The creep behaviour of the magnesium-zirconium alloy ZA at 400° and 450° C in carbon dioxide containing ≈ 200 ppm moisture

Abstract

The creep behaviour of the magnesium-zirconium alloy ZA was studied in tests of up to 5600 h duration at 400° C and up to 12 600 h duration at 450° C, in an atmosphere of carbon dioxide containing ≈ 200 ppm water. The accompanying microstructural changes were observed by optical and electron microscopy. The alloy is stronger at 450° C than at 400° C and additional strengthening obtains from prestraining at 250° C prior to creep-testing. In stress rupture tests at 200° C subsequent to creep-testing, the time to rupture and the rupture ductility are lower in specimens previously tested at 450° C than in those tested at 400° C. The increase in creep strength at 450° C, and subsequent loss of ductility, are attributed principally to the precipitation of a zirconium-rich phase, tentatively identified as ε-zirconium hydride, which forms both intragranularly (as ribbons and thin hexagonal plates) and as intergranular particles.