Applied Surface Science 363 (2016) 543–547 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Formation of core–shell structure in high entropy alloy coating by laser cladding Hui Zhang a , Wanfei Wu a , Yizhu He a,∗ , Mingxi Li a , Sheng Guo b,∗ a School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China b Surface and Microstructure Engineering Group, Department of Materials and Manufacturing Technology, Chalmers University of Technology, Gothenburg SE-41296, Sweden a r t i c l e i n f o Article history: Received 12 October 2015 Received in revised form 28 November 2015 Accepted 7 December 2015 Available online 12 December 2015 Keywords: Liquid phase separation High-entropy alloy Laser cladding Inoculant a b s t r a c t The formation of core–shell structure is an interesting phenomenon occurring during the solidification process, due to the liquid phase separation. The formation of core–shell structure in high-entropy alloys, a new class of advanced metallic materials, has not been reported previously, and thus constitutes an intriguing scientific question. Here, we firstly report the formation of core–shell structure in one laser cladded high-entropy alloy, where we show the nanosized-Y 2 O 3 powder addition, serves as the catalyst for the liquid phase separation. © 2015 Published by Elsevier B.V. 1. Introduction The core–shell structure, occurring during the macroscopic liquid phase separation, constitutes one of the most interesting solidification microstructures [1]. The opportunity to create such a special regular microstructure draws attention from researchers in the field of condensed matter and materials physics. Liquid phase separation usually takes place in immiscible alloys with a large positive enthalpy of mixing [2]. However, it is usually diffi- cult to achieve a high undercooling in these kinds of alloys, due to the low liquid–liquid interface energy [2]. For example, in the Cu–Co binary alloys, the metastable liquid phase separation only occurs when a critical undercooling (50–80 K) is reached [3]. A high undercooling can be achieved by melt processing methods, including container-less solidification (typically electromagnetic or electrostatic levitation, drop tube, and atomization) and fluxing [3]. High-entropy alloys (HEAs), or multi-component alloy with equi- or close to equiatomic compositions, are emerging novel metallic materials with many interesting structural and func- tional properties [4–7]. Investigations on the solidification behavior ∗ Corresponding author. E-mail addresses: heyizhu@ahut.edu.cn (Y. He), sheng.guo@chalmers.se (S. Guo). (including liquid phase separation) of HEAs, as a kind of new materials, can greatly enrich our understanding to the solidifi- cation of multi-component alloys [8,9]. Interestingly, there has so far no a single report on the formation of the core–shell structure in HEAs, although the segregation involving Cu (Cu- rich liquid solidifies later in the inter-dendritic region, after the dendrites are solidified), which has positive enthalpy of mixing with many alloying elements in HEAs [10], is commonly seen. In addition, it has been shown that a large undercooling is not necessarily leading to the formation of liquid phase separation in HEAs. For example, Munitz et al. did not observe the liq- uid phase separation in a Cu-rich HEA, Al 1.8 CoCrCu 3.5 FeNi, even when the undercooling reached 150 K. They reckoned that an even higher undercooling is required for the liquid phase separation to occur. The intention of the current work is to explore the pos- sibility of forming the core–shell structure in HEAs, using simultaneously two strategies. Firstly, we will use the laser cladding method to achieve the high cooling rate, which will also promote the high undercooling [3]. Secondly and more importantly, we plan to use high melting point nanosized par- ticles as inoculants, to catalyze the liquid phase separation . Our idea is inspired by the promotion of liquid phase separation by the addition of Al 2 O 3 , ZrO 2 and TiB 2 to Al–Bi and Al–Pb monotectic binary alloys [11,12]. As will be shown later, the addition of inoc- ulants indeed proved to be the key. To the best of our knowledge, http://dx.doi.org/10.1016/j.apsusc.2015.12.059 0169-4332/© 2015 Published by Elsevier B.V.