Alloys selection criteria

If some other criterion such as creep-rupture strength is of primary importance, the alloy choice may be restricted. Here it would be necessary to have thennal fatigue comparisons only for the alloys that pass the primary screening. When alloy selection reaches this stage some further cautions are in order. 

In the absence of a planar reaction front, we cannot, in general, predict the rate of selective oxidation of alloys. The criterion (9.72) is nevertheless useful, because it allows us to understand qualitatively the influence of different parameters on the oxidation behavior of binary alloys. 

That is, ttcr is directly proportional to K c/cry) since oh is a fraction of Oy. Thus, the larger the value of acr, the more attractive is the material, since cracks can be easily detected without the use of sophisticated equipment. The Ashby plot of fracture toughness versus density (Figure 8.10) indicates that of the three classes of materials selected with Criterion 1, only the engineering composites and engineering alloys provide suitable possibilities for Criterion 2. Again, of the alloys, titanium, steel, nickel, and copper alloys are the best here. 

The real power of the Ashby diagrams comes when we realize that we can combine Figures 8.9 and 8.10 to yield one, more useful diagram (Figure 8.11), namely a plot of fracture toughness versus strength. This plot shows unequivocally that the steel, nickel, and titanium alloys are the best classes of materials to select for this application. We will use Criterion 3 to narrow this field even further. 

Though there are many possibilities of the engineering alloys, let us consider three common alloys from different classes a steel, an aluminum alloy, and a titaninm alloy. The three alloys and their appropriate design properties are listed in Table 8.3. The values that are the most favorable in each category are listed in bold typeface. On the basis of Criterion 1, the best material is maraging steel, but from the viewpoints of Criteria 2 and 3 the titanium alloy is obviously superior. Cost is an additional factor that could influence the final selection. 

Although ASTM tests use temperature to accelerate attack during exposures, temperature can also be used in combination with electrochemical techniques. One example of this is the determination of critical pitting temperatures (CPT) [76,37,32] for alloy development or selection, or both. An anodic potential is applied to an electrode at low temperature (room or below) in the solution of interest and the temperature slowly increases in order to determine the temperature at which initiation of localized corrosion occurs, as signified by an increase in the current above some criterion. Such a method allows a quantitative ranking of materials in terms of resistance to pitting. 

Although it is a matter of common knowledge that stainless steel is quite prone to corrosion in fuel cells, bare substrates of different alloys were tested in past material investigations. In 1998, Hornung and Kappelt (1998) selected different iron-based materials for Solid Polymer Fuel Cell bipolar plates by using the pitting resistance equivalent (PRE = %Cr + 3.3%Mo + 30%N) as corrosion resistance criterion. The authors exposed that some iron-based materials with PRE >25 (the material compositions are not given... 

Example In the manufacture of aircraft engines and gas turbines superalloy is widely used due to its excellent performance, which to a great extent depends on the content of trace elements in the alloy. Critical control of the alloy s composition is therefore essential for the safe operation of such engines. The obvious method to avoid isobaric interferences is to select isotopes where no interferences exist. While H, Pb, and Bi meet this criterion, in case of Sn only the Sn ion is free of superimpositions, whereas Sn has serious interferences from Ars, Mo N2, and "Te". To determine the correct content of elements in superalloy on a quadrupole GD instrument also when no suitable isotope exists, both matching the sample matrix by use of external standards and multivariable linear regression have extensively been used (Fig. 15.10) [61]. 

Obviously, the parameter y is a measure of the kinetic situation when the rate-limiting step of the selective dissolution is the solid-phase diffusion mass transfer. It is clear that the increase in y contributes to the transition to the solid-phase diffusion control of the process the similar criterion was found in [4] for chronoampero- and chronopotentiometric diffusion problems of homogeneous binary alloys SD. 

---