Microstructural Evolution in Gamma Titanium Aluminides During Severe Hot-Working
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INTRODUCTION
INTERMETALLIC titanium aluminides are intended to replace the currently used nickel-based superalloys in aircraft engines because of their low density and high structural stability. Of particular interest are the so-called high niobium-containing alloys within the approximate composition range Ti-(44-47)Al(5-8)Nb-X (in at pct) with X representing small amounts of metallic and non-metallic elements.[1,2] The alloys mainly consist of the intermetallic phases c(TiAl) and a2(Ti3Al), where the volume fraction of the a2 phase is between 5 and 20 pct. The microstructures are relatively coarse as well as chemically and structurally very inhomogeneous after casting due to segregation processes mainly consisting of large lamellar colonies that are composed of c and a2 platelets and a small number of c grains.[3,4] Such coarse and inhomogeneous microstructures are particularly harmful to the alloys. Because of their structural stability, they are intrinsically brittle so that local differences in plastic flow resulting from the inhomogeneities cannot easily be balanced. Therefore, high constraint stresses can develop after very small deformation strains and generate cracks. They can propagate within colonies or grains along cleavage planes[5–7], thereby easily forming critical lengths leading to premature failure of the material. The microstructures must therefore become finer and more homogeneous. The more homogeneous the microstructures, the more damage-tolerant the materials are,
ULRICH FRO¨BEL and ANDREAS STARK, Scientists, are with the Helmholtz-Zentrum Geesthacht, Institute for Materials Research, Max-Planck-Straße 1, 21502 Geesthacht, Germany. Contact e-mail: [email protected] Manuscript submitted November 14, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
so that particularly significant weight can be saved when components with very homogeneous microstructures are used. Advantageous in this respect is also the accompanied small property variability enabling weight-saving constructions with small safety allowances. Very homogeneous microstructures must therefore be aimed at to fully exploit the potential of the alloys. This purpose can usually be achieved most effectively through hot-working by means of deformation-induced recrystallization processes ideally leading to completely transformed and homogeneous microstructures. The microstructures of the c titanium aluminides are, however, often incompletely transformed and inhomogeneous after hot-working.[2,8–11] They also show crystallographic and morphologic textures[12,13] which lead, similar to the inhomogeneities, to localized deformation. The textures reflect the plastic anisotropy of the constituent phases resulting from non-equivalent slip systems[14] or the plastic anisotropy of the lamellar morphology[15] depending on hot-working temperature. The inhomogeneities result, on the other hand, from the very same features which provide for the structural stability. These are the low dislocation mobility impeding deformation, and the low mobilities of th
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