Microyielding in iron at low temperatures

Abstract

The micro and macro-yielding of zone refined polycrystalline iron was investigated between 298 and 5°K. The-elastic limit of 5000 psi at 298°K did not increase until the temperature was below about 50°K; the value at 5°K was 10 4 psi. The stress to produce a permanent strain of 10 −6, σA, increased by 8–14 × 10 2 psi in going from 298°K to 5°K. σA did not change with a five fold change in strain rate. The macro-yield point of prestrained material increased by 12 × 10 4 psi between 298° and 5°K. The elastic limit was associated with the stress to move existing edge kinks against a Peierls-Nabarro force and also against impurities, σA was connected with another Peierls-Nabarro force which resists the motion of screw dislocations with impurities making an additional contribution. The difference between σA and the flow stress has been, in part, attributed to a difference in plastic strain rate at which each stress is measured. In addition, a different Peierls-Nabarro force may control the flow stress.