Dual-phase (DP) steels consisting of a ferrite matrix with dispersed martensite particles have attracted a significant interest due to their combination of high work hardening and ductility. A great deal of experimental work has been done to obtain a better comprehension of the relation of their mechanical behaviour to their microstructural characteristics. In the present work, a micromechanical study of ferrite-martensite DP steels is conducted. The deformation of ferrite is described by a rate-dependent crystal plasticity theory, which relates the stress-strain field equations on the grain level to the macroscopic behaviour of the material. The crystal plasticity theory assumes that slip is the only deformation mechanism. Martensite, on the other hand, is considered an elastic-plastic isotropic solid. The interfaces of the grains are taken into account through an idealized form of grain boundaries. A FORTRAN program was coupled with the finite element method to solve the stress equations of the crystal plasticity. Including the grain boundaries made it possible to examine the effect of ferrite grain size on the strength of the material. It is shown that by decreasing the grain size, the yield stress increases according to Hall-Petch equation. Additionally, the effects of the volume fraction of martensite (Vm) on the onset strain, i.e. the strain at which martensite deforms plastically, and of the distribution of martensite on the stress are studied. The former showed that the onset strain of the DP steel declines linearly with increasing Vm up to 36%, beyond which the onset strain becomes independent of V m. The latter revealed that when martensite particles are formed as islands in the ferrite grains, the material exhibits higher strength and hardening rate; compared to when martensite is distributed as large blocks among the ferrite grains.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.99778 |
| Date | January 2007 |
| Creators | Mahmoody, Sam. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Master of Engineering (Department of Mechanical Engineering.) |
| Rights | © Sam Mahmoody, 2007 |
| Relation | alephsysno: 002612685, proquestno: AAIMR32606, Theses scanned by UMI/ProQuest. |
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