Α-phase Content Research Articles

Influence of phase composition and stress on the corrosion behavior of metastable β titanium alloy Ti-5Mo-5V-6Cr-3Al in 15 wt% HCl solution

Immersion experiments, electrochemical tests under different tensile stresses, and microstructure observation using SEM, EBSD, and TEM were carried out on three Ti-5Mo-5V-6Cr-3Al alloys with different phase compositions in 15 wt% HCl solution to clarify the effects of the phase compositions and stresses on the corrosion behavior of metastable β-titanium alloys in acidic media. The microstructure with only primary α-phase on the β-matrix (No. M1) presents the smallest corrosion current density and the highest corrosion potential, indicating relatively the best corrosion resistance; the microstructure with both primary and secondary α-phase on the β matrix (No. M2) has moderate corrosion resistance; and the microstructure with secondary α-phase precipitated on the β matrix (No. M3) has the relatively poorest corrosion resistance. The results of the immersion experiments show the same regularity as the electrochemical tests. The corrosion resistance of all three microstructures decreases as the degree of stress loading increases. The apparently small amount of α-phase content in M1 is the reason for the difference in corrosion resistance between M2 and M3, M3 alloys with larger average grain sizes exhibit better corrosion resistance than M2 alloys. Under tensile loading, disruption of the surface passivation film and pitting promoted by dislocation slip are the main reasons for the reduction in the corrosion resistance of the alloy.

Synthesis of Si3N4 powder with a controllable alpha/beta ratio by sol-gel assisted carbothermal reduction and nitridation

The controllable α/β ratio of Si3N4 powder was synthesized using tetraethyl orthosilicate (TEOS), glucose, MgCl2 and YCl3·6H2O by sol-gel assisted carbothermal reduction and nitridation (CRN). This study aimed to investigate the impact of various parameters, including a H2O/TEOS mol ratio of the sol, a C/Si mol ratio, sintering aids, and the temperature of CRN on the phase transformation of α-Si3N4. The results demonstrated that the H2O/TEOS ratio significantly influenced the relative content of α and β phase in Si3N4 by changing the morphological characteristics of SiO2. When the H2O/TEOS ratio was 100, β-Si3N4 powder with an approximate β content over 88 wt% was synthesized at 1500 °C. This remarkable phase transformation was likely facilitated by the eutectic liquid phase consisting of MgSiO3/Mg2SiO4. The SEM image revealed rod-like whiskers with a length around 1.4 μm for the β-Si3N4 grains. Due to the presence of liquid phase, the synthesis of spherical-like α-Si3N4 nano-powder with an α-phase content of 91 wt% was achieved at a lower temperature of 1450 °C, with an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The size of the particles was about 70 nm due to the reduced reaction temperature. Furthermore, without the addition of any sintering aids or Si3N4 seeds, high purity α-Si3N4 with an α phase content approaching 96 wt% could be synthesized at 1500 °C, using an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The resulting α-Si3N4 powder exhibited an extremely regular hexagonal prism shape with a length of approximately 2 μm.

Aging Treatment Induces the Preferential Crystallographic Orientation of αs in the Near-α Titanium Alloy Ti60

In this article, we subjected the Ti60 alloy to solid-solution treatment at 1020 °C and aging treatment at 600 °C, respectively, achieving a bimodal microstructure. The microstructures obtained after aging treatment showed no significant difference in the primary α-phase content, size, and width of the lamellar α phase. This suggests that the final microstructure morphology is primarily determined by the solid-solution temperature, with the aging process exerting less pronounced effects on microstructural alterations. Furthermore, we investigated the effect of solid-solution and aging treatment on the crystallographic orientation evolution of the secondary α phase (αs) in the near-α titanium alloy Ti60. The αs phase displays a random orientation in solid-solution treatment sample, while it demonstrated a preferential {0 1 −1 0} orientation after aging treatment. This interesting phenomenon is attributed to the enhanced variant selection resulting from the dissolution of variant near 60° and 90° during aging. Furthermore, the αs with {0 1 −1 0} orientation nucleated at the grain boundary and coalesced into larger αs lath with increasing aging time, further contributing to the αs {0 1 −1 0} texture.

