Enhanced cavitation erosion resistance of a friction stir processed high entropy alloy
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Enhanced cavitation erosion resistance of a friction stir processed high entropy alloy Rakesh B. Nair, H.S. Arora, and Harpreet Singh Grewal Surface Science and Tribology Lab, Department of Mechanical Engineering, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pradesh 203207, India (Received: 30 October 2019; revised: 10 January 2020; accepted: 5 February 2020)
Abstract: Friction stir processing of an Al0.1CoCrFeNi high entropy alloy (HEA) was performed at controlled cooling conditions (ambient and liquid submerged). Microstructural and mechanical characterization of the processed and as-cast HEAs was evaluated using electron backscatter diffraction, micro-hardness testing and nanoindentation. HEA under the submerged cooling condition showed elongated grains (10 μm) with fine equiaxed grains (2 μm) along the boundary compared to the coarser grain (~2 mm) of as-cast HEA. The hardness showed remarkable improvements with four (submerged cooling condition) and three (ambient cooling condition) times that of as-cast HEA (HV ~150). The enhanced hardness is attributed to the significant grain refinement in the processed HEAs. Cavitation erosion behavior was observed for samples using an ultrasonication method. All of the HEAs showed better cavitation erosion resistance than the stainless steel 316L. The sample processed under a submerged liquid condition showed approximately 20 and 2 times greater erosion resistance than stainless steel 316L and ascast HEA, respectively. The enhanced erosion resistances of the processed HEAs correlate to their increased hardness, resistance to plasticity, and better yield strength than the as-cast HEA. The surface of the tested samples showed nucleation and pit growth, and plastic deformation of the material followed by fatigue-controlled disintegration as the primary material removal mechanism. Keywords: cavitation erosion; microstructure; mechanical properties; surface engineering
1. Introduction Hydraulic machinery, such as ship-propellers, pump-impellers, valves, and hydro-turbines, are severely affected by slurry and cavitation erosion leading to substantial financial losses [1–3]. Cavitation erosion, formed by the implosion of vapor bubbles resulting from rapid pressure fluctuations in the liquid, is a serious concern. During implosion, high-intensity shockwaves and microjets are produced, which impact the solid surface at a high frequency. This repeated impact of microjets and shockwaves leads to the formation of higher density pits, plastic deformation and eventual material failure [1]. The rate of material degradation is more aggressive if cavitation erosion occurs in a corrosive environment. State-of-the-art materials used in these applications show limited resistance to cavitation erosion. Therefore, advanced materials and surface modification techniques are essential measures to counter the degradation phenomenon. The recently developed high entropy alloys (HEAs) have received considerable attention as structured materials due to their promising properties and unique micr
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