Issue 27

A. Brotzu et alii, Frattura ed Integrità Strutturale, 27 (2014) 66-73; DOI: 10.3221/IGF-ESIS.27.08 66 Effects of the manufacturing process on fracture behaviour of cast TiAl intermetallic alloys A. Brotzu, F. Felli, D. Pilone Dipartimento ICMA, Sapienza Università di Roma, Roma (Italy) andrea.brotzu@uniroma1.it , ferdinando.felli@uniroma1.it , daniela.pilone@uniroma1.it A BSTRACT . The γ -TiAl based intermetallic alloys are interesting candidate materials for high-temperature applications with the efforts being directed toward the replacement of Ni-based superalloys. TiAl-based alloys are characterised by a density (3.5-4 g/cm 3 ) which is less than half of that of Ni-based superalloys, and therefore these alloys have attracted broad attention as potential candidate for high-temperature structural applications. Specific composition/microstructure combinations should be attained with the aim of obtaining good mechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperature strength for targeted applications. Different casting methods have been used for producing TiAl based alloys. In our experimental work, specimens were produced by means of centrifugal casting. Tests carried out on several samples characterised by different alloy compositions highlighted that solidification shrinkage and solid metal contraction during cooling produce the development of relevant residual stresses that are sufficient to fracture the castings during cooling or to produce a delayed fracture. In this work, crack initiation and growth have been analysed in order to identify the factors causing the very high residual stresses that often produce explosive crack propagation throughout the casting. K EYWORDS . Titanium aluminides; TiAl intermetallics; Fracture toughness. I NTRODUCTION iAl based alloys are interesting for high-temperature applications mainly in aerospace and automotive industries. Their potential is seen in low density, high specific yield strength, high specific stiffness, good oxidation resistance at room temperature (RT), resistance against ”titanium fire”, and good creep properties up to high temperatures [1]. In fact the good specific mechanical properties of titanium aluminide alloys push the development of these materials. Because of their ordered structure, intermetallics have high mechanical strength both at RT and at high temperature [2,3]. Despite that, TiAl-based alloys cannot be used as single phase alloys since they have a very low ductility at RT. The presence of a second α 2 phase (Ti 3 Al) allows control of the microstructure. As far as mechanical properties are concerned, the addition of alloying elements such as Cr, V and Mn reduces the grain size with consequent ductility improvement. Depending on alloy composition and microstructure, these alloys exhibit good workability, medium-to-good tensile properties, tensile fracture strains in the range 1-3% at RT and fracture toughness values in the range 10- 25 MPa√m [4-7]. Various TiAl-based alloys have been developed. Adding transition metals of a high melting temperatures is generally beneficial to increase the high temperature strength of these alloys [8-10]. More recently, the so-called 2 nd and 3 rd generation alloys have been developed in order to improve their mechanical properties and high temperature properties T