A study of microcreep in magnesium single crystals in the temperature range 77 degrees to 300 degrees K
Bresie, Don Alan
Roberts, John M.
Master of Science
Single crystals of 99.97% pure magnesium were grown by the Bridgman technique. A very sensitive capacitance strain gage was employed to observe microcreep in these crystals at temperatures from 77° to 300°K. The microstrain-time results at a constant applied stress for all temperatures within this range were found to fit the empirical equation where I is the resolved shear microstrain, t is time and oc and A are constants. The constant A did not appear to be a function of temperature. Although A did seem to vary with the load increment, this dependence was not well defined by the observed results. The constant infinity was a function of several parameters but it seemed to be best described experimentally as proportional to the total strain during an incremental creep curve. The effective activation volume is defined where Ta is the resolved shear stress, k is Boltzman's constant and T is the absolute temperature in °K. The effective activation volume was found to be independent of applied stress. It was proportional to temperature such that v* = 2.38 x 10-21 T cm3 for an ultrahigh purity specimen and v* = 2.28 x 10-21 T cm3 for an alloyed crystal containing 0.046% Al. Internal stress was measured by experimentally finding the applied stress at which the forward creep balances the backward strain recovery. It was found to be 93% of the applied stress.