Creep-Rupture Behavior of Die-Cast Magnesium Alloys

It is found that the relationship between the time-to-rupture t(r) and the time to the onset of tertiary creep t(ot) (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 Dobes and Milicka 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 E-SD up to 175 degrees 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 congruent to 3 typical of solid-solution alloys. The increase in n values at higher stresses up to similar to 5 is caused, probably, by stress-induced precipitation of beta-phase during long-term creep tests.