M.-X. Zhang

Phase diagrams of Ti-Al-C, Ti-Y-O, Nb-Y-O, and Nb-Al-O at 1100°C

To develop suitable coating materials for A12O3 fibers used in Ti-based metal-matrix composites, the phase diagrams of Ti-Al-C, Ti-Y-O, Nb-Y-O, and Nb-Al-0 at 1100 °C were studied. The phase equilibrium samples were prepared by hot isostatic press (HIP), annealed, and then examined by x-ray diffraction (XRD). In the Al-Ti-C ternary, the present study was concentrated on the Ti-rich corner involving Ti, TiC, α2Ti3A1, and the ternary Pphase (T3AlC). The results show that at 1100 °C, β-(Ti, Al) is in equilibrium with TiC, but not in equilibrium with the Pphase as suggested by Schuster et al. In the Ti-Y-O, Nb-Y-O, and Nb-Al-0 ternary systems, the present study shows that Ti-Y2O3, Nb-Y2O3, and Nb-Al2O3 are in equilibrium at 1100 °C.

Synthesis of in situ composites through solid-state reactions: thermodynamic, mass-balance and kinetic considerations

The applications of thermodynamics, mass balance and kinetics to the synthesis of in situ composites through solid-state reactions are discussed. The proper choice of starting materials, the principles governing the diffusion paths and the formation of desirable microstructures are considered. The use of stability diagrams for rationalizing diffusion and reactions during composite synthesis is also demonstrated. Applying these principles to the synthesis of NbS1 2 -SiC composites, it is shown that the proper starting materials are Nbc 1-r and Si. The probable microstructure is an aggregate type, composed of NbSi 2 and SiC. Preliminary experimental results of the study of bulk NbC 1-x -Si diffusion couples annealed at 1300 °C for 60 h revealed that the microstructure is indeed a two-phase mixture of NbSi 2 and SiC. Discontinous SiC particles with an average size of 1 μm are homogeneously dispersed in the NbSi 2 matrix. This microstructure is considered favorable for high-temperature structural composite application. Further characterization of mechanical and other properties for this material are currently being performed.

Interdiffusion between the ordered intermetallic compound TiAl and Mo

Abstract Diffusion experiments on Mo/γ-TiAl couples were conducted at 900, 1000 and 1100 °C for annealing times varying from 121 to 553 h. Two intermediate phases, δ-(Mo,Ti)3Al and β′-(Mo,Al)Ti, were found to develop between the Mo and γ-TiAl phases. The diffusion path was found to be Mo/δ-(Mo,Ti)3Al/β′-(Mo,Al)Ti/γ-TiAl. The growth of both the δ and β′ phases can be described by parabolic relationships. All diffusion couples were examined by electron probe microanalysis (EPMA). The Boltzmann-Matano analysis was applied to the δ and β′ phases. The interdiffusion coefficients obtained at 1000 °C are: D MoMo δ = 3.0 × 10 −12 , D AlAl δ = 2.9 × 10 −12 , D MoMo β′ = 3.2 × 10 −12 and D AlAl β′ = 4.0 × 10 −12 cm 2 /Sec . Using the Darken-type relationship between interdiffusivities and intrinsic diffusivities, the intrinsic diffusion coefficients of the β′-phase were also obtained. The intrinsic diffusion coefficients obtained at 1000 °C are: DMoβ′ = 2.0 × 10−12, DAlβ′ = 8.7 × 10−12 and DTiβ′ = 4.7 × 10−12 cm2 sec Analysis of the data for the Mo/γ-TiAl couple annealed at 1100 °C for 308 h shows that the high interdiffusion fluxes of Mo are a consequence of the lattice movements toward γ-TiAl with respect to the Matano plane. The existence of planar interfaces in this diffusion couple was also discussed. By determining the dominant diffusing species in each intermediate phase and the velocities of all three interfaces, it was shown that planar interfaces should be stable during the growth of the δ and β intermediate phases.

Oxidation of single-phase Pb-In alloys

The oxidation of several single-phase Pb-In alloys has been studied in air at room temperature using AES (Auger electron spectroscopy) combined with sputter-depth profiling. Alloy samples with indium composition between 3 and 64 at.% In, which were prepared using a microtome, were oxidized in air. The oxidation of alloys with low In contents was found to be the same as that of Pb-2.9 at.% Sn.1 Increasing the bulk composition of In increased the ratio of oxidized In to oxidized Pb in the oxide mixture, although Pb oxide was observed even on the surface of the oxide for samples up to 64 at.% In. The oxidation behavior of Pb-In alloys can be explained in terms of preferential oxidation of In due to its much greater affinity for oxygen than Pb.

Oxidation kinetics of a Pb-64 at.% in single-phase alloy

The solid-state oxidation kinetics of a Pb-64 at. % In (50 wt. %) single-phase alloy were studied from room temperature to 150°C using an AES (Auger Electron Spectroscopy) depth profiling technique. The general oxidation behavior of this alloy is different from that of a Pb-3 at.% In alloy but similar to that of a Pb-30 at.% In alloy. The oxide formed on this alloy is almost pure In oxide (In2O3) with the possible existence of some In suboxide near the oxide/alloy interface. At room temperature, oxidation of the alloy follows a direct logarithmic law, and the results can be described by the model proposed previously by Zhang, Chang, and Marcotte. At temperatures higher than 75° C, rapid oxidation occurred initially followed by a slower parabolic oxidation at longer time. These data were described quantitatively by the model which assumes the existence of short-circuit diffusion in addition to lattice diffusion in the oxide as proposed by Smeltzer, Haering, and Kirkaldy. The effects of alloy composition on the oxidation kinetics of (Pb, In) alloy are also examined by comparing the data for Pb-3, 30, and 64 at. % In alloys.

Stability of an alloy/oxide interface with oxygen ions being the dominant diffusing species in the oxide scale

Abstract The criterion for interfacial stability between an alloy and its oxide was formulated by Wagner [J. electrochem. Soc.103, 571 (1956)] for the specific case when the cations are the diffusing species in the oxide scale. However, for the oxidation of (Pb, In) and (Pb, Sn) alloys, oxygen in the predominant diffusing species in the oxide scales. The criterion formulated by Wagner is no longer applicable. In the present study, the criterion was formulated for the case when oxygen ions are the dominant diffusing species, following the approach used by Wagner. The criterion was used to determine the interfacial stability of (Pb, In)/In2O3. In order to determine the (Pb, In)/In2O3 interfacial stability, it is necessary to know the interdiffusion coefficient of (Pb, In) and the intrinsic diffusion coefficient of oxygen in In2O3. Intrinsic diffusion coefficients for In2O3 are not available and are estimated from the parabolic rate constants for the oxidation of (Pb, In) alloys. Calculations using the interdiffusion coefficient of (Pb, In) from the literature and the estimated intrinsic diffusivity of oxygen indicated that the (Pb, In)/In2O3 interface should be planar.