Microstructure and strength of quasicrystal containing extruded Mg-Zn-Y alloys for elevated temperature application

Microstructural studies have been made on alloys Mg95Zn4.2Y0.8 and Mg92.5Zn6.5Y containing icosahedral quasicrystalline phase extruded at two different temperatures of 523 K and 673 K and correlated to room temperature and higher temperature tensile strength. Icosahedral phase occur-red as particles about half micron size at grain boundaries and as nano-sized precipitates in the matrix. Solutionizing at 673 K (1 h) was employed followed by annealing at 473 K for finer precipitation in the matrix. Solutionizing produced a fine precipitation of a ternary phase tau(1) in the matrix of the Mg95Zn4.2Y0.8 alloy, replaced by icosahedral precipitates on annealing. The Mg92.5Zn6.5Y alloy containing larger volume fraction of icosahedral phase showed higher strength. Solutionizing produced a composite of icosahedral and a H-Mg25Zn58Y17 hexagonal phase composite. Yield stresses of over 250 MPa at room temperature (elongation 15-20%) and 150 MPa at 473 K (UTS 240 MPa, elongation 40%) were obtained upon post extrusion annealing. (C) 2004 Elsevier B.V. All rights reserved.