Thermal activation of flow stress transients in mild steel

A high strain rate evolution through the Lüder’s extension can be induced in mild steel wires under dead weight loading using a rapid heating (≈40°C s−1) thermal history generated by the direct resistance heating method. Using this experimental technique, the behavior of two different microstructures derived from cold drawn AISI 1018 wire was investigated, a pearlite/ferrite microstructure (ASTM no. 9) produced by a normalizing heat treatment, and a smaller ferrite grain size (ASTM no. 10) produced by recrystallization annealing. With axial and diametral extensometers estimates of the Lüders band velocity and the lower bound strain rate attained within the band were obtained. For normalized mild steel at 273 MPa, the maximum Lüder’s band velocity measured directly was 1.3 m s−1. The lower bound strain rate within the band was estimated to be about 8 s−1. Transmission electron microscopy (TEM) investigation of the dislocation structures associated with both materials showed that the strain bursts resulted in a higher dislocation density. Based on the high strain rates measured, the short duration of deformation and the dislocation structures observed, some constraints on possible mechanisms are proposed.