Effect of Temperature on the Creep of Polycrystalline Aluminum by the Cross-Slip Mechanism

The apparent activation energy for creep of polycrystalline aluminum was determined over the range of 273 deg to 350 deg K by the effect of small abrupt changes in temperature on the creep rate. A constant activation energy of 27,400 or - 1000 calmole was obtained over strains of 0.003 to 0.23, stresses ranging from 2250 to 6000 p. s. i. and strain rates varying from 0.1145 x l0exp -5 to 29.5 x l0exp -5 per minute. Metallographic studies and comparison with theory suggested that creep in this range is controlled by the rate of cross-slip of dislocations. Both X-ray diffraction analyses and room temperature tensile stress-strain data following precreep revealed that the substructure produced in this range by creep under a given stress depended only on the total creep strain being independent of the actual test temperature. As a result it was deduced that the total strain Epsilon during creep under a given stress should be a function of temperature compensated time Theta te - QRT where t is the duration of the test, Q the apparent activation energy, R the gas constant and T the absolute temperature. A number of creep tests conducted at two different temperatures verified the validity of this conclusion.