Hardness and Elastic Modulus Gradients in Plasma-nitrided 316L Polycrystalline Stainless Steel Investigated by Nanoindentation Tomography

Correlations between the grain orientations and elastic properties of plasma-nitrided polycrystalline 316L austenitic stainless steel are investigated. The grain orientations (h k l) in a delimited area were obtained from electron backscatter diffraction and related to hardness (H _hkl) and elastic modulus (E _hkl) maps obtained from large nanoindentation matrices. The influence of nitrogen concentration on the local mechanical properties has been studied by repeating these indentation matrices in the same area after successive partial removals of the nitrided layer. This nanoindentation tomography allowed the orientation, the shape and the surroundings of individual grains to be taken into account. The results show that plasma nitriding leads to a complete reversal of the elastic behaviour anisotropy: while the non-nitrided 316L austenitic stainless steel shows the typical elastic anisotropy of face-centred-cubic-type metals with a maximum of E _hkl for the 〈111〉 oriented grains, the maximum of E _hkl is observed for the 〈001〉 oriented grains in the nitrided layer. A similar anisotropy reversal is observed for the hardness H _hkl. These observations are discussed on the basis of the microstructural changes induced by the nitrogen incorporation.