Microstructure formation and mechanical behaviour of titanium aluminides during torsion

Abstract

Two-phase Ti-47Al-4(Cr, Nb, Mn, Si and B) alloys (at.%) with fine-grained globular and coarse-grained lamellar structures were deformed in torsion at 1000 °C under 400 MPa hydrostatic pressure at a maximum shear strain rate of 2 × 10⁻⁴ s⁻¹ in a Paterson-type rock deformation machine. The evolution of microstructural characteristics during thermomechanical processing was investigated by transmission electron microscopy (TEM). It is observed that the γ-TiAl + α₂-Ti₃Al lamellar structure breaks down, and a fine-grained duplex globular structure with a grain size on the order of 5 μm develops. TEM observation indicates that the phase transformation of α₂ to γ occurring during torsion shows diffusional character. Ordinary dislocations 1/2[1 1 0] and superdislocations 1/2[1 1 ] are observed. The microstructural and mechanical characteristics indicate that superplastic flow occurs during torsion of titanium aluminides.

Keywords:

• Ordered intermetallics
• torsion
• phase transformation
• transmission electron microscopy

Acknowledgement

G. H. Cao thanks the Alexander-von-Humboldt Foundation for a fellowship and sponsoring of a renewed research stay. Thanks are also to Dr F. Appel (GKSS Geesthacht, Germany) for providing the starting material, Dr E. Rybacki (Geo Research Centre Potsdam, Germany) for deforming the materials by high-pressure torsion and to T. Reiter and Mrs. H. Müller (TU Dresden) for technical assistance.