Elsevier

Materials Chemistry and Physics

Volume 200, 1 October 2017, Pages 33-49
Materials Chemistry and Physics

The influence of the fabrication route on the microstructure and surface degradation properties of Al reinforced by Al9Co2

https://doi.org/10.1016/j.matchemphys.2017.07.061Get rights and content

Highlights

  • Al-7 wt% Co has been produced by vacuum arc melting (VAM), casting, free sintering.

  • All alloys consist of Al9Co2 in an Al-matrix.

  • The VAM alloy shows the highest aqueous corrosion and sliding wear resistance.

  • Its superior behavior is mostly ascribed to the refined, eutectic microstructure.

  • Pitting corrosion and sliding wear mechanisms are proposed.

Abstract

Al-7 (wt. %) Co alloys have been prepared by conventional casting (Cast), vacuum arc melting (VAM) and free sintering (FS), with the objectives to: a) determine an effective fabrication route (in terms of low cost, ease of manufacture and property boosting) and b) exploit the benefits that the combination of a complex metallic alloy (Al9Co2) with Al at the Al-rich end of the Al-Co phase diagram can offer, corrosion- and wear-wise, whilst at the same time, specific weight and raw material costs remain at relatively low levels. All the attained microstructures consist of Al9Co2 in an Al matrix. However, the main microstructural features are strongly affected by the preparation method. The VAM alloy presents the finest microstructure, which is also highly directional. All the microstructures are analytically being discussed and explained with respect to the fabrication method. Porosity and hardness data are correlated with the fabrication route and the corresponding microstructures. All alloys, regardless of the preparation method, exhibit high resistance to localized corrosion in 3.5 wt% NaCl. The potentiodynamic and potentiostatic polarization performances are analysed. The sliding wear resistance of the Al-Co alloys is notably improved as compared to commercially pure Al and common Al-alloys. A wear mechanism is proposed and the role of Al9Co2 is clarified. The superiority of the VAM alloy with respect to the surface degradation behavior is noted and correlated with its microstructure.

Introduction

Aluminum alloys rich in transition metals present a high research interest due to a great variety of structures that combine excellent physical and mechanical properties with a low specific weight. Al-transition metal systems can form, depending on the composition, fabrication and heat treatment conditions, disordered alloys, supersaturated solid solutions, ordered intermetallic compounds, complex metallic alloys (CMA) and quasicrystals. The term CMA denotes a new group of intermetallic phases with giant unit cells (containing at least a few tens of atoms) and lattice parameters of several nanometers [1]. The CMA structure is complex and is characterized by a duality: On the scale of several nanometers, the structure is crystalline with translational periodicity of the lattice; on the atomic scale, the atoms are arranged in clusters of polytetrahedral order, whilst they are distributed quasiperiodically [2]. The quasiperiodic order is considered responsible for unusual properties including amongst other unusual surface properties. The interesting surface properties (associated with a low surface energy) include low wetting by water [3], low solid-solid adhesion [4], [5], intriguing features in solid film growth [6], oxidation resistance [6], corrosion resistance [7], friction anisotropy [8], low friction [4], [6]. The electronic structure of quasicrystals is characterized by a suppression in the density of states at the Fermi edge, known as pseudogap. Unlike the bulk pseudogap in BCC-transition metals, the pseudogap in quasicrystals is also present at their surface [9]. There is evidence that this pseudogap strongly influences the CMA/quasicrystal surface properties [6], [10].

As far as the corrosion behavior of CMAs is concerned, Thiel [6], in her review on the surfaces of quasicrystals, stated that the quasiperiodic order engenders a fundamental resistance to oxidation. However, Veys et al. [11], studying the corrosion behavior of bulk Al-Cr-Fe and Al-Cr-Cu-Fe CMAs in an acid chloride solution, concluded that it is governed by their chemical composition rather than their complex structure. Nevertheless, Beni et al. [7], reported that the anisotropy of CMAs along certain crystal orientations and the associated difference in chemical composition are responsible for the different localized corrosion resistance (acidic NaCl solution) along the (001) and (100) directions of Al-Cr-Fe CMAs.

The tribological behavior of CMAs cannot strictly be correlated with the structure complexity, since it depends on many parameters, intrinsic and extrinsic. Experimental evidence shows that the structural complexity contributes into friction decreasing [4], [10], [12]. The incorporation of CMA-particles (Al59Cu25.5Fe12.5B3 and Al71Cu9Fe10Cr10) into an aluminum matrix was shown to improve the dry sliding wear resistance [13].

Despite their unusual properties, CMA applications are still in an early stage of maturity. The reason is that being notably brittle, CMAs are regarded unsuitable for applications as monolithic alloys, but still viable as coatings and composites [14]. However, CMAs as coatings present intrinsically poor adherence to their substrates owing to their low surface energy [15]; thus, the use of a bond coat is recommended [16]. The production of two- or multi-phase structures including a soft metallic phase shows a large potential for improving the ductility of CMAs at low temperatures.

The Al-Co system includes several phases seen as approximants to decagonal quasicrystals, among which monoclinic Al9Co2 presents an intermediate structural complexity between B2-AlCo and the decagonal Al-Ni-Co quasicrystal [17]. The electronic structure of Al9Co2 includes a pseudogap near the Fermi energy, which suggests a high corrosion resistance in relation to simpler alloys [18].

