Terminal Solid Solutions Research Articles

Substrate effect on texturized microstructures in NiCr thin films

NiCr thin films with a composition of 40at.%Ni and 60at.%Cr have been sputter deposited on stainless steel and glass substrates. The microstructure and composition have been analysed by scanning electron microscopy, Rutherford-backscattering spectroscopy, X-ray diffraction and transmission electron microscopy. Evidence is found of the metastable σ phase and of the two terminal solid solutions α-Cr and γ-Ni. The microstructure is characterized by a cubic close-packed lattice alternating with a hexagonal one within the same grains, resulting in a stacking fault presence. Evidence is found that the films have a fibre texture with the axis tilted 20° with respect to the surface normal and that different substrates induce different fibre axis orientations: (111) for the stainless steel substrate and (110) for the glass one. This characteristic could be interpreted in terms of different relative concentrations of the two terminal solid solutions due to the different substrate temperature during thin film deposition.

A thermodynamic calculation of the Ni-Nb phase diagram

Abstract An optimized set of thermodynamic functions of the Ni-Nb system has been obtained using phase diagram data and thermodynamic data from the literature. The excess free energies of solution phases, liquid and two terminal solid solutions (Ni) and (Nb), were described by the Redlich-Kister formula. Two compound phases, Ni 3 Nb and Ni 6 Nb 7 , were described by the Wagner-Schottky model: Ni 3 Nb was modelled with two sublattices assuming anti-structure atoms on both sublattices, and Ni 6 Nb 7 was modelled with four sublattices after the formula (Ni,Nb) 2 (Nb,Ni) 1 (Ni) 4 (Nb) 6 , where the second atoms in the first and second brackets are anti-structure atoms. Optimization and calculation were performed alternately with different selections of data in order to get the most acceptable compromise between the scattered experimental data. The calculated phase diagram agrees very well with the experimental values from the literature.

Comparative Creep Properties of Single-Phase Intermetallics of the Ti-Al System

Creep characteristics of polycrystalline single-phase intermetallics, α 2 -Ti 3 AI and γ-TiAl, at 1100 K have been summarized to evaluate their relative strength at elevated temperature at three strain-rate levels, 10 -1 10 −4 and 10 −8 s −1 , and are compared with those of α Ti-Al terminal solid solution alloys. The strength of these intermetallics is obviously higher than that of α, random solid solutions, especially at low strain-rates. However, the difference in high-temperature long-term strength among intermetallic alloys is not significant at any strain-rate level.

Constitutional studies of molybdenum-palladium alloys using ultra-rapidly solidified samples

Phase relationships in the system Mo-Pd in the temperature range 880–1100°C were re-investigated using ultra-rapidly solidified samples. The finely divided microstructures produced using this technique were found to achieve equilibrium on annealing much more rapidly than conventionally prepared materials. Optical microscopy, X-ray diffraction methods and scanning and analytical electron microscopy were used for phase characterization. The solubility limits of the molybdenum-rich and palladium-rich terminal solid solutions were established. No intermediate phases were found at the temperatures investigated.

Room temperature section of the phase diagram of the Cu-Fe-Ge ternary system

The room temperature section of the phase diagram of the Cu-Fe-Ge ternary system was determined by means of X-ray diffraction, microscopic investigation and electron probe microanalysis. The section consists of nine single-phase regions, σ (copper), β (Cu 5Ge), η (Cu 3Ge), τ (germanium), α-Fe, δ (Fe 3Ge), γ (Fe 3Ge 2), ρ (FeGe) and μ (FeGe 2), 15 two-phase regions and seven three-phase regions. All phase boundaries of these phase regions were established. No appreciable solid solubility of iron or copper in germanium was observed. The substitution of small amounts of germanium for copper in the terminal solid solution (copper) is very advantageous to increase the solubility of iron in copper at room temperature

Crystallization of amorphous thin films during heavy-ion irradiation

Crystallization of amorphous thin films of NiZr, NiTi and FeZr during heavy-ion irradiation with 500 keV Xe + ions and without irradiation has been studied. The crystallization temperatures are found to be significantly lowered by ion irradiation. In particular, this effect is pronounced for compositions close to elemental ones, where the crystallization is found to take place through nucleation and growth of terminal solid solutions. A close agreement is found between the onset temperature of crystallization and the upper temperature limit for amorphization of crystalline elemental multilayer with the same heavy-ion irradiation.

