Creep, tensile and corrosion behaviour of Ca and/or Sb added AZ91 Mg alloys

Abstract

In the present thesis, the effects of combined additions of Ca and Sb on microstructure, tensile, creep and corrosion behaviour of AZ91 alloy fabricated by squeeze-casting have been investigated. For comparison, the same has also been studied on the squeeze-cast AZ91 alloy with and without individual additions of Ca and Sb. The composition of all the fabricated alloys are AZ91+0.59Sb (AZY911), AZ91+0.89Ca (AZX911), AZ91+0.91Ca+0.31Sb (AZXY9110), AZ91+0.89Ca+0.62Sb (AZXY9111) and AZ91+1.80Ca+0.33Sb (AZXY9120) (wt.%).
A detailed microstructural characterization of all the fabricated alloys has been done. Tensile tests of all the specimens at ambient, 423 K and 473 K temperatures have been carried out. The impression creep tests are conducted in the stress range of 300 to 480 MPa and temperature range of 423 to 523 K for a dwell time of 7200 s. A detailed microstructural analysis of the creep tested specimens has been carried out to further interpret creep behaviour of the alloys. Compressive creep tests are carried out on all the modified alloys at a temperature 473 K and stress of 70 MPa. The corrosion tests have been performed in 0.5NaCl (wt.%) solution at neutral pH with specimen exposed area of 0.5 cm2 at ambient temperature.
The results indicate that both individual and combined additions refine the grain size and β-Mg17Al12 phase, which is more pronounced with combined additions. Besides, α-Mg and β-Mg17Al12 phases, a new reticular Al2Ca and rod-shaped Mg3Sb2 phases are formed following individual additions of Ca and Sb in the AZ91 alloy. With combined additions, an additional Ca2Sb phase is formed suppressing Mg3Sb2 phase. Additions of both Ca and Sb increase yield strength (YS) at both ambient and elevated temperatures up to 473 K. However, both ductility and ultimate tensile strength (UTS) decrease first up to 423 K and then increase at 473 K. The increase in YS is attributed to the refinement of grain size, whereas, ductility and UTS are deteriorated by the presence of brittle Al2Ca, Mg3Sb2, and Ca2Sb phases. The best tensile properties are obtained in the AZXY9110 alloy owing to the presence of a lesser amount of brittle Al2Ca, and Ca2Sb phases resulted from the optimum content of 1.0Ca and 0.3Sb (wt.%). The fracture surface of the tensile specimen tested at ambient temperature reveals cleavage failure that changes to quasi-cleavage at 473 K. All the modified alloys exhibit superior creep resistance than the base AZ91 alloy. The combined additions of Ca and Sb are more effective in improving creep resistance than the individual additions. Individual Ca added AZX911 results superior creep resistance than the Sb added AZY911 is owing to the higher thermal stability of the Al2Ca phase in the former alloy compared to that of the Mg3Sb2 phase in the later one. The AZXY9120 alloy pertaining 2.0Ca and 0.3Sb (wt.%) exhibit the best creep resistance due to reduced amount of β-Mg17Al12 phase and presence of higher amount of dense network of thermally stable Al2Ca phase at grain boundaries. Post creep microstructural observation confirms the ability of the Al2Ca phase to withstand applied high stresses at elevated temperature without undergoing significant changes in its structure. The values of stress exponents and activation energies are in the range of 4.3 to 6.5 and 111.9±1.1 to 114.9±3.0 kJ/mol, which concludes dislocation climb controlled by pipe diffusion is the dominant creep mechanism for all the alloys in the temperature and stress level employed. The observed trend in creep rates with respect to the individual and combined additions of Ca and Sb in the AZ91 alloy was further confirmed by carrying out conventional compressive creep tests on all the alloys. It has been observed that the trend in creep rates, i.e., impression velocities in impression creep, and strain rates in compression creep obtained with respect to the individual and combined additions of Ca and Sb to the AZ91 alloy is the same. Corrosion resistance of all the modified alloys is better owing to the refined and reduced volume fraction of β-Mg17Al12 phase as well as grain refinement. Individual additions are better than mixed additions. The AZX911 alloy comprising individual Ca addition with a continuous network of Al2Ca phase reveals the lowest corrosion rate. Among the alloys comprising combined additions, the AZXY9120 alloy exhibits the best corrosion resistance due to higher and lower volume fraction of Al2Ca and Ca2Sb phases, respectively.
To conclude, the individual and combined additions of Ca and Sb to the AZ91 alloy resulted in improved ambient and elevated temperature tensile properties as well as impression and compression creep behaviour without deteriorating corrosion resistance. Among the modified alloys, the AZXY9120 alloy exhibited the best properties considering its targeted powertrain application at elevated temperature. In addition, the tensile and creep properties of the squeeze-cast alloys in the present investigation were superior as compared to that of the alloys developed by gravity-cast.