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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. Refereed/Peer-reviewed

Metastable alloys with transformation-/twinning-induced plasticity (TRIP/TWIP) can overcome the strength-ductility trade-off in structural materials. Originated from the development of traditional alloys, the intrinsic stacking fault energy (ISFE) has been applied to tailor TRIP/TWIP in high-entropy alloys (HEAs) but with limited quantitative success. Here, we demonstrate a strategy for designing metastable HEAs and validate its effectiveness by discovering seven alloys with experimentally observed metastability for TRIP/TWIP. We propose unstable fault energies as the more effective design metric and attribute the deformation mechanism of metastable face-centered cubic alloys to unstable martensite fault energy (UMFE)/unstable twin fault energy (UTFE) rather than ISFE. Among the studied HEAs and steels, the traditional ISFE criterion fails in more than half of the cases, while the UMFE/UTFE criterion accurately predicts the deformation mechanisms in all cases. The UMFE/UTFE criterion provides an effective paradigm for developing metastable alloys with TRIP/TWIP for an enhanced strength-ductility synergy.

Effect of multiple repair welding has been investigated on the crack susceptibility and mechanical properties of Inconel 718 (IN718) alloy casting. The results show a dumbbell-shaped fusion zone (FZ) with thick ends and thin in the middle portion, depending on the number of repair welding cycles. The width and length of heat-affected zone (HAZ) cracks are significantly larger than that of FZ cracks, indicating that HAZ cracks are quite severe in multiple repair welding of IN718 alloy. HAZ cracks play a significant role in decreasing the mechanical properties. The stress rupture life sharply reduces after repair welding. The hardness of FZ is close to that of HAZ and base metal (BM) region, and only a minor decrease in hardness was found from BM to FZ after linear fitting. With the increase in repair welding times, the residual stress formed during welding increases gradually based on finite element analysis. The repair welding cracking is mainly controlled by local residual stress and crystal orientation based on this study. Both FZ and HAZ crackings increase with the increase in the number of repair welding cycles; thus, it must be limited to three for IN718 alloy casting.

In order to investigate the deterioration of microstructure in service, the intrinsic microstructural stability and performance of DD6 during long-term aging without an external load are investigat...

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 solution treatment on the micropore and mechanical properties is investigated for the DD6 superalloy. The results show that solution treatment has positive and negative effects on the DD6...

High-strength low-alloy (HSLA) steels often comprise of Cu clusters and M2C (M: Mo, Cr) carbides as strengthening particles. In this work, three new HSLA steels with alternative strengthening phases, Fe2SiTi and Ni3Ti, are investigated by using HSLA-115 steel as the reference. To evaluate the weldability for potential fabrication using casting, welding, and additive manufacturing, freezing ranges are studied using differential thermal analysis (DTA), and CALPHAD (Calculation of Phase Diagrams) approach under equilibrium and nonequilibrium conditions. While the cooling signals in the DTA analysis are not pronounced enough for thermal analysis, the trend of freezing range change based on the nonequilibrium and equilibrium calculations are consistent with the heating signals. High-throughput calculations are performed to deduce the effect of variation of each alloying element on the freezing range. Moreover, the experimental and calculated phase fractions and compositions of the as-cast and heat-treated alloys were compared. Though CALPHAD model-prediction can provide valuable insights into the phase stability of these new alloys, there is a remarkable difference regarding phase fraction and composition of individual phases. Therefore, this study indicates that the application of the CALPHAD approach in new alloy discovery requires a careful model-validation and database calibration.

In this work we report the thermodynamic modelling of the Al-N, B-N and Al-B-N systems by the Calphad approach, making use of the ab initio calculations and the available experimental thermochemical and phase equilibria data. Gibbs energy description for the Al-B system is taken from the literature, with a modification for the model for the liquid phase. The enthalpy of formation and the heat capacity of various polymorphs of AlN and BN are estimated using the DFT and the phonon calculations, respectively. The enthalpy of mixing of the mixed nitride (Al,B)N in wurtzite and cubic structures are obtained using the SQS method. Gibbs energy of formation of the mixed nitride from the binary nitrides is also computed using the phonon calculations. These quantities along with the critically assessed thermochemical and constitutional data are used for obtaining the Gibbs energy functions of various phases in the Al-N, B-N, and Al-B-N systems. Calculated phase equilibria and thermochemical data are compared with the experimental data.