Role of powder morphology on α-phase content in plasma sprayed alumina coatings

Despite various studies addressing the retention of α phase in thermal sprayed alumina coatings, a noticeable research gap persists regarding the impact of feedstock morphology on phase retention and properties. In this work, we explored this simple way to enhance the retention of the α phase and studied its impact on mechanical and dielectric properties. To achieve this, we deposited alumina powders with two different morphologies (such as angular and spherical) with similar particle size using plasma spraying. After deposition, we quantified the retention of α phase present in the coatings using the Rietveld refinement method. From this analysis, we noticed that spherical morphology (SM) powder exhibited the highest percentage of α phase retention (82.81 %) as compared to angular morphology (AM) powder (48.8 %). In turn, SM-coating demonstrated superior mechanical properties, with improvements in hardness, elastic modulus and fracture toughness of 21.9 % 13.7 % and 25 % respectively, compared to AM-coating. This improved mechanical properties in SM-coating is attributed to high α phase retention, synergistic effect of both fully melted (FM) and partial melted (PM) regions of coating microstructure which hindered the crack propagation. Furthermore, the SM-coating exhibited superior dielectric properties, with a dielectric strength 9.9 % higher than the AM-coating. This dielectric performance is ascribed to high retained α phase and generation of discontinuous crack due to presence of PM regions in the SM-coating. These findings suggest the impact of feedstock powder morphology on coating performance and offer avenues for optimizing coating processes in diverse technological fields.

Effect of sintering aids content and powder characteristics on gas pressure sintered Si3N4 ceramics

The Si3N4 raw powder, sintering aids, and sintering process are the most critical factors affecting the thermal conductivity and mechanical properties of Si3N4 ceramics. The impact of sintering aids content and powder characteristics on gas pressure sintered Si3N4 ceramics was investigated in this study. Lower aids content facilitated phase transformation prior to densification, promoting a pronounced bimodal microstructure. Based on in-situ shrinkage behavior, liquid phase sintering of Si3N4 ceramics can be divided into three stages without clear boundaries. Between 1200 and 1400 °C, higher oxygen and metallic impurities favored particle rearrangement. Between 1400 and 1800 °C, the dissolution and precipitation process was probably controlled by the α-phase ratio and particle size. Above 1800 °C, subsequent grain growth was primarily facilitated by the Ostwald ripening process, leading to a reduced densification rate. Higher oxygen content leading to greater weight loss during sintering. Higher Fe resulted in a darker color of sintered Si3N4 ceramics. High α-phase content promoted the formation of a bimodal microstructure and ensured superior mechanical properties. Reduced levels of oxygen and aluminum were prerequisites for achieving high thermal conductivity. Powders prepared by silicon nitriding method were cost-effective materials for the preparation of high-performance Si3N4 ceramic substrates.

Effect of spherical alumina crystalline phase content and particle size distribution polydispersity on the properties of silicone rubber composites

High loading of thermally conductive fillers for enhancing thermal conductivity (TC) conflicts with proper rheological viscosity in excellent thermal interface materials (TIMs), which remains a significant challenge. The physical properties (crystalline phase, particle size and distribution, etc.) of thermally conductive filler are the base of the high load in the polymer. In this paper, the α-phase content of spherical aluminum (S–Al2O3) increases after calcination, leading to the viscosity of composites increasing sharply but was only slightly improved for the TC. The polydispersity of the S–Al2O3 particle size distribution was regulated to fill the silicone rubber under the premise of controlling the same average particle size, crystalline phase content and filling amount. Results indicated the viscosity of the sample with a polydispersity value of 0.74 was 41.21 Pa s at a shear rate of 10 s−1 at 66 vol%, which was 87.7% lower than that of the sample with a polydispersity value of 0.48. And the viscosity difference became more prominent with the increasing filling amount. However, the difference in TC between several groups of samples with different polydispersity is insignificant at 60–67 vol%, which provides practical, theoretical guidance for achieving controllable viscosity and developing low-viscosity TIMs in the processing process.

