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The present study explores the high temperature deformation behavior of FeCoNiCrAlTi dual phase high entropy alloy (henceforth referred to as DP-HEA) in the temperature range of 900–1100 °C (1173–1373 K) and the strain rate varying from 0.001 to 0.1 s−1. The as-cast sample has been characterized using scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry, which reveals the presence of two phases, disordered γ and ordered γ′. A constitutive relationship between the process parameters (stress temperature, strain, and strain rate) has been drawn using the Arrhenius-type equation to recognize the high temperature deformation behavior of the DP-HEA. The optimum thermomechanical processing window of the DP-HEA has been determined by constructing multiple contour maps based on different parameters such as efficiency, strain rate sensitivity, etc. And the optimum processing domain has been found to lie approximately in the temperature range of 1260–1300 K and SR = 10–2.3–10–2 s−1, 1325–1373 K and SR = 10–1.4–10–1 s−1 & 1173–1193 K and SR = 10–1.3–10–1.55 s−1. Finally, the stable and unstable regimes in the processing maps are correlated with the microstructure of hot deformed samples. [ABSTRACT FROM AUTHOR]

Microstructure and texture evolution of the Fe11.5Co20.6Ni40.7Cr12.2Al7.8Ti7.2 (at. pct) high-entropy alloy during the high-temperature deformation has been investigated in the temperature range of 1173 K to 1373 K and strain rate of 0.1 to 0.001 s−1. The stress–strain curve obtained from the deformation indicates significant flow softening at low temperatures. The softening at 1173 K is due to cracking, whereas high-temperature softening is attributed to dynamic recrystallization (DRX). Arrhenius-type sine hyperbolic relationship is used to carry out the flow stress analysis, and the predicted flow stress shows good agreement with the experimental results with an accuracy of (R2 = 0.955), especially when the deformation takes place at a low strain rate. The estimated strain hardening exponent, n (> 3), and activation energy, Q (> 400 kJ/mol), indicated that the deformation mechanism is dislocation controlled. Detailed microstructural and textural characterization of the hot deformed sample has been carried out using EBSD analysis. Microstructural investigation confirms that dynamic recrystallization is the primary reason behind the flow softening for the samples deformed at 1273 K and above. Strain-free recrystallized grains are found to nucleate near the grain boundary region. Furthermore, the size of the recrystallized grains increases with an increase in temperature and a decrease in strain rate. The volume fraction of the recrystallized grains is found to decrease with an increase in the Zener Holloman parameter. DRX grain was found to possess a weak texture with a low texture index. [ABSTRACT FROM AUTHOR]

High-pressure die casting (HPDC) is a widely used process with short cycle times to manufacture complex shapes of aluminium castings for the automotive industry. Most of the die casting companies use specialized commercial software to simulate molten metal filling and solidification. This approach works satisfactorily and is industrially accepted for defect prediction capability. Die cooling is an effective technique to reduce internal porosity in die cast components. But die cooling process is not completely captured in the approach used in the casting simulations. This often resulted in false positives in the defect predictions. One of the main reasons for the lack of precision in simulation is the uncertainty in assigning the boundary conditions such as heat transfer coefficients (HTC) for the die cooling channel. In this study, a coupled simulation strategy was developed to determine accurate HTC values of the die cooling channel. The computational domain was divided into two sub-domains, i.e., casting zone and die cooling zone. Two different industrially accepted simulation tools were used: (a) for casting simulation and (b) for turbulent fluid flow and heat transfer simulation in the cooling channel. A series of iterations were performed by exchanging information at the interface between the two sub-domains till the convergence was achieved. Experiments were also conducted with a thermocouple inserted in the die and the actual die temperature readings were measured. The converged simulation results agreed well with the experimental measurements. Also, the influence of flow rate on HTC was studied by conducting experiments with two different flow rates and the castings produced were analyzed with the help of CT scan analysis, micro-structure evaluation and thermal imaging. The obtained results demonstrated the efficacy of the method adopted in prescribing the values of heat transfer conditions in casting simulations. With the coupled simulation approach developed in this work, parametric studies were conducted to maximize the heat removal rate from a given die, for different flow velocities, nozzle diameters, cooling hole diameters and nozzle-to-surface spacings. [ABSTRACT FROM AUTHOR]

