On the formation and stability of precipitate phases in a near lamellar gamma-TiAl based alloy during creep

Babu, Prasath R and Vamsi, RV and Karthikeyan, S (2018) On the formation and stability of precipitate phases in a near lamellar gamma-TiAl based alloy during creep. In: INTERMETALLICS, 98 . pp. 115-125.

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Abstract

The formation, evolution and stability of metastable phases observed in the gamma-TiAl based alloy Ti-47Al-2Cr-2Nb was studied under creep deformation with stress applied at two different hard orientations in a highly textured as-cast + HIPed material. Previously we have reported that the metastable phase Ti(Al,Cr)(2) with C14 Laves phase structure forms at the gamma-alpha(2) interface which acts as sink for the alloying elements ejected from the dissolving alpha(2) phase and also expected to effectively control the interface stresses through short range diffusion and modifications in the chemical composition 1]. Ab initio density functional theory based calculations were carried out to evaluate the effect of choice of lattice position and site occupancy of aluminium atoms in the Ti (Al,Cr)(2) structure on the lattice parameter variation and thermodynamic stability. C14 with the composition 25 at. % Al was found to have lattice parameter values close to the inter-planar spacing of < 110 > (gamma) and < 1010 > alpha(2) which would have a lower misfit with C14 across the interface. From the cohesive energy calculations, Laves phase C14 with a constrained lattice parameter due to the adjoining phases, exhibits higher stability than the B2 and Ll(0) structures across a range of compositions studied. Electron diffraction simulations of C14 with a composition of 25% Al compared with the experimental data suggest that the structure C14 has taken up either a random site occupancy compared to a specific choice of ordering to minimize the interfacial stress. Though the experimental evidences do not strongly support a long-range ordering theory in C14, short-range ordering could be a tangible choice for alleviating interface misfits. The ability of C14 to assume different lattice parameters at and far from the alpha(2)-gamma interface also suggest that the C14 acts as buffer layer between alpha(2) and gamma phases in the presence of local stresses, although this is not the thermodynamically expected phase at the temperature of creep experiment.

Item Type:	Journal Article
Publication:	INTERMETALLICS
Publisher:	ELSEVIER SCI LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, OXON, ENGLAND
Additional Information:	Copy right of this article belong to ELSEVIER SCI LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, OXON, ENGLAND
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	02 Jul 2018 14:50
Last Modified:	21 Feb 2019 11:42
URI:	http://eprints.iisc.ac.in/id/eprint/60114

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