Microstructures and properties of high-entropy alloys

An assessment of the entire Al-Fe system including D03 ordering

Effects of Si additions on intermetallic compound layer of aluminum–steel TIG welding–brazing joint

The Al-rich part of the Fe-Al phase diagram between 50 and 80 at.% Al including the complex intermetallic phases Fe5Al8 (e), FeAl2, Fe2Al5, and Fe4Al13 was re-investigated in detail. A series of 19 alloys was produced and heat-treated at temperatures in the range from 600 to 1100 °C for up to 5000 h. The obtained data were further complemented by results from a number of diffusion couples, which helped to determine the homogeneity ranges of the phases FeAl2, Fe2Al5, and Fe4Al13. All microstructures were inspected by scanning electron microscopy (SEM), and chemical compositions of the equilibrium phases as well as of the alloys were obtained by electron probe microanalysis (EPMA). Crystal structures and the variation of the lattice parameters were studied by x-ray diffraction (XRD) and differential thermal analysis (DTA) was applied to measure all types of transition temperatures. From these results, a revised version of the Al-rich part of the phase diagram was constructed.

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The Al-Rich Part of the Fe-Al Phase Diagram

First-principles studies of Al–Ni intermetallic compounds

A thermodynamic description of the Al–Fe–Si system over the whole composition and temperature ranges via a hybrid approach of CALPHAD and key experiments

Thermodynamic properties of the Al–Fe–Ni system acquired via a hybrid approach combining calorimetry, first-principles and CALPHAD

The V–Si system is reassessed based on a critical literature review involving recently reported data and the present experimental data. These new data include the thermodynamic stability of V 6Si5 and the enthalpies of formation for the compounds calculated by first-principles method. Two alloys were prepared in the region of (Si)+V Si2 and annealed at 1273 K for 14 days. After X-ray diffraction (XRD) and chemical analysis of these alloys were performed, the eutectic reaction (L⇔(Si)+V Si2) temperature was determined by differential thermal analysis (DTA). Self-consistent thermodynamic parameters for the V–Si system were obtained by optimization of the selected experimental values. The calculated phase diagram and thermodynamic properties agree well with the experimental ones. Noticeable improvements have been made, compared with the previous assessments.

Thermodynamic modeling of the V–Si system supported by key experiments

NiAl and NiAl based ternary metallic systems have attracted much attention in recent years for potential use as high temperature structural materials [1–6]. Alloy development of such systems requires a thorough knowledge of the phase equilibria and thermodynamics of the systems. Using high temperature calorimeters [7–9], the enthalpies of formation for many alloy systems have been measured and the results incorporated into thermodynamic databases for modeling of their phase diagrams using software such as Thermocalc©R [10]. However, there exists some confusion regarding the value for the enthalpy of formation of NiAl. In one reference [11], comparison of published experimental data was made which are referred to different standard states, with the consequent conclusion that there is a wide discrepancy in the experimental NiAl data. In this paper, the enthalpies of formation of NiAl as a function of composition determined by two different calorimetric techniques are compared. The results are also compared to enthalpy of formation data for the stoichiometric alloy from several other sources. Two kinds of calorimeters are widely used in measuring the enthalpies of formation of intermetallic compounds with high melting points: the differential solution calorimeter and the direct synthesis calorimeter. The former is an indirect method because the compound is prepared before the enthalpy of formation is determined, while the latter is a direct one since the enthalpy of formation is determined during alloy formation. Using the differential solution calorimeter, Henig and Lukas [12] determined the enthalpy of formation of NiAl at 1100 K. During their experiment, the element Ni and the compound NiAl were dropped into the liquid aluminum separately and their heat of dissolution were measured in turn. The reactions involved in the measurement are:

The enthalpy of formation of NiAl

The Al-Ni-Ti ternary system forms the basis of several alloy systems of practical importance. This study was undertaken to provide additional experimental thermodynamic data for the modeling of the Al-Ni-Ti system. High-temperature direct reaction calorimetry has been used to determine the standard enthalpy of formation of several Al-Ni-Ti alloys. The stability of the L21 structure over the B2 structure at the stoichiometric composition of Ni0.5Al0.25Ti0.25 was estimated from the enthalpy results to be 6.8 kJ/mol. The lattice parameters and melting temperatures of these compounds are also reported.

Rongxiang Hu

Papers

A thermodynamic description of the Al–Fe–Si system over the whole composition and temperature ranges via a hybrid approach of CALPHAD and key experiments

Thermodynamic properties of the Al–Fe–Ni system acquired via a hybrid approach combining calorimetry, first-principles and CALPHAD

Thermodynamic modeling of the V–Si system supported by key experiments

The enthalpy of formation of NiAl

Enthalpy of Formation in the Al-Ni-Ti System