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The effect of Sn addition on the oxidation of the Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn (at.%) alloy (JG6) in the as cast (AC) and heat treated (HT) conditions was studied at 800 °C and 1200 °C in static air using thermo-gravimetry and microstructural analysis. The oxidation kinetics, morphology and microstructure of the oxide scale and the microstructure of the bulk of the oxidised alloy were investigated. Oxidation occurred by inward oxygen anion diffusion. The oxidation of JG6 at 800 °C and 1200 °C is compared with the oxidation of Sn-free Nb–Ti–Si–Cr–Al–Mo–Hf alloys and is found to have been improved by the addition of Sn. At 800 °C pest oxidation, which was exhibited by the heat treated Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloy (JG4-HT), was eliminated by alloying with 5 at.% Sn. The elimination of pesting at 800 °C is attributed to the nature of the Nb solid solution in the alloy which consists of Sn-rich, Si-rich and Ti lean solid solution usually surrounded by Sn-poor, Si-poor and Ti-rich solid solution. The oxide scales that formed at 1200 °C on JG6 did not separate from the base metal and consisted of Nb 2 O 5 , TiO 2 , SiO 2 , HfO 2 and TiNb 2 O 7 . TiN, instead of TiO 2 , and the (Nb,Ti) 5 (Sn 1− x Si x ) 3 phase, which is considered as a ternary phase based on Nb 5 Sn 2 Si, are formed in the diffusion zone of the alloys JG6-AC and JG6-HT after oxidation at 1200 °C. The formation of these phases in the oxidised alloys JG6-AC and JG6-HT controlled the penetration of oxygen into the base material. The better oxidation performance of JG6-AC compared to JG6-HT at 1200 °C is attributed to the formation of Nb 3 Sn in the former. It is suggested that the presence of the Sn-poor, Si-poor and Ti-rich Nb ss in the microstructure is a key to the formation of the Nb 3 Sn phase at the scale/diffusion zone interface in the JG6-AC oxidised at 1200 °C.

The phase stability and microstructures of Nb ss /Nb 5 Si 3 based in situ composites alloyed with Hf and Sn have been investigated. The Nb 5 Si 3 silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb 3 Si phase was suppressed by the addition of Hf. The structure of the Nb 5 Si 3 phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb 5 Si 3 . This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb 5 Si 3 was higher than that in the Nb 5 Si 3 . The Si concentration in the Nb 5 Si 3 phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb 5 Si 3 and Nb ss , respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb ss , the Sn addition promoted the formation of Si-rich C14 Cr 2 Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO 2 were formed below the surfaces of the heat treated alloys.

The full understanding of the Nb–Si–Cr ternary system is important for the development of Nb silicide based composites, which show great potential for high temperature applications. There was, however, disparity in experimental observations of phase equilibrium in the vicinity of the Nb-corner. Two kinds of three-phase equilibrium, Nbss+C14+αNb5Si3 and Nbss+CrNbSi+αNb5Si3, in the Nb corner of the Nb–Si–Cr system have been reported in the literature. This work aims to clarify the three-phase equilibrium near the Nb-corner, by studying phase equilibrium in the Nb–18Si–15Cr ingot. Such a composition was chosen with the assistance of CALPHAD calculations to avoid unnecessary load of work. The alloy ingot was prepared by clean melting followed by heat treatment at 1000 and 1500 °C. The C14 Laves phase formed in all the samples and was stable at both temperatures. The results confirmed that Si has the effect of stabilising the C14 Laves phase down to at least 1000 °C. The three-phase equilibrium Nbss+C14+αNb5Si3, instead of Nbss+CrNbSi+αNb5Si3, was observed in this work. The current work demonstrates that ingot metallurgy is necessary to check the reliability of the information obtained about the ternary and higher-order phase diagrams, especially for the regions where the exact phase boundaries are in question. The investigation of the selected alloy was greatly helpful to clarify the confusion of the three-phase field near the Nb corner in the Nb–Si–Cr ternary system. The work confirmed the CALPHAD prediction of phase equilibrium near the Nb corner, showing the power to combine phase diagram predictions with experimental work for cost effective alloy development.

