Scheil Simulation Research Articles

Microstructure, multi-scale mechanical and tribological performance of HVAF sprayed AlCoCrFeNi high-entropy alloy coating

Thermal spray high-entropy alloy (HEA) coatings have demonstrated potential for improving the wear resistance of conventional materials used in extreme engineering environments. In the present work, an equiatomic AlCoCrFeNi HEA coating was manufactured using the high velocity air fuel (HVAF) process. The phase and microstructural transformations in gas-atomized (GA) powder during HVAF spraying were analyzed using SEM, EDS and EBSD techniques. The tribological properties of this HEA coating sliding against an Al2O3 ball at both room temperature (RT) and 600 °C were also evaluated. The GA powder was composed of Body Centred Cubic (BCC) + ordered BCC (B2) phases, which transformed to BCC + B2 + minor Face Centred Cubic (FCC) phases during the HVAF coating process, validating the thermodynamic phase prediction projected by the Scheil simulation for non-equilibrium processing conditions. The rapid solidification and high velocity impact-assisted deformation of GA powder resulted in significant grain refinement in the HVAF coating, which ultimately improved the mechanical properties at both micro and nanoscale levels. The wear resistance of the HEA coating at RT was severely impacted by the relatively brittle BCC/B2 phase structure, leading to susceptibility to abrasive wear and surface fatigue. The wear resistance at 600 °C was slightly lower at RT due to the formation of a brittle oxide layer on the worn surface, which induced surface fatigue and aggravated mass loss of the coating.

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

Design and validation of refractory alloys using machine learning, CALPHAD, and experiments

Refractory multicomponent alloys (RMCAs) have garnered attention as potential materials for high-temperature structural applications, due to their excellent mechanical properties. However, conventional alloy design has limitations in terms of constrained compositional space and a lack of computational databases with adequate coverage. To address this, we present a design framework that leverages machine learning (ML), the CALculation of PHAse Diagram (CALPHAD) method, and experimental validation to efficiently develop refractory alloys. The present study focuses on the Mo-Nb-W ternary system. Six ternary alloys were inversely designed by means of the conditional generative adversarial network (cGAN) and fabricated via arc melting. The ternary alloys exhibit a single BCC phase which is consistent with CALPHAD calculations as well as Scheil simulations. The present interactive design loop between the ML surrogate model and experiments is demonstrated through the accurate hardness prediction, resulting in cGAN models capable of rapid exploration of the higher-order design space. The hardness of the Mo-Nb-W alloys is in the range of 5–6 GPa due to their solid solution strengthening.

Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion

The demand for high-performance aluminum components drives research into the design of novel alloys that can be processed by laser-based additive manufacturing. In recent years, the addition of grain refiners proved to be an effective strategy to reduce the hot-cracking of high-strength Al alloys. In this study, the solidification and aging behavior of an Al2139 alloy doped with additions of Zr and Ti for L-PBF was investigated. These elements favored the formation of a fine-grained structure free of cracks. The formation of Al3(Zr,Ti) inoculants was predicted by Scheil simulations and observed as cuboidal particles in the center of α-Al grains. The microstructure of the as-built material featured fine and fully equiaxed grains, which appeared comparatively finer at the edge (300–600 nm) and coarser (0.8–2.0 μm) at the center of the molten pools. In both cases, there was evidence of Cu and Mg micro-segregations at the grain boundaries. The microhardness of 109.7 HV0.5 in the as-built state was increased to 186.1 HV0.5 after optimized T4 heat treatment, responsible for the precipitation of many rod-shaped Zr- and Ti-based second phases and quasi-spherical Cu-, Mn-, and Fe-rich particles. Prolonged exposure carried out to simulate high-temperature service caused a drop in microhardness and marked modification of the microstructure, evidenced by the rearrangement and subsequent spheroidization of Cu- and Mg-rich particles at the grain boundaries.

