Correlation between ion bombardment, lattice expansion and nitrogen content after nitriding of austenitic stainless steel

Broadbeam low energy ion implantation is a versatile method for nitrogen insertion in transition metals containing alloys like austenitic stainless steel. The process is characterized by an anomalous high diffusivity leading to nitrogen enriched layers of up to several micrometers thickness coupled with an anisotropic lattice expansion of up to 12%. By using a radiation heater in addition to ion beam heating to maintain the process temperature between 350 and 450 C, controlled continuously by pyrometer, and an electronic nitrogen ion beam switch, a reduced nitrogen ion flux was realised. As a result, the XRD peak widths of the resulting expanded austenite were reduced from about 2 to 0.7-0.9 , while a strong nitrogen concentration gradient was still observed within the first micrometers. Thus, the initial broadening is less related with a range of different nitrogen concentration dependent lattice expansions through the layer. At the same time, no linear correlation between the lattice expansion and the nitrogen content was observed, invalidating the use of Vegard's law to estimate the nitrogen content from the lattice expansion.