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Arc-melted Ti100–xMo2Alx alloys (where x is 44, 46, 48, and 50 at.% Al) that were produced from pure components were examined by X-ray diffraction and scanning electron microscopy with electron microprobe analysis (SEM/EDX). The melting points and solid-phase transformations were studied by differential thermal analysis (DTA) and the mechanical properties by fracture toughness, bending, and compression tests. The CALPHAD approach was used for thermodynamic calculation of phase equilibria in the composition region under study. The cast alloys mainly consist of a lamellar structure formed by lamellas of γ-TiAl and α2-Ti3Al phases with submicron thickness and a cubic Ti55Mo4–6Al39–40 phase of A2 type (β) or B2 type (β0). In the γ-TiAl-based alloys, molybdenum behaves as a low-melting dopant, enriching the grain periphery. The standard mechanical characteristics of the alloys were determined and their structural sensitivity was analyzed. All studied alloys demonstrate excellent high-temperature strength. The yield stress is 400–600 MPa in the temperature range 20–750°C. The strength slightly increases at 300 and 600°C under dynamic strain ageing. The strain hardening parameters were established over a wide range of test temperatures. The temperature dependence of the strain hardening coefficient and strain hardening index was analyzed for alloys in different phase and structural states. In the temperature range from 20 to 600°C, the strain hardening index and coefficient were found to vary slightly. The strain hardening index increased from n = 0.6 to n = 0.95 with aluminum content changing from 44 to 50 at.%, which indicates that the strain hardening mechanism changes with variation in the alloy phase constituents.

We present the first observations of a large-scale coexistence between rod-like and lamellar eutectic growth patterns during directional solidification of a eutectic alloy. In situ experiments with real-time optical monitoring were carried out under microgravity onboard the International Space Station (ISS). We used the transparent succinonitrile-d,camphor eutectic alloy that ordinarily forms rod-like patterns. At low growth velocity, short lamellae stabilized at the contact line with a sample glass wall. In the presence of a controlled transverse temperature gradient, the coupled-growth pattern experienced a global drift along an inclined isotherm, and a stable lamellar domain spread over the solidification front. The propagative nature of the lamellar-to-rod transition was evidenced. The advance of the lamellar/rod domain boundary was determined by a slowly amplifying varicose instability of the lamellae. On a large scale, the domain boundary underwent a dynamic faceting.

In the present paper, the complete ternary B–Fe–Ti system is thermodynamically modelled based on our own experimental results and information reported in literature. Series of key experiments were performed for the binary Fe-B, Fe-Ti and the ternary B-Fe-Ti systems in order to obtain parameters of invariant reactions. For this, 19 alloys were prepared and analysed in the as-cast and annealed conditions by means of SEM/EDS, SEM/EBSD, XRD and DTA/DSC techniques. A new ternary boride Ti2FeB2 with the crystal structure of Mo2FeB2 (an ordered derivative of U3Si2 crystal structure type) was revealed to exist at temperatures from 1273 K to solidus. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental results. The calculations are shown to reproduce available experimental data entirely well.

Acta materialia 212, 116880 (2021). doi:10.1016/j.actamat.2021.116880, The solidification behavior of the CoCrFeNi high-entropy alloy and the ternary CrFeNi and CoCrNi medium-entropy suballoys has been studied in situ using high-speed video and synchrotron X-ray diffraction on electromagnetically levitated samples. In all alloys, the formation of a primary metastable bcc phase was observed if the melt was sufficiently undercooled. The delay time for the onset of the nucleation of the stable fcc phase, occurring within bcc crystals, is inversely proportional to the melt undercooling. The experimental findings are corroborated by thermodynamic calculations of stable and metastable phase diagrams for the (CoCrNi)-Fe section. Crystal-growth velocities for the CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys extracted from the high-speed video sequences in the present study are comparable to the literature data for Fe-rich Fe-Ni and Fe-Cr-Ni alloys, evidencing the same crystallization kinetics. The effect of melt undercooling on the microstructure of solidified samples is analyzed and discussed., Published by Elsevier Science, Amsterdam [u.a.]

Frontiers in Materials 7, 296 (2020). doi:10.3389/fmats.2020.00296, Published by Frontiers Media, Lausanne

Frontiers in Materials 7, 270 (2020). doi:10.3389/fmats.2020.00270, Published by Frontiers Media, Lausanne

Interaction of melt flow with solidification microstructures is of fundamental interest and technical relevance due to its effect on the selection of characteristic microstructural features and length scales. Here, we report on in-situ optical observation of both melt flow at the solidification front and microstructural evolution in polycrystalline cellular and dendritic microstructures during upwards directional solidification of a transparent succinonitrile-2.2wt.%(d)camphor alloy in a bulk rectangular sample. Melt flows at different flow intensity levels were applied and the microstructures were characterized in terms of morphology, spacing and growth direction. Under unforced or natural convection, transitions from planar to cellular and further to dendritic solid/liquid interface morphology were obtained (having cellular/dendritic spacing with similar trends as reported in literature data). Under forced convection, after a plateau, decreasing dendrite spacing with increasing shear flow intensity was observed. Also, an increasing inclination of cellular and dendritic growth in the downstream direction was established. Furthermore, for the first time a flow-mediated transition from dendritic to seaweed-type microstructures and vice-versa was observed. Finally, an oscillation of dendrite tip positions with fixed frequency was found for a certain parameter range. The results indicate the large variety of flow effects at different length scales and corresponding physical mechanism, which are discussed.