The variation of thermal oxidation resistance across certain binary systems

Isochronal oxidation tests have been carried out on numerous compositions based on copper and silver with elements forward in the Periodic Table and compositions based on aluminium and zinc passing back. Variability across the various systems is such that three composition ranges may be defined; the first is the terminal solution where the oxidation parameter, T p, rises from a value typical of a metal to that of a non-metal, a second where non-metal behaviour is the norm, and a third where T p rises in the liquid state to that of the later B element. The results, in particular the rise in the terminal solid solution, are discussed with respect to concepts such as s-d resonance, Fajans' rule, and the limits of the zone of influence of the composite divide. A postscript discusses the fall in oxidation resistance of molten aluminium when alloying elements are present.

Phase relations and transformation kinetics in the high Nb region of the Nb-Si system

Recent research has shown that in the Nb-Si system, it is possible to produce composites consisting of a brittle Nb{sub 5}Si{sub 3} intermetallic matrix and ductile Nb phase particles. These composites exhibit a good balance in mechanical properties, i.e., toughening is provided up to 1500{degrees}C. The currently accepted Nb-Si phase diagram exhibits a wide two-phase field between the terminal Nb-Si solid solution and the intermetallic Nb{sub 5}Si{sub 3} which is bounded by the eutectoid isotherm at 1783{degrees}C. The Nb{sub 5}Si{sub 3} phase shows no change in Si solubility with temperature and the terminal Nb phase shows only a small change in Si solubility at temperatures exceeding 1200{degrees}C. These solubility characteristics indicate the possibility of producing stable two-phase microstructures with a slow rate of thermally induced morphological change. The results of current research have shown that the two phases are indeed thermochemically stable and exhibit little coarsening after a heat treatment of 100 hours at 1500{degrees}C. The phase diagram shows the existence of a high-temperature {beta} phase which is a line compound having a stoichiometric composition of Nb{sub 3}Si, and the {beta} to (Nb + Nb{sub 5}Si{sub 3}) eutectoid transformation is shown at {approximately} 1783{degrees}C. A recent paper, however, shows themore » presence indicates that the {beta} {l reversible} Nb + Nb{sub 5}Si{sub 3} eutectoid isotherm is at {approximately} 1720{degrees}C instead of 1783{degrees}C. In addition to these conflicting results, the variation of Si solubility with temperature in the terminal Nb phase is not well established.« less

Metastable Phase Equilibria in Co-Deposited Ni1−xZrx Thin Films

AbstractWe determine the glass forming range (GFR) of co-deposited Ni1−xZrx (0 < x < 1) thin films by measuring their electrical resistance during in situ constant-heating-rate anneals. The measured GFR is continuous for 0.10 < x < 0.87. We calculate the GFR of Ni-Zr melts as a function of composition and cooling rate using homogeneous nucleation theory and a published CALPHAD-type thermodynamic modeling of the equilibrium phase diagram. Assuming that the main competition to the retention of the amorphous structure during the cooling of the liquid comes from the partitionless crystallization of the terminal solid solutions, we calculate that for dT/dt = 1012 K s−1, the GFR extends to x = 0.05 and x = 0.96. Better agreement with the measured values is obtained assuming a lower ‘effective’ cooling rate during the condensation of the films.

Optimization and calculation of the ZrO 2-MgO system

The experimentally determined phase diagram data in the ZrO 2-MgO system have been critically evaluated. An optimal set of thcrmodynamic functions for this system has been obtained using the selected phase diagram data from the literature. The Gibbs free energies are described by the following models: a regular solution model for the liquid and tetragonal ZrO 2 solid solution, and a subregular solution model for cubic ZrO 2 solid solution. The terminal solid solutions based on monoclinic ZrO 2 and MgO are treated as pure oxides in view of the negligible solubilities. A consistent set of parameters describing the system is presented, and comprehensive companions with the experimental data are made. It is shown that most of the experimental information is satisfactorily accounted for by the present thermodynamic description.

