Fracture toughness and fracture micromechanism in a cast AlCoCrCuFeNi high entropy alloy system U. Roy a , H. Roy b , H. Daoud c , U. Glatzel c , K.K. Ray a,n a Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur-721302, India b NDT & Metallurgy Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India c Metals and Alloys, University of Bayreuth, Ludwig-Thoma-Straße 36b, 95447 Bayreuth, Germany article info Article history: Received 30 March 2014 Accepted 10 June 2014 Available online 18 June 2014 Keywords: High entropy alloy Fracture toughness Shear band Microstructure abstract This investigation is aimed to understand crack initiation resistance and mechanism of cracking in a high entropy alloy, Al 23 Co 15 Cr 23 Cu 8 Fe 15 Ni 15 (at%). The fracture toughness of the alloy consisting of primarily Fe–Cr rich disordered and Al–Ni rich ordered BCC phases is found to be 5.4 70.2 MPa m 1/2 . Lower magnitude of fracture toughness has been attributed to nanosized microstructural constituents, in which cracking is governed by formation of shear bands, cleavage facets and herringbone type features. & 2014 Elsevier B.V. All rights reserved. 1. Introduction High entropy alloys (HEA) having usually five principal ele- ments or more are known to exhibit only one or two simple solid solutions due to their characteristic enthalpy and entropy of mixing. The significance of HEAs originates from their various combinations of high strength, good thermal stability, excellent corrosion resistance and oxidation resistance; these alloys there- fore are being considered as potential materials for various applications [1]. Examinations of mechanical properties of these alloys by earlier researchers [1,2] are usually in terms of their compressive strength and hardness; these are insufficient for assessing their potential for structural applications. Limited infor- mation is available on fatigue behavior but no report is found on fracture toughness of HEAs till date. This communication aims to report the crack initiation resistance and fracture micromechan- ism in an Al 23 Co 15 Cr 23 Cu 8 Fe 15 Ni 15 (at%) HEA; the latter being relatively more studied [3–6]. 2. Experimental details An Al 23 Co 15 Cr 23 Cu 8 Fe 15 Ni 15 alloy ingot was prepared using 99.9% pure elements by vacuum induction melting followed by casting in Cu mold. Representative microstructures of the alloy were examined using both optical and a scanning electron micro- scope (Zeiss, Model: SUPRA 40). X-ray diffraction measurements were carried out using a Bruker diffractometer (model: D8 ADVANCE) with Co K α radiation and Fe filter, operated at a voltage of 40 kV and a current of 40 mA. Hardness tests were carried out using Vickers hardness tester (Leco, model: LV 700, load 20 kgf, dwell time 10 s). Compression tests were performed using cylindrical specimens (height: 5.4 mm and diameter: 3 mm) in a 50 kN Instron (model: 4505). Fracture tough- ness values of the alloy were determined using: both (i) single edge notched bend (SENB) specimens having 3 mm  6 mm  30 mm dimensions with an EDM notch of approximately 3 mm depth following the method recommended in ASTM standard E399-12 and (ii) chevron notched rectangular bar (CVNRB) specimens having 4 mm  6 mm  30 mm dimensions with normalized initial notch length of 0.3 and chevron notch angle of 60 1 in three point bend loading mode as per Wu Shang-Xian [7] and Ray et al. [8]. All fracture toughness tests were carried out with the help of an Instron (model: 3365) of 5 kN capacity at a nominal crosshead speed of 0.2 mm/min using specimen-span length of 24 mm. The generated fracture surfaces were examined under FEGSEM (Zeiss, model: SUPRA 40) to reveal the micro mechanism of cracking in this alloy. 3. Results and discussion Microstructural study: The as-cast alloy exhibits dendritic micro- structure. Each dendrite spinoidally decomposes into Ni–Al rich and Fe–Cr rich phases and thus forms domains of Fe–Cr rich disordered Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.06.067 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 3222 283278; fax: þ91 3222 282280. E-mail addresses: kkrmt@metal.iitkgp.ernet.in, kalyankumarry@yahoo.com (K.K. Ray). Materials Letters 132 (2014) 186–189