Thermodynamic Studies on the System Nb-Al

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THE binary and ternary transition metal-aluminum systems are of particular importance in materials science and technology owing to the excellent high-temperature properties of the intermetallic phases. The Nb-Al system has three intermetallic phases out of which the Nb3Al phase is of particular interest because of its many desirable properties such as low density, high melting temperature, and high yield strength.[1] However, the brittleness of this intermetallic phase limits its application, which can be circumvented by a combination of ternary alloying and rapid solidification. Moreover, Nb3Al shows superconducting behavior having its high transition temperature, Tc = 18.8 K.[2] Accurate knowledge of the phase diagram and thermodynamic data is required in attempting to engineer the properties of the various phases in the Nb-Al system. The phase diagram of the Nb-Al system has been investigated by many researchers and compiled in the literature.[3–9] The Nb-Al system consists of five different phases, which are summarized in Table I.[6] Thermodynamic data for this system have been limited to a few experimental investigations.[10–12] Shilo et al.[10] have carried out vapor pressure measurement on the system Nb-Al in the high-temperature range 1844 to 2146 K using the Knudsen effusion method and derived the enthalpies of formation {DfHm(298.15 K)} of different phases. Meschel and Kleppa[11] have meaS.C. PARIDA, Postdoctoral Fellow is with the Bhabha Atomic Research Centre, Mumbai, India. PRATHEESH GEORGE, Graduate Student, and R. G. REDDY, ACIPCO Professor, Head and Associate Director of the Centre for Green Manufacturing are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected] Manuscript submitted July 7, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B

sured the standard enthalpies of formation of Nb3Al and NbAl3 phases by direct synthesis calorimetry. Mahdouk et al.[12] have determined the enthalpies of formation of the intermetallic phases by direct reaction calorimetry at high temperatures and compared them with the values calculated using the Miedema model.[13] Thermodynamic optimization for the Nb-Al system has been carried out by Servant and Ansara.[14] The thermodynamic data and phase diagram for the system AlNb assessed using the calculation of phase diagram (CALPHAD) approach have been documented in recent literature.[6,8,15,16] All of these assessments and theoretical calculations are based on the limited experimental high-temperature vapor pressure data of Shilo et al.[10] and the calorimetric data of Meschel and Kleppa[11] and Mahdock et al.[12] Colinet et al.[17] have theoretically studied the phase stability of the Al-Nb system using ab initio and total energy calculations starting with crystal structure information. The authors[17] have used the linear-muffin-tin-orbital (LMTO) method in the full potential (FP) approach. It is interesting to note here that the phase diagrams reported[6,8] and those calculat