Year

1996

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

In the present work, the cross-sectional transmission electron microscopy (XTEM) , traditional selected area diffraction (SAD), nanobeam electron diffraction (NBD) , high resolution electron microscopy (HREM) and glancing angle X-ray diffraction (GAXD ) have been used to investigate the surface microstructure of AISI 316 austenitic stainless steel after plasma immersion ion implantation (PI3) and plasma nitriding.

It has been established that GAXD is incapable of unambiguous characterisation of the microstructure of nitrogen implanted and nitrided austenitic stainless steel. Further, it has been demonstrated that sole reliance on GAXD could, at best, provide a very complicated and confusing picture of the microstructure and, at worst, could lead to totally false and erroneous interpretation. In stark contrast, the combination of XTEM + NBD provides a powerful technique to completely and unequivocally characterise the very fine microstructural features in this system.

Detailed microstructural characterisation by XTEM + NBD has shown that the sequence of the microstructural evolution in austenitic stainless steel after PI3 and plasma nitriding is primarily controlled by diffusion processes. In other words, the high energy implantation effects in PI3, i.e. collision cascade and radiation damage, plays only a minor role. However, PI3 offers the possibility of building up higher nitrogen concentration in the near surface region which could influence the evolution of microstructure.

More specifically, it has been shown, for the first time, that the microstructure of austenitic stainless steel after PI3 and plasma nitriding treatments is very complex, consisting of several layers of amorphous, or semi-amorphous and nanocrystalline structure with nanocrystalline precipitates such as CrN and α-ferrite. The formation of an amorphous layer in high temperature nitrogen implanted or nitrided austenitic stainless steel, either as a sole layer or located as a sublayer underneath a nano-crystalline sublayer, seriously challenges the conventional wisdom that the radiation damage is the cause of amorphisation.

It has been argued that the incorporation of high nitrogen concentration into austenitic stainless steel leads to the formation of highly supersaturated austenite which akin to a supercooled eutectic structure. Given the low diffusivity of substitutional elements, in particular chromium, the re-arrangement of atoms to allow co-precipitation of CrN and α is very difficult. Under this condition, the total energy of the system could only be lowered by amorphisation. Depending on temperature and nitrogen content, some re-arrangement of atoms could take place on a very short scale, thus resulting in formation of nano-sized CrN and a precipitates.

The precipitation of CrN and α is strongly influenced by the temperature. Above 500°C. CrN and a can directly nucleate from ϒ. Below this temperature, precipitation of CrN and α requires extensive atomic rearrangement which under restrictive diffusion condition induces the collapse of ϒ substrate into an "amorphous" phase from which very fine CrN and α precipitates nucleate. Therefore, author suggests that the solid state eutectic reaction provides the driving force for amorphisation in nitrogen implanted austenitic stainless steel. Both Bain and N-W (Nishiyama and Wasermann) relationships have been observed between CrN and α precipitates. The K-S (Kurdjumov and Sachs) relationship has also been observed between CrN and α at low temperature and low nitrogen concentration (e.g. 350°C plasma nitrided sample).

At low nitrogen concentration, Cr2N, as an eutectoid phase, can precipitate directly from y to form a nanocrystalline Cr2N + α layer. The following orientation relationship has been observed between Cr2N and ϒ:

(011)ϒ//(3211)Cr2N
[011]ϒ//(1232)Cr2N

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