Dezincification in cast and heat-treated alpha-beta brass samples

Abstract Two α-β brass alloys with varying Zn contents have been studied. The samples received for examination were heat-treated at 500 °C in order to conduct dezincification studies according to EN ISO 6509-1 and to measure the depth of dezincification from metallographic sections. According to the phase diagram, the α-phase content should increase as a result of heat-treatment at 500 °C. This phenomenon, however, is difficult to observe in the metallographic sections which were prepared from the heattreated samples. When analyzing the depth of dezincification, it was found that reduced dezincification occurred in the heat-treated samples. This is plausible since β brass is attacked by preferential corrosion. Not only was uniform layer dezincification observed in the brass alloy with the higher Zn content, but also the onset of plug dezincification. This is associated with the original microstructure of the brass which is characterized by a coarse primary grain size. In this alloy, it is clearly observed that larger areas containing alpha brass will impede the process of dezincification.

New Al-alloys with dispersed stable quasicrystal approximant phases: Overcoming the barrier of conventional casting processing and microstructure design

Multi-stage manufacturing processes, high production costs and narrow two-phase field of Al-alloys containing Al-FCC and quasicrystals or approximants phases have been barriers to the widespread use of these alloys in technological applications. In the present work, we demonstrated that new Al-alloys with dispersed stable quasicrystal approximant phases can be produced by conventional metallurgical fabrication methods. Six alloys containing different volume fractions (5% up to 40%) of the approximant α-phase were designed and produced by conventional solidification in a graphite mold. The hardness and tribological properties of Al-FCC matrix with different volume fractions of the quasicrystal approximant phase were evaluated. A wide range of wear rates, from 2.1 × 10−3 mm3/Nm to 6.3 × 10−4 mm3/Nm, could be achieved by tuning the volume fraction of the α-phase ranging from 5% up to 40%, respectively. Low coefficient of friction values around 0.3 were observed after the running-in periods, regardless of the α-phase content, indicating that the quasicrystal approximant phase played a critical role. The alloy containing 25% of α-phase was chosen to be processed by different manufacturing processes to show that the morphology of the stable approximant α-phase can also be controlled. This finding opens new perspectives for the commercial application of products derived from such light- and wear-resistant materials.

Improvement of Mechanical Properties for a Novel Zr–Ti–V Alloy via Hot-Rolling and Annealing Treatment

In this experiment, an annealing treatment was carried out for a rolled Zr–Ti–8V alloy, and the toughening mechanism of the material was thoroughly analyzed by combining advanced material characterization and other testing methods. The phase composition of the Zr–Ti–8V alloy was sensitive to the applied annealing temperature, while a series of changes in the phase composition of the alloy were induced by enforcing bigger thermal budgets. Implementing a temperature value of 450 °C led to a higher α-phase content, in striking contrast with the case where a lower annealing temperature of 400 °C was applied. The β grains that were stretched in the alloy’s rolling direction and annealed at 600 °C to 800 °C were recrystallized. As a result, the acquired configuration was equiaxed with β grains. The extracted results revealed that the alloy annealed at 450 °C showed a good strong–plastic ratio, with tensile strength and elongation of 1040 MPa and 8.2%, respectively. In addition, the alloy annealed at 700–800 °C showed good plasticity properties. From the hardness tests and friction wear experiments on all the experimental alloys, it was demonstrated that the dual-phase alloy with α + β had higher hardness and wear resistance, whereas the opposite trend was observed for the single β-phase alloy.