Rapid solidification during metal additive manufacturing (AM) leads to non-equilibrium microsegregation, which can result in the formation of detrimental phases and cracking. Most of the microsegregation models assume a Scheil-type solidification, where the solidification interface is planar and there exists a local equilibrium at the interface along with either zero or infinite solute diffusion in the respective participating phases—solid and liquid. This assumption leads to errors in prediction. One has to account for finite solute diffusion and the curvature at the dendritic tip for more accurate predictions. In this work, we compare different microsegregation models, that do and do not consider finite diffusion and dendrite tip kinetics, against experiments. We also propose a method to couple dendrite tip kinetics with the diffusion module (DICTRA®) implemented in Thermo-Calc®. The models which accounted for both finite diffusion and dendrite tip kinetics matched well with the experimental data. [ABSTRACT FROM AUTHOR]

Haynes 282 is a creep-resistant, γ′ strengthened nickel-based superalloy with excellent weldability. Additive manufacturing, especially laser powder bed fusion (LPBF), of Haynes 282 is being researched for its potential applications in aerospace and power generation sectors. LPBF enables us to achieve desired properties via microstructure control through process parameter optimization. Integrated computational materials engineering (ICME) approach can be used to tune the process parameters to obtain desired microstructure, instead of trial-and-error methods. In this report, ICME framework has been established to simulate microstructural evolution during LPBF and post-processing of Haynes 282. A dimensionless number was used to optimize the process parameters with minimal porosity. Thermal modelling was performed using a semi-analytical model and the obtained thermal data was used to simulate solidification during LPBF. The corresponding cell spacing obtained matched closely with the experimental data. The segregation pattern extracted from the phase-field model was used as input for homogenization simulation. The γ′ precipitate evolution was predicted using Langer–Schwartz theory and Kampmann–Wagner-Numerical (KWN) approach. The predicted values agreed with the experimentally measured values. The room temperature yield strength of as-built samples was comparable and that of heat-treated samples were superior to the aged wrought material. [ABSTRACT FROM AUTHOR]

Eutectic high entropy alloy with seven components is designed based on the integrated computational materials engineering (ICME) framework. The framework includes thermodynamic prediction using calculation of phase diagrams (CALPHAD), microstructure simulation using phase-field method, and experimental validation. The designed alloy shows the eutectic structure consisting of FCC and laves phase in the composition range from 8.25 to 10 at. pct Ta. The simulation and experimental results are co-related and a framework is proposed that can be used for high entropy alloy design subjected to various manufacturing processes. [ABSTRACT FROM AUTHOR]

Undercooling studies were carried out in equiatomic CuCoNi system with an aim to understand the growth kinetics and microstructural variations as a function of undercooling. The morphological change in microstructure was observed and correlated with the undercooling obtained. Non-linear variation of growth velocity with respect to the increase in undercooling was obtained from high-speed video measurements. The variation in growth velocity was compared with the dendritic growth model with a modified kinetic undercooling term. The segregation profile was predicted using multi-phase field method and compared with the experimental data. Micro-hardness variation was correlated with the undercooling obtained. [ABSTRACT FROM AUTHOR]

Aerospace and launch vehicles structures demands materials with low density and high strength. 2195 is an advanced Al-Cu-Li alloy, considered as a potential material for launch vehicle structures due to low density and high strength. However, the fusion weldability of this alloy is not as simple as conventional 2219 alloy. Porosity and solidification cracking are main concerns in 2195 alloy with respect to weldability. In this present work, weldability issues are addressed to minimise their effect. 2195 alloy is welded by gas tungsten arc welding (GTAW) and friction stir welding (FSW) using optimised process parameters. Weld tensile properties of GTA and FSW welds were evaluated in as-weld condition. Hardness survey across the welds was performed. Microstructural analysis of GTA and FSW welds were carried out. Effect of tool rotation speed and tool travel speed on mechanical properties was studied. [ABSTRACT FROM AUTHOR]

