The grain boundary hardness in austenitic stainless steels studied by nanoindentations

The strength of structural materials depends strongly on the structure and properties of grain boundaries. Grain boundaries in crystalline solids are internal surfaces and therefore likely sites for nucleation of precipitates and segregation effects. Nanoindentations are a suitable method to study the influence of grain boundaries on the mechanical properties. The necessary lateral resolution is achieved by combining with an atomic force microscope. Measurements on an austenitic steel after annealing at 650 degreesC, where the boundary is saturated with carbide precipitates, clearly show a decreasing hardness close to the interface in opposite to the general expected behaviour of strengthening. In this case segregation effects strongly influences the mechanical properties around the boundaries. These results are discussed in comparison with nanoindentation measurements on strongly ordered intermetallics like NiAl where no significant property change at the boundaries was found and with Vickers hardness measurements by Westbrook et al., where a higher grain boundary hardness was found in many metallic and intermetallic materials depending on their composition.