D. M. Dimiduk

PROGRESS IN THE UNDERSTANDING OF GAMMA TITANIUM ALUMINIDES

During the last two years remarkable improvements have been made in the properties (such as toughness and creep) and processing technology of gamma titanium aluminides, making them potentially viable engineering alloys for high-temperature structural applications. These achievements were made possible by a greater understanding of both the fundamental and the practical aspects of these aluminides, such as phase relationships, the effects of alloying elements, deformation mechanisms, microstructure evolution and processing. This article reviews the current understanding of the above specific aspects and the processing-microstructure-property relationships, and identifies pacing problems and applications.

The development of Nb-based advanced intermetallic alloys for structural applications

A new generation of refractory material systems with significant increases in temperature capability is required to meet the demands of future aerospace applications. Such materials require a balance of properties such as low-temperature damage tolerance, high-temperature strength, creep resistance, and superior environmental stability for implementation in advanced aerospace systems. Systems incorporating niobium-based beta alloys and intermetallic compounds have the potential for meeting these requirements.

Advanced intermetallic alloys—beyond gamma titanium aluminides

Recent studies have shown that Nb-base refractory intermetallic materials may be viable for service at temperatures that compete with the nickel-based superalloys in structural applications. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high operating temperatures, two-phase or multiphase intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. Exploratory efforts on these systems have indicated that damage tolerance, creep resistance and oxidation tolerance may be obtained and controlled simultaneously. In turn, these findings have led to alloy development strategies and approaches which are currently under investigation and are expected to lead to focused research on a small set of alloys. This manuscript presents an overview of recent results on selected Nb-base intermetallic systems with particular emphasis on phase equilibria and mechanical behavior.

Microstructures and Mechanical Behavior of Nb-Ti Base Beta + Silicide Alloys

Studies on Nb/Nb 5 Si 3 based in-situ composites have demonstrated an acceptable balance of low-temperature damage tolerance and high-temperature strength/creep resistance. However, catastrophic oxidation and embrittlement of these materials limit their usefulness in structural applications. Alloying studies were initiated at Wright Laboratory with the aim of obtaining incremental improvement in the overall oxidation response of the Nb/Nb 5 Si 3 system, while seeking microstructurally similar systems. The results showed that reduced metal recession rates and oxygen embrittlement can be obtained by Ti and Al additions to Nb-Si base alloys. This paper focuses on the effect of Ti and Al alloying additions to Nb-Si base alloys on phase equilibria, microstructures, temperature dependence of strength, low-temperature toughness, and environmental resistance.

Compressive creep behavior of Nb5Si3

Abstract The compressive creep data for Nb5Si3 can be described by the following relation: e = 2.773 × 10 −22 kT exp ( −28125 T σ 0.8±0.1 in the temperature range of 1200 to 1400 °C and stress regime of 52 to ~174 MPa. Based on a stress exponent of ~1, and activation energy of 234 kj mol , it is suggested that creep of Nb5Si3, in the temperature range 1200–1400 °C probably occurs by the Nabarro-Herring mechanism with bulk diffusion of Nb in Nb5Si3 as the probable rate-limiting process. However, inhibition of grain boundary sliding by precipitates at triple points is believed to assist crack nucleation and propagation, thereby limiting creep ductility in this material.

Microstructure development in gamma alloy mill products by thermomechanical processing

Gamma titanium aluminides are emerging as a revolutionary high-temperature material. The last decade led to significant engineering advances and component demonstrations for cast gamma-alloy products. However, cast processing has yet to permit the full potential of these materials to be realized, principally as the result of the limited microstructural control available. Conversely, thermomechanically processed gamma alloys are leading to a wide spectrum of microstructures, an outstanding balance of properties within the alloy class, and prospects for improved alloy durability. The manuscript succinctly describes the general field of thermomechanical processing for gamma alloys, giving particular attention to homogenization of large ingots in wrought processing. Aspects of producing fine-grained fully-lamellar microstructures, having controlled lamellar characteristics in wrought mill products are discussed. Some influences of alloy chemistry are discussed to show the feasibility of producing high-strength alloys across the gamma alloy class. Both a β-phase forming element (1 at.% Mo) and boron in the alloys are examined as grain-size controlling agents. These alloys are compared along with traditional alloys containing neither of these elements. Thermal-treatment windows are identified and discussed for producing fully-lamellar materials. When grain-size controlling agents such as boron or beta-phase are used, the lamellar transformation kinetics may be significantly altered relative to conventional gamma alloys, thus changing the thermal process path and affecting the perfection of the lamellar microstructures. These lead to concomitant changes in alloy properties. The prospects for attaining such structures and properties in large product scales are discussed.

