Issue 22

A. Brotzu et alii, Frattura ed Integrità Strutturale, 22 (2012) 20-25; DOI: 10.3221/IGF-ESIS.22.03 20 Fracture toughness of TiAl-Cr-Nb-Mo alloys produced via centrifugal casting A. Brotzu, F. Felli, D. Pilone Dipartimento ICMA, Sapienza Università di Roma, Via Eudossiana 18, 00184 Roma (Italy) A BSTRACT . Fracture toughness of a TiAl base intermetallic alloy has been investigated at room temperature. The Ti-48Al-2.5Cr-0.5Nb-2Mo (at. %) alloy produced via centrifugal casting exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with a very small quantity of residual β phases along lamellar colony boundaries. In order to determine the alloy fracture toughness compact tension specimens were tested and the results were compared with those available in literature. K EYWORDS . Titanium aluminides; Fracture toughness; Intermetallics. I NTRODUCTION amma titanium aluminides are considered to have great potentials in high temperature applications, mainly due to weight saving in combination with excellent mechanical properties at elevated temperatures [1-3] and good resistance to high temperature oxidation when alloying elements are carefully selected [4, 5]. However, their applications are hindered by relatively low room temperature ductility, poor fracture toughness and bad hot workability. In the last decades, significant improvements in the room temperature ductility and fracture toughness of these alloys have been achieved by a careful selection of alloying elements, heat treatments and thermomechanical treatments [6-9]. Over the last years β phase containing alloys have been developed and studied due to their good hot deformability. In fact the disordered β phase structure is softer than α 2 and γ phases at high temperature and then it aids thermomechanical processing of TiAl alloys [10]. Among several kinds of alloys TiAl alloys combining α 2 and γ phases show reasonable fracture toughness: after forging and heat treatment the K IC reaches a value of about 30 [11, 12]. Studies available in literature show that in lamellar structures the local resistance to crack propagation depends on the lamellae orientation. Fracture toughness tests gave toughness values ranging from 3.5 to 25.7 [13]. A new class of TiAl alloys showing an interesting balance of properties has been recently developed and obtained by thermal treating cast TiAl alloys. In these alloys each grain contains α 2 lamellae embedded in γ, creating a “convoluted” structure [14]. One of these novel alloys displays fracture toughness values of 13 at room temperature and 19 at 923 K. Considering the bad workability of TiAl intermetallic alloys, toughness tests are often carried out according to the ASTM E1820 standard by using samples tested in 3-point bending. In this work the authors explored fracture toughness and tensile strength of the Ti-48Al-2.5Cr-0.5Nb-2Mo (at. %) alloy produced via centrifugal casting and set up a methodology to obtain compact tension specimens via direct centrifugal casting. G