Molecular Dynamics Study on the Welding Behavior in Dissimilar TC4-TA17 Titanium Alloys.

Titanium alloys have become the material of choice for marine parts manufacturing due to their high specific strength and excellent resistance to seawater corrosion. However, it is still challenging for a single titanium alloy to meet the comprehensive specifications of a structural component. In this study, we have applied a molecular dynamics approach to simulate the aging phase transformation, K-TIG welding process, and mechanical properties of the TC4-TA17 (Ti6Al4V-Ti4Al2V) alloy. The results show that during the aging phase transformation process, changes in the structure of the titanium alloys are mainly manifested in the precipitation of a new phase from the sub-stable β-phase, and after the state stabilization, the α-phase content reaches 45%. Moreover, during the melting and diffusion process of TC4-TA17, aluminum atoms near the interface diffuse, followed by titanium atoms, while relatively few vanadium atoms are involved in the diffusion. Finally, the results of tensile simulations of the TC4-TA17 alloy after welding showed that stress values can reach up to 9.07 GPa and that the mechanical properties of the alloy in the weld zone are better than those of the single alloys under the same conditions. This study will provide theoretical support for the optimization of process parameters for TC4-TA17 alloy welding.

Beta-phase Stabilization and Increased Osteogenic Differentiation of Stem Cells by Solid-State Synthesized Magnesium Tricalcium Phosphate.

In this study, magnesium and strontium-doped β-tricalcium phosphates were synthesized to understand dopant impact on substrate chemistry and morphology, and proliferation and osteogenic differentiation of mesenchymal stem cells. Under solid-state synthesis, magnesium doping stabilized the β-phase in tricalcium phosphate, with 22% less α-phase content than control. Strontium doping increased α-phase formation by 17%, and also resulted in greater surface porosity, leading to greater crystal precipitation in vitro. Magnesium also significantly enhanced the proliferation of stem cells (P < 0.05) and differentiation into osteoblasts with increased alkaline phosphatase production (P < 0.05) at all time points. These results indicated that magnesium stabilizes β-tricalcium phosphate in vitro and enhanced early and late-time-point osteoconduction and osteoinduction of mesenchymal stem cells.

Excellent strength-toughness synergy in metastable austenitic stainless steel due to gradient structure formation

The effect of cold swaging and subsequent annealing on the structure and mechanical properties of a 321-type metastable austenitic stainless steel was studied. Cold swaging to a strain of 90% produced a gradient structure with a gradual decrease in the α-phase content from the edge to the center of the bar. Globular and lamellar regions of both strain-induced martensite and retained austenite were found in the center, meanwhile, a completely globular austenite–martensite dual-phase structure was formed at the edge. After annealing at 500 °C, (~30° below the reverse martensite-to-austenite phase transformation temperature) microstructure refinement due to simultaneous development of recrystallization and reverse martensite-to-austenite transformation was observed. Both ultimate tensile strength and yield strength increased noticeably after the annealing. Besides, notch toughness of the annealed specimen doubled in comparison to that of the cold-swaged condition and almost reached the level of a coarse-grained condition.

In Situ Graded Ceramic/Reduced Graphene Oxide Composites Manufactured by Spark Plasma Sintering

The present work merges two key strategies for the manufacturing of advanced ceramics, in particular, the development of functionally graded materials (FGMs) and the addition of graphene-based fillers into a ceramic matrix. A silicon nitride/reduced graphene oxide FGM composite is produced, in one step, from a single powder composition using the spark plasma sintering (SPS) technique with an asymmetric setting of the punches and die to create a continuous temperature gradient along the cross section of the powder compact. A deep microstructural and mechanical characterization has been done across the specimen thickness. The FGM composite exhibits bottom-top gradients in both the matrix grain size (150% increase) and α-phase content (89→1%). The FGM bottom surface is 10% harder than the top one and, on the other hand, the latter is 15% tougher. The presence of reduced graphene oxide sheets homogeneously distributed within the ceramic composite reduces the mechanical gradients compared to the monolithic silicon nitride FGM, although allows reaching a maximum long-crack toughness value of 9.4 MPa·m1/2. In addition, these graphene-based fillers turn the insulating ceramics into an electrical conductor material.