The solidification microstructures of the Mg–30Zn–2.5Y alloy have been studied in detail using both optical and analytical electron microscopy. CALPHAD simulation has been carried out for in-depth understanding of the solidification process. The work clarifies phase equilibrium in the vicinity of the Mg corner of the Mg–Zn–Y ternary system, showing that a ternary icosahedral (I) quasicrystal phase is in equilibrium with the (Mg) solid solution phase. The suppression of the Z-phase during solidification is attributed to low thermodynamic driving force and sluggish transformation kinetics for ordering icosahedral clusters into the crystalline structure. The current work lays the foundation for developing (Mg)–matrix composites with a ternary icosahedral phase as well as other intermetallic compounds. The work presents a case study of solidification process to demonstrate the feasibility to cast (Mg)–matrix composite containing the I-phase from the L+I semi-solid phase region, as long as the equilibrium volume fraction of the I-phase is not too high to guarantee practical slurry fluidity before casting. This would be useful for making use of primary I-phase particles as effective nucleation sites to improve structural refinement and thus maximize the benefit from the I-phase reinforcement.

The effects of Ti and Mo additions on phase selection, phase transformations and microstructure development in as cast and heat-treated Nb–Si–Cr–Al in situ composites have been investigated. The βNb5Si3 was formed in all the as cast alloys, which are classified as hyper-eutectic alloys. After heat treatment at 1500 °C, the βNb5Si3 transformed to αNb5Si3 completely or partially. The lattice parameter of the bcc Nb solid solution (Nbss) decreased with the addition of Ti and with increasing Mo concentration. The C14–Cr2Nb Laves phase was absent in the alloys with Ti addition at 24at.% in the presence of Mo (≤5 at.%). The eutectoid decomposition of Nb3Si to Nbss and αNb5Si3 was very sluggish in the alloy without Ti but it was enhanced with the addition of Ti. The partitioning of Ti between the Nbss and (Nb,Ti)5Si3 led to the formation of Ti-rich (Nb,Ti)5Si3, where the concentration of Ti was about 30.3 at.% in the heat treated microstructure. Nbss particles precipitated inside αNb5Si3 in the heat treated alloys.

Prediction of the thermodynamic stability of metallic glasses is important, as it offers one a most useful indicator for designing alloys with fully amorphous phase. A full thermodynamic database for glass forming ability (GFA) can be established by treating the glass transition as a second-order phase transformation from liquid phase, which can give good predictions of all important GFA indicators such as the reduced glass transition temperature and the thermodynamic stability of the amorphous phase. More interestingly, kinetic behaviour of the amorphous phase such as the crystallisation temperature can also be correlated to the thermodynamic stability of the amorphous phase, making the CALPHAD approach attractive for designing multi-component amorphous alloys.

Thermodynamic assessment of the Nb–Si–Al system has been carried out by evaluating the available equilibrium data, using the Calculation of Phase Diagram method. The Nb–Si system has been reassessed for good description of phase equilibria in the ternary system. Thermodynamic models for compound phases are based on the crystal structures, as well as orientations and solubility ranges of single-phase fields in the experimental isothermal sections at different temperatures. Efforts have been directed towards justifying thermodynamic data of this work against data for relevant phase structures. Structural relationships and structure maps have been considered in devising thermodynamic models and assessing lattice stabilities for compound structures.

The effects of chlorine on the high temperature (900–1115 °C) oxidation resistance of γ-TiAl-based alloys were studied by applying MnCl2 on the sample surfaces of the Ti47Al, Ti55Al, Ti47Al3V and Ti47Al2Cr2Nb alloys. The oxidation rates of these alloys decreased remarkably after treating the sample surfaces with MnCl2. The oxide scales formed on TiAl based alloys generally consisted of a TiO2-rich outer layer and a TiO2+Al2O3 mixed inner layer. After surface treatment with MnCl2, a very convoluted scale consisting of a thin discontinuous TiO2 outer layer and continuous Al2O3 inner layer with a small amount of TiO2 formed on the four alloys. At the initial stage of oxidation the continuous Al2O3 layer had formed on the MnCl2 treated samples. It is suggested that occurrence of chlorination of aluminium during the initial stage promoted the formation of the continuous Al2O3 layer.

