Peritectoid

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

DEF. A peritectoid is a three-phase reaction by which, on cooling, two solid phases react to give a single new solid phase. 

The foregoing classification is not without ambiguity. For example, it is common practice to call the reaction A - B +C° (see Fig. 6-1) induced by decreasing the temperature a phase transformation. The similar (peritectoid) reaction C = a+fi (Fig. 12-2) induced by a temperature increase, however, is named a decomposition reaction. In addition, the isothermal reaction AO = A+j02, which occurs if the intensive variable fio2 is decreased so that AO decomposes, is called a metal oxide reduction. It is thus categorized as a genuine heterogeneous solid state reaction (the... 

Figure 4.1 shows these diffusion couples below (left) and above (right) the decomposition temperature of the , phases.10 It is obvious that the phase bands of the , phases are present between the fi and 6 phases the left column of the microstructures while it is absent in the right column showing the presence of an invariant phase reaction. The accordingly invariant temperatures are given in Table 4.1. For the Nb-C system the formation of a phase band was most difficult to observe because the peritectoid temperature is rather low and the carbon diffusivity is slow. 

In Figure 5, a speculative version of a phase diagram for the system Na WOs is shown, including the possible peritectoid isotherm. In this temperature region it is clear that the system is no longer pseudobinary, so that verification of the possibility shown here requires some control of the sodium vapor pressure. The eutec-... 

In the case of racemate, the situation is more complex since one racemate polymorph can be a true racemate and the other one a conglomerate and one has to consider additional solid phase transitions such as peritectoids and eutectoids, which are not polymorphic transitions. ... 

In general, the various experimental techniques differ in sensitivity, and therefore in usefulness, from one portion of the phase diagram to another. Thus, thermal analysis is the best method for determining the liquidus and solidus, including eutectic and peritectic horizontals, but it may fail to reveal the existence of eutectoid and peritectoid horizontals because of the sluggishness of some solid-state reactions or the small heat effects involved. Such features of the diagram are best determined by microscopic examination or x-ray diffraction, and the same applies to the determination of solvus (solid solubility) curves. 

This low misfit accompanied with very low solid solubility of Zr in A1 favors the stability of the precipitates. The reason of stability was showed by the study of peritectoid reaction (998+12 °C) ((3Zr)+Zr2Al <- Zr3Al [1], These small precipitates act to inhibit recrystallization and pin grain boundaries through a Zener drag process. 

No single crystals could be manufactured due to principal difficulties (the compound is formed by a peritectoid solid-state reaction which prevents a growth from the melt). The discrepancy between the bulk spontaneous moment [(0.4-0.45))iB/U] obtained for polycrystalline samples at 4.2 K, and the microscopic moment (0.8/xB) derived from neutron diffraction (Frings and Franse 1985a) point to a high uniaxial magnetic anisotropy. This may account for the lack of saturation of the magnetic isotherms in fields up to 35 T. 

Phase diagrams are the roadmaps from which the number of phases, their compositions, and their fractions can determined as a function of temperature. In general, binary-phase diagrams can be characterized as exhibiting complete or partial solid solubility between the end members. In case of the latter, they will contain one or both of the following reactions depending on the species present. The first is the eutectic reaction is which a liquid becomes saturated with respect to the end members such that at the eutectic temperature two solids precipitate out of the liquid simultaneously. The second reaction is known as the peritectic reaction in which a solid dissociates into a liquid and a second solid of a different composition at the peritectic temperature. The eutectic and peritectic transformations also have their solid state analogues, which are called eutectoid and peritectoid reactions, respectively. 

K, when it decomposes peritectoidally into 72 and P2. There have not been any heat capacity measurements reported for 71. Ball et al. [92BAL/DIC] obtained estimates of C (NiTeo.775, cr, 298.15 K) = 45.8 (48.7) J-K -moT and 5 (NiTeo.775, cr, 298.15 K) = 71.0 (67.6) J K mol by taking a weighted average of heat capacities of the P and NiTe2 v(cr) phases. The values in brackets have been calculated with the cationic and anionic contributions to the heat capacity at 298 K and the revised Latimer entropy contributions [93KUB/ALC]. If reasonable uncertainties of 5.0 J K mol are chosen for and A5°, both methods of estimation lead to results that are consis-... 

Parts of the Th-N phase diagram were determined by X-ray, microscopic, chemical, and thermal analyses of phases equilibrated with nitrogen (up to 3 bar) at temperatures up to 3093 K. The Th-ThN portion of the phase diagram is a peritectoid-peritectic system wherein, npon heating, the face centred cubic Th phase with a N Th ratio (0.08 0.02) decomposes peritectoidally at (1878 + 20) K to body centred cnbic Th and ThN. The... 

Similar transformations, in which only solid phases occur, are called peritectoid transformations. On heating through a peritectoid point, we find ... 

On heating a homogeneous solid through a peritectoid point it forms ... 

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