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-pha se TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temper atures above T-alpha produces refined lamellar structures, whose micro structural features can be further controlled by subsequent heat-treat ment at and above 900 degrees C. The mechanical properties of the allo ys with lamellar structures depend on three factors: colony size, inte rlamellar spacing, and alloying additions. The tensile elongation at r oom 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, show ing increasing strength with decreased colony spacing. The fracture to ughness at room temperature can be substantially improved by heat-trea tment 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 m aterials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments. Copyright (C) 1996 Elsev ier Science Ltd