Controlling the grain size and texture of lamellar TiAl-alloys is essential for well-balanced creep and fatigue properties. Excellent refinement and texture mitigation are achieved in aluminum lean alloys by low boron additions of 0.2 at.%. This amount is sufficient to promote in situ formation of ultrafine borides during the last stages of body centered cubic (BCC) solidification. The borides subsequently serve as nucleation sites for hexagonal close packed (HCP) during the BCC–HCP phase transformation. Bridgman solidification experiments with alloy Ti-43Al-8Nb-0.2C-0.2B were performed under a different growth velocity, i.e., cooling rate, to evaluate the HCP grain size distribution and texture. For slow-to-moderate cooling rates, about 65% of HCP grains are randomly oriented, despite the pronounced texture of the parent BCC phase resulting from directional solidification. For high cooling rates, obtained by quenching, texture mitigation is less pronounced. Only 28% of the HCP grains are randomly oriented, the majority being crystallographic variants of the Burgers orientation relationship.

In the present work are reported ground-reference results from a series of transient solidification experiments with the organic eutectic alloys SCN-23.6 DC (wt.%) and SCN-24.2DC-0.5NPG (wt.%), which have been performed in preparation of microgravity experiments using a new multi-user facility called Transparent Alloys or DIRSOL. In 1g lab conditions we investigated the response of rod-like eutectic patterns to imposed transients, involving defined acceleration and deceleration of the growth velocity. The patterns have been recorded in-situ in oblique view using a long distance optics and were analysed for a region of interest of 640 μm x 320 μm over long periods of time. The time dependent growth velocity and the associated acceleration / deceleration were determined using images from a side-view camera. The reported results mainly refer to the spacing distribution and the hexagonal order parameter S0, dynamically attained at a defined time instant, i.e. at the moment when the local growth velocity reaches V = 40 nm s−1 along deceleration 80 → 10 nm s−1 or acceleration 10 → 80 nm s−1 paths. The major result is that the hexagonal order is deteriorating under transient growth conditions compared to steady state conditions. We could not observe an increase of order for either imposed acceleration or deceleration, mainly because spacing adjustment is accomplished by abrupt morphological transitions which destroy the previous order. The abrupt morphological transitions are clearly identified as sharp events along an experiment cycle 80 → 10 → 80 nm s−1 and could conveniently be seen in hysteresis loop diagrams. Upon deceleration the abrupt spacing adjustment event corresponds to a transition from fibrous to vermicular morphology. Upon acceleration the abrupt spacing adjustment event corresponds to spontaneous rod splitting.

Quaternary as-cast Ti 96–x Nb 2 Cr 2 Al x and Ti 93–x Nb 5 Cr 2 Al x alloys, where x = 44, 46, 48, and 50, melted from pure components (~99.9 wt.%) in a laboratory arc furnace, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), and differential thermal analysis (DTA). Bending testing and fracture toughness measurements were performed at room temperature, and compression testing was carried out in a range from room temperature to 750°C. The alloys were found to consist mainly of a superfine lamellar structure formed during decomposition of hightemperature phases. In addition, the alloys with 50 at.% Al contain grains of the γ phase, and all others have precipitates of the cubic β/β 0 phase at boundaries of the high-temperature phases because of double enrichment of the last melt drops with Cr. The tests show that the Ti 96–x Nb 2 Cr 2 Al x alloys with 46 and 48 at.% Al possess the most balanced properties.

In the present paper, the constituent binary B–Mo system is re-modelled in order to reproduce genuine homogeneity ranges of the molybdenum borides. Based on this, available descriptions of the constituent B–Ti and Mo–Ti systems as well as new experimental data reported in the literature for the ternary B–Mo–Ti system are used to model the B–Mo–Ti system in the entire composition and temperature ranges. The elaborated thermodynamic description is further applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental results. The calculations are shown to reproduce the experimental data reasonably well, however for few alloys belonging to the domain of primary (Ti,Mo)B 2 crystallization the computed solidus temperatures are about 200 K higher than incipient melting temperatures measured by pyrometry.

In the present paper, the ternary Al–Cr–Ti and binary constituent Al–Cr systems are thermodynamically re-modelled based on new experimental information reported in the literature within the past few years. Few key experiments were performed with selected ternary alloys in order to complement data on phase equilibria in the composition range of common TiAl-based alloys. Six sample compositions were prepared and analyzed in the as-cast and annealed conditions by means of SEM/EDS, XRD and DTA techniques. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental results. The calculations are shown to reproduce the experimental data reasonably well.