A study of the nucleation characteristics of discontinuous precipitation in a pro-eutectic Cu-Ag alloy

The formation of a solute-depleted matrix and a precipitate phase as a duplex product behind a boundary advancing into a supersaturated matrix is termed as discontinuous precipitation (DP) [1]. The initiation of DP naturally involves the nucleation of heterogeneous grain boundary precipitates and the motion of these grain boundaries which act as the sweeping short-circuit paths for solute transport [1, 2]. These processes, in turn, depend on the structure of individual boundaries [1, 3-5]. Studies on grain boundary and interphase interface structures have indicated that the two may not be identical in nature [6-10]. Information on the relative effectiveness of different types of interfaces in initiating DP is, however, not well documented, and this has provoked the present investigation on a Cu-7 .7a t% Ag alloy. The proeutectic alloy [11] can provide a sample surface with or without strain, grain boundaries, eutectic boundaries with various curvature and discontinuous reaction fronts as the available sites for the onset of DP. Moreover, it is possible to generate artificial interfaces in this alloy quite easily, as follows. At the precipitation temperature (say, 400 ° C) the terminal solid solutions in the Cu-Ag system are almost pure copper or silver [11]. Thus, a coating of copper on a very large-grained polycrystalline specimen of copper-rich supersaturated solid solution may provide numerous grain boundaries at the precipitation temperature, whereas a similar coating with silver may generate incoherent precipitate-matrix type interfaces. It was also proposed to study the initiation of DP at these "synthetic interfaces" to obtain a better insight of the nucleation process. About 300g Cu-7.7at % Ag alloy was prepared from 99.99% pure copper and 99.99% pure silver by induction melting. A large-grained ingot of diameter 10 mm was grown from this alloy under argon atmosphere by the vertical Bridgeman technique. Semicircular discs of thickness 5 mm were cut from the ingot by a low-speed diamond saw, and these specimens were homogenized at 775 410 ° C for 5 h under protective cover before quenching in water at room temperature. The flat faces of these specimens were polished to 0.5/~m diamond followed by electropolishing in concentrated (about 85%) H3PO4 for 20rain, using a stainless steel cathode with a d.c. potential of 2 V, which eventually removed all traces of surface strain on the specimens. One of the two semicircular faces of a selected set of samples was cleaned with acetone and coated with pure copper (1.2 # thick) or pure silver (0.3 ~m thick) by a vapour deposition technique. The strain-free

Optimization and calculation of the Hf-Ni phase diagram

An optimized phase diagram for the Hf-Ni system was constructed using experimental data from the literature. The excess free energies of solution phases, liquid and (Ni) were described by the Legendre polynomials. Eight compounds, i.e. Hf 2Ni, HfNi, Hf 9Ni 11, Hf 7Ni 10, Hf 3Ni 7, HfNi 3, Hf 2Ni 7 and HfNi 5, were modelled as line compounds. Two terminal solid solution phases, i.e. β-Hf and α-Hf, were treated as pure elements b.c.c.-Hf and h.c.p.-Hf respectively. Optimization and calculation were performed alternatively with different selections of data in the composition region 15–50 at.% Ni in order to get the most acceptable compromise between the conflicting experimental results. The data set obtained was subjected to adjustment by systematic trial and error in the ranges of 15–50 and 90–100 at.% Ni where the experimental data were insufficient for a reasonable calculation.

Formation of metastable crystalline phases in light-metal systems by rapid solidification

Abstract The experimental facts concerning the formation of extended terminal solid solutions and new crystalline intermediate phases as a result of rapid solidification of Al and Mg alloys are reviewed and the available approaches to systematization and interpretation of such results are discussed. It is argued that, while alloy chemical and thermodynamic approaches are useful for identifying alloy systems that may be susceptible to particular non-equilibrium effects, kinetic modelling is required to predict the outcome of particular conditions of rapid solidification. The recent progress in such modelling and prediction of morphology, composition, growth temperature and microstructural spacing of solid phases generated by rapid solidification at known front velocity is assessed. To conclude, the practical significance of this emerging capability to predict the consequences of rapid solidification, as applied to light alloys, is outlined.

Determination of the free energy of metastable crystalline and amorphous NbCo alloys and its application to metastable phase formation

The thermodynamic functions of the metastable and stable phases in the NbCo system are calculated on the basis of the available experimentally determined thermodynamic data by applying the CALPHAD method. In particular, the free energies of the amorphous phase and of the terminal solid solution phases are determined. They are used to describe the formation of supersaturated b.c.c. Nb Co and amorphous alloys, prepared by a liquid qenching process with extreme queching rates. In addition, the precipitation of an amorphous phase from a supersaturated Nb Co solid solution can be interpreted by the thermodynamic conditions and the kinetic constraints.