Development of composite high entropy-medium entropy alloy coating

The viability of adapting HEAs or MEAs with single-phase structures, such as either FCC or BCC phases, for industrial applications is challenging due to the trade-offs in properties between different phases. Therefore, a novel high entropy alloy-medium entropy alloy composite coating based on a hard metal concept was designed using a CALPHAD approach, followed by experimental validation that included microstructural, mechanical and tribological analysis. A harder BCC/B2-based AlCoCrFeNi HEA powder was blended with a softer and more ductile FCC-based CoCrFeNi MEA to manufacture a composite coating using the HVOF process. The phase prediction, using Scheil simulation, which depicts non-equilibrium processing conditions experienced during the HVOF process, suggested the development of BCC, B2 and FCC phases, complementing XRD phase projections. The composite coating exhibited higher hardness than either single phase AlCoCrFeNi or CoCrFeNi coatings and achieved superior wear resistance under dry sliding conditions at elevated temperature, thereby, exhibiting potential for structural applications.

Efficient alloy design of Sr-modified A356 alloys driven by computational thermodynamics and machine learning

• A reliable CALPHAD thermodynamic database of al-Si-Mg-Sr system was established. • The quantitative composition-microstructure-properties relation was constructed. • The optimal Sr additional amount in A356 alloys was designed as 0.005 wt.%. • The strengthening/toughening mechanisms of Sr-modified A356 alloys were analyzed. A356 alloys are widely used in industries due to their excellent comprehensive performance. Sr is usually added in A356 alloys to improve their mechanical properties. There have been various experimental reports on the optimal additional amount of Sr in A356 alloys, but their results are inevitably inconsistent. In this paper, a combination of computational thermodynamic and machine learning approaches was employed to determine the optimal Sr content in A356 alloys with the best mechanical properties. First, a self-consistent thermodynamic database of quaternary Al-Si-Mg-Sr system was established by means of the Calculation of PHAse Diagram technique supported by key experiments. Second, the fractions for solidified phase/structures of A356- x Sr alloys predicted by Scheil simulation, together with the measured mechanical properties were set as the input dataset in the machine learning model to train the relation of “composition-microstructure-properties”. The optimal addition of Sr in A356 alloy was designed as 0.005 wt.% and validated by key experiments. Furthermore, such a combinatorial approach can help to understand the strengthening/toughening mechanisms of Sr-modified A356 alloys. It is also anticipated that the present approach may provide a feasible means for efficient and accurate design of various casting alloys and understanding the alloy strengthening/toughening mechanisms.

Simulation of the Solidification and Microstructural Evolution in Steel Casting Processes Using the InterDendritic Solidification Tool

InterDendritic Solidification (IDS) is a thermodynamic–kinetic software combined with a microstructure tool developed to simulate the nonequilibrium solidification (non‐EQS) of steels. Herein, its main calculation module, solidification (SOL), is introduced, and some essential results of that module, such as the formation of ferrite and austenite in different types of steels during their solidification, and the formation and dissolution of precipitates during subsequent cooling and heating processes, respectively, following solidification, are presented. The non‐EQS is compared with equilibrium and poor‐kinetics solidification to demonstrate the effect of kinetics on the results using finite solute diffusion and microstructure data. The poor‐kinetics solidification is comparable with the modified Scheil simulation ignoring the solid‐state diffusion of slowly moving metallic elements. A particular emphasis is made on demonstrating how to use a postprocessing treatment to control the residual ferrite amounts in stainless steels and the extent of precipitation in particular steel. In this context, the phenomena occurring behind the results are discussed. Finally, to validate the simulations of the SOL module, its calculations are compared with numerous solidification measurements, such as the liquidus and solidus temperatures of different steels and the residual ferrite amounts in stainless steels.

Determination of Location-Specific Solidification Cracking Susceptibility for a Mixed Dissimilar Alloy Processed by Wire-Arc Additive Manufacturing

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.

Control of surface micro−structure for Al alloy/polymer joining fabricated by laser process using Al−Ti−C powders: Effect of powder composition.

Surface micro−structure for joining with thermoplastic parts was additively manufactured on an aluminum alloy (A5052) substrate by laser scanning on Al−Ti−C powder mixtures. The micro−structure consisted of particle−shaped protrusions with a few hundred micrometers. The effect of C content in Al−Ti−C powder mixtures on the micro−structure and constituent phases was investigated. The fraction of particle−shaped protrusion reached maximum when the C/Ti molar ratio was 0.8. Adhesiveness between the particle−shaped protrusions and A5052 substrate decreased drastically when the C/Ti molar ratio increased from 0.6 to 0.8. Changes in these structural factors were related to the formation of TiC and Al3Ti phases after the laser−irradiations. The A5052 substrate was joined with Polyamide−6 sheets via the micro−structure by hot−pressing. When the C/Ti molar ratio was 0.6, the joint strength reached maximum since both the fraction of particle−shaped protrusions and adhesiveness between particle−shaped protrusions and A5052 substrate were high. The constitute phases (TiC and Al3Ti phases) in the micro−structures were assessed by Scheil simulation for Al−Ti−C system to calculate the component partitioning and solidification path during solidification. These results were used to discuss the relation between the surface micro−structure and powder compositions.

