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On the formation and stability of precipitate phases in a near lamellar γ-TiAl based alloy during creep
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Indian Institute of Science, Bangalore, India.
2018 (engelsk)Inngår i: Intermetallics (Barking), ISSN 0966-9795, E-ISSN 1879-0216, Vol. 98, s. 115-125Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The formation, evolution and stability of metastable phases observed in the γ-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 γ-α2 interface which acts as sink for the alloying elements ejected from the dissolving α2 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>γ and <10-10>α2 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 L10 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 α2-γ interface also suggest that the C14 acts as buffer layer between α2 and γ phases in the presence of local stresses, although this is not the thermodynamically expected phase at the temperature of creep experiment.

sted, utgiver, år, opplag, sider
Elsevier, 2018. Vol. 98, s. 115-125
Emneord [en]
B2 and C14 Laves phases, Creep deformation, Density functional theory, Gamma titanium aluminide, Interface stress, Phase transformation, Transmission electron microscopy
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Identifikatorer
URN: urn:nbn:se:kth:diva-228726DOI: 10.1016/j.intermet.2018.04.017ISI: 000435053700014Scopus ID: 2-s2.0-85046703106OAI: oai:DiVA.org:kth-228726DiVA, id: diva2:1210820
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QC 20180529

Tilgjengelig fra: 2018-05-29 Laget: 2018-05-29 Sist oppdatert: 2018-07-02bibliografisk kontrollert

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