Based on novel experimental data the thermodynamic description of the ternary Al–C–Ti system was subject to optimization using the CALPHAD approach (Thermo-Calc/PARROT). The reaction scheme, the projections of the liquidus and solidus surfaces, as well as a number of isothermal sections and isopleths were calculated using the proposed thermodynamic description and compared with the experimental results. The calculations were shown to adequately reproduce the experimental data. The main feature of the phase equilibria in the system is the existence of three ternary compounds P (Ti3AlC), H (Ti2AlC) and N (Ti3AlC2) forming peritectically from the liquid phase and TiC1−x carbide at 1907, 1865 and 2013 K, respectively. It is shown that these three compounds are thermodynamically stable in a wide temperature interval.

Eutectic alloys from the ternary system Al-Cu-Ag are excellent model alloys for the investigation of coupled eutectic growth, not only because most materials properties are well known but also because the system offers a rich variety of crystal structures and crystal orientation relationships (ORs) being associated to distinct minima of the solid-solid interface energy. This paper describes three research topics specifically related to bulk lamellar Al-Al2Cu eutectics, e.g. the maze-to-lamellar transition during early growth, the role of fault lines during lamellar spacing selection close to the pinch-off limit and the onset of eutectic cell formation above the constitutional supercooling limit. These topics are central to the microgravity experiments SETA presently being prepared for the MSL / SQF.

International audience; Phase equilibria and phase transformations in the ternary Al-B-Nb system were investigated by XRD, SEM/WDS and SEM/EDS techniques, DTA, and optic pyrometry. No ternary compound, including the ternary phases reported in literature, was found. It was experimentally measured that the Al-Nb phases epsilon (NbAl3), sigma (prototype sigma CrFe) and delta (prototype Cr3Si) have evident ternary extensions with maximal solubility of boron amount to 1.2, 10.4 and 3.2 at.%, respectively. The Al solubility in niobium borides is also significant. The obtained results were used to elaborate the thermodynamic description of this ternary system based on recently published thermodynamic models of the constituent binary subsystems using the CALPHAD approach (Thermo-Calc/PARROT). The Al-B-Nb reaction scheme, projections of the liquidus and solidus surfaces as well as a number of isothermal sections and isopleths were calculated using the proposed thermodynamic description and compared with the experimental results. The calculations were shown to reproduce adequate the experimental data. The main features of the phase equilibria in the range of melting/solidification are the existence of two quasibinary sections, NbAl3-NbB and NbB2-AlB12, eutectic L-E - epsilon + sigma + (NbB) at 1850 K and cascades of invariant U-type reactions.

The eutectic spacing (λ) was investigated in three alloys of succinonitrile–neopentylglycol–(D)camphor–aminomethylpropanediol (SCN–NPG–DC–AMPD) system over a wide range of temperature gradients (G) and growth velocities (V), both in thin ⩽5 μm (two-dimensional (2-D)) samples and in bulk (three-dimensional (3-D)) samples. The compositions were selected such that fraction of rod-forming phase (f) was equal to 11.5 vol.% for the invariant SCN–12.2 wt.% NPG alloy, 23.3 vol.% for the univariant SCN–24.2 wt.% DC–0.5 wt.% NPG alloy and 29.8 vol.% for the bivariant SCN–33.0 wt.% DC–6.0 wt.% NPG–3.0 wt.% AMPD alloy. For high G/V conditions significant deviations from growth at minimum undercooling were found with spacing selection closer to the pinch-off limit. The experimental results show that confining the 3-D eutectics to 2-D growth conditions invariably leads to larger values of the mean rod spacing, but to smaller values of the mean rod diameter. The 2-D/3-D ratio of the square of mean rod diameters η = d 2 D R / d 3 D R 2 was found to be independent of the phase fraction of the rod-forming phase f and equal to η = 0.80 ± 0.02. The 2-D/3-D ratio of square of rod spacing k = λ 2 D R / λ 3 D R 2 was found to dependent on f following k = ( 2 f ) - 1 or else k = η 3 8 π f - 1 . These findings are discussed in terms of the Jackson–Hunt model of eutectic growth and simple geometrical considerations.

Lamellar eutectic growth was investigated in the transparent organic alloy neopentylglycol-(D)camphor of eutectic composition (NPG-45.3 wt% DC) using bulk (3D) and thin (2D) samples. Two types of eutectic grains were observed in the polycrystalline samples, either with lamellae well aligned to the direction of solidification or inclined at an angle of 21.5±1.5°. The well aligned grains were used for determining lamellar spacing as function of growth velocity V and temperature gradient G . Based on these data the Jackson–Hunt constant was evaluated to be K JH =1.60±0.15 μm 3 s −1 . For low growth velocity experiments the contact angles for (DC) and (NPG) lamellae at eutectic triple junctions were also evaluated, being θ (DC) =50.9±4.1° and θ (NPG) =41.8±4.7°, respectively. Using these values, as well as phase diagram data and the Gibbs–Thomson coefficients, the chemical coefficient of diffusion of (D)camphor in the eutectic liquid at eutectic temperature 53 °C was estimated to be D L =97±15 µm 2 s −1 .

