METASTABLE PARAEQUILIBRIUM STATES ACCESSIBLE DURING GAS-METAL INTERACTION

Abstract

Mankind’s search for materials with superior properties, in terms of the purposes it deems important, has been a constant accompanying factor during the progress of civilization. With extensive exploration of materials in a state of thermodynamic equilibrium having already been carried out, un-explored materials in states of thermodynamic metastability are being vouched for as possible sources for breakthroughs. Significant progress is being made in relevant areas such as bulk nanostructures, metallic glasses, high entropy alloys and so on. The purpose of this thesis is to carry out a thermodynamic analysis of the states of metastability in solid solutions that can be accessed in substitutional-interstitial element based metallic solid solutions. Classically, rapid quenching of materials from high temperatures has been the most common strategy to sample metastable states. However, in this thesis an alternative ‘isothermal’ strategy is employed for the same purpose. Isothermal thermo-chemical treatments involving gas-solid interaction such as nitridation, carburization and oxidation are chosen as model processes, in which the source of interstitial elements are the respective gas atmospheres. These treatments are usually carried out at ‘low temperatures’ i.e. at temperatures wherein the diffusivity of interstitial elements in the solid state remains significant, while diffusivity of substitutional elements is negligible. Because of this, a situation ripe for sampling metastable substitutional/interstitial element based solid solutions gets established. This strategy to study metastable states is much more convenient, as the processes can be carried out at isothermal conditions, and thus offer better reproducibility of results. The aspects of metastability of the gas-solid interaction that is specifically focussed on in this thesis are: (i) metastable interstitial element supersaturation of solid solutions; (ii) and the response of those supersaturated solid solutions to internal composition fluctuations. The chapter 1 of this thesis contains an elaborate review of literature, spanning over almost half a century, pertaining to the nitridation/carburization/oxidation of binary/ternary iron based alloys and stainless steels. Emphasis is particularly laid on describing a number of experimental observations made in literature that do not comply with predictions of equilibrium thermodynamics, and thus require non-equilibrium thermodynamic analysis. Following this, chapter 2 builds the basic thermodynamic framework that is foundational to all the analysis carried out for the different gas-solid interactions considered in this thesis. It can broadly be understoodflowing gas atmospheres, thermodynamic criterion for interstitial element based metastable gassolid paraequilibrium and finally the thermodynamic criterion for the stability of metastable solid solutions to infinitesimal composition fluctuations. In chapter 3, the non-classical chemical pathways traversed by binary ferritic iron based alloys during gas phase nitriding is discussed. Evidence is shown for the first time for the occurrence of in-situ isothermal spinodal decomposition during controlled nitridation of ferritic Fe-Cr alloys, prior to equilibrium CrN precipitation. Recognition of this phenomenon has led to the explanation of the following experimental observations: dependence of nitridation kinetics of ferritic Fe-Cr alloys on N diffusion and the paradox of Cr-nitrides precipitation sequence not following the equilibrium thermodynamic predictions. Additionally, literature says that a group of alloy systems (Fe-V, Fe-Ti) exhibits nitridation kinetics governed by interstitial N diffusion while not showing clear evidence for alloying element nitride precipitation while another group of alloy systems (Fe- Mo, Fe-Al and Fe-Si) exhibit nitridation kinetics governed by substitutional alloying element diffusion with clear evidence for alloying element nitride precipitation. All these observations can also be understood by considering the existence of a region of immiscibility in the respective ternary Fe-Me-N phase diagrams and whether this region of immiscibility can actually be sampled by the system under the imposed conditions of nitriding. Based on this consideration, all the investigated alloy systems seem to neatly fall into three different classes. Such a clear understanding of the nitridation characteristics has evolved for the first time. Therefore, recognizing the role of the ‘regions of immiscibility’ in alloy systems in realizing rapid kinetics during nitridation treatments is shown to open up a new, unexplored alloy design strategy for the development of steels with favorable nitriding response. In addition, the surface grain orientation dependent N uptake kinetics observed in certain alloy systems has also been explained in terms of energetically favored spinodal fluctuations occurring along elastically soft <100> directions, thereby causing the N uptake kinetics to depend on the angle between the surface normal and the nearest <100> crystallographic direction. Chapter 4 is aimed at considering the colossal interstitial (N/C) supersaturation of ferritic as well as austenitic stainless steels during low temperature nitridation/carburization treatments that has lately gained much technological significance. Available thermodynamic models to calculate the N/C paraequilibrium solubility limits have failed to explain the levels of colossal N/C supersaturation observed in several cases of nitrided ferritic/austenitic stainless steels. In this chapter, it is shown that such a colossal interstitial supersaturation is not opposed to the concept of metastable N/C paraequilibration of the solid phase with the nitriding/carburizing atmosphere. Rather, it is shown that it is a consequence of triggering spinodal decomposition during the process of establishment of paraequilibrium. Therefore, a thermodynamic model is developed for the first time that considers the paraequilibrium between spinodally decomposed ferrite/austenite fractions and the nitridation/carburization atmosphere. This modification in the thermodynamic model has led to the successful explanation of the colossal N/C supersaturation in ferritic and austenitic stainless steels; predictions of the new thermodynamic model are validated by experimental data in literature. Chapter 5 seeks to explain a large number of experimental reports in literature of the features of the natural, air-formed oxide film on stainless steel surfaces and the minimum of 12 at.% Cr required for Fe-Cr alloys to have stainless character. The features include a nano-scale chemical heterogeneity within the oxide film involving complete segregation of Cr atoms to the sub-surface regions of the oxide film. For the first time, the occurrence of a natural air atmosphere induced surface-directed spinodal decomposition is revealed in these alloys. In other words, spinodal decomposition of the O-supersaturated Fe-Cr-O ferritic solid solution is shown to be occurring as a precursor to stable oxide nucleation. This has led to explanation of the rapid compositional segregation observed at even room temperature conditions, where all kinetic mechanisms are severely hindered. Most interestingly, the minimum 12 at.% Cr necessary for stainless character of Fe-Cr alloys is observed to coincide with the spinodal limit boundary, implying that it is this Cr content that is actually required to trigger spinodal decomposition in these alloys at room temperature; further confirming the correlation of spinodal decomposition with passivation characteristics of stainless steels. In chapter 6, the entire work is summarized in perspective of the gas-solid interstitial element paraequilibrium induced spinodal decomposition which serves to be the common thread unifying all the so-called anomalous observations previously reported. The broader implications of the work carried out in the current thesis are discussed, based on which scope for future work in such frontier areas of materials research such as hydrogen storage, thin film technologies etc. are outlined. to be comprised of three distinct sections viz. the thermodynamic description of metastable,