İkizlemenin yüksek manganlı östenitik çeliklerin plastik deformasyonu ve niti şekil hafıza alaşımlarının süperelastisitesi üzerindeki krıtik rolünün sayısal ve deneysel incelemesi

Several micro-deformation mechanisms are simultaneously activated during the plastic deformation of high-Mn austenitic steels, which does not allow for a clear understanding of the work hardening response of this special class of steels. The major motivation of this thesis was to provide an explanation of the complicated work hardening mechanism in these materials, specifically promoting twinning among other micro-deformation mechanisms with the aid of accelerated loading. With this motivation, numerical analyses were carried out utilizing molecular dynamic simulations with very high strain rates. The initial analyses were focused only on the effect of the grain boundary misorientation angle on the deformation response of pure metallic materials, specifically copper and iron. The results demonstrated the insufficiency of numerical effort as a result of very small samples to be simulated and much more mechanisms take role in the plastic deformation of these materials in addition to grain boundary effects. A comparison between the current simulation results, and the available experimental findings and numerical results at macroscale in the literature has demonstrated that further experimental and numerical analyses should be carried out for clarification. Therefore, the study was extended to a more experimental area where the composition and temperature dependencies of deformation response of high-manganese austenitic steels were investigated under three different loading schemes. First, high-velocity impact loading was utilized in order to examine the microdeformation mechanisms activated. Secondly, the loading velocity was increased via custom compression test setup. Then, an ultra-high velocity loading scenario was applied which was split-Hopkinson tension load. In order to detect the effect of composition, different types of steels, i.e. Hadfield, TWIP, and, XIP steels, with changed alloy contents were tested. On the other hand, for the examination of the effect of temperature, three different temperatures, namely -170 °C, room temperature, and 200 °C were selected as testing temperatures. The promotion of twinning over other mechanisms due to applied accelerated loading was investigated thoroughly with numerous methods which are scanning electron microscopy, transmission electron microscopy, confocal laser scanning microscopy, and, optical microscopy. In addition, a thermal perspective was adopted for impact tests as in situ thermographical analyses were conducted to detect the temperature change related to the hit of impact hammer. Various mechanisms such as slip, formation of more than one twin variant, nano-twins inside primary twins and voids were activated in Hadfield steel, while the deformation was twin-dominated in TWIP steel at all temperatures, which stems from the increase in stacking fault energy (SFE) due to the higher Mn content. Moreover, nano-twin formation within one primary twin system was observed for TWIP steel at subzero temperatures. The XIP steel with the highest SFE, on the other hand, deformed mostly by slip at elevated temperatures, although twins and nano-twins dominated the microstructure as the temperature decreased to room temperature, and then to -170 °C, respectively. The current set of results lay out the roles of temperature, deformation velocity and alloy content on the microstructure evolution of high-Mn steels, which altogether can be tailored to improve the work hardening capacity of this class of materials. In addition to high-manganese austenitic steels, another class of materials, i.e. nickeltitanium (NiTi) shape memory alloys, was investigated due to the significant role of twinning on material properties. Specifically, NiTi alloys have three properties, namely one- and twoway shape memory effects and pseudoelasticity, which all take place by solid to solid phase diffusionless transformation via twinning and detwinning upon the application of either load or temperature change. Among these three, the pseudoelasticity was taken into consideration in the current work. First, the fatigue performance of micro-scale pseudoelastic NiTi wires was investigated under cyclic bending loading. The current findings demonstrated that the change in the scale from macroscopic to microscopic promotes the formation of the B19' phase and significantly hinders stabilization of the R-phase. Second, a rather application-based study was also conducted on NiTi orthodontic archwires. The NiTi orthodontic wires were examined in terms of their biocompatibility via mimicking the actual contact state of wires around brackets in intraoral environment. These wire-bracket contacts made the simulation of mechanical loading at the wires possible during ex situ experiments. An undeformed wire and another set which was bound to brackets on a plaster dental model were immersed into artificial saliva solution for a period of 31 days. Post-immersion electron optical analyses were carried out on the wires, while inductively-coupled plasma mass spectroscopy was conducted on the solutions. The carbon-rich product formation was detected on both undeformed and bound wires after immersion. This formation was observed to be randomly distributed on the undeformed wire, while it was localized on certain locations with equivalent intervals on the bound wires. This localization of corrosion products was then attributed to the preferential formation at the wire-bracket contact regions due to higher surface energy attained and micro-scale cracks formed at these regions after optical microscopy analyses. Furthermore, the limited amount of Ni or Ti ion release into the solutions was attributed to the suggested blocking of micro-cracks with these corrosion products. The current set of results revealed the importance of the simulation of chemical and mechanical conditions of the intraoral environment simultaneously for a more reliable and realistic investigation of the biocompatibility of NiTi orthodontic wires.