Peritectics

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. 

Binary Alloys. Aluminum-rich binary phase diagrams show tliree types of reaction between liquid alloy, aluminum solid solution, and otlier phases eutectic, peritectic, and monotectic. Table 16 gives representative data for reactions in tlie systems Al—Al. Diagrams are shown in Figures 10—19. Compilations of phase diagrams may be found in reference 41. 

Al—Cr. Although no commercial ahoys are based on this system, chromium [7440-47-3] Cr, is an ingredient of several complex and commercially significant ahoys (Fig. 16. The Cr is added for control of grain stmcture. The Al—Cr system has a peritectic portion at 661°C where soHd solubiHty is 0.7% Cr and Hquid solubiHty is 0.4% Cr. Free2ing of binary ahoys containing >0.4% Cr produces coarse primary crystals (metallic inclusions) of AlyCr [12005-37-7] which may adversely affect mechanical properties. In complex ahoys, the Hquid solubiHty can be reduced such that formation of primary AlyCr crystals may occur at significantly lower Cr contents. 

Cu—Zn, peritectic (III) RBCuZn same as Class 2 same as Class 2 same as Class 2... 

Copper Alloys. Tellurium is alloyed with copper for various purposes. Frequently the tellurium is added to molten copper as a copper teUuride (46.3% Te) master alloy, taking advantage of the peritectic melting point of 1051°C. 

Teaching yourself phase diagrams, part 3 eutectics, eutectoids and peritectics... 

Eutectics and eutectoids are important. They are common in engineering alloys, and allow the production of special, strong, microstructures. Peritectics are less important. But you should know what they are and what they look like, to avoid confusing them with other features of phase diagrams. 

Many phase diagrams show another feature. It looks like an upside-down V (i.e. a A) touching a horizontal line. It is a peritectic reaction, and the tip of the A is a peritectic point (see Fig. A1.43). 

DEF. A peritectic reaction is a three-phase reaction by which,... 

The iron-carbon diagram (Fig. A1.37) has a peritectic point. Ring it on the diagram. 

The copper-zinc system shown in Fig. A1.38 has no fewer than five peritectic reactions. Locate them and ring the peritectic points. (Remember that when a single-phase field closes above at a point, the point is a peritectic point.)... 

Figure A 1.52 shows the Ti-Al phase diagram (important for the standard commercial alloy Ti-6% Al-4% V. It shows two peritectic reactions, at each of which liquid reacts with a solid phase to give an intermetallic compound, (a) Ring the peritectics and give the (approximate) chemical formula for the two compounds, (b) Shade all... 

J. Tiaden, B. Nestler, H. J. Diepers, I. Steinbach. Physica D 115 11, 1998 G. J. Schmitz, B. Nestler. Simulation of phase transitions in multiphase systems, peritectic solidification of YBaCuO-superconductors. Mater Sci Eng B 53 11, 1998. 

FSeSeF) and (Se=SeF2) trapped at low temperature Se2Cl2 yellow-brown liquid mp —85°, bp 130° (d) ( -)Se2Br2 blood-red liquid bp 225° (d) (a-SeBr, mp 4-5°) a-Te4l4 black mp 185° (peritectic) iS-Tel black... 

By way of example, the Cu-Zn phase diagram shown in Fig. 20.42 exhibits a number of different intermediate phases (j8, 7, 6, etc.) and a number of peritectic reactions and a eutectoid reaction. In many instances it is not necessary to consider a complete phase diagram. Thus Fig. 20.43 illustrates the Al-rich end of the Al-Cu phase diagram and is used below in a discussion... 

The peritectic transformation generally has little effect on the structure, properties or corrosion resistance of steels at room temperature an exception to this occurs in the welding of certain steels, when 6-ferrite can be retained at room temperature and can affect corrosion resistance. Furthermore, since most steels contain less than about 1 -0 oC (and by far the greatest tonnage contains less than about 0-3%C) the eutectic reaction is of relevance only in relation to the structure and properties of cast irons, which generally contain 2-4%C. This discussion, therefore, will be limited to the eutectoid reaction that occurs when homogeneous austenite is cooled. 

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