We have determined nonisothermal diffusive transport coefficients of a succinonitrile-(d)camphor mixture with a composition of c = 0.239 wt.-frac. (d)camphor at a temperature of 318.2 K, which is close to the eutectic point. The employed experimental techniques are optical beam deflection in a Soret cell and photon correlation spectroscopy. The diffusion coefficient is D = (1.43 ± 0.04) × 10-10 m2 s-1, the thermodiffusion coefficient is DT = (2.00 ± 0.06) × 10-12 m2 s-1 K-1, and the Soret coefficient is ST = (1.40 ± 0.02) × 10-2 K-1. Camphor migrates toward the lower and succinonitrile migrates toward the higher temperatures. While the diffusion coefficient is in good agreement with the literature, the Soret coefficient has been determined for the first time. Our analysis shows that a significant concentration shift can be established in the liquid mixture in the presence of a temperature gradient. The mixture has a negative separation ratio, which leads to convective instabilities if heated from above.

Alloys from the succinonitrile–(D)camphor (SCN–DC) system are widely used as model alloys for the in situ investigation of solidification using light optical microscopy, but literature on the binary phase diagram is contradictory with respect to the solubility limit of DC in (SCN). Phase equilibria of the system were therefore revisited experimentally and critically assessed in the present work. The results prove that the maximum solubility of DC in the succinonitrile solid solution (SCN) is far less 1 wt% and the volume fraction of the (DC) phase in the eutectic solid is 23.3%. On this basis and on recently reported experimental data the CALPHAD description of the SCN–DC system was re-optimized.

An experimental method for the in-situ observation of solidification in eutectic metallic alloys growing at moderate temperatures, i.e. 500–600 °C was developed. It is based on unidirectional solidification of thin samples (200 to 700 μm thick) contained in rectangular quartz glass tubes allowing for in-situ observation of growth on the sample surface by means of light microscopy using a long-distance microscope. The method is illustrated for two-phase eutectic growth in Al–17.5Cu–1.0Ag, Al–11.8Si and Al–11.8Si–0.025Sr as well as for three phase growth in Al–13.3Cu–5.7Si, all at%. Coupled growth of lamellar Al–Al2Cu eutectic in Al–17.5Cu–1.0Ag was investigated in more detail with regard to spacing selection: experiments were performed in a temperature gradient of 38±1 K cm−1 using different, but constant withdrawal velocities ranging from 0.025 to 1.8 μm s−1. Measured spacing was compared with literature data and discussed with reference to the Jackson–Hunt theory and the recent amendment proposed by Akamatsu, Faivre, Karma and Plapp. In-situ observations were complemented by post-mortem metallographic analysis using SEM and EBSD in transverse sections.

In the present article, a thermodynamic description of the ternary Al–Fe–Ta system is proposed, being obtained by CALPHAD modelling based on assessed experimental information from literature and our own experimental results. Dedicated experiments were performed to complement literature data on phase equilibria and thermodynamic properties. As-cast and annealed samples were prepared and analysed by means of SEM/EDS, XRD, DTA and DSC techniques, as well as isoperibolic dissolution calorimetry. The experiments revealed that three binary phases have large ternary extensions: the C14 Laves phase based on TaFe2 dissolves up to about 56 at.% Al, the μ or D85 phase dissolves up to about 39 at.% Al and the σ phase based on Ta2Al dissolves up to 12 at.% Fe. The ternary ordered Heusler phase L21 (the MnCu2Al crystal structure type) is modelled as a stable phase such that it forms through a second-order ordering process at 1122 K from the ordered B2 phase of composition Ta0.04Fe0.50Al0.46 involving the substitution of Al with Ta atoms and reaches TaFe2Al stoichiometry at ambient temperatures. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental results. The calculations are shown to reproduce reasonably the experimental data.

A new experimental method for in-situ observation of microstructure formation in top view during unidirectional solidification of bulk, transparent, organic alloys is presented. This method allows observing growth patterns over an extended interface area with high resolution and minimal optical aberrations. With (D)camphor – neopentylglycol – succinonitrile (DC–NPG–SCN) alloys a series of unidirectional solidification experiments were performed in order to validate the set-up. By means of multi-focus exposition eutectic cells were observed over a depth of several millimeters, followed by 3D reconstruction of their shape. The method also allows capturing the integral interface dynamics and measuring its relative undercooling.

Ternary as-cast Ti55−x Nb x Al45, Ti53−x Nb x Al47, Ti55−x Ta x Al45, and Ti53−x Ta x Al47 alloys, where x = 0, 4, 8, and 12, and Ti45Nb8−x Ta x Al47 and Ti47Nb8−x Ta x Al45 quaternary alloys, where x = 2, 4, and 6, melted in a laboratory arc furnace from pure components (~99.9 wt.%) are studied by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), differential thermal analysis (DTA), and compression testing. The alloys are found to consist mainly of a superfine lamellar structure of decomposition β → α → α + γ → α2 + γ and contain tiny islands of the γ phase and also the β phase at 12 at.% Nb or Ta. The CALPHAD approach is used to obtain a thermodynamic description of the Al − Nb − Ta − Ti system, which adequately reproduces the experimental data. The compression tests performed at room temperature show that the ternary alloys with 45 at.% Al are characterized by higher strength but lower plasticity than the alloys with 47 at.% Al. For the Ti46Nb8Al46 and Ti46Ta8Al46 alloys, high strength in the temperature range 20 – 800°C and a drop in plasticity to 2 – 4% at 200 – 600°C are revealed. Among the quaternary alloys examined, the best combination of strength and plasticity (σ02 = 1043 MPa, σult = 1612 MPa, epl = 12.4%) is exhibited by the Ti47Nb4Ta4Al45 alloy.

