Formation Of Intermetallic Phases Research Articles

Thermodynamic measurements and ab initio calculations of the indium-lithium system

The limiting enthalpy of the solution of liquid indium in liquid tin was measured at 723 K. The calorimetric method was applied to determine the standard enthalpy of the formation of intermetallic phases and alloys from the In-Li system. The measurements were done at 747 K and 756 K. The structures of prepared alloys were confirmed by the X-ray diffraction measurements. Besides that, the ab initio calculations allowed the modeling of the formation energies, the volume thermal expansion, the heat capacity under constant pressure, and the elastic properties of the intermetallic phases. The theoretical formation energies show good agreement with the experimental findings. The analysis of the phonon dispersion indicates an instability of the InLi phase in the Fd-3m space group. A further investigation on the atomic arrangement in the case of the equiatomic ratio is suggested.

Distribution of the temperature in friction stir welding of aluminum and copper alloys

Friction stir welded are studied joints of pure copper M1 and aluminum alloy AD1. A technique for measuring the temperature was developed at the points of the weld. The results of measurement temperature are given in friction stir welding at various speeds of rotation of the welding tool (800-1000 rpm), welding speed (25-61 mm/min). The temperature reached in copper is about 60 K higher than in aluminum. The higher cooling rate of copper is due to the greater thermal diffusivity than that of aluminum. The maximum temperature in the copper reached 769 K, which was lower than the melting point of the eutectic in the Al-Cu system (821 K). The residence time above the temperature of formation of intermetallic compounds (623 K) is much shorter than the duration of the thermal cycle (50 s). With an increase in the welding tool rotation frequency from 800 to 1000 rpm, the maximum temperature near the pin increases from 650 to 769 K. At a tool rotation frequency of 800 rpm, the maximum temperature in copper achieved in the cycle is lower than the transition temperature to the superplastic state and is 764 K high heating and free cooling rates (50–145 K/s and 25–50 K/s) of the weld on the side of the copper alloy predetermine the short-term (from 5 to 15 s) stay of the metal in the temperature range of formation of intermetallic phases.

On the rate of microstructural degradation of Al-Ta-Ti-Zr refractory metal high entropy superalloys

Refractory metal high entropy superalloys (RSA) are being developed for high temperature structural applications. However, recently the microstructures of several RSA have been shown to be unstable, precipitating potentially deleterious intermetallic phases during prolonged exposure at elevated temperatures. For such instability to be acceptable it is necessary for the intermetallic phase formation to be sufficiently sluggish so as to not compromise alloy performance over the time scales likely to be experienced in service. However, the rate of intermetallic phase formation in RSA are not yet known. To address this issue, here, two quaternary Al-Ta-Ti-Zr refractory superalloys that contain all of the key microstructural features of more compositionally complex RSA have been studied following exposures of 1, 10 and 100 h at temperatures between 700 and 900 °C and compared to those previously obtained following 1000 h exposures. Critically, needles of an Al-Zr-rich intermetallic phase were observed following only 1 h exposure at 900 °C, demonstrating rapid formation kinetics. Furthermore, the microstructures continued to coarsen from 100 to 1000 h, indicating low morphological stability. Higher Al content increased both the prevalence of the Al-Zr-rich intermetallic phase and raised the solvus of the desirable B2 phase. Consequently, this work highlights additional challenges in RSA development for elevated temperature applications.

Electron-Beam Welding Cu and Al6082T6 Aluminum Alloys with Circular Beam Oscillations

In this study, we present the results from electron-beam welding operations applied on copper and Al6082T6 aluminum alloys. The influence of beam-scanning geometries on the structure and mechanical properties of the welded joint is studied. The experiments were conducted using a circle oscillation mode with an oscillation radius of 0.1 mm and 0.2 mm. The beam deflection was set to 0.4 mm with respect to the side of the aluminum alloy, and the beam power was set at 2700 W. The phase composition of the obtained welded joints was studied by X-ray diffraction (XRD). Scanning electron microscopy (SEM) was used for the investigation of the microstructure of the joints. The chemical composition was investigated by using energy-dispersive X-ray spectroscopy (EDX). The mechanical properties were studied by micro-hardness investigations. The fusion zone of the weld seam contains three phases—an aluminum matrix, an ordered solid solution of copper and aluminum in the form of CuAl2, and pure copper. Electron beam-scanning geometries have significant influences on the structure of the weld. Increasing the beam oscillation’s radius leads to a decrease in intermetallic phases and improves homogeneity. The measured microhardness values in the fusion zone are much higher than the ones measured in the base metals due to the formation of intermetallic phases. The microhardness of the weld joint formed using an oscillation radius of 0.2 mm was much lower compared to the one formed using an oscillation radius of 0.1 mm.

