P. Tsakiropoulos

On the oxidation behaviour of MoSi2

Abstract 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.

Thermodynamic reassessment of the Mo–Si and Al–Mo–Si systems

The thermodynamic properties and phase diagrams of the Mo‐Si and Al‐Mo‐Si systems are assessed, and a complete thermodynamic description is obtained. The calculation results show good agreement with the experimental data. The high temperature phase equilibria involving the C40 and C11b MoSi2 structures in the Mo‐Si binary and Al‐Mo‐Si ternary systems have been clarified. # 2000 Elsevier Science Ltd. All rights reserved.

A generalized law of mixtures

The mechanical properties of two-phase composites are predicted using a rigorous continuum mechanics analysis and an equivalent microstructural transformation approach. This leads to a generalized law of mixtures which is contrasted with the classical linear law of mixtures which requires some explicit assumptions. The generalized law of mixtures enables prediction of a variety of mechanical properties of a two-phase composite with any volume fraction, grain shape and phase distribution. It is shown that the classical linear law of mixtures is a specific case of the generalized law of mixtures. Examples are given from continuous Cu-W composites, the particulate Co-WC system, Al/SiCp composites, α-β Ti-Mn alloys and α-β Cu-Zn alloys for the predictions of properties such as Youngs modulus, yield strengths, flow stresses, the overall friction stresses and the overall Hall-Petch coefficients. It is shown that the theoretical predictions by the generalized law of mixtures are in very good agreement with the corresponding experimental results drawn from the literature, for both continuous fibre composites and particulate reinforced systems.

On the ω phase formation in Cr-Al and Ti-Al-Cr alloys

Abstract The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr2Al phase (C11b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.

The intermediate and high-temperature oxidation behaviour of (Mo,X)Si2 (X = W, Ta) intermetallic alloys

Abstract The oxidation behaviour of (Mo,X)Si 2 (X = W, Ta) alloys was studied at low (773 K) and high temperatures (1073, 1673 K). W and/or Ta additions did not suppress pest oxidation which was shifted slightly to temperatures higher than 773 K by Ta. Mo and Si were detected in the oxides formed in pest oxidation. All alloys exhibited low oxidation rates at T > 1000 K. A silica scale was formed at T > 1600 K. The scale thickness depended on alloy composition with the thinner scale formed on the alloys with Ta addition. Formation of Mo 5 Si 3 beneath the scale was not detected.

ω-phase formation in V[sbnd]Al and Ti[sbnd]Al[sbnd]V alloys

Abstract ω-phase formation in V-50 at.% A1 alloy and Ti[sbnd]Al[sbnd]V alloys has been studied using electron diffraction. It is shown that both electron and size factors are important in affecting the magnitude of the deviation of diffuse ω maxima from the commensurate ω maxima positions. It is suggested that the addition of the electron-deficient element Al to the later transition metal V will destabilize the high-temperature β-phase and favour ω formation. It is expected by analogy that the w transformation may occur in other later transition-metal based alloy systems such as Nb[sbnd]Al, Mo[sbnd]Al, Cr[sbnd]Al and Mn[sbnd]Al. Structural models have been proposed to interpret the heavy streaking effects in β + ω and B2 + ω structures.

Extension of the Hall-Petch relation to two-ductile-phase alloys

Abstract The Hall-Petch relation, developed originally for single-phase alloys, has been extended to alloys containing two ductile phases. The extended Hall-Petch relation can separate the contributions from the different kinds of boundary to the overall efficiency of grain and phase boundaries as obstacles to dislocation motion. The overall friction stress and the overall Hall-Petch coefficient for the two-ductile-phase alloys have been derived as functions of the Hall-Petch constants of the different constituent phases and their microstructural parameters. The extended Hall-Petch relation has been applied to α-β Ti-Mn alloys, α-β Cu-Zn alloys and α-γ Fe-Cr-Ni stainless steels to evaluate the efficiency of the phase boundaries as obstacles to dislocation motion. It is concluded that phase boundaries are not always the strongest obstacles to dislocation motion as in the case of α-γ Fe-Cr-Ni stainless steels.

An experimental study of bonding and crystal structure modifications in MoSi2 and MoSi2+xAl (x=10 to 40 at% Al) via Auger parameter shifts and charge transfer calculations

Abstract The alloying behaviours of as-cast MoSi2 and MoSi2+xAl alloys have been studied using high-energy XPS with Cr Kβ radiation. The charge transfer occurring upon alloying was calculated using the variations in the Auger parameters of Mo, Si and Al between alloyed and unalloyed conditions and the linear potential model of Thomas and Weightman and the non-linear potential model of Cole, Gregory and Weightman. In MoSi2 there was a significant increase in the Auger parameter of Si, while the shift in the Auger parameter of Mo was negligible. The charge transfer towards the Si atoms was close to zero and is smaller compared to theoretical calculations. It is concluded that the atomic bonding between Mo and Si is of a covalent p–d character. In MoSi2+xAl alloys, similar observations were made for Mo and Si, while the Auger parameter of Al was reduced. Donation of electronic charge by Al atoms is possible; covalent bonds of Al with Mo are formed. The plasmon loss structures of the Si 1s and Al 1s peaks showed reduced intensity in the alloys relative to the pure metals. This was attributed to more strongly bound valence electrons. The opposite was the case for the Mo 2p3/2 peak. The substitution of Si by Al atoms is confirmed, in agreement with previous studies.

A study of charge transfer in vapour deposited Mg—V crystalline alloys via changes in Auger parameters

Abstract An analysis of variations in the Auger parameters of vapour deposited Mg—V alloys with nominal compositions 0.48, 2.95, 8.9 and 15 at.% V has been undertaken in order to investigate the electronic changes that take place upon alloying Mg with V. The presence of vanadium in the Mg—V system required certain assumptions to be made in the calculations as a result of the electronic configuration of the valence band of V. Charge transfer from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. The application of Thomas and Weightmans approach is shown to have advantages over the use of core ionization energy shifts in combination with the potential model. With the use of the Auger parameter there is no need for explicit energy referencing of the core levels and, moreover, in the improved model the Madelung or surface dipole contributions are not required unlike in the potential model. An at...

Study of alloying behaviour in metastable Mg-Ti solid solutions using Auger parameter measurements and charge-transfer calculations

Abstract The Auger parameter in vapour-deposited Mg-Ti alloys (Mg-4.2 at.% Ti, Mg-7.6 at.% Ti, Mg-15.1 at.% Ti, Mg-20.7 at.% Ti, Mg-25.3 at. % Ti and Mg-30.2 at.% Ti) has been measured using high-energy X-ray photoelectron spectroscopy with a Cr Kβ (hv = 5946.7 eV) X-ray source. The charge transfer occurring upon alloying was estimated using the Thomas-Weightman model. The data were used to investigate changes occurring in the electronic structure upon alloying. Changes in the Auger parameter of the Mg-Ti alloys suggested that, in contrast with Mg, Ti is better screened in the presence of a core hole in the alloy than in the pure form. The Mg-to-Ti charge transfer in the Mg-Ti alloys was smaller than in the Mg-V alloys studied previously. This is in agreement with the Pauling electronegativities of V and Ti.