The Effect of Grain Size on High Temperature Plastic Deformation of Polycrystalline Pure Iron

Abstract

The effect of grain size on the tensile deformation behavior of high-purity polycrystalline iron was studied over the temperature range from 25 to 900°C, by means of a newly devised equipment with which the metallic materials could be quenched by hydrogen gas immediately after hot tensile deformation at various strain rates from 1 to 10⁻⁷ l/sec. The following characteristics of high temperature deformation behavior of pure iron were observed above 500°C where the effect of the blue-brittleness phenomena disappeared: (1) the steady state flow, (2) no grain size dependence of flow stress in high strain range, and (3) the marked increase in total elongation and the decrease in strain at tensile strength.
The relation between the grain size and yield stress or flow stress at a given strain was approximately represented by the Hall-Petch equation from 25 to 800°C. The temperature dependence of yield stress originates from the temperature dependence of friction stress (σ_i) and of grain size effect (K_y·D^−1⁄2). The K_y⁄σ_i-ratios were almost constant (0.18±0.02) between 400 and 800°C, and this fact indicates the yield stress largly depends upon the grain size even at 800°C. The temperature dependence of σ_i was similar to that of the yield stress of iron single crystal.
The grain size dependence of flow stress shows a complicate behavior with the change of temperature and strain. This behavior is explained qualitatively in terms of the work hardening model rather than the dislocation pile-up model. The grain size dependence of steady state flow stress was hardly observed. This seems to be due to the fact that a uniform subgrain size was produced at high temperature and high strain range regardless of the initial grain size.