New approach for multi-material design: Combination of laser beam and electromagnetic melt pool displacement by induced Lorentz forces

Multimaterial structures are a promising solution to reduce vehicle weight and save fuel or electric energy in automotive design. However, thermal joining of steel and aluminum alloys is a challenge to overcome due to different material properties and the formation of brittle intermetallic phases. In this study, a new joining approach for producing overlap line-shaped joints is presented. The lower joining partner (EN AW 5754) is melted by a laser beam, and this melt is displaced into a line-shaped cavity of the upper joining partner (1.0330) by induced Lorentz forces. The melt solidifies in the cavity to a material and form-fitting joint. This approach needs no auxiliary joining elements or filler materials. Previous investigation to produce spot-shaped joints by using this approach showed that quality and reproducibility were limited by known melt pool dynamics of aluminum alloys (keyhole collapses). For line-shaped joints, the melt displacement can take place behind the keyhole. This allows the displacement process to be spatially uncoupled from the influence of keyhole collapses. The study shows that this improved the process stability and the quality of the joint. The created line-shaped joints were microstructurally characterized by transversal sections. Intermetallic phases were identified by electron backscatter diffraction and EDX analysis. The detected intermetallic phases consist of a 5–6 μm compact phase seam of Al5.6Fe2 and a needle-shaped phase of Al13Fe4. Tensile shear tests were carried out to quantify the load capacity. It was possible to create a joint with a load capacity of about 2 kN.

Mechanical properties of Al-Nb in situ metal-matrix composites fabricated by constrained high pressure torsion at 10 GPa and subsequent annealing

Mechanical alloying of dissimilar metals by means of severe plastic deformation techniques is known to be a perspective method of obtaining in situ metal matrix composites with advanced properties. Recent investigations found out that besides severe mixing of elements, intensive formation of non-equilibrium intermetallic phases takes place. Presence of intermetallic particles in the composite affects both its physical and mechanical properties. In this work we have realized the fabrication of Al-Nb composites in one step by means of high-pressure torsion technique. A non-trivial phenomenon of growth of tensile strength with increasing number of revolutions during HPT was revealed. It was explained by more homogeneous mixing of components and precipitation of intermetallic phase in the vicinity of Al-Nb interphases. The microstructural observations and X-ray diffraction analysis allowed to reveal Al3Nb intermetallic phase in the as deformed composites, while the equilibrium temperature of formation of this phase is above 600°C. Varying the number of revolutions and post deformation annealing allows obtaining composites with different fractions of intermetallic particles. This approach can be considered as a way for the development of composites with a tailorable complex of physical and mechanical properties.

Effect of Copper on the Formation of L12 Intermetallic Phases in Al–Cu–X (X = Ti, Zr, Hf) Alloys

Transition metal trialuminides of the Al3X type of groups 4 and 5 of the periodic system have reduced density, high melting points, and corrosion resistance. Aluminides with a cubic lattice of the Al3Sc type can be used as a nucleating phase for aluminum alloys. However, low plasticity and a tetragonal lattice limit their application. In this work, we stabilized the metastable cubic lattice of Al3X-type aluminides by replacing aluminum with copper. The conditions for the formation of L12 metastable aluminides in the Al–Cu–TM (TM: Ti, Zr, Hf) alloys were studied using a wide range of copper concentrations. A high concentration of copper (hypereutectic alloys) is the one of the necessary conditions for the formation of (Al1−xCux)3Ti, (Al1−xCux)3Zr, (Al1−xCux)3Hf aluminides. With an increase in the copper concentration, the number of metastable aluminides sharply increased. The process of their formation strongly depended on the sequence of dissolution of the corresponding components in the melts. The low volume fraction of precipitated titanium aluminides was the result of insufficient supersaturation of α-Al with titanium (at the peritectic temperature) compared to that for alloys with zirconium and hafnium. Under identical synthesis conditions in the crystal lattice of metastable aluminides formed in experimental Al–Cu–Ti, Al–Cu–Zr, Al–Cu–Hf alloys, copper was found to substitute up to 8, 10, and 13 at.% of aluminum, respectively. The crystallographic and dimensional similarities of the lattices in metastable transition metal aluminides and in α-Al suggest their usefulness as modifying additions in aluminum-based alloys.

