Creep Behaviour of Titanium Aluminides and its Relation to Phase Distributions and Dislocation Structures

Doktorsavhandling, 2001

The need for more efficient gas turbines with lowered fuel consumption and thereby lowered emission has recently lead to increased interest in low-density, .gamma.-based TiAl intermetallic alloys as a replacement of e.g. Ni-based superalloys and Ti-based al-loys. The capacity of Ti-Al alloys at elevated temperatures, having high strength-to-weight ratio, is substantial. Other applications are thought of with similar arguments. The creep and fatigue properties are of primary concern in this context. The present project is part of a large European consorted action (COST 501 and 522) on the properties and use of .gamma.-based TiAl alloys in gas turbines with high efficiency. A series of .gamma.-based TiAl alloys with the nominal composition Ti-48Al-2W-0.5Si (at-%) was studied. The material was cast, hot isostatically pressed and heat treated in two successive steps into the shape of bars of different size simulating turbine blades. De-pending on composition, geometry and thermal treatments, different mi-crostructures were obtain-ed: Nearly lamellar, duplex or pseudo-duplex structures; the latter micro-structure de-velops under conditions of comparatively high Si-contents. In the tempera-ture and stress ranges employed in the studies (700 - 850°C; 225 - 325 MPa), the nearly lamellar microstructure exhibits considerably better creep resistance than the duplex mic-rostructure. Low cycle fatigue studies reveal the duplex microstructure to have the best fatigue resistance. On the other hand, pseudo-duplex alloys combine good creep with good low cycle fatigue performance. A common characteristic of all micro-structures is the absence of stable secondary creep. The dislocation structure in creep strained material has been studied with specialised TEM techniques like large-angle convergent beam electron diffraction. It allowed de-tailed identification of ordinary dislocations, superdislocations and twinning structures in the crystallographically ordered, tetragonal .gamma.-phase. The relation be-tween creep rates and dislocation structures is demonstrated, as is the role of dy-namic recrystallisation and lamella breaking taking place during creep in early stage two. The important role of temperature on the creep rate asked for studies on the thermal transport behaviour. Use of newly developed high precision techniques for measur-ing anisotropic conductivity allowed interpretation of the anisotropic transport prop-erties in terms of distribution and orientation of the phases contained in the micro-structure.