Cohesive Interfaces for crack resistant nanosteels

Only technical advances that reduce energy consumption and foster green energy sources can enable a sustainable environment for future generations. For this purpose materials in automotive applications must fulfill two criteria. First, high-strength is needed to enable light-weight design and thereby lower energy consumption. Second, materials need to endure hydrogen exposure for usage of hydrogen-based powertrains. Combining both requirements, so high-strength with hydrogen- resistance represents to date the holy grail of materials engineering. This project aims to overcome this challenge by using grain boundary design at the nanoscale. In a first step, we synthesize nanostructured iron by deforming the material to multiple thousands of percent of shear strain. This procedure subdivides the material in nanospaced portions called grains, which are delineated by the so-called grain boundaries. In such high-strength nanomaterials the grain boundaries represent the weak spots, where fracture initiates and consequently early failure of the material occurs. When hydrogen atoms are introduced during application, they migrate to the boundaries, weaken their connections and thereby accelerate materials failure. Our clue to retard materials failure is to introduce the element boron into the nanostructured iron sample. We know from theoretical simulations that once boron atoms are allocated at the grain boundaries it strengthens their connections and thereby fights the detrimental effect of hydrogen. This assumption is validated by performing cyclic bending experiments inside a scanning electron microscope. Thereby, we focus not only on the sample lifetime, but also reveal changes of the fracture behavior due to boron in detail. The high resolution of the microscope allows tracking of the failure procedure of the microsamples having dimensions smaller than a human hair with and without hydrogen atmospheres present. Correlating the failure process to the recorded mechanical properties uncovers fracture mechanisms and enables to optimize the boron-content for high sample life-time. In a second step, the boron- concept will be transferred to pearlitic nanosteels, which belong to the strongest structural materials to date. Succeeding to improve their resistance against failure also in the vicinity of hydrogen, provides materials design criteria enabling both, light-weight design and hydrogen resistance.