Phase Transformations During Homogenization of Inconel 718 Alloy Fabricated by Suction Casting and Laser Powder Bed Fusion: A CALPHAD Case Study Evaluating Different Homogenization Models

Estimating the required homogenization time during post-heat treatment after alloy fabrication is critical for both casting and additive manufacturing (AM). In this work, three CALPHAD-based modeling approaches to estimate the homogenization time in order to dissolve the Laves_C14 phase into γ matrix are evaluated for Inconel 718 alloys made by suction casting and laser powder bed fusion. These values are compared with the homogenization at 1180 °C for different durations. The compositions of the γ matrix obtained from experiments are used as inputs for the first model. The first model involves single-phase diffusion simulations using the diffusion module (DICTRA) implemented in the Thermo-Calc software package with composition profile for the Laves_C14 phase either determined from experiments or calculated by the lever rule. The second model uses Scheil simulations for predicting the segregation profiles, which are used as inputs for single-phase simulations. In the third model, moving boundary simulations using DICTRA are performed using the composition of Laves_C14 phase from the lever rule. The homogenization time determined using the first model matches reasonably well with the experimental observation for the AM alloy. The second model is imprecise as the segregation from Scheil calculation is not reliable for AM alloy. The last model is inaccurate due to lack of mobility data for atomic diffusion in the Laves_C14 phase. The predicted homogenization times for the cast alloys using these models do not match with the experimental values. This necessitates the need for determining the mobilities for Laves_C14 phase for improving the accuracy of DICTRA simulations to estimate the homogenization time.

Effect of Al addition and homogenization treatment on the magnetic properties of CoFeMnNi high-entropy alloy

The effect of Al addition to CoFeMnNi on the phase evolution and magnetic properties was studied for vacuum arc-melted AlxCoFeMnNi (x = 0, 0.3, 0.7, 1) alloys. These alloys were subsequently homogenized at 1050° C for 50 h and water quenched. The CoFeMnNi and Al0.3CoFeMnNi alloys showed single-phase FCC in as-cast and homogenized conditions. The Al0.7CoFeMnNi alloy showed BCC + FCC phases in as-cast condition, and the phase fraction of FCC phase increased upon homogenization. The AlCoFeMnNi alloy had B2 phase in as-cast condition and had BCC + FCC phases after homogenization. Scheil simulation was performed to predict the phase evolution during casting, and calculation of phase diagram (Calphad) approach was used to predict the phase evolution during homogenization. CoFeMnNi alloy was ferromagnetic in as-cast condition, whereas it exhibits spin or cluster glass behaviour after homogenization. The Al0.3CoFeMnNi alloy remained paramagnetic in both as-cast and homogenized condition. In Al0.7CoFeMnNi alloy, the saturation magnetization increased from 20 to 60 emu/g upon homogenization. In the as-cast alloys, AlCoFeMnNi had a maximum saturation magnetization of 126.2 emu/g, and upon homogenization, the saturation magnetization decreased to 94.3 emu/g due to the phase change associated with it. The BCC/B2 phase with equiatomic or near-equiatomic composition had a high saturation magnetization. The change in magnetic properties is correlated to the phase change associated with Al addition and homogenization.

Experimental phase diagram, thermodynamic modeling and solidified microstructure in the Mo–Ni–W ternary system