Influence of La addition on Fe-rich intermetallic phases formation and mechanical properties of Al-7Si-4Cu-0.35Mg-0.2Fe alloys prepared by squeeze casting

Brittle β-Al5FeSi (β-Fe) intermetallic phases directly affect the mechanical properties of Al–Si alloys; thus, it is essential to modify these phases. Scanning electron microscope analysis showed that rare earth Lanthanum (La) additions (0, 0.15, 0.30 and 0.50 wt%) had a refining effect on β-Fe phases in Al–7Si–4Cu-0.35Mg-0.2Fe (wt.%) alloys prepared by squeeze casting. The underlying refinement mechanisms of rare earth La on the β-Fe were discussed in terms of experimental observations, nucleation thermodynamics and growth kinetics. For nucleation thermodynamics, La additions could induce the formation of La-rich intermetallic phases Al4Cu2SiLa with orthorhombic structure. Because β-Fe nuclei had a good lattice matching and epitaxial relationship with the Al4Cu2SiLa phases, the Al4Cu2SiLa provided energetically favorable nucleation sites for β-Fe and greatly promoted the nucleation of β-Fe in alloys. Meanwhile, the added La atoms could restrain the diffusion of atoms to the solid/liquid interface front and alter the β-Fe interfacial structure and energy, therefore restricting the β-Fe growth. Furthermore, β-Fe/Al4Cu2SiLa grew into side-by-side and cross-like morphologies, and its growth behaviors were controlled by solute concentration and anisotropy of the solid-liquid interfacial energy in a cooperative eutectic reaction. In addition, different fracture behaviors from brittle fracture to mixed ductile fracture and quasi-cleavage brittle fracture were also illustrated with increasing La addition, and these fracture behaviors were influenced by the characteristics of brittle La-rich and β-Fe intermetallic phases. When the addition of La was increased to 0.3 wt% and 0.5 wt%, it occurred quasi-cleavage brittle fracture, which was caused by the precipitation of coarse plate-like La-rich phases.

Effect of Ta addition on the sinterability of spark plasma sintered W-Ni-Fe alloy

The purpose of this research is to investigate and characterize the W-Ni-Fe-Ta compositions by solid-state sintering using the Spark Plasma Sintering (SPS) technique at a relatively lower temperature. The sinterability of W-Ni-Fe ternary tungsten heavy alloy (WHA) with the addition of Tantalum in predetermined compositions was sintered using SPS at 1100°C under 30 MPa pressure. The effect of Ta addition on WHA densification, microstructure, and mechanical properties was studied. Ni was found to act as an activator in the sintering of tungsten. SEM and energy dispersive spectroscopy revealed an even distribution of Ta in the W and Ni-Fe matrix. The data of hardness and ultimate tensile strength were explained using grain size, contiguity, and the formation of brittle intermetallic phases. W-7Ni-3Fe-5Ta alloy composition has the highest ultimate tensile strength of 870 MPa, 189.2 HV0.5kgf hardness, and 83.42% relative sintered density. During the tensile test, W grain cleavage fracture, W-W decohesion, matrix tearing, and W matrix intergranular types of fracture surface were observed.

Effect of addition of Nb on phase stability of equiatomic CrFeMoV alloy

Multiprincipal alloys with better high temperature properties and irradiation resistance are being explored as potential candidates for nuclear applications. This work is aimed at investigating the effect of Nb addition on the phase stability of CrFeMoV alloy. Microstructure of the as cast CrFeMoNbV alloy showed a dendritic morphology. Microsegregation in the cast alloy has been understood on the basis of partition coefficient and difference in melting point of the constituent elements. Structural analysis through Precession Electron Diffraction (PED) and 3D Automated Diffraction Tomography (ADT-3D) showed the formation of HCP intermetallic Laves phase of Fe2Nb type in a BCC matrix. The formation of this dual phase structure has been understood through in depth empirical calculations and CALPHAD simulations. Addition of Nb is shown not only to increase the lattice strain through increase in atomic size difference, but also increases the enthalpy contribution, which favors the formation of intermetallic Laves phase.

