Browsing by Author "Liapina, Tatiana"

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Phase transformations in interstitial Fe-N alloys
(2005) Liapina, Tatiana; Mittemeijer, Eric J. (Prof. Dr. Ir.)
The improvement of the properties of iron and steel by heat-treatment procedures is a very important field in materials science. The design and controlled improvement of such procedures requires knowledge of the static properties (crystal structure, mechanical properties) of the phases involved, of the kinetics of the transformations between the various phases (e.g. diffusion processes) leading to specific microstructures, and of the implications which the formed microstructures have on the properties of the work-pieces. Nitriding is a prominent thermochemical heat-treatment procedure leading to various types of surface property improvements of the treated iron and steel. Some of these improvements are related with the formation of a hard, abrasion and corrosion resistant surfacial compound layer consisting of different iron nitride phases. Although nitriding is widely applied, many questions regarding even very basic properties of these compound layers and the relevant nitride phases are still open. Some of these open questions related with the behaviour of iron nitrides and, in particular, of iron nitride compound layers occurring below the usual process (i.e. nitriding) temperatures are addressed to in this thesis, as relevant e.g. for the cooling procedure after nitriding. The most important iron nitrides occurring in iron-nitride compound layers are the gamma'- and epsilon-phases in the Fe-N system. Both the epsilon-Fe3N(1+x) and the gamma'-Fe4N can be described as interstitial phases with nitrogen partially occupying the octahedral sites with long-range ordering. It is well known that the hexagonal lattice parameters of the epsilon-phase vary strongly with the nitrogen content and until now only a simple one-to-one relation was assumed. However, it is shown in this work that for relatively low nitrogen contents of epsilon-iron nitrides (around Fe3N) the cooling rate upon going down from an elevated annealing temperature to room temperature has a significant effect on the lattice parameters. X-ray and neutron diffraction analysis revealed that the lattice parameter values observed after fast cooling are affected by the higher degree of nitrogen disorder at elevated temperature, thus changing the c/a ratio. New relations between the lattice parameters of epsilon-iron nitrides and the nitrogen content are suggested for different types of cooling. Slow cooling after nitriding or annealing of epsilon-iron nitrides of low nitrogen content at temperatures lower than the typical nitriding temperatures (573 K – 693 K compared to 700 K – 900 K) leads to formation of gamma'-Fe4N and an enrichment of the epsilon-Fe3N(1+x) with nitrogen due to the strongly temperature-dependent gamma'+epsilon/epsilon phase-boundary composition. The mechanism of gamma'-formation in epsilon-iron nitride powders is not yet well known. In this work the investigation by TEM of the decomposition upon annealing (633 K, 673 K) of initially homogeneous epsilon-Fe3N powders revealed that this gamma'-formation occurs in only a few powder particles in a grain-like form. Moreover, diffraction line-profile analysis (synchrotron radiation) revealed N transport occurring from particle to particle, leading to inhomogeneities of N content in the epsilon-phase. Typical iron nitride compound layers are constituted of an epsilon-sublayer adjacent to the surface and of a gamma'-sublayer adjacent to the layer/substrate interface. In this case the shift of the N equilibrium which occurs during slow cooling after the nitriding or upon afterwards annealing, at temperatures lower than the nitriding temperature, induces phase transformations in the compound layer. It was shown that gamma'-iron nitride formation can occur by ‘backwards growth’ of the existing gamma'-sublayer at the cost of the epsilon-sublayer increasing N concentration in the epsilon-layer. Another process, which may additionally occur in the compound layer upon annealing, is diffusion of N from the epsilon-phase through the gamma'-sublayer due to a small N concentration gradient in the gamma'-layer. This leads to formation of gamma' not only at the epsilon/gamma'-interface but also at the gamma'/alpha-interface increasing the overall double-layer thickness. The preferential occurrence of the different processes is controlled by the diffusion coefficients of N in the epsilon- and gamma'-phases. The observed evolution of the epsilon- and gamma'-sublayer thicknesses upon annealing has been modeled. This was done by numerical simulation of the diffusion processes in the compound layer, which involved fitting of volume-diffusion coefficients of N in the epsilon- and gamma'-phases on the basis of the experimentally observed time-dependent sublayer thickness changes. This allowed for the first time determination of these diffusion coefficients in the corresponding range of annealing temperatures.