Dispersion of stable quasicrystal phase by precipitation during extrusion in a Mg-Zn-Al alloy: Microstructure and mechanical properties

Precipitation of stable icosahedral quasicrystalline phase (i) has been recently reported in a Mg-6Zn-3Al (in wt%, ZA63) alloy on ageing after solutionizing treatment of cast alloy to dissolve dendritic i-phase. This offers an opportunity to disperse this phase in a ZA alloy through a solutionizing treatment followed by extrusion, thereby precipitating the i-phase during extrusion. By this process, the i-phase can be dispersed in the alloy uniformly, instead of dispersion from the cast structure by extrusion. In this study, a cast ingot of ZA63 alloy was solutionized at 400 degrees C, followed by extrusions at 255 degrees C and 300 degrees C. Extrusion at 255 degrees C resulted in larger amount of i-phase precipitation (about 4.5 mass%), in which spherical precipitates of size about 100 nm or less were distributed in the matrix and boundaries of grains of average size 2.4 +/- 1.0 mu m, exhibiting true yield strengths of 263.7 +/- 1.4 MPa in tension and 255.4 +/- 5.0 MPa in compression (with elongation to failure of about 18% in both cases). Extrusion at the higher temperature of 300 degrees C resulted in fewer precipitates (about 1.4 mass%) and larger average grain size of 12.9 +/- 6.1 mu m and i-phase precipitates of size 500 nm which were mostly at the grain boundaries, and consequently lower true yield strengths of 214.4 +/- 1.3 MPa in tension and 159.3 +/- 0.3 MPa in compression. The strength after extrusion at 300 degrees C increased by 48 MPa in tension and 38 MPa in compression on ageing at 250 degrees C for 20 min. Ageing precipitate phases were identified to be i-phase, Mg4Zn7 and possibly ternary ??????-phase. Phase equilibrium and change in morphology of phases during ageing has been discussed. The peculiar nature of work hardening during tensile deformation has also been discussed.