Univariant and invariant phase equilibria for systems that display second-order transformations such as chemical and magnetic ordering are arranged consistently aiming to construct complete Scheil’s reaction schemes. For this purpose it is assumed that univariant phase boundaries representing second-order (or higher-order) transformations are nothing else than phase fields collapsed into infinitely thin thickness. This implies that second-order transformations can be formally treated like first-order transformations. Adequate notations and representation guidelines are proposed for the Scheil’s scheme. Examples are given for binary Al-Fe and ternary Al-Fe-Ta systems using recently obtained thermodynamic descriptions.

In the present paper a thermodynamic description of the entire ternary Al–Ta–Ti system is proposed, being obtained by CALPHAD modelling. Dedicated experiments were performed in order to complement literature data on phase equilibria reported mainly for the temperature interval from 1273 to 1723 K. As-cast and annealed samples were investigated by means of DTA, pyrometry after Pirani–Alterthum, SEM/EDS, SEM/EBSD and XRD techniques in order to prove the phase relations in ranges of invariant equilibria at melting, α 2 + γ alloys and low-temperature ternary phases. The experiments revealed that two ternary phases exist in the Al–Ta–Ti system: the orthorhombic O-phase (Ti 2.17 Ta 0.77 Al 1.06 , Pearson symbol oC16 ) and the ordered B8 2 -phase (Ti 3.00 Ta 0.89 Al 2.11 , Pearson symbol hP6 ) are thermodynamically stable below 1123 K and 1171 K, respectively. The obtained and literature data were employed throughout the optimization work, using the PARROT module of Thermo-Calc. The elaborated thermodynamic description was applied to calculate selected phase equilibria as to provide a comparison between calculated and experimental data. The calculations are shown to reproduce correctly experimental data. They revealed the presence of a miscibility gap for the ternary (βTi,Ta,Al) solid solution in the temperature interval from 1461 to 813 K.

The lamellar α 2 (Ti 3 Al) + γ(TiAl) microstructure of intermetallic Ti–46Al–8Ta (at.%) alloy is thermodynamically unstable and transforms to α 2 + γ + τ type during long-term ageing at 750 °C. A new ternary τ-phase with B 8 2 type structure (space group P6 3 /mmc, Pearson symbol hP 6) is identified by XRD analysis and its occurence is included to thermodynamic modelling of Ti–Al–Ta phase diagram by CALPHAD approach. The lattice parameters of the coexisting γ, α 2 and τ phases change during ageing.

Phase relations in the binary Al–Ta system were re-assessed taking into account recent experimental data. Alloys of different composition were prepared and analyzed by means of DTA, DSC, pyrometry after Pirani–Alterthum, SEM/EDS and SEM/EBSD techniques in order to prove the phase relations in the system. Both, as-cast and annealed samples were investigated. A new thermodynamic description of the binary Al–Ta system was obtained by CALPHAD modeling of the Gibbs energy of all accepted individual phases, taking into account the above experimental data as well as assessed published data. Applying the results, calculations of the phase diagram and thermodynamic properties were performed with the Thermo-Calc software. They are shown to reproduce properly experimental data.

Casting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via β-(Ti), e.g. a pathway that avoids peritectic growth of α-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic α (Ti)...

The thermodynamic description of the ternary system Al–B–Ti is obtained by modelling the Gibbs energy of all individual phases in the system using the CALPHAD approach. The model parameters have been evaluated using the computer optimization technique PARROT based on the available descriptions of the constituent binary systems Al–B, B–Ti and Al–Ti recently published and relevant experimental information on phase equilibria for the ternary system. For the ternary system Al–B–Ti the reaction scheme, projection of the liquidus, solidus and solvus surfaces, selected vertical and isothermal sections are calculated using the proposed thermodynamic description. An acceptable agreement between the calculations and experimental data is achieved.

The thermodynamic description of the entire ternary Al–Nb–Ti system is obtained by CALPHAD modelling of the Gibbs energy of all individual phases, taking into account experimental data on phase equilibria and thermodynamic properties published and complemented by own experiments. The description includes a re-evaluation of the constituent binary Al–Nb system. Selected equilibrium calculations were performed with the Thermo-Calc software using the proposed description. They are shown to well reproduce experimental data on both, phase equilibria and thermodynamic properties in the entire Al–Nb–Ti system.

The grain refinement achieved with low boron additions to selected TiAl-based alloys was investigated in unidirectional solidified samples by means of EBSD-orientation mapping of the α(Ti) phase, retained by quenching. The analysis shows that excellent grain refinement is achieved during the solid state transformation of the body centred β(Ti) into the hexagonal close packed α(Ti). It leads to a fairly random orientation of the α(Ti) grains which indicates that heterogeneous nucleation of α(Ti) on borides is at the origin of refinement. The borides were identified to be orthorhombic (Ti,Nb)B monoboride ribbons with the crystal structure of NbB. The refinement is severely impeded by peritectic growth of α(Ti): peritectic α(Ti) grains almost completely invade the microstructure by imposing their crystal orientation through all subsequent solid state transformations.