Deformation and Stresses During Alkali Metal Alloying/Dealloying of Sn-Based Electrodes

Abstract Enhancement of energy density and safety aspects of Li-ion cells necessitate the usage of “alloying reaction”-based anode materials in lieu of the presently used intercalation-based graphitic carbon. This becomes even more important for the upcoming Na-ion battery system since graphitic carbon does not intercalate sufficient Na-ions to qualify as an anode material. Among the potential “alloying reaction” based anode materials for Li-ion batteries and beyond (viz., Na-ion, K-ion battery systems), Si and Sn have received the major focus; with the inherently ductile nature of Sn (as against the brittleness of Si) and the considerably better stability in the context of electrochemical Na-/K-storage, of late, tilting the balance somewhat in favor of Sn. Nevertheless, similar to Si and most other “alloying reaction”-based anode materials, Sn also undergoes volume expansion/contraction and phase transformations during alkali metal-ion insertion/removal. These cause stress-induced cracking, pulverization, delamination from current collector, accrued polarization and, thus, fairly rapid capacity fade upon electrochemical cycling. Unlike Si, the aforementioned loss in mechanical integrity is believed to be primarily caused by some of the deleterious first-order phase transformations and concomitant formation of brittle intermetallic phases during the alloying/de-alloying process. Against this backdrop, this review article focuses on aspects related to deformation, stress development and associated failure mechanisms of Sn-based electrodes for alkali-metal ion batteries; eventually establishing correlations between phase assemblage/transformation, stress development, mechanical integrity, electrode composition/architecture and electrochemical behavior.

Microstructure, mechanical property and corrosion behavior of biomedical Zn-Fe alloy prepared by low-temperature sintering

Zn is recognized as a novel orthopedic implant due to its excellent biocompatibility and favorable degradation behavior. Nevertheless, its poor strength and hardness fail to meet mechanical requirements of orthopedic implants. In this study, Fe with excellent mechanical properties was successfully alloyed with Zn at low temperature of 350 ℃ via rapid spark plasma sintering. Alloying Fe activated grain boundary pinning effect where pinning force at the grain boundary inhibited grain growth, and significantly refined grain. The grain refinement reduced stress concentration, and induced large resistance to dislocation and cracks growth, which improved mechanical properties of Zn-Fe alloys. Moreover, the formation of intermetallic FeZn13 phases contributed to precipitation strengthening. As a result, compression yield strength and hardness of Zn-Fe alloys were increased to 135.2 MPa and 64.6 Hv, respectively. Additionally, Zn-Fe alloys exhibited good biocompatibility to MG-63 cells in cytotoxicity assay. All these results demonstrated that Zn-Fe alloys were a great potential candidate as orthopedic implants.

Interfacial formation of intermetallic Ni-Al-Ti systems formed by induction heating

Intermetallic systems of Nickel (Ni), Aluminium (Al), and Titanium (Ti) are candidates for lightweight materials that offer high-temperature resistance. Combustion synthesis has been widely studied to produce intermetallic and coating deposition by exploiting the heat released by the combustion. An underlayer is often used to enhance the adhesion of the coating to the substrate. The interaction of the coating and the underlayer during heating is, therefore, crucial for achieving a good adhesion quality. This work aimed to investigate the microstructure and properties of the interfacial formation across the NiAl coatings and Ti underlayers formed by combustion synthesis. Induction heating was used to initiate the heating and reaction process with heating rates of 46.6, 57.0, and 85.5 K/s. The microstructure was characterized by Scanning Electron Microscopy (SEM) equipped with an Energy Dispersive Spectroscopy (EDS) detector, whereas the formed phases were identified using X-ray Diffraction (XRD) tests. The hardness distribution was measured by the Vickers microhardness test. The result shows that NiAl with Al-rich and Ni-rich were formed in the coating region. The average thickness of the coating increases by approximately 200, 300, and 400 µm with a heating rate of 46.6, 57.0, and 85.5 K/s, respectively. The different thicknesses of the coating can be attributed to the migration of Ni/Al from the coating to the underlayer zones. The microstructure observed in the underlayer confirms the formation of several intermetallic phases of Ni-Ti and Ti-Al systems. The infiltration of Ni and Al elements from Ni and Al to Ti sides was responsible for generating a reaction between Ni-Ai-Ti. The formation of Ti2Ni–Ti3Al phases in the underlayer increases with the heating rate. The hardness across the coating, interface, and underlayer increases with the heating rates. The heating rate of 46.6, 57.0, and 85.5 K/s results in the hardness of the interface by 669.1, 804.8, and 967.7 HV, whereas the underlayer increases by 680.1, 772.7, and 978.7 HV, respectively. The increased content of the Ni-Al-Ti system, which are AlNi2Ti and Ti2Ni–Ti3Al phases, was attributed to the increased hardness of the interface and underlayer. This work improves the understanding of second reactions across the interface while fabricating coatings that apply an underlayer.

