Active Research Projects
- M. Efe (PI), “Formability of Dual-Phase Steels at High Temperatures and Industrially Relevant Deformation Rates”, 2015-2018, 3501 – National Young Researcher Career Development Program, Funded by: The Scientific and Technological Research Council of Turkey (TÜBİTAK)
Completed Research Projects
- M. Efe (PI), “In-Situ Analysis of Magnesium Sheet Forming,” 2014-2016, Career Integration Fellowship, Funded by: The Scientific and Technological Research Council of Turkey (TÜBİTAK)
- M. Efe (PI), “In-Situ Observation and Control of Microstructure and Surface Defect Evolution during Magnesium Sheet Forming,” 2014-2018, Marie Curie Career Integration Grant, Funded by European Commission Research Executive Agency
Small-scale and multi-axial testing of sheet metals, particularly of lightweight aluminum and magnesium alloys are becoming important as these materials exhibit forming behavior sensitive to their unique microstructural features and strain paths. As an alternative to large-scale standard tests, in this project we introduce a novel biaxial tensile test apparatus utilizing miniature cruciform samples. The compact and portable apparatus includes a custom-built optical microscope and high-resolution digital image correlation (DIC) equipment for in-plane and in-situ strain measurements at the microstructure scale.
With our cruciform test setup, it is now possible to test the samples until their fracture limit without the formation of local necking and premature fracture. The predetermined strain path stays constant until the fracture. This way the local necking is eliminated and the actual material behavior is observed under large strains. In the literature, most of the micro-scale strain maps are limited to the small strains. Our surface preparation technique and imaging setup, however, allow the strain maps to be plotted under the large strains. Therefore, we are able to capture the strain localizations to the microstructural features and the defects originating from the strain localizations.
In-Situ Analysis of Magnesium Sheet Forming
Strain localizations in aluminum originate from the orientation differences between the individual grains, resulting in strain accumulations at the grain boundaries. They become the first sites to fail in the microstructure and micro-cracks propagate through the grain boundaries leading to the ultimate sample failure. The localizations are also strongly sensitive to the strain path. In magnesium, strain localizes to the grain boundaries under the uniaxial tension, but the twins accumulate strains when the strain path is switched to the biaxial tension. Under uniaxial tension, cracks propagate through the grain boundaries, whereas twinning-induced shear bands form under biaxial tension. Cracks propagate through the shear bands and result in catastrophic fracture. The shear bands also cause a nonuniform microstructure.
The results obtained in this project have two major applications: First, is the standardization and commercialization of the cruciform tests. In this project, we showed that the cruciform test is on par with the standard tests when measuring the formability of materials. They also have certain advantages, such as simple sample design, frictionless setup, and in-plane deformation. Second, the results on aluminum and magnesium sheets should lead to a better design of the forming processes and the processed materials for improved defect control. We now know what is responsible from the localizations and how they lead to the defect formations. The starting texture of the formed sheets and the strain paths in the forming processes can be optimized for improved formability and defect-free sheet products.
Formability of Dual-Phase Steels at High Temperatures and Industrially Relevant Deformation Rates
Dual-phase (DP) steels offer a combination of high strength and formability at room temperature, increasing their importance and uses in recent years. In industrial forming methods, however, forming conditions become adiabatic as the deformation (strain) rate increases. This leads to a significant temperature increase during the process, which may decrease the forming limits through microstructural changes and softening of the flow stress. On the other hand, DP sheets may be externally heated at low strain rates, as warm forming is shown to be helpful in reducing springback in other advanced high strength steels. This study aims to establish the deformation behavior of DP steels at high temperatures either through external heating or adiabatic heating. The plastic flow of ferrite and martensite phases will be monitored at the microstructure scale through digital image correlation (DIC) and infrared thermal imaging (IRT) techniques as opposed to current millimeter scale tests. The local flow and temperature data will be correlated with microstructure and texture, which will reveal the source of any flow instabilities (or lack thereof). In addition, mechanical properties such as yield strength, total elongation and strain hardening coefficient will be -1 obtained at practically relevant deformation rates (1-10 s ) and temperatures (25-300 oC). These results are expected to serve as a database for the models and simulations predicting the forming limit diagrams and mechanical behavior through -4 -3-1 microstructure, as they will be significantly different than the results obtained from the conventional slow strain rate (10 -10 s ) and room temperature tests. Furthermore, steel sheet manufacturers and end-users in automotive industry will also be informed and encouraged for the use of new and realistic mechanical properties obtained by this project in their forming applications and simulations.
Ultrafine-Grained Tungsten as A Plasma-Facing Component In Fusion Devices
Low energy helium and hydrogen (< 200 eV) ions can cause severe surface degradation in tungsten components exposed to the plasma in magnetic-confinement nuclear fusion reactors. The accumulation of the ions in a few micrometer surface layer results in blisters, pores, bubbles and fuzz, which can lead to macroscopic dust emission into the fusion plasma and disturb the operation of the reactor. Recently, nano- and ultrafine-grained tungsten with high grain boundary area has been proposed to mitigate this kind of irradiation damage. In this study, we demonstrate surface deformation by machining as an alternative and convenient method to refine the microstructure of pure tungsten to the ultrafine region. We control the strain, strain rate and the temperature during machining to achieve a distribution of nano and ultrafine grains in the microstructure. With increasing strain, and strain rates the grain boundaries also become sharper with higher misorientation. When compared with the commercial, fine-grained tungsten, the nano- and ultrafine grains with high grain boundary angle act as sinks for the irradiation induced defects and increase the threshold for the severe surface damage.