The thermodynamic description of the ternary system B–Nb–Ti is obtained by modelling the Gibbs energy of all individual phases in the system using the CALPHAD approach. The model parameters have been evaluated by means of a computer optimization technique based on the available descriptions of the constituent binary systems B–Nb, B–Ti and Nb–Ti and relevant experimental information on phase equilibria for the ternary system. The thermodynamic descriptions of both systems with boron, B–Nb and B–Ti, were published in the first part of the work. For the ternary system B–Nb–Ti several vertical and isothermal sections as well as the projection of the liquidus and solidus surface are calculated using the elaborated thermodynamic description. A tolerable agreement between the calculations and experimental data is achieved.

We study the directional-growth patterns of the transparent nonfaceted alloy succinonitrile–( d )camphor (SCN–DC) at eutectic concentration in thin and bulk samples using real-time observation methods. We measure in situ the minimum-undercooling spacing λ m and the small-spacing stability limit λ c of the thin, quasi-lamellar patterns. We find λ m 2 V ≈ 10.2 μ m 3 s - 1 and λ c ≈ 0.65 λ m . This last result is contrary to the common conjecture that λ c = λ m in lamellar patterns, but in agreement with previous experimental and numerical studies in the CBr4–C2Cl6 eutectic. In bulk samples, we observe the growth of rod-like patterns in real time. We show that these patterns do not usually achieve long-range hexagonal order after long solidification times, although their average spacing reaches a constant value, which is close to λ m , after a relatively short transient.

The temperature of phase transformations, as well as the enthalpy of melting of 29 different alloys from the Bi–In–Sn system were measured by DSC using a Perkin–Elmer DSC-7 instrument. The following tie-triangles were characterized in terms of analysing the chemical composition of the phases in equilibrium, using energy dispersive X-ray analysis (EDX): (Bi)–BiIn–(Sn) at 77 °C, BiIn 2 -β-γ at 59 °C including the composition of the liquid phase at 59 °C and the tie-line BiIn 2 -γ at 59 °C. A new thermodynamic description is presented for the ternary Bi–In–Sn system in the entire composition range. The parameters for the thermodynamic models of the constituent binary systems Bi–In, Bi–Sn and In–Sn are adopted from earlier assessments, and those for the Bi–In–Sn system are optimized in this study using experimental data on phase equilibria and data on the enthalpy of melting of different alloys available in the literature and measured in the present work. Several vertical and isothermal sections as well as the liquidus surface and selected thermodynamic properties are calculated using the thermodynamic description. They show reasonably good agreement with reported experimental data.

Several key experiments were performed to determine the temperature of transformations of organic alloys from the ternary systems 2-amino-2-methyl-1,3-propanediol–(D)camphor–succinonitrile (AMPD-DC-SCN) and 2-amino-2-methyl-1,3-propanediol–neopentylglycol–succinonitrile (AMPD-NPG-SCN) by means of differential scanning calorimetry (DSC). The phase diagrams of these ternary systems were assessed via the CALPHAD approach using Thermo-Calc and the experimental data on phase equilibria were determined by the above key experiments. Good agreement between the experimental and the calculated data was achieved for both ternary systems. Combining the obtained thermodynamic descriptions with those obtained for two other ternary systems, AMPD–DC–NPG and DC–NPG–SCN, published earlier, the thermodynamic description for the entire quaternary system was created. In order to identify the phases involved in solid–liquid equilibria and the nature of coupled eutectic growth in quaternary alloys, several unidirectional solidification experiments were performed: two quaternary alloys with composition 37.5SCN–44.5DC–16.5NPG–1.5AMPD (wt%) and 34SCN–44DC–19NPG–3AMPD (wt%) were used for the experiments on monovariant eutectic growth. We find that in the last alloy the monoclinic NPG phase is fully suppressed by the addition of AMPD and three solid phases show coupled eutectic growth at the solid/liquid interface: the hexagonal phase (DC solid solution), the FCC phase (NPG–AMPD solid solution) and the BCC phase (SCN solid solution). The quasi-2D eutectic patterns obtained in a thin sample show the stacking sequence –BCC–Hex–FCC–Hex–. The volume ratio of Hex:BCC:FCC phases equals 4:3:2. In the quaternary alloy with 1.5 wt% AMPD the monoclinic NPG phase is present at the solid/liquid interface and the eutectic growth is rather irregular.

Technical Al-Si alloys always contain sufficient amounts of Fe and Mn, especially alloys made from scrap. During casting, Fe-containing intermetallics, such as Al-Fe, Al-Fe-Si and Al-Fe- Mn-Si phases, are formed between the aluminum dendrites. Fe and Mn-rich intermetallic phases are well known to be strongly influential on mechanical properties in Al-Si alloys. In the present work the influence of controlled fluid flow conditions on the morphology and spatial arrangement on intermetallic phases in cast Al-Si alloys is characterized. A binary Al-7wt.%Si and a ternary Al- 7wt.%Si-1wt.%Fe alloy was solidified under and without the influence of a rotating magnetic field (3mT at 50Hz) over a range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient G of 3K/mm. The scientific results reached so far indicate a strong influence of the electromagnetic stirring on the primary dendrite and secondary dendrite arm spacings.

