Alloy systems order-disorder transformation

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,... 

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. 

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. 

Earlier work on systems such as Ni-Al-Cr reported in Sanchez et al. (1984b) used FP methods to obtain information on phases for which there was no experimental information. In the case of Ni-base alloys, the results correctly reproduced the main qualitative features of the 7 — 7 equilibrium but cannot be considered accurate enough to be used for quantitative alloy development. A closely related example is the work of (Enomoto and Harada 1991) who made CVM predictions for order/disorder (7 — 7 ) transformation in Ni-based superalloys utilising Lennard-Jones pair potentials. 

Martensitic transformations in alloys are essentially order-disorder displacive transitions that take place very rapidly, because atomic diffusion does not occur. The discussion of the formation of martensite in the Fe-C system, in Section 8.2.5, is an example. This transition is the transformation of a cubic phase containing excess carbon in interstitial sites into a tetragonal phase. As any one of three cubic axes can be elongated, three orientations of the martensite c axis can occur. This is a general feature of martensitic transformations and the different orientations that can arise are called variants or domains of the martensitic phase. These variants are simply twins (see Section 3.4.10). 

Physical metallurgy is a rather wide field of applications of Mossbauer spectroscopy and it is possible to enumerate only the main topics phase analysis, order-disorder alloys, surfaces, alloying, interstitial alloys, steel, ferromagnetic alloys, precipitation, diffusion, oxidation, lattice defects etc. Alloys are well represented by the iron-carbon system, the mechanism of martensite transformation, high-manganese and iron-aluminium alloys, iron-silicon and Fe-Ni-X alloys. 

Transformations from disordered to ordered solid solutions do also occur in some further binary alloy systems, namely, Au-Cd, Au-Cr, Au-Mn, Au-Nb, Au-Pd, Au-V, and Au-Zn [1-3]. The martensitic transformations associated with the ordering in the Au-Cd and Au-Zn systems are relevant for shape memory applications and are also accompanied with considerable strengthening effects. The transformations in the other alloy systems listed above are, in part, relevant for particular functional applications, but little is known about the impact of the transformations on (mechanical) properties. 

The SME process can be illustrated by the Cu—Zn system, one of the first SMAs to be studied. A single orientation of the bcc P-phase on cooling goes through an ordering process to a B2 phase. In a disordered alloy, the lattice sites are randomly occupied by both types of atoms, but on ordering the species locate at particular atomic sites, yielding what is called a supedattice. When the B2 phase is cooled below the Mp it transforms to... 

A very similar transformation of the original hep adlayer to a surface alloy coverage with the same Tl-Tl interatomic distances and [V3 x V3]R30° symmetry has been observed in the system Tl/Ag(lll) during extended polarization of the incompletely formed first T1 adsorbate layer. As in the system Pb/Ag(lll), there is strong evidence that the transformations proceed from the boundaries of the peripheral adsorbate-free domains inwards on the terraces. However, in contrast to the system Pb/Ag(lll), the transformed coverages include both ordered and disordered domains, and then-desorption results in the formation of monoatomic pits in the substrate with widths of ca. 3 to 10 nm [3]. These pits diminish and finally vanish within a few minutes by coalescence and lateral displacement, at a rate that can be increased markedly by positive shift of the substrate potential. 

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