Influence of heat input on intermetallic formation in dissimilar autogenous laser welding between Inconel 718 and AISI 316L steel

Intermetallic formation during dissimilar welding of Inconel and stainless steel significantly hinders producing sound-quality weld. In the present work, a high-power CO2 laser welding system is employed to join Inconel 718 and AISI 316L stainless steel without using any filler materials. The finite element based thermal model provides the temperature distribution in the weld zone and consequently the solidification parameters (G/R and G.R). The micro-segregation and intermetallic formation in laser welding are correlated with these solidification parameters. The segregation of principal alloying elements triggers the formation of intermetallic phases during solidification. The presence of various phases like Laves, NbC, and TiC in the fusion zone reduces with increase in heat input. The amount of these intermetallic phases present in the weld zone is estimated by XRD analysis. With an increase in heat input, the mode of solidification changes from columnar to equiaxed dendrites that is characterized at reduced value of solidification parameter (G/R). The intermetallic Laves particle diminishes significantly (more than 90%) with a reduction of G/R parameter. In equiaxed dendrites, the segregation and formation of these secondary particles reduces to more than 40% compared to the columnar dendritic structure. Overall, the reduction in the intermetallic particles improves the joint efficiency up to 100% with an elongation of 14.2% for the equiaxed structure of the laser welding system.

Titanium nitride concentration and coordination effect with hexagonal boron nitride enhancing the reciprocating wear behavior of AZ91D alloy-based composites

The influence of TiN hard ceramic concentration and coordination effect of hBN solid lubricant particulates on reciprocating wear and friction behavior of AZ91D alloy-based magnesium matrix composites is investigated in this work. A unique stir-ultrasonication-squeeze casting procedure was employed to fabricate the composites. The fine dispersion of reinforcing particulates in matrix was examined using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). The X Ray Diffraction (XRD) analysis confirmed the existence of reinforcements in composites and formation of intermetallic phases demonstrating the thermal stability of matrix reinforcement materials. The hardness of fabricated AZ91D/5 wt.% TiN (AT), AZ91D/5 wt.% hBN (AH), AZ91D/5 wt.% TiN/5 wt.% hBN (ATH), and AZ91D/10 wt.% TiN/5 wt.% hBN (ATTH) composites were increased by 18.37%, 8.14%, 11.68%, and 25.2% in comparison with as-cast alloy “A”. The tribo-properties of single and dual ceramic particles reinforced materials were investigated using a linear reciprocating tribometer at an unchanged sliding distance of 200 mm and stroke length of 10 mm under varied loads (7.5, 15, 22.5, and 30 N), reciprocating frequencies (2, 4, 6, and 8 Hz), and temperature conditions (50°C, 100°C, 150°C, and 200°C). Compared to material “A,”“ATTH” composite performs well in terms of wear behavior, with performance increasing by 42.63% and 54.33% respectively, under ambient and enhanced conditions. At low load, increased frequency accelerates material removal from the plate’s surface due to direct abrasion and plowing action caused by rough asperities on the counterface. The increase in applied load under high-frequency settings increases the pin’s contact pressure with the sample surface, which raises the temperature and causes the shallow grooves to severe delamination. At increased temperatures, the wear characteristics of intense oxidation, plastic deformation, and significant delamination were discovered on manufactured materials.