In this work, the isothermal section at 900 °C, a liquidus projection and invariant reaction temperatures in the Mo–Ni–W ternary system were established using X-ray diffraction (XRD), Differential Scanning calorimeter (DSC) and electron probe micro-analysis (EPMA). Based on the experimental phase diagram data from the present work and the literature, the Mo–Ni–W system was assessed by means of the CALPHAD (CALculation of PHAse Diagrams) method. The liquid, fcc-(Ni), bcc-(Mo) and bcc-(W) phases were described with a regular solution model. The sub-lattice models were used to express Gibbs energies of the intermediate phases of MoNi3, MoNi, (Mo, W)Ni4, WNi and W2Ni. The present modeling covers the entire composition and an extensive temperature ranges of the Mo–Ni–W ternary system, and a set of self-consistent thermodynamic parameters were finally obtained. Comprehensive comparisons between the calculated and measured phase diagram data show that most experimental information is satisfactorily accounted for by the present thermodynamic description. Scheil solidification simulations for a few representative as-cast alloys were performed. It was found that the solidified microstructures can be reasonably described by the Scheil simulation. The liquidus projection and reaction scheme of the Mo–Ni–W system were generated by the presented modeling.

Solidification Characteristics and Segregation Behavior of Cu-15Ni-8Sn Alloy

The solidification characteristics and segregation behavior of Cu-15Ni-8Sn alloy were systematically investigated in the present study. The solidification characteristics were revealed with the assistance of a solidification quenching experiment, DTA analysis and Scheil simulation. The solidification microstructure of Cu-15Ni-8Sn alloy was characterized by SEM, and the results indicated that the as-cast microstructure of Cu-15Ni-8Sn alloy mainly consists of Sn-depleted α-Cu(Ni,Sn) matrix, Sn-rich γ phase and lamellar (α + γ) structure. It has been demonstrated that the solidification process begins with the nucleation and growth of primary Sn-depleted α1 phase ($$ L \to \alpha_{1} $$ at 1114 °C) and terminates with the divorced eutectic reaction ($$ L_{2} \to \alpha_{2} + \gamma $$ at 868 °C). During the subsequent cooling process, the discontinuous precipitation ($$ \alpha_{2} \to \alpha_{1} + \gamma $$) takes place in the temperature range from about 700 °C to 600 °C. In addition, the macrosegregation behavior of Sn in Cu-15Ni-8Sn alloy was investigated by adopting vertical unidirectional solidification and measuring the cooling curves at different positions. The results indicated that an inverse macrosegregation of Sn solute exists in the as-prepared ingot, which mainly segregates at the chill surface of the alloy ingot. Namely, the Sn content is higher than 8 wt pct at the chill surface and about 8 wt pct inside the ingot for Cu-15Ni-8Sn alloy.

Formation of intermetallic phases during solidification in Al-Mg-Mn 5xxx alloys with various Mg levels

In the present study, four Al-Mg-Mn 5xxx alloys with different Mg levels (2-5 wt.%) were investigated for better understanding the evolution of intermetallic phases formed during solidification. Optical and scanning electron microscopes, electron backscattered diffraction and differential scanning calorimetry analyses in combination with thermodynamic calculation were used to identify various intermetallic phases. Results showed that the most dominant intermetallic phases are Al6(Mn,Fe), α-Al(Fe,Mn)Si, Al3Fe, Alm(Mn,Fe) and Mg2Si in experimental Al-Mg-Mn alloys, which is greatly dependant on the Mg levels. It is found that Chinese script α-Al(Fe,Mn)Si is the dominant iron-rich intermetallic phase for the alloys containing 2-3 wt.% Mg, while blocky Al6(Mn,Fe) and needle-like Al3(Mn,Fe) become the major phases for the alloy containing 4 wt.% Mg. Further increasing Mg content to 5 wt. %, the dominant phase transfers to blocky Al6(Mn,Fe) intermetallic. Meanwhile, the morphology of primary Mg2Si is changed from well-branched to plate-like with increasing Mg contents. In addition, β-Al3Mg2 and τ-Al6CuMg4 eutectic phases have been observed in the alloys with 3-5 wt. % Mg. A comparison on various intermetallic phases from the Scheil simulation and the actual as-cast microstructure is provided.

