Microscopic Rotation Behaviours of Crystals in Polycrystalline Bcc High Purity Iron

A series of tensile tests of polycrystalline high purity iron were carried out in order to trace microscopic orientations in different grains during plastic deformation. Crystal rotations were classified into two stages: one was the initial stage to the maximum strength or onset of necking, and the other was the subsequent stage from necking to fracture. Before the necking, two types of grain were observed: one rotated toward the [011] direction and the other did not rotate. During necking or plastic instability, the grains near the necking rotated toward the [111] direction. Activation of only a single slip system at the early stage did not follow the Taylor model which assumes multiple slip systems were active during deformation. In addition, it was observed that one grain was divided into two grains because a part of the grain rotated and the rest of the grain hardly rotated. Precise prediction of deformation of bcc metals requires improvement of the Taylor model by considering the transition of a single slip system to a multiple slip system, and a constraint on crystal rotation caused by adjacent grains. Necking due to tensile deformation was observed in grains oriented toward the [011] direction and local areas close to the necking or plastic instability finally rotated toward the [111] direction when the specimen fractured. Such rotation was also observed in a single crystal tensile test, because the lattice was released from various constraints on rotation.