Influence of tantalum's crystal phase growth on the microstructural, electrical and mechanical properties of sputter-deposited tantalum thin film layer

High melting point refractory tantalum (Ta) metal is frequently grown into thin film layer for many applications in biomedical implants, microelectronic devices and micro-level mechanical systems. Tantalum growth mechanism is still in debate and the inconsistent crystal phase outcome has puzzled many, though it is certain that the properties of the grown film are highly dependent on the formed crystal phase configuration. The microstructure, surface morphology, crystal orientation and residual stress of the sputter-deposited Ta thin films using direct current (DC) magnetron sputtering technique are studied at 0.4 Pa – 2 Pa of sputtering pressure, 100 W – 250 W of DC sputtering power on bare silicon and silicon dioxide substrate and 10 min – 50 min of sputtering duration. α-phase Ta is preferably grown at high sputtering time (high thickness) and high DC sputtering power (high growth rate). The sputtering pressure affects the thin film's microstructural porosity while the sputtering power controls the crystallisation's quality. Both sputtering pressure and power affect the generated argon plasma in the DC magnetron sputterer where α-phase Ta is preferably formed at high plasma. The presence of 3 μm silicon dioxide underlayer makes no difference compared to bare silicon substrate. Our study reveals that β-phase Ta is grown first irrespective of sputtering conditions and then transformed into α-phase Ta after reaching a certain thickness. The grown major α-phase content promotes small thin film sheet resistivity (58.9 μΩcm – 86.1 μΩcm) and is suspected to be the dominant factor that increases the compressive stress within the thin film layer and reduces the adhesion of Ta layer onto the substrate surface. The study has given a new insight in controlling the conductivity and adhesion level of Ta thin film based on the grown phase layer.

Morphology and phase composition of lead dioxide coatings: Influence of methanesulfonate ions

Abstract Depending on the concentration of methanesulfonate in the electrolyte and the hydrodynamic conditions of the process, it is possible to synthesize high-quality films up to 2 mm thickness, free from internal stresses, with reliable adhesion to the substrate in the current density range of 2–180 mA cm−2. The change in the composition of the methanesulfonate electrolyte and the deposition conditions affect the phase composition of lead dioxide. The main difference from oxides obtained from nitrate solutions is the significant α-phase content, which can vary from 17 to 90%. The deposits are polycrystalline and are characterized by smaller crystals. All the results obtained by the X-ray diffraction method are in satisfactory agreement with the scanning electron microscopy data.

Advantages of drawing and rolling metals with pulse current

The results of studies on the drawing of copper and steel wires using pulsed current in the deformation zone are presented. The analysis of their structure and physicomechanical properties is given. As a result, the drawing forces of copper and steel wires are reduced by 30-35%, the electrical resistance is reduced by 18-20%, a more perfect axial texture appears, the ductility of the wires increases compared to warm drawing. In the steel wire, the α-phase content sharply decreases, which is predominantly released in the surface layer by several microns. We propose methods for the modernization of rolling equipment for technology using pulsed current. The efficiency of using a high-density pulsed current on a rolling mill for producing stainless steel from an initial billet of 2 mm thick and 100 mm wide 0.3 mm thick tape without intermediate annealing and oxide film is shown.

