The Effect of Strain Rate, Temperature and Grain Size on the Lower Yield Stress and Flow Stress of Polycrystalline Pure Iron

Abstract

The lower yield and flow stresses of polycrystalline pure iron specimens with a wide range of grain sizes from 10.3 μ to 102 μ were measured by tensile test at the temperatures between 77°K and 473°K at the strain rates from 10^-7 sec^-1 to 1 sec^-1. The observed stress was decomposed into a friction part (al) and grain size depending part (k·D^-1/2) in the Hall-Petch relation and they were discussed separately. It was found that the lower yield stress did not satisfy the Hall-Petch relation at each temperature and strain rate investigated. The lower yield stress of coarse grained specimens below D^-1/2=7 mm^-1/2 showed a usual grain size dependence, but that of fine grained specimens showed a dependence about 3 times as large as the former. The thermal component of lower yield stress of the specimen with abnormal grain size dependence had larger temperature and strain rate dependences than that of the specimen with normal dependence.
The relation between the activation energy or activation volume for low temperature deformation and thermal shear stress was not affected by the grain size. Therefore it was concluded that the lower yield stresses of both fine and coarse grained specimens were controlled by the same thermally activated rate process (the Peierls-Nabarro mechanism).
The strain rate dependence of the lower yield stress of pure iron was continuous in a very wide range of strain rate. The relation between the thermal component of lower yield stress (σ₁*) and strain rate (ε) was well approximated by the equation, σ₁*^1/2=r′₁ log ε+C₄′ at all the temperatures tested.(r₄′ and C₄′ are constants which depend on temperature).