| dc.description.abstract | Medium- and high-entropy alloys (M/HEAs) offer a much larger and richer design space than conventional alloys, providing new opportunities for discovering unexplored physics and functionalism. Some of these alloys exhibit an outstanding combination of high strength and ductility, which has been linked to the activation of various deformation modes triggered by low and medium-energy stacking faults. Although there are many recent works uncovering the microscopic and macroscopic features of these materials, there is a striking paucity on the investigation of single crystalline M/HEAs that would allow for a superior understanding of the fundamental deformation mechanisms. Therefore, exact atomistic and microstructural reasons for the exceptional mechanical response of these alloys are not fully understood. With this need, the present study is focused on the fabrication and characterization of M/HEA single and polycrystals to explore the underlying deformation mechanisms providing outstanding mechanical properties.
In the present work, NiCoCr, Fe40Mn40Co10Cr10, CoCrFeMnNi and Fe50Mn30Co9Cr9 + 2C at.% M/HEAs have been studied for a multitude of reasons. The orientation dependence of tensile deformation in NiCoCr MEA and Fe40Mn40Co10Cr10 HEA was investigated in [001], [011], [111] and [123] oriented single crystals. Several microstructural investigations were performed to reveal the major mechanisms controlling the deformation stages. Later, using extensive microstructural characterization of bulk single crystal experiments, it was discovered that the extraordinary mechanical properties of NiCoCr arise from the existence of short range atomic order (SRO), which triggers the simultaneous activation of deformation twinning-induced plasticity and martensitic transformation-induced plasticity (TWIP and TRIP), and leads to dynamic hardening against dislocation motion due to continuously refining length scales as a result of the nano twin and nano-martensite boundaries. By controlling the degree of SRO through aging at high temperatures, the level of twinning and strain-induced phase transformation was increased, further enhancing the ductility levels in NiCoCr. These results indicate that the interplay of SRO and plasticity could be exploited to trigger various deformation modes to discover new M/HEAs with unprecedented mechanical properties.
However, the yield strength levels of some of the M/HEAs are still low as compared to other high strength materials. Therefore, the present work also aims to reveal the effect of interstitial solid solution hardening, grain refinement on the post processing mechanical properties using the equal channel angular pressing (ECAP), and precipitation hardening. An experimental study was performed on the CoCrFeMnNi HEA single crystals with and without carbon addition. Carbon addition shows a simultaneous improvement in strength and ductility. In the carbon doped single crystals, more twining density suppressed the neck instability and significantly enhanced the ductility. ECAP was employed to improve the mechanical behavior of the most well-known HEA, CoCrFeMnNi, through microstructural refinement. Very high yield strength levels around 1 GPa were obtained after ECAP via grain refinement, high dislocation density, and the activation of TWIP/TRIP that are anomalous at high temperatures. Finally, the superior mechanical behavior of medium entropy Fe-30Mn-8.5Al-0.9Si-0.9C-0.5Mo (wt.%) lightweight steel was reported owing to nanoscale ordered κ-carbide precipitates. However, the precipitate distribution in the microstructure was not always homogeneous, which caused an anisotropic mechanical behavior. In this study, reasons for the anisotropic mechanical properties were attributed to the chemical segregation bands.
Finally, data reporting on the fatigue properties of M/HEAs is only available scarcely in polycrystalline materials and there is no single crystalline study exploring orientation dependence of cyclic loading of HEAs so far. In this study, the cyclic response of single crystalline Fe50Mn30Co9Cr9 + 2C at.% and polycrystalline CoCrFeMnNi HEAs was also investigated during strain-controlled low cycle fatigue tests under fully reversed push-pull loading at room temperature. The microstructural evolution during cyclic loading was compared to the microstructure under quasi-static monotonic loading. The orientation dependence of the fatigue life and cyclic hardening was presented. The fatigue life and cyclic hardening behavior were governed by the refined grain size, high density dislocation walls, and the annihilation of existing dislocations resulting in the formation of cell structures in the polycrystalline specimens. | en |
