientation relationship of globular α grains is consistent with the precursor (widmanstätten microstructures). The current study shows that the number and crystal orientation of widmanstätten microstructure in prior β grains can be tailored well, laying the foundation for manipulation of the globular grains.

Detection and effects of lack of fusion defects in Hastelloy X manufactured by laser powder bed fusion

Laser Powder Bed Fusion (L-PBF) is increasingly used for the manufacturing of complex metal parts. A major hurdle for L-PBF to manufacture critical parts is the lack of knowledge about the effects of inner material defects on the material properties. This paper presents a new method to manufacture specimens with inner lack of fusion (LOF) defects and to detect and localize them simultaneously by the in-process monitoring tool Optical Tomography (OT). The presence of defects at locations indicated by OT was verified through Computed Tomography analyses. Correlative metallographic preparation of the processed specimens showed LOF defects with their main extent within the build plane and sharp edges. Moreover, microstructural investigations revealed a fine globular grain structure and coarse dendritic segregations in the area, influenced by the defect. Correlations between the LOF defects analyzed in the microstructure and corresponding OT indications were established. Tensile tests and high cycle fatigue tests were performed on defective and non-defective material to evaluate the effects of LOF defects on the material properties of Hastelloy® X manufactured by L-PBF. While a minor influence of the LOF defects on static material properties was identified, the fatigue life of the defective specimens was significantly reduced.

Microstructure and Mechanical Properties of a Ni-Based Superalloy Thin Film Investigated by Micropillar Compression

The microstructure and local micromechanical properties of a Ni-based superalloy thin film produced by magnetron sputtering using ERBO/1 sputter targets were investigated. The thin film consists of columnar nanograins (an average size of ~ 45 nm) with mostly < 111 > orientation. Inside the nanograins, very fine nanotwins with an average thickness of ~ 3 nm are present. In-situ micropillar compression tests, complemented by nanoindentation, were conducted to evaluate the mechanical characteristics. The microhardness and Young’s modulus of the thin film correspond to ~ 11 and 255 GPa, respectively, the critical strength to ~ 4 GPa. The plastic deformation of the micropillars occurs through the formation of a shear band initiating at the top of the pillar. Inside the shear band, globular grains with random orientation form during the deformation process, while the regions near to the shear band remained unaffected.

Revealing the correlation of microstructure configuration and mechanical properties of Al–Si–Mg alloy reinforced by C-doped TiB2 and SiC

A new Al–Si–Mg alloy was prepared and refined by C-doped TiB2 (C-TiB2) and strengthened by SiC, and the effects of microstructure configuration on the mechanical properties of the alloys were investigated in this study. The results show that the eutectic Si and reinforcing particles distribute in a network configuration in the cast Al–Si–Mg alloy with globular grains. In addition, the distribution of the eutectic Si and reinforcing particles is streamlined along the extrusion direction after hot extrusion. The streamlined distribution of eutectic Si and reinforcing particles is beneficial to the ductility of the alloy, the elongation of which is almost twice that of the Al–Si–Mg alloy with a network configuration that possesses higher strength. The reinforcing phase with a network configuration can better withstand the stress and prevent the deformation of the matrix, thus improving the strength of the alloy. By contrast, the reinforcing phase with a streamlined configuration provides conditions for the plastic deformation of the matrix and hinders the contact of microcracks in the matrix, which increases the elongation of the alloy.

A Ti‐Doped Chemical Vapor Deposition Diamond Device as Artificial Synapse for Neuromorphic Applications

AbstractThe great demand of multifunctional portable electronic products in daily life and the need of a large integration of memories with logic devices and sensors, have increased the interest in the identification of suitable materials for neuromorphic computing applications. Major innovations in this direction have been achieved by exploring materials belonging to different fields of applications and taking advantage of already consolidated deposition methods. Despite the great interest in the field and the large use in complementary applications such as sensing electrodes, neural and cellular interfaces, the use of diamond‐like materials in neuromorphic applications is still limited to a few examples. Herein, the development of a synaptic element based on high‐quality polycrystalline diamond layers containing Ti inclusions showing a marked and reproducible resistance switching behavior is reported. Realized by means of a hybrid chemical vapor deposition‐powder flowing technique, this titanium doped diamond shows a 3D polycrystalline organization that is characterized by globular grains of a few microns. The coupling of Raman spectroscopy, X‐ray photoelectron spectroscopy, and X‐ray diffraction analyses confirms the good quality of diamond phase and convincingly points out the inclusion of the titanium species within the diamond lattice.