Phase transformations in Nb-Al-Ti alloys

Phase relationships as well as morphological and crystallographic features in Nb-rich Nb-Al and Nb-Al-Ti alloys have been investigated. The phase boundaries involving the bcc and Nb3Al (A15 structure) were experimentally determined and several isothermal sections of the Nb-rich corner of the Nb-AI-Ti phase diagram established. The present findings show that (a) the solubility of Al in Nb is considerably less than that reported previously, (b) the high-temperature bcc phase undergoes an ordering transformation to the B2 structure, and (c) the ω phase also forms in these alloys. The sequence of decomposition of the high-temperature bcc phase during isothermal decomposition in the bcc + Nb3Al phase field has been systematically studied in these alloys. A wide variety of morphological features were found to be associated with the Nb3Al precipitates that formed in the bcc/B2 matrix during isothermal heat treatments. The lengthening kinetics of the plate-shaped Nb3Al precipitates were also studied.

Dislocation structures and deformation behaviour of Ti-50/52Al alloys between 77 and 1173 K

Abstract Coarse grained, binary alloys of Ti-50at.%Al and Ti-52at.%Al, containing low (approximately 250 wt.ppm) levels of interstitials, have been deformed in compression over a range of temperatures (77–1173 K). The resulting flow stress-temperature profiles in both the alloys consist of three distinct regimes: (I) 77–600 K (normal); (II) 600–1073 K (anomalous); (III) above 1073 K (normal). The fine structure of dislocations in the deformed samples was observed using weak-beam microscopy. It appears that the 1 2 〈110] dislocations are the most difficult to move at 77 K and room temperature (RT). At low temperatures, deformation is controlled by twinning and the 〈011] superdislocations. The mobility of the 1 2 〈112] dislocations is intermediate between the 〈011] and 1 2 〈110] dislocations. As the deformation temperature increases, all the dislocations become relatively mobile, but the 〈011] dislocations become increasingly blocked as they adopt a thermally activated non-planar configuration and thereby give rise to a flow anomaly. Finally, at very high temperatures, the 〈011] superdislocations decompose to 1 2 〈110] unit dislocations and 1 2 〈112] superdislocations which are sufficiently mobile such that the strength decreases following the flow stress peak.

Anisotropic analysis of dislocation line energies and possible dislocation core dissociations in MoSi2

Abstract The line energy factors of(110)[001], [331], [11],[11];(100)[010];(103)[331], [010]; and (011)[100], [111] dislocations as well as the elastic interaction forces between two 1/4<331] and two 1/4<111] partials, in MoSi2, are calculated using anisotropic elasticity theory. The line energy factors are found to be relatively large (170–250 GPa) and isotropic, whereas the non-radial interaction forces are found to be a small fraction of the radial forces. The atomic configurations around planar faults on the {110), {013) and {116) planes are analysed using the embedded atom method technique. Results of such calculations are used to energetically rank the possible core dissociations of 1/2<331], 1/2<111], <110], and <100] dislocations on the {110), {013) amd {116) planes in MoSi2. Collectively, these results suggest that the core structures of 1/2<331], 1/2<111], <110] and <100] dislocations are expected to be complicated and non-planar, similar to dislocation cores in b.c.c derivative B2 structures. T...

Effect of Solutes on Phase Stability in A13 Nb

Alloying transition metal trialuminides such as Al 3 Nb with appropriate ternary elements may stabilize the ordered cubic Ll 2 structure relative to the DO 22 structure. This may enhance the symmetry related contributions to plastic flow, thereby aiding in overcoming the low ductility in the trialuminide systems. The present investigation was directed towards exploring the potential for obtaining new ternary Al 3 Nb-based intermetallics with the Ll 2 structure. The effects of various ternary elements on the phase stability of A1 3 Nb were studied by SEM, XRD, and EPMA. Results of the A1 3 Nb-X phase relationship studies are presented. Preliminary microhardness results on the ternary DO 22 Al 3 Nb-X phases are also reported. The predictive capabilities of Pettifors structural stability scheme are evaluated in the light of the present investigation.