Alloy two-phase

Polyphase Alloys. The two-phase alloys have a rather wide range of properties resulting from variations within the stmcture. If the second phase is distributed in critical depression, the hardness and strength are at a maximum and the ductility is at a moderate level. Tensile strength may be 415—825 MPa (60,000—120,000 psi) yield strength, 170—585 MPa (25,000—85,000 psi) and elongation, 10—40%. 

You can get the relative amounts of each phase in a two-phase alloy from the phase diagram. 

C. Nishimatsu and J. Gurland, Experimental Survey of the Deformation of a Hard-Ductile Two-Phase Alloy System, IVC-Co, Brown University Division of Engineering Technical Report Number 2. September 1958. 

In principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

Alloys of copper and zinc can be obtained by combining the molten metals. However, zinc is soluble in copper up to only about 40% (of the total). When the content of a copper/zinc alloy contains less than 40% zinc, cooling the liquid mixture results in the formation of a solid solution in which Zn and Cu atoms are uniformly distributed in an fee lattice. When the mixture contains more than 40% zinc, cooling the liquid mixture results in the formation of a compound having the composition CuZn. The solid alloy consists of two phases, one of which is the compound CuZn and the other is a solid solution that contains Cu with approximately 40% Zn dissolved in it. This type of alloy is known as a two-phase alloy, but many alloys contain more than three phases (multiple-phase alloys). 

Figure 2.26. Isothermal section of the Al-Bi-Sh phase diagram at 200°C. In the triangle marked hy the asterisk, three phases (coincident with the two elements A1 and Bi together with the compound AlSh) are observed. In the other triangle two-phase alloys are formed. A few tie-lines are shown. The alloy marked hy , for instance, contains the compound AlSb together with a...

The points of these segments represent the AmixG of two-phase alloys. In the composition range between the maximum and the spinodal (xs) a two-phase alloy, such as a mixture xul + xu2, has therefore an overall free energy lower than that of any single-phase alloy of an intermediate composition, which is therefore unstable. 

A general treatment of a diffusion-controlled growth of a stoichiometric intermetallic in reaction between two two-phase alloys has been introduced by Paul et al. (2006). A reaction couple in which a layer of Co2Si is formed during inter-diffusion from its adjacent saturated phases was used as a model system. In the discussion it has been emphasized that the diffusion couple is undoubtedly one of the most efficient and versatile techniques in solid-state science it is therefore desirable to have alternative theories that enable us to deduce the highest possible amount of information from the data that are relatively easily attainable in this type of experiments. 

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

The variation of resistivity with composition is also expressed in an empirical fashion. For two-phase alloys consisting of phases a and P, the rnle of mixtnres can be nsed to approximate the alloy resistivity from the individnal metal resistivities ... 

This approach can be applied to segregation phenomena in other two-phase alloys, such as reported recently for polycrystalline Fe99Pdi with a small fraction of ordered FePd nuclei [86]. Positive entropy and enthalpy characterize the observed increase in segregation level with temperature. 

The key to the interpretation of the powder patterns of alloys is the fact that each phase produces its own pattern independently of the presence or absence of any other phase. Thus a single-phase alloy produces a single pattern while the pattern of a two-phase alloy consists of two superimposed patterns, one due to each phase. 

Other points could be found in a similar manner. For example, if the same series of alloys were equilibrated at temperature T2, a parameter curve similar to Fig. 12-8(b) would be obtained, but its inclined branch would be shorter and its horizontal branch lower. But heat treatments and parameter measurements on all these alloys are unnecessary, once the parameter-composition curve of the solid solution has been established. Only one two-phase alloy is needed to determine the rest of the solvus. Thus, if alloy 6 is equilibrated at Tj and then quenched, it will contain a saturated at that temperature. Suppose the measured parameter of a in this alloy is a,. Then, from the parameter-composition curve, we find that a of parameter contains y percent B. This fixes a point on the solvus at temperature T2. Points on the solvus at other temperatures may be found by equilibrating the same alloy, alloy 6, at various temperatures, quenching, and measuring the lattice parameter of the contained a. 