(Digital Presentation) Influence of Li2s on the Volume Expansion of Sn Particles during Lithiation of Tin Sulfides

In recent years tin sulfides, particularly SnS and SnS2, have attracted significant attention in the energy storage research community. Their high theoretical specific capacities (1111 mAhg-1 for SnS and 1209 mAhg-1 for SnS2) make this group of materials good candidates for anode active materials for Li-ion batteries. Moreover, their low cost and environmental benignity further contribute to the appeal. During lithiation, tin sulfides undergo a conversion reaction with Li in which Li2S and Sn are formed. Further lithiation triggers the alloying reaction between Sn and Li, resulting in the formation of various Sn-Li intermetallic phases. The newly formed Sn-Li phases have a significantly higher unit cell volume than their parent phases, causing cracking of particles on repeated alloying and dealloying. Ultimately, the continuous cracking of particles leads to pulverization of anode material, reduced contact between the particles and capacity fade.In our work, we explored the influence of Li2S formed during the conversion reaction on the stability of the solid electrolyte interphase (SEI) layer and the overall cycling performance of SnS and SnS2 materials. The precipitation reaction was used to synthesised both SnS and SnS2 materials. Furthermore, SnS2 was also synthesised via the widely used hydrothermal method. In all cases, SnClx ·yH2O (x= 2; y= 2 for SnS and x= 4; y= 5 for SnS2) was used as a Sn source and thioacetamide as a S source. The physio-chemical properties of the as-synthesised materials were examined by powder X-ray diffraction (PXRD) and structural parameters were extracted using Rietveld analysis of the measured patterns. Furthermore, the sample morphologies were characterized by scanning electron microscopy (SEM) and the crystallinity of the samples was further investigated by transmission electron microscopy (TEM). The presence of surface impurities which may formed during material synthesis, was examined by X-ray photoemission spectroscopy (XPS).The cycling performance of SnS2 prepared via the precipitation reaction was compared to SnS2 prepared via hydrothermal method as well as to SnS prepared via precipitation reaction. In this way we were able to draw comparisons between materials with the same chemical composition but different particle size and morphology as well as materials with different chemical composition and very similar morphology.The length changes of SnS and SnS2 electrodes during cycling were investigated by the in-situ dilatometry technique, and several ex-situ methods such as ex-situ XRD and ex-situ SEM were utilized to investigate the role of crystal chemistry and particle morphology on electrochemical performance post-mortem. The results showed that on alloying, the electrode comprised of partially amorphous SnS2 synthesised via the precipitation expands the least. Additionally, the cycling data show that partially amorphous SnS2 electrochemically outperformed both SnS and highly crystalline SnS2. This may be attributed to two effects. Firstly, thicker Li2S layers are generally formed on particles with lower surface areas, thereby better restraining the volume changes of Sn during cycling. Secondly, due to the intercalation step prior to the conversion reaction in SnS2, it is postulated that Li2S and Sn are homogeneously distributed on the atomic scale, which further helps to minimize the volume expansion during cycling.

Investigations on a ternary Mg-Ca-Si wrought alloy extruded at moderate temperatures

• A Mg-Ca-Si alloy was extruded at 300 °C and 350 °C without any heat treatment. • Extrusion at 300 °C leads to bi-modal microstructure: DRX grains and non-DRX areas. • Elongation to break (>8 %) is comparable for both processing temperatures. • Increased YS is correlated to lower grain size and higher texture intensity. Wrought Magnesium alloys are lightweight structural materials with excellent specific strength. Recently, Ca has received increased attention as a main alloying element in Mg alloys providing advantages in processing and microstructural evolution. Especially the synergetic use with additional elements such as Al, Zn or Sn has shown improved performance in extruded profiles, as it promotes the formation of intermetallic phases and a stable, fine-grained microstructure. Superior mechanical properties can be achieved with these alloys when processed at moderate temperatures. Here we present an investigation into the processing of an Mg-Ca-Si alloy, discussing the alloying principle as well as microstructural and mechanical behavior.

