Deformation characteristics of changing strain paths on high formability steels

Vrettou, Anastasia (2024). Deformation characteristics of changing strain paths on high formability steels. University of Birmingham. Ph.D.

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Abstract

Understanding the mechanisms that govern deformation of metallic sheets is crucial for their forming applications, such as the manufacturing of structural vehicle components. This thesis focuses on the effect of strain path and Strain Path Changes (SPCs) that can influence the mechanical properties and microstructure of two high formability steels. A DX54 low carbon steel, and a TWIP steel are studied. Both alloys are used in the automotive industry, with the first having broad use due to its high ductility and low cost, while the second has a higher cost but exhibits exceptional work hardening and high ductility.

The effect of abrupt SPCs was examined on the DX54 low carbon steel via in-situ Synchrotron X-Ray Diffraction (SXRD) and ex-situ via Electron Backscatter Diffraction (EBSD). A two-step deformation process was used, including cold rolling followed by uniaxial tension. Both pre-strain direction and tensile directions as well as the pre-strain magnitude were varied. The role of texture changes and their association with microstructure development was examined. The deformation process resulted in a significant ductility loss for highly pre-strained specimens. The ductility decrease was more pronounced when SPCs were abrupt (i.e. a 90\degree \ change in the loading direction). Results also showed a large texture dependence of the mechanical properties and the microstructure evolution. This was an important result as prior studies had only considered the effect from a small number of grains and had underestimated the significance of texture.

An investigation of the TWIP steel behaviour under a variety of strain paths required the development of a biaxial testing device. A miniaturised testing setup was developed in Chapter 4, which allowed specimens to be subjected to near uniaxial, plane strain or equibiaxial strain paths. Such studies are seldom reported. Strain measurements were performed via Digital Image Correlation (DIC). The effect of non-proportional deformation was also examined for two, two-step processes: uniaxial pre-straining followed by equibiaxial tension and cold rolling followed by equibiaxial tension. Interrupted tests were conducted to facilitate microstructure examination via EBSD. Results highlighted the influence of strain path on microstructure evolution, with cold rolling leading to the highest Geometrically Necessary Dislocation (GND) density development and twin generation. Twin formation also showed notable grain orientation dependence, with the [011]||normal direction being the least favourable for twin formation.

Strain path sensitivity of the TWIP steel was additionally examined via in-situ neutron diffraction and ex-situ EBSD to provide microstructural information in the mesoscale. Five strain paths were examined, proportional and non-proportional incorporating uniaxial, and equibiaxial deformation states. Results from proportional strain paths showed that equibiaxial deformation led to higher dislocation densities and decreased twinning activity, due to the enhancement of the [011||normal direction fibre, which is unfavourable for twinning. Furthermore, results indicated that whilst twin generation during equibiaxial deformation is limited compared to uniaxial deformation, twin growth is promoted for the former strain path.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):