Abstract
In this work the structure and corrosion behavior of quasicrystalline cast Al69Co21Ni10 and Al72Fe15Ni13 alloys in 5-% sodium chloride solution (рН 6.9–7.1) were investigated. The alloys were cooled at 5 К/s. The structure of the samples was studied by methods of quantitative metallography, X-ray analysis, and scanning electron microscopy. Corrosion properties were determined by potentiodynamic method. Stationary potential values were measured by means of long-term registration of (Е,τ)–curves using ПІ–50–1 potentiostat and ПР–8 programmer with three-electrode electrolytic cell. A platinum electrode served as counter electrode and silver chloride – as reference electrode. The made investigations confirm the formation of stable quasicrystalline decagonal D-phase in the structure of Al69Co21Ni10 and Al72Fe15Ni13 alloys. In Al69Co21Ni10 alloy, at room temperature D-phase coexists with crystalline Al9(Co,Ni)2 phase, and in Al72Fe15Ni13 alloy – with Al5FeNi phase. Comparison of Vickers hardness of these phases exhibits the following sequence: H(D-AlCoNi)>H(D-AlFeNi)>H(Al5FeNi)>H(Al9(Co,Ni)2). In 5-% sodium chloride solution, the investigated alloys corrode under electrochemical mechanisms with oxygen depolarization. Compared with Al72Fe15Ni13 alloy, Al69Co21Ni10 alloy has more negative value of stationary potential (–0,40 V and –0,48 V, respectively), and its electrochemical passivity region extends due to the inhibition of anodic processes. For both alloys, transition to passive state in the saline solution is observed. A corrosion current density, calculated from (E,lgi)-curve, for Al69Co21Ni10 alloy amounts to 0.12 mА/сm2 and for Al72Fe15Ni13 alloy – to 0.14 mА/сm2. After immersion in the saline solution for 8 days, pits are revealed on the surface of the alloys in areas, mainly where the phase boundaries and flaws are located. The number and size of pits are smaller on the surface of Al69Co21Ni10 alloy as compared with those on the surface of Al72Fe15Ni13 alloy. The lower corrosion resistance of Al72Fe15Ni13 alloy may be explained by the presence of iron-containing phases in its structure. Based on obtained results, the Al69Co21Ni10 alloy has been recommended as coating material for rocket-and-space equipment working in marine climate.
Highlights
The ternary Al–Co–Ni and Al–Fe–Ni alloys are the most interesting stable quasicrystalline materials having decagonal rotation symmetry [1,2,3,4,5,6,7,8]
Two ternary compounds are found in the structure: the crystalline Al9(Co,Ni)2 phase and decagonal quasicrystalline (D) phase with the composition Al72Co9.5Ni18.5, as energy-dispersive spectroscopy (EDS) measurements show
As the D-phase is observed at room temperature the stability of this phase with respect to decomposition into its neighboring phases is clearly confirmed
Summary
The ternary Al–Co–Ni and Al–Fe–Ni alloys are the most interesting stable quasicrystalline materials having decagonal rotation symmetry [1,2,3,4,5,6,7,8]. Quasicrystals have many attractive properties, such as high hardness, low electrical and thermal conductivities, low surface energy, accompanied by a low coefficient of friction, reasonable oxidation and strong corrosion resistance, and unusual optical properties which have not been observed for crystalline alloys [9,10,11,12,13] These properties can only be used for technological applications in the form of thin film coatings [14,15,16,17,18] or reinforcement particles in metal matrix composites [19,20,21,22,23] in order to circumvent their intrinsic brittleness. Aim of this paper is to investigate structure of quasicrystalline and crystalline phases observed in the cast Al–Co–Ni and Al–Fe–Ni alloys and compare their corrosion characteristics in aqueous sodium chloride solution
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