Publications: (Journals: 3; Conferences: 3)

1. NSF Grant Number : MET DMR-0312172PI : Dr. R. G. Reddy Institution : The University of Alabama Title : Fundamental Studies on Intermetallic Aluminides Results from the EMF studies indicated that the activities of Al and Nb follow negative deviation from ideality showing strong interaction between the constituents. The intermetallic phase NbAl3 has been found to be the most stable phase from the Gibbs energies of mixing data. The activation energies for the oxidation of Ti-Al-Nb alloys calculated from 750 to 1100oC show two distinct slopes with a break at 900oC which suggest that the mechanism of oxidation are different in the low and high-temperature regions. Further, the X-ray diffraction patterns of the oxide layers supported that the predominant oxide layer at low temperature is Al2O3 and at high temperature is TiO2. (i) To investigate the thermodynamic properties of intermetallic aluminides in the Ti-Al-X (X= Nb, Mo, etc.) systems by EMF method using a solid state galvanic cell; (ii) To study the stability of various intermetallic phases in different environments such as air, oxygen, and sulfur dioxide. Thermodynamic properties Nb-Al system which is sub-system of Ti-Al-Nb have been investigated. Five selected alloys in the Nb-Al system with XAl = 0.03, 0.19, 0.32, 0.73 and 0.90 have been prepared and characterized by XRD, SEM and EDAX. A solid-state galvanic cell (EMF cell) with CaF2 solid electrolyte has been designed with ultra-high purity gas purification system and standardized to test the performance by using (Ni-NiF2) and (Fe-FeF2) coexistence electrodes. After standardization, e.m.f. measurements have been carried out for the Nb-Al system in the temperature range from 873 to 1078 K and thermodynamic properties of alloys and intermetallics have been calculated form e.m.f. data. Oxidation studies on the ternary alloys of Ti-Al-Nb (atom % 75-21-4) have been carried out in oxygen at 1atm. pressure from 750 to 1100oC using Perkin Elmer TGA 7 HT apparatus. The oxidation products have been characterized by SEM, EDAX and XRD techniques. The TGA data have been used to calculate the activation energies of oxidation at different temperatures. The SEM, EDAX and XRD figures have been used to elucidate the mechanism of oxidation. a b d C The successful completion of this project will enhance our fundamental understanding of the phase stability in advanced materials under the influence of diverse environments. The thermodynamic data will enable calculation of correct phase diagrams through CALPHAD approach. The oxidation data will enable the engineering of the growth of user-defined layered materials which will pave way for a new technology of synthesizing newer materials. Figures: (a) Activity of Al and Nb in Nb-Al alloys; (b)XRD patterns of oxidation products of Ti(75)-Al(21)-Nb(4) alloy; (c) and (d) SEM images of oxidation products of Ti(75)-Al(21)-Nb(4) alloys at900 and 1100oC, respectively. Publications: (Journals: 3; Conferences: 3) 1. D. Livingston, D. Mantha, R.G. Reddy, High Temp. Mater. Proc., Vol. 24(4), 2005, pp.259-267.