Tensile properties and fracture toughness of TiAl alloys with controlled microstructures

The objective of this study is to improve the mechanical properties by careful control of both microstructure and alloy additions in two-phase TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temperatures above T-alpha produces refined lamellar structures, whose microstructural features can be further controlled by subsequent heat-treatment at and above 900 degrees C. The mechanical properties of the alloys with lamellar structures depend on three factors: colony size, interlamellar spacing, and alloying additions. The tensile elongation at room temperature is strongly dependent on lamellar colony size, showing increasing ductility with decreased colony size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing increasing strength with decreased colony spacing. The fracture toughness at room temperature can be substantially improved by heat-treatment at 1320 and 1350 degrees C. The tungsten addition at a level of 0 . 2%, improves the tensile strength, whereas the silicon addition at a level of 0 . 3%, reduces the castability of the TiAl alloys. TiAl materials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments. Copyright (C) 1996 Elsevier Science Ltd