The dependence of the lower yield strength in iron and steel on grain size and temperature

Experimental results obtained from the literature on the dependence of the lower yield strength on grain size and temperature in α-Fe at temperatures between 90° and 373°K are shown to substantiate the validity of the relation, σ _aF = σ _i +V _c/NθKl ²)^1/m . Values ofm, the dislocation velocity-stress exponent, and its temperature dependence show reasonable agreement with results obtained from stress relaxation, double strain-rate cyclic and premacroyield tests in polycrystalline iron and Ti-stabilized iron after homogeneous and inhomogeneous deformation. An apparent activation enthalpy of 0.73 ev was determined and an enthalpy of 0.86 ev was computed which agree well with values reported for homogeneous deformation in α-Fe. The results obtained indicate that the dislocation mechanisms controlling inhomogeneous and homogeneous deformation in α-Fe at low temperatures are the same. As temperature is lowered, the marked increase in lower yield strength in α-Fe is related to a strong decrease in θ, the average dislocation velocity at unit effective stress.