Flow Stress and Dislocation Structures in Polycrystalline Molybdenum

The flow stress of a polycrystalline molybdenum of commercial purity was determined in the temperature range 20° to 940°C, and the result was discussed in comparison with the dislocation structure obtained by transmission electron microscopy.The flow stress decreases markedly with a rise in temperature between 20° and 350°C. In this temperature range, except at 200°C, the work-hardening rate is almost independent of deformation temperature, while above 500°C it decreases as the temperature increases. The increase in the dislocation density is proportional to the strain in an early stage of deformation, but the rate of increase in the dislocation density becomes smaller at the strain where well-defined cell-structures are observed. The non-uniformity of the dislocation distribution increases as the deformation temperature rises between 20° and 500°C, but it is almost independent of strain once dislocation tanglings are formed. The increase in flow stress as the deformation proceeds is proportional to the square-root of dislocation density in a dislocation-concentrated region. The proportionality factor α′ is about 0.19 between 20° and 900°C, except at 200°C. The flow stress, dislocation density and the relation between the two show anomalous behaviours at temperatures near 200°C.