The two-phase alloy mentioned in Prob. 12-1, after being quenched from a series of temperatures, contains a having the following measured parameters ... 

Dealloying occurs when one component of an alloy is lost preferentially. Thus, brass is an alloy of zinc (a rather active metal) and copper (a rather noble metal). Consequently, the zinc tends to be lost in preference to the copper. Often the copper will form a seal over the surface, preventing further corrosion, but if conditions do not allow this, then the corrosion can penetrate into the component, removing most of the zinc. The result is a porous copper component, which has little mechanical strength, and the problem is often discovered when the component fractures. Similarly, one component of a two-phase alloy can... 

Tungsten metal exhibits outstanding thermal properties, which makes it attractive for a broad range of applications. However, for certain applications, its electrical and thermal conductivity, sensitivity toward oxidation, and poor workability are unsatisfactory. These limitations have led to the development of two-phase alloys, in which the useful properties of tungsten are combined with those of the additive. 

As-cast Ti-3Al-2Si alloy has a typical polycrystalline microstructure of a solid solution of P-transformed Ti (a-Ti) with single particles of the secondary silicides (Fig. la). Additions of Zr promote the precipitation of the silicide (Fig. lb) and arising the first portion of eutectic (Fig.lc). Ti-3Al-2Si-5Zr and Ti-3Al-2Si-15Zr are two-phase alloys consisting of a-Ti and Ti5Si3 and a-Ti and (Ti,Zr)2Si phases, respectively (Table 1). 

Plain carbon steels may be heat treated to have dispersions of small, round, isolated iron carbides in the continuous iron matrix. The amount of carbide is usually less than 10% of the structure. With two-phase alloys such as this, the carbide may become anodic in some environments and cathodic in others. Predict the progress of corrosion if the carbide is (a) anodic and (b) cathodic. Be reasonably specific in describing changes at the surface. 

If the two metals that form the alloy are insoluble in one another, then they will exist as two separate phases, often in alternate layers such as observed in tin-lead alloys or cast irons, where the carbon is often found as minute tadpole like shape (flakes) adjacent to the pure iron. These types of two-phase alloys are extremely difficult if not impossible to shape by hot or cold working. Fortunately, these alloys have a melting point well below that of the parent metals and are very suitable to shape by casting into moulds. This is the reason why iron 4.5% carbon alloys were called cast irons. These alloys have two important limitations in that first, they are very brittle when subjected to impact loads, and second, their corrosion resistance is inferior to pure metals or single-phase alloys. 

The major alloy of tin recovered from archaeological sites is pewter. This can be divided into those containing lead and lead-free alloys. The former could have a lead content ranging from 67 % (equivalent to plumbers solder) down to 15%. The French in Elizabethan times kept the lead of their wine goblets to below 18% as above this, the wine would become tainted As the lead and tin are insoluble in one another, they are classed as a two-phase alloy and articles could only be manufactured by casting. The lead-tree pewter was invariably an alloy of tin with a small amount of copper (0.5-7% for pewter recovered from the Maty Rose). The copper dissolved in the tin crystal structure resulted in a single-phase structure, which was considerably harder than pure tin. Hence this class of pewter could be subjected to a limited amount of mechanical working to achieve the final shape. 

Fig. 1. Specific yield strength (0.2 % proof stress in compression per unit weight density at 10 4s 1 strain rate in compression) as a function of temperature for the D022 phase Al3Nb [112, 113], the Heusler-type phase Co2TiAl [67], the Laves phasesTiCr15Si05 andTaFcAl [67,114], the two-phase alloy NbNiAl-NiAl with 15 vol.% NiAl in the Laves phase NbNiAl [67,114], and the hexagonal D8g phase Ti5Si3 [100] in comparison to the superalloy MA 6000 (in tension) [115] and the hot-pressed silicon nitride HPSN (upper limit of flexural strength) [116],...

---