Creep, Wear And Corrosion Behaviour Of Novel Magnesium Alloys And Composites

Abstract

In the present investigation, MMCs have been fabricated using the creep-resistant AE42 magnesium alloy as matrix and reinforcing it with saffil short fibres (essentially δ-Al2O2) and SiC particles in various combinations. These MMCs have been investigated for their creep, wear and corrosion behaviour. The above properties of the matrix AE42 alloy have also been investigated for comparison. Further, laser surface melting has been carried out on a creep-resistant MRI 230D Mg alloy and the corrosion and wear behaviour of this alloy before and after laser surface melting has been investigated. The creep tests on the AE42 alloy were carried out in the temperature range of 1500 to 2400C at the stress levels ranging from 40 to 120 MPa and the composites were tested in the temperature range of 1750C to 3000 at the stress levels ranging from 60 to 140 MPa both in the longitudinal direction (LD) and in the transverse direction (TD). Wear tests were conducted on a pin-on-disc set-up under dry sliding condition at a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10 to 40 N for the AE42 alloy and the composites, which were tested both in LD and TD, and for a constant sliding distance of 1km in the load range of 5 to 20 N for the MRI 230D alloy before and after laser melting. All the materials were subjected to electrochemical corrosion tests in a 5 wt.% NaCl solution having ph value 11 for 22 hours. All the composites in both LD and TD exhibit lower creep rate as compared to the AE42 alloy and it is higher in TD than in LD. The creep resistance of the hybrid composites, in which saffil short fibres are partially replaced by SiC particles, is observed to be comparable , i.e., of the same order of magnitude , to that of the composite reinforced with Saffil short fibres alone at all the temperatures and stresses employed in both LD and TD. Wear rate of all the composites in both LD and Td is found to be lower than the alloy at all the loads employed and it is higher in TD than LD, Wear rate progressively decreases with the partial replacement of Saffil short fibres by Sic Particles, and is lowest for the composites reinforced with 10 vol.% Saffil short fibres and 15 vol.% Sic particles in both LD and TD. It is 34% and 35% lower than the 20% Saffil composite at 40 N load in LD and TD, respectively. The Ae42 alloy exhibits the best corrosion resistance and the addition of the Saffil short fibres and/or Sic particles in the AE42 alloy deteriorates its corrosion resitance. The composite reinforced with Saffil short fibres alone exhibits slightly better corrosion resitance than the hybrid composites. However, there is no systematic trend of corrosion resistance with SiC particles content. The laser surface melting is found to improve the corrosion, hardness and wear resistance of the MRI 230D alloy. High temperature climb of dislocation is found to be the dominant creep mechanism in the AE42 alloy in the stress and temperature range employed. Various glide and climb of dislocation are found to be the dominant creep mechanisms for all the composites in both LD and TD in the stress and temperature range employed. The presence of SiC particles in the hybrid composites improves the wear resistance in both LD and TD since these particles remain intact and retain their load bearing capacity even at the highest load employed in the present investigation. They promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads in case of the composites in LD. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites in both LD and TD. It is accompanied by severe plastic deformation of surface layers in case of the alloy, the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of the composites in Ld, and the fracture and pull-out of the Saffil short fibres in case of the composites in TD. The lower corrosion resistance of all the composites is not caused by the galvanic coupling between reinforcements and matrix, and is related to the microstructural changes, such as, distribution of precipitates and the nature of the film formed at the surface. The improved corrosion resistance following laser surface melting is due to the absence of the Al2Ca phase at the grain boundary, microstructural refinement and increased solid solubility, particularly of Al, owing to rapid solidification; the improved hardness and wear resistance is due to grain refinement and solid solution strengthening. To conclude, the creep resistance of the hybrid composites is comparable, wear resistance is better and corrosion resistance is slightly inferior to the composite reinforced with Saffil short fibres alone. Therefore, from the commercial point of view, the use of the hybrid composites, replacing a part of the expensive Saffil short fibres by cheap SiC particles, is beneficial. The laser surface melting is beneficial for the corrosion and wear resistance of the MRI 230D alloy.