The glass transition in metallic alloy systems can be modelled thermodynamically, using the Calphad approach, as a second-order transition from the supercooled liquid phase, giving good predictability for glass transition temperatures and the thermodynamic stability of the amorphous phase in intermetallic alloy systems. The resultant thermodynamic database can also be used for calculating crystallisation temperatures for the glass devitrification, and the Calphad approach is a powerful tool for designing bulk metallic glass alloys.

Structural models of long-period out-of-phase superstructures are discussed on the basis of diffraction physics. The paper analyses the structural factors of one-dimensional out-of-phase superstructures of both constant periods and incommensurate arrangements, showing excellent agreement with experimentally observed diffraction patterns.

Hf, Mo and W are useful elements for developing refractory intermetallic alloys of high modulus and creep resistance at high temperatures, while little is known about thermodynamic properties in the Hf–Mo and Hf–W systems. In this work, full thermodynamic descriptions of the Hf–Mo and Hf–W systems are obtained by assessing the available equilibrium data, using the CALPHAD method. The Laves phases in the two binary systems are treated using the two-sublattice model to consider their noticeable solubility ranges, showing good agreement with the experimental phase diagram data.

The Re–Si system is assessed thermodynamically, using the CALPHAD method. The calculated phase diagram and thermodynamic properties are in good agreement with available experimental data. Calculated enthalpies and entropies of fusion are compared with available data for other transition metal silicides, against melting points, showing good agreement with the general trends. This is a useful approach for thermodynamic assessment of alloy systems, where experimentally measured thermodynamic data are limited. The stability of the amorphous phase in this system has also been discussed.

The oxidation behaviour of MoSi 2 has been analyzed on the basis of thermodynamics and kinetics. Thermodynamically speaking silica is in equilibrium with any of the stable molybdenum silicides. At low temperatures, the limited bulk diffusion in MoSi 2 makes it impossible to sustain a sufficient supply of silicon for the formation of a continuous silica layer. This leads to the simultaneous-oxidation of silicon and molybdenum, forming a scale of mixed oxides. The formation of molybdenum oxides leads to ruptures owing to the significant volume change and accelerates the oxidation, leading to the pest phenomenon.

Nanometre-scale β–FeSi2 precipitates were introduced in a single crystal silicon substrate by low-dose ion-beam synthesis (IBS). The crystallographic relationship between these nanometre β precipitates and the silicon substrate has been studied by high resolution electron microscopy (HREM). The results show that the orientation relationship (OR) between the nanometre β precipitates and the silicon substrate is [100]β//[110]Si and (001)β//(11)Si, with abnormally large strain between the precipitates and the substrate. This OR is important for the formation of 90°-OD boundaries within β–FeSi2 grains. Also, the relationship between various reported low-index ORs has been analysed and a new low-index OR is predicted.

Conventional and high energy X-ray photoelectron spectroscopy (XPS) with Al Kα and Cr Kβ radiation, respectively, were used to calculate the Auger parameters of the elements in the V–50 at% Al and Nb–17 at% Al alloys. The shifts of the Auger parameters of the elements of interest between unalloyed and alloyed conditions were used to calculate values of charge transfer occurring upon alloying. The results were related to thermodynamic predictions and the microstructures of the two alloys. The ordering tendency in the Nb–17 at% Al alloy, shown by thermodynamic modelling and the microstructural studies, was attributed to substantial electron transfer from Al to Nb. The small charge transfer from Al to V in the V–50 at% Al alloy was attributed to the lack of ordering in this alloy.

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)2 Laves phase. The L12 τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.

Phase transformations in arc melted Ti-40Al-10V ingots have been studied. Solidification started with the bcc β phase. The room temperature microstructure consisted of B2,ωord (ordered ω), γ and α2 phases. The α2 phase was generally found in bands of (α2 + γ) and was richer in V and Ti than the γ phase. Orientation and interfacial relationships have been determined. The fine structures of the γ phase and different morphologies of the (α2 + γ) structure have been studied by electron microscopy. It is shown that the phase transformation sequence in the alloy is L → L + β → β → α + β → α + B2 → α2 + γ + B2 → α2 + γ + B2 + ωord.