rd
3 International Conference of Engineering Against Fracture (ICEAF III) 26-28 June 2013, Kos, GREECE
Microstructure and Corrosion Behaviour of Aluminium Alloys Containing Complex Metallic Alloy Phases A. K. Sfikas1, A. Lekatou1*, A.E. Karantzalis1, D. Sioulas1 1
Dept. of Materials Science & Engineering, Univ. of Ioannina, 45110 Ioannina, Greece *
[email protected]
ABSTRACT Al-7 (wt %) Co alloys have been prepared by casting, arc melting (AM) under argon and free sintering, with the scopes to investigate an effective fabrication method (in terms of low cost, ease of manufacture and property boosting) and exploit the corrosion-wise benefits of a single intermetallic phase and the ductility-wise benefits of the Al-rich end of the Al-Co phase diagram. The preparation method strongly affects the attained microstructure. A notably refined morphology predominates in the microstructure of the AM alloy. All alloys, regardless of the preparation method, exhibit low resistance to localised corrosion in 3.5% NaCl. The passivation of the intermetallic phase (Al9Co2) plays a dominant role in the passive-like behaviour of the alloys. AM alloy has displayed the best corrosion performance mostly due to the supersaturation of the Al matrix with Co and the uniform, refined and ultra dense intermetallic network. Keywords: Complex Metallic Alloys, Al-Co intermetallics, arc melting, free sintering, cyclic polarization, chronoamperometry.
1 Introduction The term CMA (Complex Metallic Alloys) describes a new category of intermetallic compounds that possess high structural complexity, large lattice parameters and a high number of atoms per unit cell 1. During the last decade, CMAs have attracted significant attention due to properties, such as low surface energy, low friction coefficient, high hardness, high electrical resistivity, low thermal conductivity, good oxidation resistance and high hydrogen sorption capacity 2. CMAs may find a wide spectrum of applications including anticorrosion, wear resistant, heat insulation applications etc. 3. However, CMA applications are still in an early stage of maturity. Major reasons for this, are the high cost of the production techniques and their low ductility. The method that shows the largest potential for improving the ductility of CMAs at low temperatures is the production of two- or multi- phase structures including a soft metallic phase 4. Several types of CMAs exist, depending on the nature of the constitutive elements and their respective concentrations. The most widely studied so far are based on aluminium 3. Al13Co4 has gained special research interest as a potential CMA structure due to a pronounced yield-point effect and weak work hardening at high strain 1,4,5. Intermetallic compounds of aluminium have attracted great research interest regarding hightemperature and structural applications, since they combine good high temperature mechanical properties, low density, high melting points 6-8 and good high temperature oxidation resistance 8. Alloy design/property work has focused on the aluminides of Ni 9 Fe 6 and Ti 7. Only few design/property studies have been devoted to Co-aluminides due to their limited (until recently) application potential. The onset, nevertheless, of three different research frontiers, has given a new boost to the Al-Co system research and new application perspectives. These new research areas are: Rapid Solidification Processes (RSPs) 10, Hydrogen Fuel Cell technologies 11 and CMA– Quasi Crystals 1, 4, 5. The corrosion behaviour of bulk Al-CMAs, such as Al-Cr-Fe, Al-Cu-Fe-Cr, Al-Cu-Fe, Al-CuCo-Si, Al-Fe-Cr has been shown to be governed by their chemical composition rather than their complex structure 12-14. Little information is available on the corrosion behaviour of Al-Co alloys. Corrosion studies are concentrated on the Al-Co-Ce system, where Co contributes to a high pitting potential, high repassivation potential and high rest potential 15-17. Previous effort 18 by the authors has shown that the alloy Al-32 (wt%) Co displays a remarkably good corrosion performance in 3.5% NaCl. The particular composition contains both Al13Co4 and Al9Co2 in an (Al) matrix. The present study deals with a hypereutectic composition in the Al-Co
* Corresponding author
A. K. Sfikas, A. Lekatou, A. E. Karantzalis, D. Sioulas
system, of modest Co content, in order to exploit the corrosion-wise benefits of a single intermetallic phase and the ductility-wise benefits of the Al-rich end of the Al-Co phase diagram. The Al-7Co alloy has been fabricated by three different techniques, of relatively low cost, with the scope to investigate an effective way of fabricating these alloys in terms of ease of manufacture and property boosting.
2 Experimental An Al-7 wt% Co alloy was produced by three different techniques: casting, arc melting under argon and powder metallurgy (free sintering). The fabricated alloys are, hereafter, denoted as “Cast”, “AM” and “FS”, respectively. The cast alloy was prepared by adding Co powder (400 mesh, 99.5% purity) and KBF4 into a melt of Al-6061 (850 oC). In the case of arc melting, appropriate mixture of Al powder (325 mesh, 99.97% purity) and Co powder was introduced into the furnace and was melted at about 2200-2500 °C. In the case of free sintering, the same mixture of Al powder and Co powder was pressed at 30 MPa for 5 min and then heated for 30 min at 300 °C and 4 h at 500 °C. The microstructure of the alloy was examined by optical microscopy (Leika 4000) and scanning electron microscopy (Jeol 5600). Microporosity was determined by the use of Image J software. XRD patterns were obtained by a Bruker D8 Advance diffractometer. Small rectangular coated coupons ground to 1000 grit and peripherally mounted in PTFE were immersed in aerated 3.5% NaCl, at 25 oC (exposed surface ~1 cm2). A standard three electrode cell was employed (reference electrode: Ag/AgCl, counter electrode: Pt). Potentiodynamic polarization was carried out (ACM Gill AC galvanostat, scan rate: 10 mV/min) after 4 h of immersion in open circuit. Corrosion current densities were determined by Tafel extrapolation 18. Cyclic polarization was conducted to assess the susceptibility of the alloys to localized corrosion. Chronoamperometry was carried out to study the progress and stability of the surface films. The nature of the latter was investigated by Raman Spectroscopy –RS- (RM 1000 RENISHAW, Nd:YAG laser excitation wavelength of 532 nm, laser power of 60 mW, power incident on the sample surface of 6 mW, focused spot diameter of ~(2-3) μm).
3 Results and discussion 3.1 Microstructure characterization Figure 1 illustrates the XRD spectra of the fabricated Al-Co alloys. All specimens exhibited similar patterns that revealed the presence of Al (αAl) and Al9Co2 CMA, in compatibility with the AlCo phase diagram 19. Figure 2 shows the respective microstructures under optical microscope. Some porosity is evident, especially in the case of FS alloys. The alloy microporosities were measured as: (0.39±0.09) % (CAST), 0.35±0.08 (AM), 1.20±0.28 (FS). The CAST alloy microstructure (Figure 2a & d) roughly agrees with that predicted from the AlCo phase diagram, (approximately 78.5% eutectic Al, 2.5% eutectic Al9Co2, 19% primary Al9Co2). A notably refined eutectic morphology predominates in the microstructure of the AM alloy. A likely explanation is that rapid solidification largely suppresses the pre-eutectic stage not allowing the primary Al9Co2 crystallites to grow. This possibility is enhanced by the presence of Al9Co2 droplets of less than 2 μm diameter (Figure 3). Some planar solidification is also indicated in Figure 2b (far left); rapid solidification of hypereutectic alloys, often adopts a planar mode which, being unstable, eventually results in coupled eutectic growth 20. In this mode of growth, primary Al dendrites may nucleate and grow at the fastest solidification rates instead of the equilibrium primary intermetallic phase 21, 22. A very interesting observation is the detection of appreciable quantities of Co in solid solution with Al (Figure 3c), despite the negligible solubility of Co in Al under equilibrium conditions (