Analyzing the influence of the investment casting process parameters on microstructure and mechanical properties of open-pore Al–7Si foams

Aluminum alloy foams with high specific compressive strength and good energy absorption capacity are widely used as structural materials and shock absorbers. The investment casting process offers an interesting opportunity for the production of open-pore aluminum foams. However, foam casting differs from casting of solid parts, and there is a lack of practical knowledge about the microstructure tuning of the ultra-thin cast struts. Our study focuses on the influences of the casting conditions such as mold temperature on the microstructure and consequently on the mechanical properties of aluminum-silicon alloy foams. We investigated microstructural features using various metallography techniques and defined mechanical properties under uniaxial compression test. Here, we observed improvements in the strut microstructure together with decrease of the mold filling by reduction of the mold temperature. Microstructure improvements involve transformation of a single dendritic aluminum grain in each strut to globular grains along with a more homogeneous distribution of the silicon particles across the strut cross-section. These changes bring higher ductility and energy absorption efficiency to the foams, which are evident from the smoother plateau of the stress-strain curves. Decline of the mold filling, on the other hand, has negative effect on the overall mechanical properties.

A Study of the Essential Parameters of Friction-Stir Spot Welding That Affect the D/W Ratio of SSM6061 Aluminum Alloy.

This study aimed to investigate how the depth-to-width (D/W) ratio of the welding area affects the welding quality of the SSM6061 aluminum alloy via the friction-stir spot welding (FSSW) process. The results showed that a higher D/W ratio directly results in better mechanical properties. If the D/W ratio value is high (at 1.494), then this leads to higher tensile shear strength at 2.25 kN. On the other hand, if the D/W ratio values are low (at 1.144), then this reduces tensile shear strength to 1.17 kN. The fracture surface behavior on the ring zone also affects the characteristics of ductile fracture. During Vickers hardness analysis, the hardness profiles are in the shape of a W; the maximum hardness was 71.97 HV, resulting from the rotation speed of 3500 rpm and the dwell time of 28 s, where the hardness of the base metal was at 67.18 HV. Finite element (FEM) analysis indicated that the maximum temperature during simulation was 467 °C in the region near the edge shoulder tool, which is 72.96% of the melting point. According to FEM simulation, the temperature under the tool pin region was 369 °C. The generated heat was sufficient to induce changes in the microstructure. For microstructure changes, the globular grain took on a rosette-like form, and coarse grains were observed in the thermal mechanical affect zone (TMAZ) and in the nugget zone (NZ), transforming in the mix zone. Hooks, kissing bonds, voids, and porosity are the defects found in this experiment. These defects indicate a discontinuity in the NZ that leads to worse mechanical properties. During examination via SEM and energy dispersive X-ray (EDX) analysis, the recrystallization structure from β-Mg2Si IMCs to Al3Mg2 and Al12Mg17 IMCs was observed. The size was reduced to an average width of 1-2 µm and an average length of 2-17 µm. Simultaneously, the oxides from the ambient atmosphere present during welding showed dominant partial elements from SiO2, MgO, and Al2O3.

Establishment and application of grain morphology distribution model for quantitative analysis of titanium alloys with typical microstructures

A novel probability model of grain morphology distribution considering the orientation and size of non-spherical grains, including P01(r),P11(r),P02(r),P33(r),P12(r), is established based on the nearest-surface function, and it is used to reveal the dispersion and uniformity degree of different morphology grains in titanium alloys. Especially, the distribution probability of lamellar α intersection is introduced instead of the distributed relationship of different α lamellae, and the distributed relationship between lamellar and globular grains is transformed into the distribution of globular grains by the equivalent circle method. Then, several corresponding distribution probabilities are selected in terms of describing different microstructure states (equiaxed, basketweave, duplex microstructures). The model is applied to the quantitative analysis of TC4 alloy with different morphology grains. The results show that the distribution of α grains with different morphology in the β matrix becomes more inhomogeneous with increasing height reduction and cooling rate. Moreover, the larger the globular α size is, the greater the aggregation degree and the smaller the uniformity of other α grains around it is. The reliability of probability model is verified by the comparison between the predicted and the measured frequency distributions for the nearest neighbor surface distance r.