Phase relations in Ti-Al-Nb alloys at 1200°C (original) (raw)

Phase equilibria and solid state transformations in Nb-rich Nb–Ti–Al intermetallic alloys

Intermetallics, 2000

The solidi®cation pathways and phase equilibria at 1100, 900 and 700 C in seven Nb-rich alloys of the Nb±Ti±Al system were examined. The as-cast microstructures revealed that all alloys had undergone b solidi®cation, extending the range of the b phase ®eld further into the ternary system than previously reported. Changes in the atomic site occupancy preferences with alloy composition were examined for the B2 ordered b 0 phase through atom location by channeling-enhanced microanalysis (ALCHEMI). A b to d massive-like transformation observed in cast material, was reproduced in continuously cooled homogenized samples. The formation of a new phase, y, based on the I4 1 /amd body centered tetragonal structure was observed below 1100 C. In addition, a metastable hP18 structure, produced through an o-type transformation of the b 0 phase, was also observed. #

Effect of thermomechanical processing on evolution of various phases in Ti-Nb alloys

Bulletin of Materials Science, 2011

This paper deals with the effect of thermomechanical processing on microstructural evolution of three alloys, viz. Ti-8Nb, Ti-12Nb and Ti-16Nb. The alloys were hot rolled at 800°C and then subjected to various heat treatments. Samples from hot-rolled alloys were given solution-treatment in β and α + β phase fields, respectively followed by water quenching and furnace cooling. The solution-treated alloys were subsequently aged at different temperatures for 24 h. Phases evolved after various heat treatments were studied using X-ray diffractometer, optical, scanning and transmission electron microscopes. The alloy Ti-8Nb exhibits α and β phases while the alloys Ti-12Nb and Ti-16Nb show the presence of α ″, β and ω phases in the as-cast and hot-rolled conditions. The β solution treated and water quenched specimen of the alloy Ti-8Nb displays α″ phase while the alloys Ti-12Nb and Ti-16Nb exhibit α″, β and ω phases. The alloy Ti-8Nb shows the presence of α, β and ω phases while those of Ti-12Nb and Ti-16Nb display the presence of α, α ″, β and ω in α + β solution treated and water quenched condition. The observation of ω phase in solution treated condition depends on the cooling rate and the Nb content while in the aged specimens, it is governed by aging temperature as well as the Nb content.

A Study of the Kinetics of Phase Transformations in Nb-Ti-Al Alloys

The work reported herein was performed by two Ph.D. students Keith J. Leonard and Joseph C. Mishurda, under the supervision of the PI. The phase equilibria and solid state transformations within fifteen Nb-rich Nb-Ti-Al alloys were investigated. The alloys ranged in composition between 15 and 40 at.% Al with Nb:Ti ratios of 1:1.5 to 4:1. Examination of the as-cast microstructures revealed that all alloys solidified from the ß phase field, with subsequent solid-state transformations occurring within four of the alloys during cooling. The range of primary ß phase solidification was determined to extend beyond the limits of previous liquidus projections. The high temperature ß phase field was verified in each alloy through quenching experiments. The ß phase exhibited B2 ordering at room temperature with the order-disorder transition temperatures evaluated for select alloys. The site occupancy preferences within the ß phase were evaluated through the ALCHEMI technique, which determined that Ti substitution occurred for Nb on Nb sublattice sites with the degree of sublattice partitioning found to depend upon alloy composition.

Properties and structural characteristics of Ti–Nb–Al alloys

Materials Science and Engineering: A, 2005

The use of Al as an ␣ stabilizer in different Ti-Nb alloys has been investigated for its effect on some properties and changes in structural characteristics. Quenched alloys with 10-40% Nb and 2-15% Al were analyzed in terms of their phase transformations, as well as elastic modulus, density and internal friction. It was found that the rhombic distortion introduced in the ␣ -HCP phase transforms into another martensitic ␣ structure. Above 5% Al, metastable ␤-BCC is formed and eventually predominates. The appearance of another metastable phase, , was verified in alloys between 2 and 5% Al with less than 35% Nb. These structure transformations are a consequence of the obstructing effect of Al on the redistribution of Ti and Nb. The elastic modulus increases with Al and decreases with Nb percentage due to different developed structures, especially at lower Al content. Small fluctuations in the generally increasing variation of the density with Nb, for Al percentages, were attributed to the phase transformations. The internal friction was relatively large for the martensitic, ␣ or ␣ , structure but decreases significantly with the appearance of ␤ phase.

Processing and Properties of Nb-Ti-Based Alloys

Superalloys 1992 (Seventh International Symposium), 1992

The processing characteristics, tensile properties, and oxidation response of two Nb-Ti-Al-Cr alloys were investigated. One creep test at 650°C and 172 MPa was conducted on the base alloy which contained 40Nb-40Ti-lOAl-1OCr. A second alloy was modified with 0.11 at. % carbon and 0.07 at. % yttrium. Alloys were arc melted in a chamber backfilled with argon, drop cast into a water-cooled copper mold, and cold rolled to obtain a 0.8~mm sheet. The sheet was annealed at 1100°C for 0.5 h. Longitudinal tensile specimens and oxidation specimens were obtained for both the base alloy and the modified alloy. Tensile properties were obtained for the base alloy at room temperature, 400, 600, 700, 800, 900, and 1000°C, and for the modified alloy at room temperature, 400, 600, 700, and 8OOOC. Oxidation tests on the base alloy and modified alloy, as measured by weight change, were carried out at 600, 700, 800, and 900°C. Both the base alloy and the modified alloy were extremely ductile and were cold rolled to the final sheet thickness of 0.8 mm without an intermediate anneal. The modified alloy exhibited some edge cracking during cold rolling. Both alloys recrystallized at the end of a 0.5-h annealing treatment. The alloys exhibited moderate strength and oxidation resistance below 600°C, similar to the results of alloys reported in the literature. The addition of carbon produced almost no change in either the yield strength or ductility as measured by total elongation. A small increase in the ultimate tensile strength and a corresponding decrease in the reduction of area below 600°C were observed. Carbon addition also served to marginally refine the grain size after annealing. The results of this study and those of similar alloys reported in the literature suggest that 40Nb-40Ti-lOAl-1OCr forms a good base alloy suitable for alloying for improvement in its oxidation and high-temperature strength properties.

Phase equilibria at 1100°C in the Nb–Ti–Al system

Materials Science Engineering a Structural Materials Properties Microstructure and Processing, 2002

The phase equilibria at 1100°C has been examined as part of a larger investigation of the phase equilibria and transformations within the Nb-Ti-Al system. Fifteen alloys ranging in composition from 15 to 40 at.% Al, with Nb:Ti ratios of 1:1.5 up to 4:1, were prepared by arc-melting. The alloys were homogenized and solution treated in the b (bcc) solid solution phase field prior to heat treatment at 1100°C, and the microstructures characterized by optical microscopy, X-ray diffraction (XRD), electron probe microanalysis, scanning and transmission electron microscopy (TEM). The phase equilibria will be discussed along with a comparison with earlier experimental work.

HIGH-TEMPERATURE BEHAVIOUR OF Ti-Al-Nb-Ta INTERMETALLICS

2011

Ti-Al intermetallics have gained significant attention as possible replacements of nickel superalloys in hightemperature applications in automotive, aerospace and energetic industry. The favourable properties of Ti-Al intermetallics are improved by alloying; keeping their low density these materials gain high strength, good structural stability, high-temperature oxidation and creep resistance. One of the common alloying elements used in this application is niobium. It improves the high-temperature properties especially the oxidation resistance. In present time the tantalum is studied as well with same or even better results.