Creep, Tensile, Wear and Corrosion Behaviour of SiC Nanoparticles Reinforced Squeeze-Cast AZ91-Ca-Sb Magnesium Alloy

Abstract

In the present thesis, the influence of SiC nanoparticles additions on the creep, tensile, wear, and corrosion behaviour of the AZ91-2.0Ca-0.3Sb (wt.%) alloy fabricated by squeeze-casting has been investigated. For comparison, these properties are also investigated in the AZ91- 2.0Ca-0.3Sb alloy without nanoparticles addition. The nominal compositions of the fabricated alloy and nanocomposites are AZ91-2.0Ca-0.3Sb (AZXY9120), AZ91-2.0Ca-0.3Sb-0.5SiCnp (NC1), AZ91-2.0Ca-0.3Sb-1.0SiCnp (NC2), and AZ91-2.0Ca-0.3Sb-2.0SiCnp (NC3) (wt.%). A detailed microstructural characterization of the alloy and nanocomposites was carried out. An impression creep testing setup was employed for carrying out the creep tests in the stress and temperature range of 300-480 MPa and 448-523 K, respectively. The alloy and nanocomposites were tensile tested at ambient and elevated temperatures of 298, 423, and 473 K. The dry sliding wear tests were conducted employing a pin-on-disc setup at normal loads of 10, 20, 30 and 40 N at a sliding velocity of 1.2 m/s for a sliding distance of 1000 m. The corrosion responses of the alloy and nanocomposites in a 0.1 M NaCl solution at ambient temperature and pH 7.0 were evaluated by immersion, hydrogen evolution, and potentiodynamic polarization scan. The results indicate that the α-Mg, β-Mg17Al12, Al2Ca and Ca2Sb phases are present in both the alloy and nanocomposites. The additions of SiC nanoparticles refine the grain size, reduce the volume fraction of the β-Mg17Al12 phase, and increase the amount of Al2Ca phase, which is more pronounced with an increase in the nanoparticle content. All the nanocomposites exhibit superior creep resistance than the unreinforced alloy. The best creep resistance is obtained in the NC3 nanocomposite. The values of stress exponents and the activation energies are in the range of 4.5 to 6.2, and 101.9 ± 2.5 to 115.5 ± 3.2 kJ/mol suggesting the governing creep mechanism for the alloy and nanocomposites is dislocation climb controlled by pipe diffusion. The post creep microstructural observation confirms that the β-Mg17Al12 phase in the alloy is rigorously fragmented, and aligns in the direction of material flow, which deteriorates its creep resistance. In contrast, the presence of Al2Ca phase network and the SiC nanoparticles increase the dislocation pile-ups and dislocation tangling resulting in superior creep resistance of all the nanocomposites. All the nanocomposites illustrate greater yield strength (YS) and ultimate tensile strength (UTS) in contrast to the alloy at all the temperatures employed. The YS, UTS and elastic modulus of both the alloy and nanocomposites decline, whereas the work to fracture increases with a rise in temperature. Among the nanocomposites, the NC3 demonstrates the best tensile properties. All the nanocomposites display superior strain hardening response than the alloy, and the maximum strain hardening is perceived in the NC3 nanocomposite. The improved tensile properties of the nanocomposites are ascribed to the reduced grain size, the increase in dislocation density owing to CTE mismatch between the alloy and the SiC nanoparticles, the Orowan strengthening as well as the presence of a relatively higher amount of Al2Ca phase in the nanocomposites. The contributions to the improvement of strength of the nanocomposites in decreasing order of their influence are the Orowan strengthening, the strengthening due to CTE mismatch, and the Hall-Petch strengthening. The fracture surfaces of the tensile specimens tested at 298 K confirm the presence of transgranular cleavage fracture which remains unchanged at 473 K as well. The wear rate is lower for all the nanocomposites compared to the alloy. All the nanocomposites demonstrate the lower specific wear rates than the alloy. Among the nanocomposites, the NC3 exhibits the best tribological performance. The values of the coefficient of friction are lower for the nanocomposites than the alloy. The abrasion, adhesion, oxidation, and delamination are the dominant wear mechanisms. The 3D topography depicts that the addition of nanoparticles to the alloy results in the reduced surface roughness during the wear tests, confirming the superior wear behaviour of the nanocomposites compared to the alloy. All the nanocomposites demonstrate a superior corrosion resistance than the alloy, and the NC3 nanocomposite exhibits the highest corrosion resistance. The improved corrosion performance of the nanocomposites is attributed to the decrease in the potential difference between α-Mg and β-Mg17Al12 phases, reduced quantity of β-Mg17Al12 phase, and an increased amount of Al2Ca phase following the SiC nanoparticles additions. To conclude, all the nanocomposites display the superior creep, tensile, wear and corrosion response compared to the alloy. Among the nanocomposites, the NC3 nanocomposite illustrates the best creep, tensile, wear and corrosion performance. Therefore, the use of the nanocomposites would be more beneficial than the alloy.