Second phase effect

Song et al. [7,8] measured the polarisation curves for the a-phase and the P-phase as shown in Fig. 3.9. The a-phase has a pitting potential about 15 mV 

7 (a) Macro-galvanic corrosion, (b) Micro-galvanic corrosion [2,8]. 

8 The microstructure of as-cast AZ91 (a typical two-phase alloy) consists of primary oc-Mg and the eutectic micro-consituent which contains eutectic-a plus the p phase (the intermetallic Mgi7Ali2) [2,8]. 

In addition to the 3-phase, the most potent cathodes in Mg-Al alloys were thought to be the iron-rich phases the iron-aluminium intermetallic phase FeAl is one of the most detrimental cathodic phases in Mg-Al alloys on the basis of its potential and low hydrogen over-voltage [22]. AlMn [17,18] is also detrimental, and Mg2Si [18] seems to have no influence, while Mg2Pb facilitates localised corrosion and leads to a negative difference effect. 

The matrix a-phase in Mg alloys is typically anodic to the second phases and is preferentially corroded. Song et al. [7,8] suggested that the primary a and eutectic a-phases, which have different aluminium contents, have different electrochemical behaviour. Both the primary and eutectic a can form a galvanic corrosion cell with the P phase, as illustrated in Fig. 3.10. There are therefore two kinds of corrosion morphology  

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Bhattacharyya AJ, Maier J. Second phase effects on the conductivity of non-aqueous salt solutions soggy sand electrolytes. Adv Mater. 2004 16 811-4. 

High temperature (>1000°C) properties of the HPSN s is controlled by the second phase [9-11]. In order to improve the high temperature properties (e.g. creep, subcritical crack growth, oxidation) of the HPSN materials, the effects of the weak amorphous second phase must be minimized. Research, done by others in order to improve the second phase effects, involved ... 

As demonstrated in [53] it is convenient to incorporate the geometrical phase effect by adding the vector potential in hyperspherical coordinates. Thus we found that the vector potential gave three terms, the first of which was zero, the second is just a potential term... 

In polycrystalline materials, ion transport within the grain boundary must also be considered. For oxides with close-packed oxygens, the O-ion almost always diffuses much faster in the boundary region than in the bulk. In general, second phases at grain boundaries are less close packed and provide a pathway for more rapid diffusion of ionic species. Thus the simplified picture of bulk ionic conduction is made more complex by these additional effects. 

The effect of a second phase is demonstrated in the copper—aluminum system, where increasing aluminum concentration causes the alloy system to change to a polyphase alloy. By obtaining a fine dispersion of the phase, the yield strength is increased from 225 to 480 MPa (33,000—70,000 psi). 

This computation is also referred to as calculating the zinc equivalent of the alloy. The increase in strength in this alloy series is caused by increased amounts of beta phase in the stmcture. The silicon brasses show similar hardening effects accompanying a second phase. Typical mechanical properties and electrical conductivity for various cast alloys are shown in Table 2. 

The second phase, becoming effective January 1, 2000, will require approximately 2000 utilities to reduce their emissions to a level equivalent to the product of an emissions rate of (1.2 lbs of S02/mm Btu) X (the average mm Btu of their 1985-1987 fuel use). In both phases, affected sources will be required to install systems that continuously monitor emissions in order to track progress and assure compliance. 

So far the plate theory has been used to examine first-order effects in chromatography. However, it can also be used in a number of other interesting ways to investigate second-order effects in both the chromatographic system itself and in ancillary apparatus such as the detector. The plate theory will now be used to examine the temperature effects that result from solute distribution between two phases. This theoretical treatment not only provides information on the thermal effects that occur in a column per se, but also gives further examples of the use of the plate theory to examine dynamic distribution systems and the different ways that it can be employed. 

Thermal changes resulting from solute interactions with the two phases are definitely second-order effects and, consequently, their theoretical treatment is more complex in nature. Thermal effects need to be considered, however, because heat changes can influence the peak shape, particularly in preparative chromatography, and the consequent temperature changes can also be explored for detection purposes. 

It has been seen that iron has an adverse effect because it forms a second phase (insoluble) material in the alloy which acts as an effective local cathode. Sequestering is the technique of adding an alloying addition that will cause an alternative intermetallic compound with iron to form. This compound might form a dross to be removed mechanically. Alternatively the new intermetallic compound could be a less effective cathode in which case removal would not be necessary. 

Figure 14. Charge-discharge curves for the upper plateau in the LirSi system inside a matrix of the Li2 f)Sn phase at 415 °C. The upper panel shows the effect of current density, whereas the lower panel shows that the potential overshoot related to the nucleation of the second phase is mostly eliminated if the electrode is not cycled to the ends of the plateau (441.

The UL plastics program is divided into two phases. The first develops information on a material s long- and short-term properties. The second phase uses these data to screen out and indicate a material s strong and weak characteristics. For example, manufacturers and safety engineers can analyze the possible hazardous effects of potentially weak characteristics, using UL standard 746C. 

Two observations on the correlations can be made. First, these results tend to invalidate one of the major objections to the application of the thermal-ignition theory to composite propellants, namely that heterogeneous interfacial reactions within the solid phase are not possible. Secondly, the effect of pressure on propellant ignitability can be qualitatively explained. 

This method exemplifies a broad class of processes that proceed via transfer of reacting species between two liquid phases. Such processes may require a catalyst that can combine with species present in one phase and effect their transfer in this form to the second phase where the main reaction occurs. Starks23 has termed such a process phase-transfer catalysis and has demonstrated its utility in reactions involving inorganic anions. For example, he has shown that the rates... 

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