Calphad-computer Coupling of Phase Diagrams and Thermochemistry最新文献

The phase transformations and pressure-volume dependencies of the Ti–45Al alloy with respect to pressure have been investigated by means of in-situ observation using multi anvil-type high-pressure devices and synchrotron radiation. Under hydrostatic compression from 0 to 10.1 GPa, about 2.3 vol % of γ transforms continuously to α₂. Lattice parameters as well as volume fractions of these two phases have been determined as functions of pressure. Bulk moduli estimated using Birch-Murnaghan's equation of state are 146.2 GPa for the γ phase, 136.7 GPa for the α₂ phase, and 145.6 GPa for their two-phase mixture of Ti–45Al alloy. First-principles have also applied to investigate bulk moduli of two single phases, and the deviation between calculations and measurements is discussed and attributed to mainly phase transformation. The present study provides useful insights into thermodynamics of α₂ and γ phases under high pressure.

First-principles calculations have increasingly become an essential tool for providing additional thermodynamic data for assessing thermodynamic databases using the CALPHAD methodology. As computational power increases and becomes easily accessible, first-principles calculation results have become more presented along with the experimental procedures to determine the thermochemical properties of the phases within a target system. This can help accelerate the alloy development process, even in complex multi-component systems. By using only first-principles calculation data for the both thermodynamic description and interaction parameters of end-members, an Al–Ni–Ti ternary thermodynamic database is constructed. By relying on the existing materials database for ground state stability of the known compounds, the constructed Al–Ni–Ti thermodynamic database can mostly capture all the features related to the solid-state phases appear in the ternary phase diagram comparable to that of the published database from an experimental data assessment.

Phase relations in the C–Fe–Zr ternary system were investigated using the experimental data obtained through the combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) techniques. Isothermal sections of the C–Fe–Zr system were experimentally determined at 1173 K, 1273 K, 1373 K, and 1473 K, and no ternary compound was found in this system. A self-consistent set of thermodynamic parameters of the C–Fe–Zr system were obtained for the first time using the CALculation of PHAse Diagram (CALPHAD) method, and the calculated results showed good consistency with the experimental data. It can provide reliable thermodynamic information of molten corium composition as one of necessary subsystems.

The hot-dip 55 wt%Al–Zn-1.6 wt%Si-(0–3)wt.%Mg alloy coatings were experimentally investigated, and the solidification behaviors and hot cracking susceptibility were simulated by means of CALPHAD (CALculation of PHAse Diagrams) method. The scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA) and glow discharge spectrometer method (GDS) were utilized to determine the microstructures and the distribution of elements of the Al–Zn–Mg alloy coatings with different Mg contents. It is discovered that with the increase of Mg content, the percentage of the eutectic microstructure scales up, and the surface quality of the alloy coating is improved. Meanwhile, the bending properties of Al–Zn–Mg coatings with different Mg contents still requires further improvement according to the present bending test. Subsequently, the equilibrium solidification processes of the coatings were calculated using thermodynamic approach. In general, the calculated results reflect the solidified phases and the precipitated temperatures according to theory of equilibrium solidification. However, there still exist discrepancies between the thermodynamic calculation results and the observed experimental results during the practical galvanizing process, because the cooling rates were not taken fully into consideration. Consequently, the kinetic analysis was carried out to obtain the secondary dendrite arm spacing under different cooling rates. The cracking susceptibility index (CSI) was also calculated to predict the hot workability of the Al–Zn–Mg alloy coating. In summary, the appropriate increase of the cooling rate turns out be the effective approach to benefit the microstructure and the corrosion resistance of the Al–Zn–Mg coatings in the practical galvanizing process, and the coating is not suitable for the hot stamping process.

According to Webb et al. (1986) by heating the Ni${}_3$In (hP8) compound at 800–1200 $°C$ under a pressure ($P$) of 6.5 GPa, a cP4 phase was formed, which reverted to hP8 when annealed at low pressure. A striking X-ray result is that the atomic volume ($V$) of cP4 was higher than that of hP8. Webb et al. determined the $V$ versus $P$ relations in samples quenched to room temperature and reported that the compressibility of the cP4 phase was significantly larger than that of the hP8 phase. These various findings have been theoretically analyzed in the current work by using ab initio density-functional-theory (DFT) calculations, including an account of the vibrational and electronic contributions to the thermodynamic properties. Calculations of the $V$ versus $P$ relations are used to reassess the compressibility relation between the cP4 and hP8 phases, and Gibbs energy calculations to characterize the relative stability of these structures. In particular, detailed comparisons are reported with the only information available on the thermal properties of the hP8 phase, viz., the Gibbs energy estimates obtained in CALPHAD-type phenomenological modeling of the Ni–In equilibrium diagram. The key qualitative result of the current work is that cP4 should be considered as a high-temperature phase, which might be stabilized by heating to the temperature chosen to anneal the hP8 material under pressure. On this basis it is suggested that the seemingly anomalous relative stability relations discussed by Webb et al. might be a consequence of considering only the effect of pressure on the cP4/hP8 relative phase stability.

