Microstructural Correlation of Creep, Tensile and Corrosion Behaviour of AZ91 Magnesium Alloy with Bi, Ca and Sr Additions

Abstract

In the present investigation, five new alloys AZ91+1.0Ca (AZX911), AZ91+0.5Bi (AZB910), AZ91+1.0Ca+0.5Bi (AZXB9110), AZ91+1.0Ca+1.0Bi (AZXB9111), AZ91+2.0Ca+0.5Bi (AZXB9120) (wt.%) have been fabricated by squeeze-cast. The first part of the thesis investigates the influence of combined additions of Ca and Bi on the microstructure, creep, tensile, and corrosion behaviour of the squeeze-cast AZ91 alloy. The same is also studied on the AZ91 alloy with and without single additions of Ca and Bi for comparison. Another three new alloys AZ91+0.5Bi+0.25Sr, AZ91+0.5Bi+0.5Sr, AZ91+1.0Bi+0.5Sr (wt.%) have also been fabricated by squeeze-cast. The second part of the thesis investigates the influence of combined additions of Bi and Sr on the microstructure and creep behaviour of the AZ91 alloy. The creep behaviour of all the alloys is evaluated using impression creep tests in the temperature and stress ranges of 423 to 523 K and 300 to 480 MPa, respectively. The tensile tests of the Ca and Bi added AZ91 alloys are performed at 298, 423, and 473 K with a strain rate of 8.33×10-5 s-1. The corrosion behaviour of the Ca and Bi added AZ91 alloys is studied by immersion, hydrogen evolution, and electrochemical corrosion tests at 0.5 wt.% NaCl solution (pH 7) at room temperature. Both the single and mixed contents of Ca and Bi in the AZ91 alloy refine the grain size of α-Mg and reduce the volume fraction of the β-Mg17Al12 phase considerably. The effect is more noticeable in combined additions than in individual additions. The reticular-shaped Al2Ca and needle-shaped Mg3Bi2 phases additionally form with the α-Mg and β-Mg17Al12 phases because of the sole Ca and Bi additions in the AZ91 alloy. The Al2Ca and Bi3Ca5 phases are formed when Ca and Bi are added together, suppressing the Mg3Bi2 phase formation. The modified AZ91-based alloys containing Ca and/or Bi exhibit improved creep behaviour than the AZ91 alloy at all the stress and temperature levels tested. The individual additions of the elements in the AZ91 alloy show a higher creep rate than the combined additions. The individual Ca addition is better than Bi addition for resisting creep deformation in the AZ91 alloy as the Al2Ca phase in the AZX911 alloy has superior thermal stability compared to that of the Mg3Bi2 phase in the AZB910 alloy. The AZXB9120 exhibits the best creep performance owing to the lower volume fraction of the β-Mg17Al12 phase and the existence of a larger quantity of thermally stable Al2Ca and Bi3Ca5 phases. The values of stress exponents and activation energies conclude that the dominant creep mechanism for all the alloys is dislocation climb aided by pipe diffusion. The microstructural investigation following creep indicates that the β-Mg17Al12 phase is broken into small pieces. In contrast, the thermally stable Al2Ca, Mg3Bi2, and Bi3Ca5 phases preserve their continuity, which results in piled-up dislocations and tangling of dislocations in the interior of the α-Mg grains that leads to the improved resistance to creep deformation of the modified AZ91 alloys. The values of yield strength (YS) are higher, and ductility is lower of all the modified alloys. The ultimate tensile strength (UTS) of the modified AZ91 alloys is lower except at 473 K. The UTS values decrease with an increase in test temperature for all the alloys. The improved YS of the modified alloys is owing to reduced grain size. The brittle Mg3Bi2, Al2Ca, and Bi3Ca5 phases in the modified alloys reduce their UTS and ductility. The transgranular cleavage fracture at 298 K changes to quasi-cleavage fracture at 473 K. Several dislocations piled up around the β-Mg17Al12 and Al2Ca phases are seen. All the modified alloys exhibit better corrosion resistance than the base AZ91 alloy. The AZX911 alloy unveils better corrosion resistance than the AZB910 alloy owing to the Al2Ca phase formation. The combined Ca and Bi added AZ91 alloys acquire better corrosion resistance than the individual Ca or Bi added AZ91 alloys. The AZXB9120 and AZXB9111 alloys exhibit the lowest and the highest corrosion rates among the combined additions. The combined Bi and Sr additions form the Al4Sr and Sr2Bi phases besides the α-Mg and β-Mg17Al12 phases and improve the creep resistance of the AZ91 alloy. The AZ91+1.0Bi+0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and activation energy values of all the alloys confirm the pipe diffusion-controlled dislocation creep as the governing creep mechanism. The post creep microstructural study reveals several dislocations pile-ups around the Al4Sr and Sr2Bi phases resulting in improved creep resistance of the modified AZ91 alloys. To conclude, the additions of Ca and/or Bi improve the creep, tensile, and corrosion behaviour of the squeeze-cast AZ91 alloy. The effect is more significant with combined additions. The combined Bi and Sr additions also improve the creep behaviour of the AZ91 alloy. Therefore, the additions of Ca, Bi, and Sr to the AZ91 alloy are beneficial.