A eutectic high-entropy alloy with seven components (Fe, Ni, Cr, V, Co, Mn, and Nb) was designed via the melt route guided by CALPHAD predictions. Configurational entropy estimated for the two-phase microstructure qualifies it to be referred to as a high-entropy alloy. When the Nb content exceeded 9.7 at. pct, the microstructure changed from hypoeutectic with primary face-centered cubic phase to hypereutectic with primary Laves phase. [ABSTRACT FROM AUTHOR]

We present experimental results, analytical calculations and phase-field simulations for undercooled Ni–Fe–Si alloy system. Undercooling experiments are performed using flux encapsulation along with in situ measurement of recalescence speed using a high-speed camera followed by microstructural characterization. Dendrite growth calculations are performed using a modified Boettinger, Coriell and Trivedi theory to incorporate constitutional undercooling due to multiple segregating elements and a modified kinetic undercooling term. Phase-field simulations are performed using a multi-component phase-field model to generate dendrites in this alloy. High growth velocities are observed and the analytical calculations are in good agreement with experiments. The microstructure evolution from the phase-field simulations indicates that there is a difference in solute segregation during growth of dendrites. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'. [ABSTRACT FROM AUTHOR]

In this study, weld solidification cracking behaviour of AA2195 Al-Cu-Li alloy was studied and compared with conventional AA2219 and AA2014 aluminium alloys. Cracking susceptibility was evaluated using varestraint test and Gleeble® hot ductility test and the slope of liquidus temperature as function of liquid fraction was also evaluated. Solidification cracking susceptibility of AA2195, AA2219 and AA2014 alloys was ranked based on the above methods. Consistent trend in cracking susceptibility was observed in all the methods where AA2195 and AA2219 alloys showed highest and lowest cracking susceptibility, respectively. [ABSTRACT FROM AUTHOR]

The present investigation reports phase and microstructure evolution during solidification of novel Ni-rich Ni-Ti-based alloys, NiTi, NiCuTi, NiCuCoTi, and NiCuCoTiTa during suction casting. The design philosophy of the multicomponent alloys involves judicious selection of alloying elements such as Cu, Co, and Ta in the near NiTi eutectic alloy by replacing both Ni and Ti so that phase mixture in the microstructure remains the same from the binary to quinary alloy. The basic objective is to study the effect of addition of Cu, Co, and Ta on the phase evolution and transformation in the Ni-rich Ni-Ti-based alloys. The detailed electron microscopic studies on these suction cast alloys reveal the presence of ultrafine eutectic lamellae between NiTi and NiTi phases along with dendritic NiTi and TiNi phases. It has also been observed that in the binary (NiTi) alloy, the ordered NiTi (B2) phase transforms to trigonal (R) phase followed by NiTi martensitic phase (M-phase), i.e., B2 → R-phase → M-phase during solid-state cooling. However, the addition of alloying elements such as Cu, Co to the binary (NiTi) alloy suppresses the martensitic transformation of the ordered NiTi (B2) dendrite. Thus, in the ternary and quaternary alloys, the ordered NiTi (B2) phase is transformed to only trigonal (R) phase, i.e., B2 → R-phase. The secondary precipitate of TiNi has been observed in all of the studied alloys. Interestingly, NiCuCoTiTa quinary alloy shows the disordered nature of NiTi dendrites. The experimentally observed solidification path is in good agreement with Gulliver-Scheil simulated path for binary alloy, whereas simulated solidification path deviates from the experimental results in case of ternary, quaternary, and quinary alloys. [ABSTRACT FROM AUTHOR]

In this investigation, crystal-melt interface growth velocities of Ni-Zr alloys are investigated at different undercoolings using molecular dynamics simulation. Ni, Ni-0.1 at.% Zr and Ni-1 at.% Zr are considered and free solidification approach is adopted to measure the interface growth velocity. Further, the anisotropy in growth velocity is studied by considering three different crystalline planes, viz., (100), (110) and (111), interfacing with the liquid. MD simulations are carried out using LAMMPS software and the Finnis-Sinclair interatomic potential developed by Wilson and Mendelev is used to describe the interatomic interactions. It is observed that addition of small amount of impurity (0.1 % Zr) increases the interface growth velocity of pure Ni during solidification. However, with further increase in the solute content (1 % Zr), interface growth velocity decreases. In all the cases, (100) is the fastest growing interface while the close packed (111) has the slowest velocity. Further, the calculated interface velocities are compared with the predicted ones from the theoretical model of Broughton-Gilmer-Jackson. [ABSTRACT FROM AUTHOR]

