Creep-rupture behavior of die-cast magnesium alloys

Abstract

It is found that the relationship between the time-to-rupture t_r and the time to the onset of tertiary creep t_οt (t_r/t_ot) in die-cast Mg-9Al alloy is equal to 1.3 and independent of stress, test temperature and diecasting parameters over the range examined. It is shown that the introduction of creep-rupture strain into Monkman-Grant relation modified by DobeÅ¡ and MiliÄka demonstrates only the deviation range of the minimum creep rate from the average creep rate. The apparent activation energy E_c of creep is below the value of magnesium self-diffusion energy ESD up to 175°C. At higher temperatures, E_c > E_SD, and creep deformation may be controlled by diffusion processes (dislocation climbing, etc.). The established magnitude of stress exponent n in the power law (the modified Arrhenius rate equation) within the stress range from 20 to 50 MPa is close to the value of n ≅ 3 typical of solid-solution alloys. The increase in n values at higher stresses up to ~ 5 is caused, probably, by stress-induced precipitation of β-phase during long-term creep tests.