Weldability of Nickel/Aluminum Dissimilar Materials Laser Welding Part I: Microstructure Development, Solidification Behavior and Mechanical Properties of Polycrystalline Nickel-based Superalloy

Weldability-related issues, including asymmetrical weld pool shape, γ phase multicomponent microstructure development, solidification behavior and mechanical properties, of γʹʹ precipitation-strengthened polycrystalline nickel-based superalloy are experimentally analyzed during keyhole laser welding repair. The crucial relationship between them provides weldability improvement opportunities and defect-free high quality weld for feasible aerospace materials laser processing. Attractive neck transition region of weld is more vulnerable to coarser dendrite than bottom part in the vicinity of fusion boundary, and this location is also liable to abundant Laves/γ eutectic phase formation because of heat and solute accumulation. The dominant chemical inhomogeneity brings about microstructure anomalies and irregular morphology to deteriorate hot cracking resistance. Because of thermometallurgical factors of selective alloying partition in the molten pool, partition-resistant Cr is chemically accumulated in dendrite core, while partition-susceptible Ti, Al, Mo and Nb are enriched in high-supersaturation interdendrite region, which consequently result in serious segregation and extensive Nb-rich intermetallic phase formation across solidification interface. Dendrite refinement and aggressive Laves/γ eutectic phase formation suppression are not simultaneously satisfied by low heat input under nonequilibrium solidification conditions. Therefore, mechanical properties are only partially improved by diffusion-limited microstructure refinement as result of phase instability. In other words, reductions of tensile strength and ductility are attributed to brittle Laves/γ eutectic phase formation during terminal stage of solidification. The severity of eutectic reaction thermodynamically depends on nail-shaped keyhole profile, solidification conditions and solute redistribution ahead of solid/liquid interface. Additionally, viable control of weld metallurgical and mechanical properties progressively encourages optimal combination of laser power and welding speed to interdendritically minimize disadvantages of eutectic phase formation by a variety of welding conditions optimization.

Development and thermal stability of Cr10Mo25Ta25Ti15V25 refractory high entropy alloys

Refractory high entropy alloys (RHEAs) are promising materials for extreme environment applications, such as high-temperatures (>1000 °C), corrosion and irradiation. In this study, a new CrMoTaTiV RHEA system has been designed using CALPHAD modeling and thermophysical parameter optimization, and produced by vacuum arc melting. Microstructure at room temperature as well as microstructural stability and mechanical properties at high temperature have been investigated. It has been found that BCC alloys are formed with dendritic structures in which interdendritic regions are enriched with Ti, V and Cr, and depleted with Ta and Mo. Transmission electron microscopy analysis has shown dislocation tangles, loops and networks as well as the formation of nano-grains some of which are determined to be Ti-rich FCC phases. Besides, there is a secondary BCC formation below 1000 °C together with a Ti-rich FCC phase without any HCP or intermetallic phase formation. High strength and ductility of this alloy at the temperatures up to 1000 °C compared to the other RHEA systems have been attributed to its unique microstructure. This clearly shows the promise of this alloy system for high temperature applications due to its stability, high strength and possible oxidation resistance.

Inconel 718-CoCrMo bimetallic structures through directed energy deposition-based additive manufacturing

Bimetallic structures and coatings through additive manufacturing (AM) have demonstrated a high degree of freedom for tailoring properties depending on the application. In this study, Inconel 718 and CoCrMo were used as both are common alloys and exhibit unique properties, such as high-temperature oxidation, wear, and fatigue resistance. Using directed energy deposition-based metal AM, bimetallic structures containing these two alloys were manufactured, and the resulting structures exhibited no intermetallic phase formation, cracking, or porosity. Scanning electron microscopy and energy dispersive spectroscopy revealed a smooth elemental transition between the two compositions. Hardness testing showed a linear transition in the interfacial zone, validating no brittle intermetallic phase formation. Compression testing and fracture surface analysis revealed that the failures were not dependent on the interface properties. High-temperature oxidation showed no distinct effect on the interface, a firmly attached chromium oxide layer on the Inconel 718 side and a loosely attached chromium oxide layer on the CoCrMo side. There was also evidence of pit formation on the Inconel 718 surface, but not on the CoCrMo. These findings confirm a stable bimetallic system in which one of the two alloys can be used on the other material to improve the structure’s high-temperature oxidation or wear/corrosion resistance.