Microstructure in rapidly solidified AlTi alloys

A. Majumdar, B. C. Muddle

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Rapidly solidified ribbons (50-70 μm thick) of AlTi alloys containing 2, 5 and 10 wt.% Ti have been melt spun under a helium atmosphere and the resulting microstructures characterized using analytical electron microscopy. As a result of the high cooling rates (about 105 K s-1) achieved, the formation of the coarse primary equilibrium intermetallic phase β (b.c.t., DO22 Al3Ti) in this system is suppressed. Ribbons of Al2Ti ald Al5Ti alloys comprise two microstructural zones in sections normal to the ribbon plane. The zone adjacent to the substrate surface is exposed to the largest degree of undercooling and is a single-phase, f.c.c. solid solution (α) of titanium in aluminium, with a grain size of 1-3 μm and a maximum titanium content of 4.0±1.0 wt.% in the Al5Ti alloy. Nearer the free surface, there is a two-phase structure comprising cuboidal particles (0.1-0.3 μm) of an ordered cubic, metastable intermetallic phase β′(L12, a = 0.404 nm) in α-Al solid solution. The metastable phase contains approximately 20 at.% Ti. It forms as primary particles directly from the melt and these particles subsequently provide effective nucleation sites for supersaturated α solid solution. The microstructure in rapidly solidified Al10Ti ribbons is uniform through the ribbon thickness and comprises particles of both the metastable β′ and the equilibrium β phases in an α matrix. Epitaxial growth of the metastable phase is commonly observed to surround particles of the equilibrium phase which are incompletely dissolved during the melt spinning cycle. This rim is effective in subsequent nucleation of several grains of the α solid solution, each sharing an identity orientation relationship with the metastable cubic phase.

Original languageEnglish
Pages (from-to)135-147
Number of pages13
JournalMaterials Science and Engineering A: Structural Materials: Properties, Microstructure and Processing
Issue number1-2
Publication statusPublished - 15 Sept 1993

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