Phase Separation During Film Deposition

ABSTRACTWe report a study of the microstructure and surface morphology of co-deposited Al-Ge films. At 200 °C and above, the terminal solid solutions are obtained, whereas at lower temperatures, metastable amorphous or crystalline phases coexist with the Al-rich terminal phase. For film thickness below 200 nm, lateral phase separation is observed with surface grooving which reflects the bulk microstructure. Analysis of the data with a model for the diffusion process shows that the results are consistent with a surface diffusion mechanism. For thicker films, there is a transition into a layered microstructure in which Al segregates to the surface. This transition is explained in terms of the surface and interfacial energies of the phases.

Ductile-Phase Toughening in Niobium-Niobium Silicide Powder Processed Composites

AbstractThe Nb-Si system offers a possibility of ductile-phase toughening of the brittle Nb5Si3 intermetallic with the terminal niobium-silicon solid solution. Powder composites have been made in which the volume fraction of the terminal Nb-Si phase is systematically varied in a matrix of Nb5Si3 in order to study the extent of toughening. The Nb-Si solid-solution phase was observed to exhibit cleavage failure under both as hot pressed and heat treated conditions, thereby limiting the toughening attained by the presence of this phase. Hot working of the composite results in a dramatic improvement in toughness because of a change in the plastic behavior and fracture mode of the terminal Nb-Si phase from brittle cleavage to a mixed mode of cleavage and ductile microvoid growth and coalescence (dimpled) fracture.

Microstructures and Mechanical Properties of NiAl+Mo In-Situ Eutectic Composites

AbstractThe quasibinary NiAI-Mo system exhibits a large two-phase field between NiAl and the terminal (Mo) solid solution, and offers the potential for producing in-situ eutectic composites for high-temperature structural applications. The phase stability of this composite system was experimentally evaluated, following long-term exposures at elevated temperatures. Bend strengths as a function of temperature and room-temperature fracture toughness data are presented for selected NiA1-Mo alloys, together with results from fractography observations.

Thermochemistry of phases in the system MgGa2O4-Mg2GeO4

The enthalpies of solution in molten 2PbO · B2O3 of phases synthesized at one atmosphere in the system MgGa2O4-Mg2GeO4 have been measured. A spinel solid solution, which is stable at 1400 °C from the MgGa2O4 end-member to 27 mole percent Mg2GeO4, shows endothermic heats of mixing of up to 10 kJ/mole at the solubility limit. The spinelloid phase, Mg3Ga2GeO8, is energetically less stable than a mixture of terminal spinel solid solutions (0.73 MgGa2O4·0.27 Mg2GeO4(sp)+Mg2GeO4(sp)), by 3.63±3.64 kJ/mole. This indicates that the spinelloid is a high-entropy phase. The volume of the spinel solid solution, MgGa2O4-Mg2GeO4, shows a positive deviation from Vegard's Law. Modeling of the cation distribution in the solid solution indicates that this ΔV is due to a change in the spinel type from inverse towards normal as the Mg2GeO4 content increases.

Phase formation during ion beam mixing of transition-metal multilayers

Abstract Evaporated films of alternating layers of early and late transition metals have been ion beam mixed with inert-gas ions for the study of the phase formation during ion irradiation. The mixing was performed with 500 keV Ar+ and Xe+. In addition, a few co-deposited amorphous films were irradiated, using 200 keV Ne+ and 500 keV Xe+ ions. All the ions fully penetrate the films of approximate thickness 800 A. The microstructure was investigated by transmission electron microscopy and X-ray diffraction. The homogeneity and composition were checked by Rutherford backscattering spectrometry. The glass-forming ranges of Fe-Zr, Co-Zr, Ni-Zr, Ni-Ti and Ni-Nb were studied as functions of composition, temperature (30–750 K) and ion mass. Besides the amorphous phase, extended terminal solid solutions and, at higher temperatures, some unidentified compounds were observed. The measured glass-forming ranges are discussed in relation to thermodynamic parameters such as the heat of mixing and equilibrium solid solu...

Application of the CALPHAD method for the prediction of amorphous phase formation

The method of calculating the thermodynamic functions of an alloy system using the CALPHAD approach is described. In particular, the thermodynamics of the undercooled liquid and the amorphous phase are considered, taking into account the influence of the specific heat of the liquid phase. From the free energy functions metastable phase diagrams are predicted, assuming equilibria between the terminal solid solutions and the liquid or amorphous phase. In addition, T 0 lines are calculated for the solution phases. The results are compared with the experimentally observed concentration range of amorphous phase formation after melt spinning or mechanical alloying of the elemental powder mixtures.