PSI - Issue 23

Ayan Ray et al. / Procedia Structural Integrity 23 (2019) 299–304 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The impact of the emergence of high entropy alloys (HEAs), at times called as multi-principal element alloys (MPAs), is well witnessed only for the last fifteen years or so through a burst of international reports. Following the earliest report of Yeh et al. (2004), the initial ventures on understanding these alloys were primarily directed in the search of newer systems which often indicated or promised high hardness, strength, wear and corrosion resistance. However, attempts to harness their oft-observed high strength and hardness towards possible applicability as useful structural components require information related to their mechanical properties and understanding about their deformation, creep, fatigue and fracture behaviour. This awareness has brought forward a few review articles (Diao et al., 2017, Laplanche et al., 2018, Li et al., 2019, Li et al., 2018) on these areas in recent years. The fracture resistance of HEAs can range from approximately 5 MPam 1/2 (Roy et al., 2014) to over 200 MPam 1/2 (Gludovatz et al., 2014). Li et al. (2018) have commented that fracture resistance of HEAs appears to depend on the crystal structures of the microstructural cons tituents and their values are in ascending order for BCC to ‘BCC+FCC’ to FCC alloys. On the other hand, attempts towards understanding the operative deformation mechanisms in HEAs through thermal activation analyses by Laplanche et al. (2018) infer that higher activation volume in HEAs indicate lower resistance to dislocation motions and easy plastic flow. But the existing knowledge is still limited due to the lack of understanding on the role of sub-structural features on their deformation and fracture behavior. The motivation in this study is to direct attempts for examining the deformation and fracture behavior of some typical body centered cubic (bcc) and face centered cubic (fcc) HEAs containing Al-Cr-Co-Cu-Fe-Ni in order to answer some unanswered queries related to the applicability of HEAs as structural components. 2. Experimental details Two varieties of high entropy alloys consisting of Al, Cr, Co, Cu, Fe and Ni were obtained as courtesy of the University of Bayreuth, Germany. These alloys were prepared by vacuum induction melting and then cast in Cu mould. The compositions of the alloys correspond to Al 23 Cr 23 Co 15 Cu 8 Fe 15 Ni 15 (bcc) and Al 8 Cr 17 Co 17 Cu 8 Fe 17 Ni 33 (fcc) and possess the crystal structures as shown in the parenthesis; henceforth the alloys will be referred to as HEA-B and HEA-F respectively. Representative microstructures of the alloys were examined using optical (Model: Leica DM 2500M) and scanning electron microscopes (Model: Zeiss EVO 60) after suitable metallographic preparation of the samples followed by etching using dilute molybdenum acid with H 2 O, HCl, and HNO 3 to reveal the microstructures. For revealing post deformation substructural features TEM examinations were carried out using a Tecnai microcope (model: 20T G2 FEI). Specimens for TEM studies were first thinned down to ~100  m by mechanical polishing followed by electro polishing in a twin jet electro polisher; details are available in an earlier report (Sarkar et al., 2014). The constituent phases of the two alloys were determined by a Bruker diffractometer (model: D8 ADVANCE) using Co Kα radiation and Fe filter (operating parameters: voltage -40 kV and current -40 mA). Estimation of the conventional mechanical properties included microhardness measurements and determination of the compressive strength. The former was done using metallographically polished specimens at a load of 20gf for dwell time of 15 s in a Leco hardness tester (Model: LV 7000); compression tests were done using cylindrical specimens (5.4 mm x 3 mm  ) in a 50kN Instron (model: 4505) machine. The average microhardness is based on 25 readings whereas the average compressive strength is estimated using the results on three specimens. Fracture toughness values of the two HEAs were determined using both single edge notched (SENB) specimens in three point bend (TPB) loading mode as per ASTM standard E399 and by using chevron notched rectangular bar (CVNRB) specimens following Shang-Xian (1984) and Ray et al. (2009). The specimen dimensions for SENB and CVNRB tests were 3 mm×6 mm×30 mm and 4 mm×6 mm×30 mm, respectively. The TPB tests were carried out on specimen-span length of 24 mm using an Instron (model: 3365) machine of 5 kN capacity at a nominal crosshead speed of 0.2 mm/min. The fractured surfaces were examined immediately after the tests by SEM to understand the micro mechanism of cracking in the alloys. On the other hand, thermal activation parameters like activation volume and internal stresses were determined using stress relaxation (SR) and strain rate change (SRC) tests in compressive deformation using cylindrical specimens identical to that used for compression tests. The SR tests were done for 600 s duration at different strain levels with initial loading corresponding to nominal strain rate of 10 -3 s -1 . The SRC tests