Year

2011

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Ti-stabilised interstitial free steel subjected to room temperature route BC 8 passes Equal Channel Angular Pressing (ECAP), was cold rolled (CR) to 25, 50 and 95% thickness reductions at ambient temperature and 25% at liquid nitrogen temperature to induce further microstructural refinement in an already ultrafine grained (UFG) microstructure. Subsequently, thermal stability of the ECAP and 95% CR microstructures was investigated by isothermal annealing at 525, 600 and 710 °C.

With greater CR reduction the ECAP microstructure is refined monotonically with a concurrent increase in high angle grain boundary (HAGB) fraction. The convergence of high and low angle boundary spacings results in an increased contribution via boundary strengthening to the yield and ultimate tensile strengths. The ECAP texture components rotate around the transverse direction up to 50% CR while a strong α-fibre rolling texture develops after 95% CR. The tensile fracture mode evolves from ductile to brittle type between ECAP and 95% CR concurrent with reduced work hardening capacity.

The bulk stored energy values estimated by calorimetry correspond to energy release from all strain sources in the material volume as well as Ti precipitation during annealing whereas the local stored energy estimates by microhardness, Electron Back-Scattering Diffraction (EBSD) and X-ray line broadening allude only to changes in dislocation content or internal stresses. High apparent activation energy suggests sluggish recrystallisation of the steel due to excess Ti in solid solution. During annealing, homogeneous coarsening of the ECAP microstructure via continuous recrystallisation is followed by abnormal growth during the final stages of softening. On the other hand, the 95% CR microstructure coarsens only homogeneously following normal growth via continuous recrystallisation with a progressive decrease in HAGBs. The evolution of room temperature tensile characteristics after annealing, from stress-drop soon following yielding, then Lüders banding and finally, to a return to continuous yielding and increased work hardening, is related to a coarsening microstructure and changing area fraction of HAGBs. Atom Probe Tomography (APT) indicates no dissolution of pre-existing Ti4C2S2 and FeTiP precipitates during ECAP or 95% CR. All fine precipitates detected by APT irrespective of deformation or annealing, exhibit a variety of non-stoichiometric compositions whereas near equilibrium composition was found in an order of magnitude coarser precipitates observed by TEM.

The correlations between room temperature shear punch and tensile testing were established for UFG IF steel undergoing coarsening via annealing. Lastly, this study demonstrates that irrespective of ECAP and 95% CR, the strength-ductility balance in the present IF steel after annealing is a function of both, fine grain size (~2.5–3 μm) and large area fraction of HAGBs (~65 – 70%).

Share

COinS