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

An Investigation of Creep Resistance in Grade 91 Steel through Computational Thermodynamics

Abstract This study was conducted to understand the relationship between various critical temperatures and the stability of the secondary phases inside the heat-affected-zone (HAZ) of welded Grade 91 (Gr.91) steel parts. Type IV cracking has been observed in the HAZ, and it is widely accepted that the stabilities of the secondary phases in Gr.91 steel are critical to the creep resistance, which is related to the crack failure of this steel. In this work, the stabilities of the secondary phases, including those of the M23C6, MX, and Z phases, were simulated by computational thermodynamics. Equilibrium cooling and Scheil simulations were carried out in order to understand the phase stability in welded Gr.91 steel. The effect of four critical temperatures—that is, Ac1 (the threshold temperature at which austenite begins to form), Ac3 (the threshold temperature at which ferrite is fully transformed into austenite), and the M23C6 and Z phase threshold temperatures—on the thickness of the HAZ and phase stability in the HAZ is discussed. Overall, the simulations presented in this paper explain the mechanisms that can affect the creep resistance of Gr.91 steel, and can offer a possible solution to the problem of how to increase creep resistance at elevated temperatures by optimizing the steel composition, welding, and heat treatment process parameters. The simulation results from this work provide guidance for future alloy development to improve creep resistance in order to prevent type IV cracking.

Selective laser melting enabled additive manufacturing of Ti–22Al–25Nb intermetallic: Excellent combination of strength and ductility, and unique microstructural features associated

To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti–22Al–25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ∼1090 MPa and excellent ductility ∼22.7%; both values are higher than most conventionally fabricated Ti–22Al–25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation and phase constitution. It is proposed that high-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during solidification, particularly regarding the O phase's formation, has also been clarified using in-situ laser heating, high-temperature synchrotron X-ray diffraction and Scheil simulation. It is demonstrated that the O phase formation involves both displacive transition (B2 → B19) and chemical ordering (B19 → O). The metastable B19 phase (as an intermediate stage) may be formed by shearing cubic B2 phase along (110)[1−11]direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.

Analysis of microstructure formation in cast Zn alloys derived from computational thermodynamics of the Zn–Al–Cu–Mg system

A self-consistent thermodynamic description of the Zn–Al–Cu–Mg quaternary alloy system is developed. The as-cast microstructures of key Zn–Al–Cu–Mg samples in the low-alloyed range are studied experimentally. These are interpreted by computational thermodynamic analysis applying both Scheil and equilibrium simulations. Contradictions concerning the variation of apparent fraction of primary (Zn) phase, the eutectic structure and visibly larger fractions of eutectoid structure with increasing Mg content can be resolved by these detailed calculations. Two series of dedicated experimental work found in this journal on as-cast microstructures of such quaternary Zn alloys in the high-alloyed range, including also thermal analysis data, were also analyzed. The viability of this computational thermodynamic approach is demonstrated by providing a detailed analysis going beyond the explanations and interpretations in the original publications. The coverage of a larger Zn alloy composition range suggests a predictive capability of this approach.

Selective Laser Melting Enabled Additive Manufacturing of Ti-22Al-25Nb Intermetallic: Excellent Combination of Strength and Ductility, and Unique Microstructural Features Associated

To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti-22Al-25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ~1090 MPa and excellent ductility ~22.7%; both values are higher than most conventionally fabricated Ti-22Al-25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation analysis and phase constitution analysis. High-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during SLM has also been clarified using in situ heating, high-temperature synchrotron X-ray diffraction and Scheil simulation, particularly regarding the O phase's formation. It is demonstrated that O phase is formed by shearing primary cubic B2 phase along (110)[111] direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.

Effect of Ta and Nb additions in arc-melted Co-Ni-based superalloys: Microstructural and mechanical properties

The current work characterizes the microstructure and mechanical properties of two arc melted Co-Ni-based superalloys containing in atomic percent: 1) Co-40Ni-10Al-7.5W-10Cr-3Ta-0.06B-0.6 C and 2) Co-40Ni-10Al-7.5W-10Cr-3Nb-0.06B-0.6 C. These two materials named 3Ta and 3Nb, respectively, were characterized by X-Ray diffraction analysis, differential scanning calorimetry, scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction and transmission electron microscopy. Furthermore, compression tests were performed from room temperature up to 1000 °C. The 3Ta and 3Nb alloys showed different as-cast microstructures that were confirmed by microstructural characterization and Scheil simulations. The phase transformation temperatures of minor phases were measured and calculated using a thermodynamic approach and some discrepancies observed on the calculated γ′-solvus temperatures reflect that the thermodynamic description of γ′ phase does not take into account the strong partition effect of Ta, Nb and Ti into γ′. As shown by DSC results, the γ′ phase was stable for both alloys up to 1000 °C, being the main reason for the 3Ta and 3Nb alloys to keep the yield stress above 500 MPa.