The Isothermal Oxidation Behavior of a Ti3Al-Nb-V-Mo Alloy
The isothermal oxidation behavior of Ti-24Al-14Nb-3V-0.5Mo alloy in synthetic air from 700°C to 1000°C was studied. The main emphasis was focused on the effect of microstructure obtained by different processes of melting and hot rolling, i.e. large and duplex of 2+B2, on the oxidation resistance. Regardless of the microstructure, the overall oxidation process of the specimens exposed at 700°C was dominantly controlled by the beneficial effect of Nb-addition due to the formation of a continuous -Al2O3 layer in the scale. However, both specimens showed poor oxidation resistance above 900°C, especially for the as-rolled specimen, because of higher volume fraction of the 2-phase. The spallation occurred in both specimens at 1000°C, which was probably relative to the formation of Ti2AlN between the thick scale and the substrate except for the TiO2 and -Al2O3.
Intermetallic Compounds for Strong High-Temperature Materials: Status and Potential
Materials of high strength have long been sought, but various needs - particularly those for aerospace applications - have added new dimensions: First, for use in aircraft and spacecraft parts of the least weight to do a given job are desired, i.e. materials of high specific strength (strength per unit density) or high specific stiffness (elastic modulus per unit density). Second, for inherently hot parts such as propulsion systems and their housings, strength and weight at elevated temperature are critical. The efficiency and total thrust of a jet engine increase with the peak temperature of the working fluid, and current limits are set by thermal constraints on materials. This paper reports that there are three features of the high- temperature intermetallics that give promise for better materials. First, many unexplored materials are available - about 400 binary compounds that melt above 1400{degrees}C. Second, the strong unlike-neighbor bonding that is responsible for ordering gives rise to high elastic moduli, and high moduli also imply high strength. Finally, some intermetallics have increasing strength with temperature (d{sigma}/dT {gt} 0) over a significant range. This behavior was first seen in Ni{sub 3}Al, later in other compounds of the same Ll{sub 2} structure, in B2more » and DO3 (cubic) compounds, and in tetragonal Be{sub 12}Nb(D2{sub b}), and TiAl (Ll{sub 0}). This property of d{sigma}/dT {gt} 0 is of obvious use in giving high strength at elevated temperature; it is equally important in not giving very high strength at low temperatures and thereby diminishing the chances of brittle failure at ambient temperatures.« less
Thermodynamic aspects of TiAl and TiSi2 oxidation: The Al-Ti-O and Si-Ti-O Phase diagrams
Thermodynamic data from the literature were used to calculate the activity variations of Ti and Al in the Ti-Al system at 700, 900, and 1100°C and of Ti and Si in the Ti-Si system at 800, 1000, and 1300°C. The activity data were then applied to estimate the stable oxide phase in equilibrium with the metal phase, assuming that the activity of the oxide phase was unity. It was found that in the Ti-Al-O system the stable oxide changed from TiO to Al2O3in the existence region of the TiAl phase, while in the Ti-Si-O system a comparable change from Ti-oxide to SiO2occurred in the existence region of the Ti5Si3phase. The significance of these results to the oxidation of intermetallics in the Ti-Al and Ti-Si systems is discussed briefly.