Thermal shock resistance of precursor derived Si-Hf-C-N(O) foams at temperatures varying from 800o-1000oC subjected to multiple thermal cycles was investigated. The as-synthesized foams possessed i...

Thermal interface material (TIM) that can exist as liquid at the service temperature enables efficient heat transfer across two adjacent surfaces in electronic applications. In this work, the thermal conductivities of different phase regions in the Ga-In system at various compositions and temperatures are measured for the first time. A modified comparative cut bar technique is used for the measurement of the thermal conductivities of InxGa1-x (x=0, 0.1, 0.214, 0.3, and 0.9) alloys at 40, 60, 80, and 100oC that are the temperatures commonly encountered in consumer electronics. The thermal conductivity values for the liquid and semi-liquid (liquid+β) Ga-In alloys are higher than the TIM currently used in consumer electronics. These measured quantities, along with the available experimental data from the literature, served as input for the thermal conductivity parameter optimization using the CALPHAD (CALculation of PHase Diagram) method for the pure elements, solution phase, and two-phase region. A set of self-consistent parameters for the description of the thermal conductivity of the Ga-In system is obtained. There is good agreement between the measured and calculated thermal conductivity values for all the phases. Hence, it can be envisaged that liquid/semi-liquid Ga-In alloys can be considered as a potential TIM in consumer electronics due to its high thermal conductivity.

For the first time, cyclic re-austenitization is carried out for additively manufactured high-strength low-alloy (HSLA) steels in order to refine the microstructure by reducing the prior austenite grain (PAG) size. In this work, HSLA-100 steels processed using laser powder bed fusion (LPBF) technique are subjected to several cycles of re-austenitization using quenching dilatometry. Microstructure characterization for every cycle revealed the presence of bainite, martensite and martensite/austenite (M/A) islands. From the analysis of the dilatometry curves and extensive microstructure characterization, it was found that till the 2nd cycle of re-austenitization, both PAG size and martensite start (Ms) temperature get reduced, while the amount of bainite transformed decreased and the retained austenite content increased. Concomitantly, the highest microhardness along with peak nanohardness of the constituent phases was achieved at the 2nd cycle. Conversely, from the 3rd cycle, the microhardness, as well as the nanohardness of the constituent phases, are found to decrease due to an increase in the PAG size. This behavior is in contrast to the general tendency where a saturation limit is reached after the peak refinement is achieved. It is found that retained austenite can act as a pinning particle to obstruct the PAG boundary movement and its fraction is found to decrease from the 3rd cycle. Hence, the increase in PAG size after the 3rd cycle can be attributed to the destabilization of effective pinning particles to hinder the PAG boundary movement during the re-austenitization., 30 pages, 12 Figures

Gibbs energy model parameters for Si-Zr-N system were obtained using Calphad approach coupled with ab initio calculations. The enthalpies of formation of α and β Si3N4 in Si-N system and Zr5Si3N end-member in Si-Zr-N system were calculated using density functional theory (DFT). The finite temperature thermodynamic properties were calculated using quasiharmonic approximation (QHA). The computed heat capacities were fitted to appropriate expressions valid down to 0 K. The ab initio thermochemical data obtained in the present work and the experimental thermochemical and constitutional data from literature were used for the thermodynamic optimization of Si-N and Si-Zr-N systems. The calculated phase equilibria and thermodynamic properties are in good agreement with the input data.

Electrolysis of molten fluorides is one of the promising methods for the recovery and recycling of rare earth metals from used magnets. Due to the dearth of phase equilibria data for molten fluoride systems, thermodynamic modelling of LiF-DyF3-NdF3 system using the CALPHAD approach was carried out. Gibbs energy modelling for LiF-NdF3 and LiF-DyF3 systems was performed using the constitutional data from literature. Ab initio calculations were used to obtain enthalpy of reaction of LiDyF4, an intermediate phase that is found to exist in the LiF-DyF3 system. Differential thermal analysis was carried out for selected compositions in the NdF3-DyF3 system, in order to determine liquidus and solidus temperatures. The Gibbs energy parameters for the limiting binaries determined in this work is used for modelling the Gibbs energy functions of equilibrium phases in the ternary system. Selected compositions of LiF-NdF3-DyF3 were subjected to DTA in order to validate the calculated phase temperatures involving melt.

