Transition between external and internal

Transition between external and internal oxidation of alloys 37... 

The principal features and theoretical models for the transitions between external and internal oxidation of alloys have been studied under relatively simple oxidation conditions. Wagner [2] was the first to develop a theoretical... 

He and his co-workers [14-19] studied the oxidation behaviours of Ag-In, Co-Cr, Ni-Al and Ni-Cr binary alloys under ultra-low oxygen pressure atmospheres in which the solvent in the alloys does not oxidize. They found that continuous external oxide scales could form on aU alloy surfaces, especially for the specimens after a short exposure. More importantly, the formation and growth of external oxide scales may or may not be accompanied by the formation of internal oxides in the alloy matrix. Based on these experimental results, they proposed that the transition between the oxidation models of an alloy should be from external to internal oxidation [18,19]. In this chapter, their works on this subject will be summarized and discussed briefly to get a better understanding of the transition between external and internal oxidation of alloys. 

Comparing the two important theories developed by Wagner, it can be found that, for the first type of transition, these two theories actually describe the same criterion, i.e. the criterion for such a transition is equal to the criterion for the formation of a stable external oxide scale. However, these two theories are established on different bases. The first is based on the nucleation and growth of oxides in the alloy substrate, and the second is based on the growth of oxides on the alloy surface. Therefore, in order to obtain a better imderstanding of these two theories and the oxidation transition behaviour of an alloy, a theory describing the transition between external and internal oxidation of alloys should be established. This model should be based on full consideration of the nucleation and growth of oxides both on the alloy surface and in the alloy substrate. 

Ni-Cr and Co-Cr alloys are typical single-phase binary alloys. Ni-6Cr, Ni-8Cr, Co-5Cr and Co-lOCr alloys were selected to study the first type of transition between external and internal oxidation under atmospheres with ultra-low oxygen partial pressures Pq in which the solvent Ni or Co in the alloys does not oxidize [14,19]. 

The criterion for the minimum solute concentration, Ng, of a binary A B alloy required for the second transition process has been derived based on thermodynamic and kinetic analyses. Using this equation the effects of oxygen partial pressure, addition of rare earth elements, surface micro-crystallization, and the gettering effects on the transition between external and internal oxidations can be explained. 

Due to the weak coupling between external and internal degrees of freedom, the internal (rotational and vibrational) temperature of the HD" " ions (see Section 18.7.2) is at 300 K, in thermal equilibrium with the vacuum chamber, with a significant (>5%) population for rotational levels up to / = 6. Indeed, 12 transitions between 1391 and 1471 nm, from lower rotational levels 7 = 0 to 7 = 6 were observed using diode laser spectroscopy. A telecom-type diode laser with a linewidth of 5 MHz on... 

The transition phenomena between external and internal oxidation of alloys are very complex. For simplification, such transitions can be divided into two main types according to the composition of the oxides formed on or in the alloys [6]. In the first type of transition, the external oxide scale and the internal oxide particle have the same composition, as shown in Fig. 3.1. This type of transition takes place between the growth processes of internal oxide particles in the alloy substrate (Fig. 3.1a) and an exclusive oxide scale on the alloy surface (Fig. 3.1b), under the condition that only the solute metal in the alloy can oxidize. In the second type of transition, the external oxide scale has a composite structure, as shown in Figs 3.2a and 3.2b. The composite oxide scale (AO + BO) represents mixtures, compounds or layers of AO and BO under different oxidizing conditions. This type of transition takes place between the formations of the internal oxide particles beneath the composite oxide scale (Fig. 3.2a) and a composite oxide scale on the alloy surface (Fig. 3.2b), under the condition that oxidation of all elements in the alloy can occur. In addition, there is a possibility of another type of transition between the growth processes of a composite oxide scale (Fig. 3.2b) and an exclusive oxide scale on the alloy surface (Fig. 3.2c) [7]. 

Fig. 1. 35. Mechanism for the control of the activity of transcription factors. Regulatory DNA-binding proteins can occur in binding active and binding inactive forms. The transition between the two forms is primarily controlled by the mechanism shown schematically above. Activation or inactivation of transcription factors is determined by signals that can be of an internal or external origin. Furthermore, the amount of available transcription factor can also be regulated via its degradation rate or rate of expression.

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