The present work is aimed at understanding the phase evolution of hypereutectic Al-rich Al-Cu-Ni in situ composites consisting of ultrafine eutectic with micron scale dendrites. The hypereutectic AlCuNi (x = 0, 5) are synthesized by vacuum arc melting technique under high purity argon atmosphere. Detailed X-ray diffraction and electron microscopic (SEM and TEM coupled with EDS) studies have been carried out to identify the phases in these alloys. Thermodynamic calculations performed using CALPHAD approach using ThermoCalc software predict the equilibrium solidification path. The AlCuNi alloy exhibits ternary quasi-peritectic reaction of L + AlNi → (Al) + AlNi at invariant point, which is supported by two univariant reactions i.e. L + AlNi → AlNi and L → (Al) + AlNi above and below the invariant point respectively. [ABSTRACT FROM AUTHOR]

Compositional instability and phase formation in Ni-Fe-Ga Heusler alloys are investigated. The alloys are synthesized into two-phase microstructure. Their structures are identified as fcc and L, respectively. The γ-phase formation could be suppressed with higher Ga-content in the alloy as Ga stabilizes austenite phase, but Ga lowers the martensite transformation temperature. The increase of Fe content improves the magnetization value and the increase of Ni from 52 to 55 at. pct raises the martensite transformation temperature from 216 K to 357 K (−57 °C to 84 °C). Magnetic properties and martensitic transformation behavior in Ni-Fe-Ga Heusler alloys follow opposite trends, while Ni replaces either Fe or Ga, whereas they follow similar trends, while Fe replaces Ga. Modulated martensite structure has low twinning stress and high magneto crystalline anisotropic properties. Thus, the observation of 10- and 14 M-modulated martensite structures in the studied Ni-Fe-Ga Heusler alloys is beneficial for shape memory applications. The interdependency of alloy composition, phase formation, magnetic properties, and martensite transformation are discussed. [ABSTRACT FROM AUTHOR]

Melt spinning technique was used to synthesize Ni2MnGa ferromagnetic shape memory alloy ribbons. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis of the ribbon synthesized at lower wheel speed (20 m/s) reveal the formation of very fine clusters of austenitic phase of Ni2MnGa. However at higher wheel speed (30 m/s) the formation of martensite and nanoparticles of Ni2MnGa with a size range of 10–20 nm in the amorphous matrix is observed. Also an amorphous phase was observed at higher wheel speed in some areas of the ribbon. Annealing (1000 °C, 1 h) of the ribbon synthesized at higher wheel speed resulted in martensite and γ (gamma) phases. Amorphous phase, Ni2MnGa nanoparticles, and the martensite phase are analyzed in detail. [ABSTRACT FROM AUTHOR]

Continuous CO2 laser welding of an Fe-Cu dissimilar couple in a butt-weld geometry at different process conditions is studied. The process conditions are varied to identify and characterize the microstructural features that are independent of the welding mode. The study presents a characterization of the microstructure and mechanical properties of the welds. Detailed microstructural analysis of the weld/base-metal interface shows features that are different on the two sides of the weld. The iron side can grow into the weld with a local change in length scale, whereas the interface on the copper side indicates a barrier to growth. The interface is jagged, and a banded microstructure consisting of iron-rich layers could be observed next to the weld/Cu interface. The observations suggest that solidification initiates inside the melt, where iron and copper are mixed due to convective flow. The transmission electron microscopy (TEM) of the weld region also indicates the occasional presence of droplets of iron and copper. The microstructural observations are rationalized using arguments drawn from a thermodynamic analysis of the Fe-Cu system. [ABSTRACT FROM AUTHOR]