Although bimetallic structures can offer unique solutions to engineering problems with varying functionality, joining two dissimilar alloys using additive manufacturing involves challenges such as cracking and segregation. In this work, bimetallic structures of Inconel 740H superalloy and P91 steel are processed using wire-arc additive manufacturing (WAAM) for the first time. P91 steel is directly deposited over 740H with a cooling time of 20 minutes in between the two depositions. During microstructure characterization, a large gradient zone with coarse grains spanning for ∼2200 μm is observed. Moreover, the microhardness of the gradient zone is the least in comparison with the P91 and 740H deposits. No deleterious phase formation is found; however, intergranular cracks are observed along the gradient zone. In order to deduce the reason for cracking, non-equilibrium solidification simulations are performed for the mixed compositions of P91 and 740H in the gradient zone. The reasons for crack formation in the gradient zone are the sudden change in the volumetric coefficient of thermal expansion due to the formation of MC carbides as well as the development of local strains from the thermal residual stresses. Therefore, altering the deposition sequence and introducing gradient layers with mixed composition between the P91 and 740H deposits will be beneficial in achieving defect-free builds. Through the integration of experiments and simulation, this work gains valuable insights into the processability of dissimilar alloy manufacturing using the WAAM technique.

Thermodynamic modelling of Ti-Zr-N system is performed using Calphad method coupled with ab initio calculations. The energies of formation of stable and metastable end-members of sublattice formulations of solid phases in Zr-N system and enthalpy of mixing of the mixed nitride (Ti, Zr)N ( δ ) are calculated using ab initio method. Phonon calculations are used to compute the Gibbs energies of stoichiometric ZrN and the mixed nitride δ . With the aid of experimental thermochemical and constitutional data from the literature along with the results of ab initio calculations, thermodynamic optimization is carried out to obtain the Gibbs energy model parameters.

A novel post-heat treatment design is implemented for additively manufactured copper-bearing high-strength low-alloy (HSLA) steels by understanding the processing-structure-property relationships. Hot isostatic pressing (HIP) is adopted to reduce the porosity from 3% to less than 1% for the HSLA-100 steel processed using laser powder bed fusion (LPBF). Quenching dilatometry is employed to design the parameters for the HIP cycle with an optimized cooling process. In order to achieve the maximum amount of martensite, drop cooling after HIP is found to be more suitable than controlled cooling. A subsequent cyclic re-austenitization is introduced to achieve effective grain refinement to compensate for the coarsened microstructure after HIP. The re-austenitization effectively leads to a 60% reduction in the prior austenite grain (PAG) size. The microstructure of the as-built and HIP HSLA steels before and after cyclic re-austenitization consists of martensite, bainite and martensite/retained austenite (M/A) islands. Tempering heat treatment is applied after HIP, to induce strengthening due to precipitation hardening, that is optimized through microhardness and microstructure characterization. The peak hardness is achieved at 5 h of aging and the microstructure consists of tempered martensite, bainite, and M/A islands. A significant increase in the tensile yield strength, as well as the ductility, is achieved in comparison with the as-built alloy with the tailored microstructure obtained after the application of the designed post-heat treatment to HSLA-100 steels processed using LPBF. As LPBF of HSLA steel is attempted for the first time, this work demostrates the significance of post-heat treatment design for the AM technique.

Polymer derived Si-Hf-C-N(O) foams were produced by the pyrolysis of polysilazane containing varying amounts of hafnium tetra n -butoxide. X-ray tomography was used to determine the cell size distribution. The ablation resistance of the foams was studied by subjecting it to different oxyacetylene flame temperatures. After ablation, fragmentation was not observed and the mass ablation rate was found to be minimal for the foams produced from highest vol% of hafnium tetra n -butoxide. The foams remained X-ray amorphous after ablation and the presence of Hf O and Si O bonds were confirmed using ATR spectroscopy. For comparison, a reasonably dense spark plasma sintered pellet was also ablated and the evolution of monoclinic hafnia was confirmed using X-ray diffraction. The microstructural characterization exhibited three different zones on the ablated surface for sintered sample and the ablation mechanisms were understood using thermodynamic calculations.