The enthalpies of formation of β and γ alloys of the Ag–Cu–Zn system were determined by dissolution calorimetry. The melting and solid-state transformation temperatures as well as the enthalpies of the order/disorder and β/χ transformations were measured by differential scanning calorimetry. Thermodynamic descriptions are presented for the binary Ag–Zn system and for the ternary Ag–Cu–Zn system in the entire composition ranges. The thermodynamic model parameters of the constituent binaries Ag–Cu and Cu–Zn are taken from earlier assessments. Those for Ag–Zn and the Ag–Cu–Zn system are established based on relevant experimental data available in the literature completed with experimental data obtained in the present work. Several vertical and isothermal sections as well as the liquidus surface and thermodynamic properties are calculated using the evaluated parameters and show reasonably good agreement with experimental data available.

The partial enthalpies of dissolution of pure Ni,Wand Ta in liquid ternary Ni –Ta –W alloys have been determined at (1773 ± 5) K using a high temperature isoperibolic calorimeter. Measurements were performed in Ni-rich alloys (from 80 to 100 at.% Ni) along sections with constant Ta .W atomic ratios 1 : 0, 2 : 1, 1 : 2, and 0 : 1. The partial enthalpies and thereby the integral enthalpy of mixing of these ternary alloys are calculated from the partial enthalpies of dissolution using SGTE Gibbs energies for pure elements as reference. The obtained thermochemical data confirm that in the investigated Ni-rich alloys the binary interactions between Ta and W as well as the ternary Ni –Ta –W interactions are negligibly small. Due to this the variation of the integral enthalpy of mixing of the ternary alloys is well described as linear combination of the constituent Ni –Ta and Ni –W binaries. Such behaviour of the ternary liquid alloys is related to a very low probability of new ternary stable phases to occur in solid state. This prediction is confirmed by differential thermal analysis, scanning electron microscopy, and transmission electron microscopy of the as-solidified and annealed samples obtained as last alloy compositions in the series of calorimetric dissolution.

Coupled, regular eutectic growth of α(Al) and Al2Cu from ternary Al-Cu-Ag liquid alloys is investigated with focus on the formation and the characteristics of eutectic cells in unidirectionally solidified, polycrystalline, bulk samples. The topologic anisotropy of the lamellar eutectic leads to destabilization along the lamellae with elongated cells being intermediate to stable cells, irrespective of the crystallographic orientation of the phases. The formation of stable cellular patterns with elongated or regular cell structure is explained with reference to the crystal orientation of the phases α(Al) and Al2Cu, measured by electron backscatter diffraction (EBSD).

The temperature and enthalpy of transformations of organic alloys from the ternary system 2-amino-2-methyl-1,3-propanediol–(D)camphor–neopentylglycol (AMPD–DC–NPG) were measured by means of differential scanning calorimetry (DSC). The phase diagram of this ternary system was assessed via the CALPHAD approach using Thermo-Calc based on the thermodynamic descriptions of the constituent binaries published earlier and on the thermodynamic and phase equilibrium data measured in the present work for ternary alloys. Good agreement between the experimental and calculated data was achieved. Experiments and calculations show that the AMPD–DC–NPG exhibits a nonvariant quasiperitectic reaction at 336.1 K and composition AMPD-17.5 mol% DC-42.56 mol% NPG, with all solid phases being nonfaceted. Unidirectional solidification experiments were performed with three different alloys selected such that each exhibits a small amount of the primary BCC (AMPD solid solution), the FCC (NPG solid solution) and the hexagonal (DC solid solution) phase, respectively, in order to verify the nature of the phases involved in solid–liquid equilibria and the nature of coupled growth in this ternary system. We find that in all these cases once steady state growth is reached two solid phases grow at the solid–liquid interface with a rod-like coupled growth pattern: The hexagonal phase (DC solid solution) and the FCC phase (NPG solid solution).

The temperature and enthalpy of transformations of organic alloys from the ternary systems (D)camphor–neopentylglycol–succinonitrile (DC–NPG–SCN) were measured by means of differential scanning calorimetry (DSC). The phase diagram of this ternary system was assessed via the CALPHAD approach using Thermo-Calc based on the thermodynamic descriptions of the constituent binary systems published earlier and the thermodynamic and phase equilibrium data for the ternary alloys measured in the present work. Proper agreements between the experimental and the calculated data were achieved. Experiments and calculations show that the DC–NPG–SCN exhibits a nonvariant eutectic reaction with the eutectic point at 297.0 K, 26.3 mol%DC and 14.8 mol%NPG. This eutectic reaction involves the ordered monoclinic crystals (OCs) of NPG and the orientationally disordered crystals (ODICs) of both, SCN (BCC_A2 phase) and DC (hexagonal phase). Unidirectional solidification experiments were performed with the nonvariant eutectic DC–NPG–SCN alloy in order to verify the phases involved in the solid–liquid equilibrium and the nature of eutectic growth. We find that in the eutectic alloy the eutectic growth is irregular: the extremely facetted monoclinic phase (NPG) grows as thin and long needle-like crystals that protrude far into the liquid and are then decorated with nonfacetted eutectic of BCC_A2 (SCN) and hexagonal (DC) lamellae, which mainly grow perpendicular to the growth direction of the monoclinic (NPG) needles. Few unidirectional solidification experiments were performed with dilute ternary alloys with composition close to the univariant eutectic reactions that extend from the constituent binary systems NPG–DC and SCN–DC, respectively. The destabilisation of planar eutectic interfaces due to constitutional supercooling were qualitatively monitored.

