Ordered alloys order-disorder transformation

In the examples given below, the physical effects are described of an order-disorder transformation which does not change the overall composition, the separation of an inter-metallic compound from a solid solution the range of which decreases as the temperature decreases, and die separation of an alloy into two phases by spinodal decomposition. 

Tanner, L.E, and Leamy, H.J. (1974) The microstructure of order-disorder transitions, in Order-Disorder Transformations in Alloys, ed. Warlimont, H. (Springer, Berlin) p. 180. 

Even when complete miscibility is possible in the solid state, ordered structures will be favored at suitable compositions if the atoms have different sizes. For example copper atoms are smaller than gold atoms (radii 127.8 and 144.2 pm) copper and gold form mixed crystals of any composition, but ordered alloys are formed with the compositions AuCu and AuCu3 (Fig. 15.1). The degree of order is temperature dependent with increasing temperatures the order decreases continuously. Therefore, there is no phase transition with a well-defined transition temperature. This can be seen in the temperature dependence of the specific heat (Fig. 15.2). Because of the form of the curve, this kind of order-disorder transformation is also called a A type transformation it is observed in many solid-state transformations. 

The Type N thermocouple (Table 11.60) is similar to Type K but it has been designed to minimize some of the instabilities in the conventional Chromel-Alumel combination. Changes in the alloy content have improved the order/disorder transformations occurring at 500°C and a higher silicon content of the positive element improves the oxidation resistance at elevated temperatures. 

Dilatometric methods. This can be a sensitive method and relies on the different phases taking part in the phase transformation having different coefficients of thermal expansion. The expansion/contraction of a sample is then measured by a dilatometer. Cahn et al. (1987) used dilatometry to examine the order-disorder transformation in a number of alloys in the Ni-Al-Fe system. Figure 4.9 shows an expansion vs temperature plot for a (Ni79.9Al2o.i)o.s7Feo.i3 alloy where a transition from an ordered LI2 compound (7 ) to a two-phase mixture of 7 and a Ni-rich f c.c. Al phase (7) occurs. The method was then used to determine the 7 /(7 + 7O phase boundary as a function of Fe content, at a constant Ni/Al ratio, and the results are shown in Fig. 4.10. The technique has been used on numerous other occasions,... 

Magnetically soft Fe-Ni alloys can have their properties altered by heat treatment. The compound NisFe undergoes an order-disorder transformation at about 500°C. Since the susceptibility of the ordered phase is only about half that of the disordered phase, a higher susceptibility is realized when the alloy is quenched from 600°C, a process that retains the high-temperature, disordered structure. Heat treatment of Fe-Ni alloys in a magnetic field further enhances their magnetic characteristics (see Figure 6.61), and the square hysteresis loop of 65 Permalloy so processed is desirable in many applications. A related alloy called Supermalloy (see Table 6.19) can have an initial susceptibility of approximately one million. 

Through control of an ordered—disordered transformation, the yield strength of these alloys increases at elevated temperatures above the room-temperature yield strength. One composition, for example, exhibits a yield strength of 480 MPa (70,000 psi) at ca 750°C compared to its room temperature value of 345 MPa (50,000 psi). The alloys also show good resistance to radiation-induced swelling (see Fusion ENERGY). These alloys can be... 

As an example of these conditions of higher order stabihty we may mention the order-disorder transformation in the alloy of equimolecular proportions of gold and copper. The affinity of the change is given by the approximate equation (c/. 19.55)... 

To describe these transformations, De Bonder has made systematic use of the concept of the degree of advancement or extent of reaction, denoted by The state of systems studied here can be defined in general by two physical variables such as the volume and temperature and one parameter for each physicochemical change that can occur in the system. The concept of extent of reaction or extent of change can be applied not only to chemical reactions and phase changes which can be represented by stoichiometric equations, but also to changes such as the order-disorder transformation in alloys for which no chemical equation can be written. 

Starting with an ordered alloy at a low temperature two types of order-disorder transformation may be recognized ... 

We shall now give some examples of order-disorder transformations observed in alloys of two types, XY and X3Y. 

Order-disorder transformations have been observed in the following alloys CU3AU, Cu3Pd, Cu3Pt, Ni3pe, and Fe3Al. In the first four cases the atoms occupy the... 

The prediction of a critical temperature is an important feature of the theory of the order-disorder transformation, for it is to be expected that at this temperature many of the physical properties of the alloy will display sharp changes. In particular, anomalies in the specific... 

In the Ni-Fe system at room temperature, the a phase extends from 0 to 7% Ni, then a. Fy mixtures from 7 to 50% Ni, and the y phase from 50 to 100% Ni. y-Phase alloys in the Ni-Fe system, known as Permalloys, exhibit a wide variety of magnetic properties, which may be controlled precisely by means of well-established technologies. Initial permeabilities up to 10 in an extremely wide temperature range, as well as coercive fields between 0.16 and 800 A/m, can be obtained (Chin Wemick, 1980). Induced anisotropy of 65-85% Ni alloys can be drastically varied by field annealing and mechanical deformation (slip-induced anisotropy) an order-disorder transformation occurs for Ni3Fe finally, preferential orientation can be induced in 50%Ni-50%Fe. 

Table 6.2. Order-disorder transformation types in soft ferromagnetic alloys.

Fig. 6.6. The four basic ordered lattices observed in order-disorder transformations in ferromagnetic alloys (a) the L2o lattice (b) the Llo lattice (c) the Llj lattice and (d) the DOj lattice. (Adapted from Chen, 1977.)...

There are a number of displacive transitions mentioned in this book. The order-disorder transformation of hydrogen atoms in hydrogen bonds in ferroelectric ceramics (Section 11.3.5) is one example. Displacive transitions that involve a change from an ordered arrangement of atoms to a random arrangement are commonly found in alloys. A subgroup of such order-disorder transitions, martensitic transitions, which can be used to produce shape-memory alloys, are considered in Sections 8.3.2 and 8.3.3. 

Jos] Josso, E., Order-Disorder Transformations in Ternary Alloys (in French), Rev. Metall, 49, 111-Til (1952) (Experimental, Phase Relations, Magn. Prop., Meehan. Prop., 15) [1952Pal] Palmer, E.W., Wilson, F.H., Constitution and Properties of some Iron-Bearing Cupro-Nickels , Trans. Amer. Inst. Min. Met Eng., 194, 55-64 (1952) (Experimental, Meehan. Prop., Morphology, Phase Relations, 8)... 

De Fontaine, D. (1994) Cluster approach to order-disorder transformations in alloys. Solid State Physics, 47,33-176. 

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