Alloys single-phase binary

A system which can easily be treated in this way is a single phase binary alloy. For preparation, however, let us consider an A crystal with a vacancy flux driven across it. In view of the fact that jA +y v = 0 in the steady state lattice system, the vacancy flux induces a counterflux of A, which shifts the whole crystal in the direction of the surface where the vacancy source is located. The shift velocity vb is yV... 

For a single-phase binary alloy A-B, the two anodic partial reactions of metal dissolution are... 

We will study and follow only single-phase binary alloys (with two components) for which it was possible to set up some models, which already show the complication of the phenomena compared to oxidation of pirre metals. 

For these single-phase binary alloys we can distinguish four modes of oxidations, which differ from each other by the nature and the localization of oxidized phases ... 

Selective oxidation of single-phase binary metal alloys... 

Consider a single-phase binary alloy (Al, A2) forming a sohd solution of substitution of two metals Al and A2. One carries out the oxidation of this alloy by a gas G2 under conditions such as only the Al element is oxidized to form on the surface of alloy a layer of the oxidized compound AIG. It is said that Al tmdergoes a selective oxidation. That will be, for example, the case of nickel-platimtm alloys with selective oxidation of nickel into NiO, example of which is used [KUB 49], [LAL 73], [THO 49], [WAG 52] to model selective oxidation as we will see further. We suppose that the gas imdergoes a dissociative adsorption. 

Oxidation of single-phase binary alloys with miscibility offormed oxides... 

The use of a percolation approach clarifies substantially the selective dissolution of single-phase binary alloys. Computer simulations support the existence of a tight relationship between dealloying thresholds and site percolation thresholds. However, a careful survey of the literature over the last 10 years leads to the striking constatation... 

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 phase diagram for a binary alloy (Fig. A1.13) shows single-phase fields (e.g. liquid) and two-phase fields (e.g. liquid plus A). The fields are separated by phase boundaries. When a phase boundary is crossed, a phase change starts, or finishes, or both. 

In contrast, the selective dissolution or leaching-out by corrosion of one component of a single-phase alloy is of considerable practical importance. The most common example of this phenomenon, which is also referred to as parting , is dezincification, i.e. the selective removal of zinc from brass (see Section 1.6). Similar phenomena are observed in other binary copper-base alloys, notably Cu-Al, as well as in other alloy systems. 

The general thermodynamic treatment of binary systems which involve the incorporation of an electroactive species into a solid alloy electrode under the assumption of complete equilibrium was presented by Weppner and Huggins [19-21], Under these conditions the Gibbs Phase Rule specifies that the electrochemical potential varies with composition in the single-phase regions of a binary phase diagram, and is composition-independent in two-phase regions if the temperature and total pressure are kept constant. 

Because of the interest in its use in elevated-temperature molten salt electrolyte batteries, one of the first binary alloy systems studied in detail was the lithium-aluminium system. As shown in Fig. 1, the potential-composition behavior shows a long plateau between the lithium-saturated terminal solid solution and the intermediate P phase "LiAl", and a shorter one between the composition limits of the P and y phases, as well as composition-dependent values in the single-phase regions [35], This is as expected for a binary system with complete equilibrium. The potential of the first plateau varies linearly with temperature, as shown in Fig. 2. 

Iridium-rhodium alloys were prepared in electric arc furnace in argon atmosphere by smelting refined powders of rhodium and iridium of 99.95 mass% purity. The composition of the alloys was controlled by the atomic adsorption analysis using the Perkin-Elmer spectrophotometer with HUS-72 analyzer. X-ray phase analysis indicated that all binary systems had a single-phase FCC-structure characteristic of... 

Fig. 12.4. Schematic illustration of conserved and nonconserved order parameters for characterizing the internal state of a binary alloy (adapted from Chen and Wang (1996)) (a) disordered phase (rj = 0) with uniform composition cq, (b) two-phase mixture consisting of disordered phases (rj = 0) with composition Cq, and c, (c) ordered single phase (ri = 1) of single composition ci with an antiphase domain boundary.

---