Solution Precursor Plasma Spraying of Cr-Doped Al2O3 Thermochromic Coatings

Solution precursor plasma spraying (SPPS) presents a modern route to deposit coatings directly from liquid feedstock. In this study, unique ability of SPPS to intermix different precursors on the atomic scale was employed to deposit pure and Cr-doped Al2O3 (technically, synthetic ruby) from several mixtures of aluminum and chromium nitrates. Highly porous coatings (> 60% porosity) were deposited by high-enthalpy WSP-H torch. Homogeneous distribution of Cr atoms in Al2O3 resulted in coatings color ranging from pink to grayish-green correlated with increasing Cr content and change of lattice parameters as observed by x-ray diffraction. High content of over 80 wt.% of α-alumina phase was reached for all coatings. For the Cr-doped Al2O3 coatings, distinct and fully reversible thermochromic behavior was observed by temperature-resolved colorimetry. Further increase in α-phase content (up to 95 wt.%) and coating densification were achieved by additional surface remelting by plasma torch.

Effect of Si particle size and NH4Cl additive on combustion synthesis of α-Si3N4

High α-phase content Si3N4 powders were fabricated by combustion synthesis method using coarse Si powder with NH4Cl additive. The effects of Si particle size and NH4Cl additive on the macrokinetic feature, phase composition, microstructure of combustion synthesized α-Si3N4 powder were investigated. Without the addition of NH4Cl, the content of α-Si3N4 in the products first decreases under a critical Si particle size of 7.8 μm, and then increases with increasing Si particle size. With the addition of 6 wt% NH4Cl, high α-phase content Si3N4 powder (>90 wt%) was obtained by using coarse (19.6 μm) Si powder. The addition of NH4Cl reduces the maximum combustion temperature and retention time of liquid Si by promoting the gasification of Si, which lead to the increase of α-Si3N4 content in the product. The use of coarse Si powder is of great significance for the low-cost production of high-quality α-Si3N4 powder.

Increasing α-phase content of alumina-chromia coatings deposited by suspension plasma spraying using hybrid and intermixed concepts

The novel method of hybrid suspension plasma spraying of dry coarse aluminum oxide powder with chromium oxide suspension using hybrid water/argon-stabilized (WSP-H 500) plasma torch was utilized for the deposition of coatings with very high α-phase content reaching up to 90%. The deposition mechanism and phase composition were compared with those of coatings deposited from i) intermixed alumina-chromia suspension and ii) alumina suspension doped with chromium nitrate nonahydrate solution. All deposition routes showed alternative ways of preparation of novel multimaterial coatings. It was demonstrated that the chromia addition and the deposition route play the crucial role in the pronounced formation of the thermodynamically stable α-phase.

The Effect of EBM Process Parameters on Porosity and Microstructure of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy.

In this article, the authors discuss the results of studies into the processing of Ti-5Al-5Mo-5V-1Cr-1Fe near-β titanium alloy (Ti-55511) by electron beam melting (EBM), an additive manufacturing technique. Due to its high flexibility in shaping mechanical properties, Ti-55511 alloy is commonly used in aircraft components such as landing gear or airframes. In this study, Ti-55511 powder was used and its properties were described as regards chemical composition and particle size distribution in order to assess its suitability for EBM processing and repeatability of results. 20 sets of processing parameters were tested in the energy input range between 10 J/mm3 and 50 J/mm3 (cathode current, 4.5 mA-19.5 mA; scanning speed, 1080 mm/s–23400 mm/s). Four types of top surfaces were obtained, namely, flat, orange peel, with single pores, and with swelling. Best results were obtained for the energy of 30 J/mm3: flat top surface and relative density in excess of 99.9%. Analysis of chemical composition showed that aluminum loss was below the specification minimum for the analyzed parameter sets. Scanning speed most significantly affected aluminum content: the lower the scanning speed, the higher the aluminum loss. Analysis of microstructures showed the dependence of lamellar α-phase volume fraction on the process parameters used. For low scanning speed, the determined α-phase volume accounted for about 78%. Higher scanning speed resulted in a decrease of the α-phase content to 61%. The dimensions of the lamellas and the amount of the α-phase strongly effected hardness results (360 HV to 430 HV).