THE COMBINED EFFECT OF GRAIN-SIZE AND STRAIN-RATE ON THE DISLOCATION SUBSTRUCTURES AND MECHANICAL-PROPERTIES IN PURE ALUMINUM

The effect of grain size on the development of dislocation substructur es has been studied as a function of strain rate. Pure aluminum rods w ith grain diameters of 70, 278, and 400 mum were deformed in tension a t room temperature to various percent strains at strain rates of 0.01, 0.25, 2.5, and 5/min. It has been confirmed that the smaller grain si ze results in higher flow stress in this strain-rate range. The cell s ize strengthening described by the modified Hall-Petch (MHP) equation is applicable to samples with 70 and 278 mum grain sizes at all four s train rates used in this study, while 400 mum grain sizes show deviati on from this because of inhomogeneities developed in the microstructur e. The influence of strain rate on the slope of the MHP plots, for a g rain size of 70 mum. is such that at lower strain rates, the slope doe s not change much, but at higher strain rates, there is an increase in the slope value. At all strain rates, the values of slopes from the M HP plots of the smaller grains are higher than for the larger grains.