Anomalies in Deformation Behaviour of TiAl Intermetallic

Abstract

An explanation was proposed for a nonmonotonic temperature dependence of the yield stress σy(T) in TiAl having two extrema where σy(T) changes its temperature behaviour. The comparison of σy(T) curves for TiAl and typical curves for other materials (b.c.c. metals, semiconductors, Ni3Al-type intermetallics) allowed reconstructing the shape of the potential relief for dislocations in TiAl. The shape of the relief reflects existence of two types of dislocation traps (shallow and deep ones) and two types of potential barriers: low and high barriers for the capture of dislocations in shallow and deep traps respectively. The deformation behaviour of TiAl was described over the whole temperature interval allowing for the capture of dislocations in traps and their release therefrom. Expressions determining extremums of σy(T) were derived. Possible dependences of the work hardening rate θ(T) in the region of the anomalous trend σy(T) were analysed. Conditions of the anomalous behaviour of θ(T) were ascertained. It was proposed that the shape of the potential relief of a dislocation changed in the effective range of a microcrack. The capture of dislocations in deep traps, which is stimulated by concentration of stresses near a microcrack, and the inhibited release of dislocations from the traps up to relatively high temperatures are viewed as a possible cause of TiAl brittleness. A model of the deformation behaviour of intermetallics after prestraining was proposed. This model enables one to describe two alternatives: observation or absence of the stress macrojump after prestraining.

Keywords

• tial
• mechanical properties
• yield stress
• work hardening rate
• flow stress anomaly
• brittleness
• microstructure
• dislocation
• peierls relief
• thermally activated blocking