Diffusion coefficient is an important physical property to control the microstructure, thus the establishment of atomic mobility databank for Ti-based alloy systems is vital to the design of novel Ti alloys. In the present work, ternary Ti–Mo–Nb diffusion couples within the single bcc phase were prepared and measured after annealing at 1273 K for 25 h, and interdiffusion coefficients of bcc Ti–Mo–Nb alloys at 1273 K were determined from the experimental composition profiles by using Matano–Kirkaldy method. Subsequently, the atomic mobility parameters of bcc Ti–Mo–Nb system under the CALculation of PHAse Diagrams (CALPHAD) framework were assessed by using recently proposed diffusion models, which were verified by the comparisons between the experimental interdiffusion coefficients/composition profiles and the model-predicted results. Moreover, diffusion models were carefully discussed for the ternary assessment. Finally, relationships among atomic mobility, kinetic coefficient, and diffusion coefficients were discussed and demonstrated via ternary contour maps. The present work contributes to the development of diffusion databank for computational design of biomedical Ti alloys.

Based on the available experimental data reported in the literature, the La–Co and Ce–Co binary systems were re-assessed thermodynamically using the CALPHAD method in this work. The calculated phase diagrams and thermodynamic properties of the La–Co and Ce–Co binary systems are well consistent with the experimental results. Furthermore, the La–Co–Fe and Ce–Co–Fe ternary systems were calculated by combining the re-assessed La–Co and Ce–Co binary systems in this work with the previous assessments of the La–Fe, Ce–Fe and Co–Fe binary systems. The calculated liquidus projections, isothermal sections and vertical sections in the La–Co–Fe and Ce–Co–Fe ternary systems are in good agreement with the experimental results. The reasonable thermodynamic parameters of the La–Co–Fe and Ce–Co–Fe ternary systems were obtained in this work, which would be fundamental to developing a thermodynamic database of the multi-component RE-TM-Fe alloy systems, and then to designing novel RE₂Fe_17-xTM_x magnets with light rare-earth metals La and Ce.

To determine the homogeneity range of the bcc phase in the Fe–Mo–V system, the isothermal section of the Fe–Mo–V system at 1373 K was constructed by analyzing phase constituents of annealing samples using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Two groups of diffusion couples A and B with the terminal alloys located in the V-rich bcc phase region were prepared and annealed at 1473 K for 96 h and 1373 K for 288 h, respectively. Based on the obtained concentration profiles, the ternary diffusion behaviors of the V-rich bcc phase in the Fe–Mo–V alloys were investigated by the electron probe microanalysis (EPMA) technique combined with the Whittle and Green method. Depending on DICTRA software, the atomic mobility parameters for the bcc phase of the Fe–Mo–V system were optimized. The experimental concentration profiles and diffusion paths in the Fe–Mo–V alloys can be well reproduced using the atomic mobility and thermodynamic parameters.

The columnar-to-equiaxed transition (CET) is known to impact crack formation during the additive manufacturing of metallic alloys. While previous experiments have shown that CET is tunable via its alloying elements, a rigorous multicomponent model to demonstrate the impact of multi-alloying components on CET is still lacking. In this study, we developed a multicomponent model by fully coupling the phase diagram of the kinetic interface condition. Building upon the binary model reported by Gaumann et al. our model replaces the restrictive approach of calculating the non-equilibrium partition coefficient and liquidus slopes with kinetic phase diagram calculation. The extended multicomponent model was validated by comparing it with the Al–Cu results reported by Gaumann et al. CET transition curves were computed for two Al–Cu–Mg–Si–Zn alloys manufactured using laser powder bed fusion. The results are in qualitative agreement with our own and previously reported experimental results. These findings suggest that the proposed multicomponent CET model is a valuable tool for designing AM alloys and optimising processing parameters.

Critical assessment of the thermodynamic data for pure copper was carried using careful analysis of the existing experimental data. An extended Einstein model was used for the crystalline phase and the two state model was applied for the liquid phase. Special attention is paid in this work to the precise description of the following thermodynamic functions: S^o₂₉₈, H^o₂₉₈–H^o₀, the melting temperature, and the entropy and enthalpy of fusion. In order to fullfill the need for a precise evaluation of S^o₂₉₈ we needed to use an additional technique, which allows the experimental heat capacity and enthalpy data for the solid phase to be approximated accurately from 0K up to the melting point. Relative stabilities of the BCC_A2 and HCP_A3 phases were derived.