The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.

Epitaxial columnar grain growth is a prevalent microstructural feature in the additive manufacturing (AM) of metal components such as Inconel, with cubic unit cell crystal lattice structure (face centered cubic (FCC) or body centered cubic (BCC)). These columnar grains evolve from the partly molten grains in the substrate or the solidified metal. This work proposes an efficient model to simulate the competitive growth of epitaxial columnar dendritic grains. The proposed model tracks the dynamic changes in the dendrites emanating from discrete points along the solid/liquid interface of a quasi-steady melt pool (MP). These dynamic changes include convergence and divergence of growing dendrites. The model is extended to predict the microstructure of large 3D parts and experimentally validated by comparing the simulation results for laser powder bed fusion (L-PBF) and wire-arc additive manufacturing (WAAM) processes. The microstructure and pole figures are predicted for Inconel 718 samples produced by L-PBF and Inconel 740H samples produced by WAAM processes. The model predictions compare well with the observed microstructure and pole figures results for both the L-PBF and WAAM processes.

In this study, the effect of Fe in precipitation strengthened Al-Co-Cr-Fe-Ni-Ti complex concentrated alloys (CCAs) is investigated. The amount of Fe significantly affected the γ′ morphology, microstructural stability, lattice misfit, solvus temperature and mechanical behaviour. The decrease in the Fe content from 11.5 (Al7.8Co20.6Cr12.2Fe11.5Ni40.7Ti7.2) to 0 (Al7.8Co20.6Cr12.2Ni52.2Ti7.2) at% changed the precipitate morphology from spherical to cuboidal upon 50h ageing at 900°C. Also, the lattice misfit and solvus temperature of the alloy increased from 0.097 to 0.41% and 1109 to 1199°C, respectively, with the complete removal of Fe from the alloy system. Coarsening studies at 900°C (up to 550h) revealed that the γ′ coarsening was driven by evaporation and condensation (EC) mechanism in Al7.8Co20.6Cr12.2FexNi52.2-xTi7.2 (x=11.5, 5 and 0, in at%) alloys. The coarsening rate constants (Kr) were calculated for the three alloys; among them, the alloy without Fe showed the least value (2.83 × 10−27 m3 s−1). The obtained results on the effect of Fe in L12 strengthened CCAs offer a new platform for the development of advanced high-temperature alloys.

Water repellency, exemplified in nature, has fascinated the materials community. This behaviour is utilised to biomimic self-cleaning effects for technological applications. Tropical leaves of three species, namely Moringa oleifera, Euphorbia milii and Caesalpinia ferrea, are investigated in this study. Wettability measurements show that C. ferrea leaf was superhydrophobic with a contact angle greater than 150°, while the other two leaves were hydrophobic with contact angles greater than 140° but less than 150°. Surface morphological studies reveal a structure with irregular wax structures on all the three leaves, unlike the regular wax structures such as tubules or platelets typically observed in most of the superhydrophobic leaves. The surface features were quantified on C. ferrea leaf surface due to its reduced wettability among the three leaves. The parameters such as height, mid-width, and radius of curvature of the cell and thickness of the wax film were determined. This study will possibly provide a new insight into developing self-cleaning surfaces with irregular nanostructures as opposed to conventional regular nanostructure patterns.

In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach based on special quasirandom structures (SQS) was used for calculating the enthalpy of mixing of the fcc solid solution. The vibrational contribution to the heat capacities of NiZr, NiZr2, Ni3Zr and Ni7Zr2 phases were calculated using the quasiharmonic approximation (QHA) and the corresponding electronic contribution was obtained using an approach based on Mermin statistics. The total heat capacities for these phases were fitted to appropriate expressions and integrated to obtain the Gibbs energy functions valid down to 0 K. The calculated thermochemical properties along with critically selected experimental constitutional and thermochemical data served as input for the thermodynamic optimisation of the system. The calculated phase equilibria and the thermodynamic properties using the optimised Gibbs energy functions are in good agreement with the input data. The calculated congruent melting points of NiZr and NiZr2 phases are close to the recent experimental data. The Ni10Z7 phase forms by a peritectic reaction, which is also in agreement with the experimental data.