Creep-rupture behavior of die-cast magnesium alloys
Keywords:Die-Cast Mg alloys, Creep rupture, Monkman-grant relation, Apparent activation energy
AbstractIt is found that the relationship between the time-to-rupture tr and the time to the onset of tertiary creep tÎ¿t (tr/tot) 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 Ec of creep is below the value of magnesium self-diffusion energy ESD up to 175Â°C. At higher temperatures, Ec > ESD, 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 â‰… 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 Î²-phase during long-term creep tests.
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