An experimental method is presented for the in situ observation of microstructure evolution during solidification or solid phase transformations in metallic alloys with low melting temperature. It is based on the direct observation of a thin metallic sample embedded between polished glass plates using a light microscope. The sample is placed in a Bridgman type furnace that enables unidirectional solidification under well defined conditions. As examples we show microstructures obtained during coupled, lamellar eutectic growth in two alloys with compositions close to the non-variant eutectic T E = 332 K from the Bi–In–Sn system for different solidification conditions. Stable lamellar patterns with an ABCB stacking sequence are found to be dominant for 2D-samples (∼10 μm in thickness), where A ≡ γ-Sn, B ≡ BiIn 2 and C ≡ β-In phase. The spacing of this stacking sequence was determined as a function of growth rate. The results are compared with the data for bulk (3D) samples of the same composition, taken from the literature. The direct observation of the microstructures at the solid/liquid interface allows the verification of the validity of two different descriptions of the Bi–In–Sn phase diagram proposed earlier, namely a description with eutectic and a second with peritectic reactions involving BiIn 2 , γ-Sn, β-In and liquid. All observed solidification patterns are consistent with the eutectic description.

The temperature and enthalpy of transformations of organic alloys from the binary systems neopentylglycol–succinonitrile (NPG–SCN) and 2-amino-2-methyl-1,3-propanediol–succinonitrile (AMPD–SCN) were measured by means of differential scanning calorimetry (DSC). The phase diagrams of these binary systems were assessed via the CALPHAD approach using Thermo-Calc by simultaneously optimizing the thermodynamic and phase equilibrium data measured in the present work. Proper agreements between the experimental and calculated data for the phase diagrams as well as for the thermochemical properties were achieved. Experiments and calculations show that both the NPG–SCN and the AMPD–SCN systems exhibit a non-variant eutectic reaction with the eutectic point at 90.45 mol% SCN (318.0 K) and at 97.39 mol% SCN (325.7 K), respectively. In the NPG–SCN system the temperature of the eutectic reaction is about 3 K higher than the temperature of the transformation from the ordered crystals (OCs) to the orientationally disordered crystals (ODICs), whereas the eutectic reaction in the AMPD–SCN involves the OCs of AMPD and the ODICs of SCN. Unidirectional solidification experiments were performed with selected NPG–SCN and AMPD–SCN alloys in order to verify phases involved in solid–liquid equilibria and the nature of eutectic growth in these systems. We find that eutectic growth in NPG–SCN eutectic alloy occurs with both solid phases being non-facetted and with a rod-like eutectic structure. The eutectic as well as some hypo-eutectic alloys from the AMPD–SCN system show irregular eutectic growth with a non-facetted BCC_A2 phase of SCN and a facetted monoclinic phase of AMPD.

The thermodynamic description of the Ag–Al–Cu system is obtained by modelling the Gibbs energy of all individual phases in the system using the CALPHAD approach. The model parameters have been evaluated, by means of a computer optimisation technique, based on the descriptions of the constituent binaries proposed in the first part of the work and relevant experimental information for ternary alloys both from literature and own experimental measurements. Several vertical and isothermal sections, the liquidus surface and some thermodynamic properties are calculated using the evaluated parameters. A good agreement between the calculations and the experimental data is achieved.

The temperature and enthalpy of transformation of organic alloys from the binary systems neopentylglycol-( d )camphor (NPG-DC) and 2-amino-2-methyl-1,3-propanediol-( d )camphor (AMPD-DC) were measured by means of differential scanning calorimetry (DSC). The phase diagrams of these binary systems were assessed via the CALPHAD approach using Thermo-Calc by simultaneously optimizing the thermodynamic and phase equilibrium data measured in the present work. Proper agreements between the experimental and calculated data for the phase diagrams as well as for the thermochemical properties were achieved. Experiments and calculations show that both the NPG-DC and the AMPD-DC system exhibit a nonvariant eutectic reaction with the eutectic point at 36.2 mol% DC and 326.0 K and at 9.3 mol% DC and 362.0 K, respectively. In each system the temperature of the eutectic reaction is higher than the temperature of the transformation from the ordered crystals to the orientationally disordered (plastic) crystals. Unidirectional solidification experiments were performed with several alloys in order to verify the nature of eutectic growth: We find that in both systems eutectic growth occurs with both solid phases being non-facetted and with a lamellar or rod-like eutectic structure. Due to the optical activity of DC its distribution in the solid samples is well detectible in polarised light.