Besides Al-Co CMAs, the Al-Co system is involved in the research of hydrogen fuel cell technologies [19]. Al-Co alloys have been shown to be efficient hydrogen catalysts by both experimentation [20] and ab initio density-functional simulations [21].

Notwithstanding the above promising application perspectives, Co-aluminides have much less been studied than Nisingle bond, Tisingle bond and Fe-aluminides [22], most likely due to their limited (until recently) potential. Little information is thereby available, regarding processing of Al-Co alloys. Reported fabrication methods include gun quenching [23], melt spinning [23], [24], [25], die casting [25], rotating-water atomization and mechanical alloying [26], arc melting under Ar [27], [28], stir casting [27].

Very limited information can be found on the corrosion behavior of Co-aluminides as single phases [28], [29] or as alloy constituents [27], [30]. Palcut et al. [28], [29], investigating the corrosion performance of Al-(24–29 at% Co) alloys consisting entirely of aluminides, observed galvanic coupling between nobler and less noble intermetallic phases and pitting in 0.6 mol/dm3 NaCl. Lekatou et al. [30] found that Al-(7–20 wt% Co) alloys, composed of various amounts of Al9Co2 in a (Al,Co) matrix, exhibited low susceptibility to localized corrosion in 3.5 wt% NaCl. It is also established that aluminides of transition metals can form Al2O3-based passivating layers in various electrolytes; these layers may incorporate oxidized species of the transition metal [31], [32], [33], [34], [35].

Literature on the wear performance of Al-Co alloys is even scarcer. The most relevant information concerns the wear behavior of cast and rapidly solidified aluminum alloys in-situ reinforced by intermetallic compounds. In-situ particulate reinforcements of Al, such as Al3Ti [36], (Al2O3 + Ti(Al1-x,Fex)3) [37], TiB2 [38], AlB2 [39], (Al12W + Al5W + Al3(Ti,W)) [40], Al3Zr [41], ZrB2 [42], (ZrB2+TiB2) [43] generally have a positive contribution to the wear resistance of Al. The applied load, the amount of the reinforcement phase, the sliding speed and the rotational speed have been identified (by Taguchi analysis) as the most significant factors affecting the wear rate of in-situ Al-matrix composites [39], [43], [44].

Within the above framework, the corrosion and wear behaviors of Al-CMA systems present great research interest, especially when considering some major issues regarding these systems and their constituents. More specifically, the formation of two- or more phase structures based on a soft metallic phase shows the largest potential for improving the ductility of CMAs. Furthermore, CMAs are claimed to have excellent surface properties. It is also well known that aluminum alloys exhibit poor tribological properties (aluminum being a light metal) and moderate resistance to aggressive environments [45].

As far as the Al-Co system is concerned, previous work by the authors showed that Al-7 wt% Co (a low Co composition −3.3 at% Cosingle bond) in the Al-Al9Co2 side of the Al-Co phase diagram [46] had slightly higher corrosion resistance, as compared to Al-Co alloys of higher Co content (Al-(10–20) wt% Co) [30]. The good corrosion performance of Al-7 wt% Co is a promising result considering that this alloy may combine the benefits arising from the high amount of base Al (e.g. toughness and ductility) with the benefits arising from the dispersion of a CMA/Co-aluminide phase (e.g. low surface energy, high strength, high corrosion and wear resistance).

Therefore, the present work explores the efficiency of fabrication of Al-7 wt% Co by three cost effective techniques (vacuum arc melting, stir casting and free sintering) with respect to the attained microstructures and the response to aqueous corrosion and sliding wear. Emphasis is given on the clarification of the main surface degradation mechanisms, bearing in mind the scarcity of literature on the corrosion and wear performance of CMA-reinforced alloys, as well as Co-aluminides and Co-aluminide reinforced alloys.

Section snippets

Fabrication

Al-7 wt% Co alloys have been produced by three different techniques: stir casting, vacuum arc remelting and powder metallurgy (free sintering). The fabricated alloys are, hereafter, denoted as “Al-7Co_Cast”, “Al-7Co_VAM” and “Al-7Co_FS”, respectively.

Al-7Co_Cast was prepared by adding Co powder (−37 μm, 99.5% purity) and KBF4 (fluxing agent to promote the intimate contact between Co and molten Al by dissolving surface oxides [47]) into a melt of Al-1050 (commercially pure Al), at 850 °C. First,

Microstructure characterization

Fig. 1 illustrates the XRD spectra of the fabricated Al-Co alloys. All specimens exhibit similar patterns that reveal the presence of αAl and monoclinic Al9Co2, in compatibility with the Al-Co phase diagram [46]. Fig. 2 shows the microstructures of the transverse sections of the alloys under SEM.

Conclusions

  • Al-7 wt.% Co alloys have been prepared by conventional stir casting (Al-7Co_Cast), arc melting under argon (Al-7Co_VAM) and free sintering (Al-7Co_FS). All alloys are composed of Al9Co2 within an Al-matrix. The microstructure of Al-7Co_Cast consists of coarse blades of pre-eutectic Al9Co2 within Al. Al is primary and/or eutectic containing eutectic needles or rods of Al3(Fe,Co) and Al9(Co,Fe)2. The microstructure of Al-7Co_VAM is mostly eutectic. Colonies of parallel stripes of Al9Co2

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