PSI - Issue 28

Konstantinos Tserpes et al. / Procedia Structural Integrity 28 (2020) 1644–1649 Tserpes, Bazios, Pantelakis, Michailidis/ Structural Integrity Procedia 00 (2020) 000–000

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most of the available techniques are still unable to produce materials in large quantities. To this end, for the characterization of nanocrystalline materials, testing methods involving small volume samples such as the nanoindentation method are preferable. Nanoindentation was extensively used in the last two decades for characterizing the mechanical performance of nano- and micro-structured materials (Doerner and Nix, 1986; Pharr and Cook, 1990; Cheng and Cheng, 1998; Tao et al., 2018; Wagih, 2016; Ogasawara, Chiba and Chen, 2006). Specifically, Berkovich indenters allow precise measurements by varying the force of nanoindentation ( P ) in relation to the depth of penetration ( h ). Observations through experimental nanoindentation tests were made on many materials so as to determine material features such as residual stresses and/or hardness (Doerner and Nix, 1986; Pharr and Cook, 1990; (Bolshakov, Oliver and Pharr, 1996; Dao et al., 2001; Gram, Carpenter and Anderson, 2015; Han et al., 2015; Zhang et al., 2018). In recent years, nanoindentation was also used for characterizing nanocrystalline materials (Trelewicz and Schuh, 2007; Cavaliere, 2009; Wei, Shu, Du and Zhu, 2005; Lai et al., 2012; Fougere et al., 1995). Aside to nanoindentation tests, analytical and numerical models (Venkatesh, 2000; Tunvisut, O'Dowd and Busso, 2001; Harvey, Ladani and Weaver, 2012; Cavaliere, 2009; Li et al., 2016) were developed aiming to minimize the test campaign and to assist the evaluation of experimental findings. The objective of the present work is twofold: to conduct nanoindentation tests on Tungsten – Cooper nanocrystalline alloys and to develop a numerical model based on the finite element method to simulate the nanoindentation behavior In the present investigation, the Tungsten-Copper alloy system was used. The alloy consisted of 75% Tungsten and 25% Copper in weight. For this material composition, two different Tungsten-Copper alloys with a different microstructure were produced, a coarse-grained alloy, hereafter noted as cW-Cu alloy and a nanostructured alloy, hereafter noted as W-Cu alloy. Both alloys were manufactured using the same powders originating from the same powder batch. The cW-Cu specimens were fabricated through the simple mixture of as-received commercial powders of Tungsten and Copper elements without any grain refinement process involved. In addition, the processes of cold pressing, hot isostatic pressing, and heat treatment were applied as to obtain consolidated specimens. On the other hand, the high energy ball milling method was utilized in order to mill Tungsten and Copper powders to achieve a nanocrystalline morphology. Similar to the cW-Cu specimens, cold pressing, hot isostatic pressing, and heat treatment were additionally applied to the W-Cu specimens. An extensive description of the produced materials’ microstructures can be found in (Bazios, Tserpes and Pantelakis, 2020). The above-said production of the two alloys was performed by the MBN Nanomaterialia (Treviso, Italy). 2.2. Nanoindentation test The nanoindentation tests were carried out using a FISCHERSCOPE H100 nanoindentation apparatus (see Fig. 1), with an accuracy of 0.1 mN and a maximum possible indentation load up to 1000 mN following strictly the ISO 14577 standard (ISO, 2007). The maximum indentation depth fluctuations measured on the same sample are mainly induced by different contact conditions between the indenter tip and the tested surface due to roughness (Bouzakis et al., 2002). The roughness effect can be confronted with the execution of an appreciable number of measurements to attain stabilization of the maximum indentation depth mean value (Bouzakis et al., 2002). In this work, 50 nanoindentation tests were performed on each specimen to eliminate roughness effects. of the materials. 2. Experimental 2.1. Materials