High-temperature structural intermetallics

During the past decade considerable research efforts have been directed towards achieving balanced engineering properties of two-phase γ-titanium aluminide alloys for future applications as structural materials. For optimization of mechanical properties such as yield and creep strengths, tensile ductility and fracture resistance, a basic understanding of the temperature dependent micromechanisms of plasticity and fracture, and their interplay with various microstructural constituents is required. In this review article, the current knowledge on dislocation types and slip systems, the development of deformation substructures, factors controlling the mobility and multiplication of dislocations, interface related plasticity, solid solution and precipitate strengthening mechanisms as well as microscopic aspects of creep and fracture will be addressed. These topics will be related to specific microstructures and associated engineering properties.

Microstructure and deformation of two-phase γ-titanium aluminides

The deformation behaviour of intermetallic superlattice compounds

Attractive elevated-temperature properties and low density make the titanium aluminides very interesting for both engine and airframe applications, particularly in the aerospace industry. The challenge to the materials scientist is to maintain these characteristics while building-in “forgiveness”. The basic phase diagram and crystal structure of both the Ti3Al and TiAl phases are reviewed, followed by a consideration of chemistry-processing-microstructure-deformation/fracture-mechanical property relationships in monolithic material. Conventional and innovative synthesis methods are presented, including use of hydrogen as a temporary alloying element. Composite concepts as a method to enhance not only “forgiveness” but also elevated-temperature behaviour are discussed. Environmental effects are evaluated prior to consideration of present and projected applications of both monolithic and composite material. It is concluded that while the titanium aluminides in monolithic form can be used now in non-demanding applications, much further research and development is required before this material class can be used in critical applications, especially in composite concepts.

Synthesis, properties and applications of titanium aluminides

Novel design concepts for gamma-base titanium aluminide alloys

Polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition have been grown and deformed in compression at room temperature. The yield stress and deformation behaviour depend strongly on the angle between the twin boundaries and the compression axis rather than on the crystallographic orientation of the compression axis. The yield stress is high when compression is perpendicular or parallel to the twin boundaries, and is generally very low for specimens where the twin boundaries are at an intermediate angle to the compression axis. The ratio of the highest to the lowest values of yield stress is almost 8:1. This large difference in yield stress has been found to be related to the difference in the deformation mode; for compression perpendicular or parallel to the twin boundaries, shear deformation always occurs across them while, for compression at an intermediate angle, shear deformation is parallel to the boundaries. The former mode of deformation is much harder than the...

M. Hosomi

Papers

Deformation of polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition