Fracture toughness of the eutectic alloy Al 3 Nb-Nb 2 Al C. Triven ˜o Rios a , P. Ferrandini b, * , R. Caram a a Universidade Estadual de Campinas/FEM/DEMA C.P.6122, 13083-970 Campinas, SP, Brazil b Depto de Engenharia Meca ˆnica, Centro Universita ´rio da FEI, Av. Humberto A. Castelo Branco, CP 901-09850-901, 3972, Sa ˜o Bernardo do Campo, SP, Brazil Received 14 January 2003; accepted 9 February 2003 Abstract Presenting high fracture toughness is a decisive condition to any structural material, and when considering brittle alloys, the Vickers indentation method to determine fracture toughness is an interesting alternative. Like many other intermetallics, the Al 3 Nb–Nb 2 Al eutectic alloy shows high strength at high temperatures and low fracture toughness at room temperature. Al 3 Nb–Nb 2 Al samples, both in the as-solidified condition and in the directionally solidified condition, had their hardness and fracture toughness determined by the Vickers indentation method. Lower values of hardness were found when higher loads were used, and fracture toughness was found to be about 2.0 MPa m 1/2 . The as-solidified condition is harder and less tough, and when fracture occurs, cracks always develop by cleavage. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Crystal structure; Deformation and fracture; Hardness; Intermetallic compounds; Microstructure; Mechanical properties 1. Introduction The maximum resistance to the propagation of an existing flaw, e.g., a crack, is known as the fracture toughness. For brittle materials, fracture toughness is always called K IC . A structural component will be subjected to a certain stress level; if it presents a crack, there is a specific value of the combination stress– crack length that makes the crack to propagate. Fracture toughness is used to obtain that value exactly, thus avoiding crack propagation which may lead to catastrophic failure. The indentation method to deter- mine fracture toughness demands a Vickers hardness indentation which will crack the material. The method is simple, quick, and has been used for ceramics for more than 20 years [1]. Recently, it has been used in the analysis of metal – ceramic composites and inter- metallics [2,3]. When brittle materials are indented, they crack. Two kinds of cracks can be used in order to determine fracture toughness [4]: Palmqvist cracks which are typical of tougher materials, such as WC and WC/Co, and diagonal cracks [5]. Blendell con- cluded that the universal behavior of the relation between fracture toughness and ratio of crack-to- indent size can be expressed as: K IC ¼ 0:0285H 0:6 v E 0:4 a 0:5 logð8:4a=cÞ; where H v is the Vickers hardness (GPa), E is the Young’s modulus, a is half the length of the diagonal 0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00245-3 * Corresponding author. Tel.: +55-11-4353-2900; fax: +55-11- 4109-5994. E-mail address: peterf@fei.edu.br (P. Ferrandini). www.elsevier.com/locate/matlet Materials Letters 57 (2003) 3949 – 3953