Microstructure and Impression Creep Behaviour of the Bi and Sr Added Squeeze-Cast AZ91 Mg Alloy

Abstract

The effect of combined additions of Bi and Sr on microstructure and impression creep behaviour of the squeeze-cast AZ91 alloy have been carried out. For comparison, the same has also been investigated on the as-cast AZ91 alloy without any addition. The results indicate that the combined additions of Bi and Sr refine the grain size and β-Mg17Al12 phase, which is more pronounced in the AZ91-1.0Bi-0.5Sr (wt.%) alloy. Besides α-Mg and β-Mg17Al12 phases, both a rod-shaped Mg3Bi2 and an irregular shaped Al4Sr phases are newly formed following the combined additions of Bi and Sr in the AZ91 alloy. All the alloys have been impression creep tested in the temperature range of 423-523 K and stress range of 300-480 MPa. All the modified alloys show superior creep resistance than the base AZ91 alloy. Among the modified alloys, the AZ91-1.0Bi-0.5Sr alloy is the most effective in improving creep resistance owing to the presence of relatively greater volume fraction of the thermally stable Mg3Bi2 and Al4Sr phases. Post creep microstructural analysis approves the capability of both the Al4Sr and Mg3Bi2 phases to endure applied high stresses at higher temperature without undergoing noteworthy changes in its structure. The values of stress exponents (n) and activation energies (Q) are in the range of 4.0-5.7 and 104.1 ± 5.9 to 107.0 ± 3.7 kJ/mol, respectively, which suggest that the governing creep mechanism for all the alloys is dislocation climb aided by pipe diffusion at all the temperature and stress level employed in the present investigation.