EFFECT OF FREQUENCY ON CYCLIC CREEP OF POLYCRYSTALLINE ALUMINUM AT ROOM-TEMPERATURE

A study was made of the effect of cyclic frequencies from zero to 1 Hz on the cyclic creep behaviour with different extents of unloading in commercial purity aluminium. There exists an optimum frequency corresponding to the lowest steady state creep rate, the value of which exhibits the trend of decreasing with attenuating stress amplitude at a given cyclic peak stress. Both the cyclic frequency and the stress amplitude strongly affect the threshold stress for the transition from cyclic creep acceleration to retardation but do not change the average internal stress. Expression for the threshold stress were derived as a function of thermal activation parameters. When the value of V(c) is near that of V(s) (V(c) and V(s) are the apparent activation volume of cyclic and static creep, respectively) the threshold stress varies inversely as (V(c)/V(s)-1); otherwise, it is inversely proportional to 1-V(s)/V(c). The effect of grain size on cyclic creep is also discussed.