Cooling alloys

The otiier type of noncrystalline solid was discovered in the 1980s in certain rapidly cooled alloy systems. D Shechtman and coworkers [15] observed electron diffraction patterns with sharp spots with fivefold rotational synnnetry, a syimnetry that had been, until that time, assumed to be impossible. It is easy to show that it is impossible to fill two- or tliree-dimensional space with identical objects that have rotational symmetries of orders other than two, tliree, four or six, and it had been assumed that the long-range periodicity necessary to produce a diffraction pattern with sharp spots could only exist in materials made by the stacking of identical unit cells. The materials that produced these diffraction patterns, but clearly could not be crystals, became known as quasicrystals. 

Very large Ni2Al3 phases having NiAl3 peritecticallv grown on them in slowly cooled alloy chunks Leach very much faster than. 

Very small Ni2Al3 phases with much more NIAI3 nucleation in the melt than peritectic growth in very rapidly cooled alloys Leach very much faster than. 

The structure of a vapor-quenched alloy may be either crystalline, in which the periodicity of the unit cell is repeated within the crystallites, or amorphous, in which there is no translational periodicity even over a distance of several lattice spacings. Mader (64) has given the following criteria for the formation of an amorphous structure the equilibrium diagram must show limited terminal solubilities of the two components, and a size difference of greater than 10% should exist between the component atoms. A ball model simulation experiment has been used to illustrate the effects of size difference and rate of deposition on the structure of quench-cooled alloy films (68). Concentrated alloys of Cu-Ag (35-65%... 

The lamellar form of eutectic is only capable of resolution under high magnifications in slowly cooled alloys, while, under the same conditions of cooling, the reticular form is visible under low magnifications. ... 

NaCl solution at 105 °C, which compares to about 35 MN/m3/2 for Kiscc for the less-severely sensitized alloy. The air-cooled alloy exhibited transgranular stress corrosion and the severely-sensitized material cracked intergranularly as shown in Fig. 7.108. 

Above 60% Fe the iron atoms favour one of the Al sites (B, say) at the expense of the other but the B/D occupations are more nearly equal for alloys quenched from 800 °C than for slowly cooled alloys. For this reason the quenched alloys are referred to in the literature as [FeAl]-type and the slowly cooled alloys as [D03]-type, although L2X would be a more accurate description of both. For example, the alloy Fe70Al30 in the quenched condition has the following site occupation... 

Quenched, flash-dried samples have considerably higher Brinell hardness than the aircooled ones. Figure 4.8b evidences that even air-cooled alloy samples with 1.5—1.7 wt% Sb content meet the hardness requirements (Table 4.1), if they contain additions of As, Sn, Se, Cu and S as well. 

Wang (1976) splat-cooled alloys in the Pr-Y system and found that by splat-cooling, the hep-phase range was extended to compositions that, under equilibrium conditions, should have the Sm-type structure. The Sm-type structure was pushed toward higher concentrations of praseodymium. Wang observed that extensions of the hep-phase fields are limited by a c/a ratio of 1.597. 

Points 1, 6, 7, 8, 9 are slowly-cooled alloys. Points 2, and 3 are quickly-cooled alloys 4 is technical zinc 5 is chemically pure zinc. As can be seen alloys prepared in the same way (slowly-cooled) have their points connected by one smooth curve. 

The CoGa alloy has been the most studied experimentally. The concentrations of vacancies and antisite atoms versus Co content are shown for this compound in Figure 7. It can be seen that the number of defects is higher in alloys quenched from 1173 K than in slowly cooled alloys (Berner et al., 1975 Van Ommen et al., 1981). An important question is the following are these defects true structural vacancies (i.e. are they stable at OK ), or are they frozen-in thermal vacancies It is generally believed that the broken lines of Figure 7 represent the structural vacancies, and that the full curves represent the total (structural phis thermal) defect concoitrations. Thus the observed vacancy concentration... 

The recent work of Day has confirmed that this modified fracture is the result of a change in the mode of growth and not the result of a change in nucleation behaviom as was earlier pro-posed. - " The coarse silicon structure of slow-cooled alloys is altered if rapid growth rates are used. A similar effect is produced by adding a modifier (sodium) to the slow-cooled alloy. These are illustrated in Fig. 5.10. The silicon morphology is basically the same I n both the growth rate-modified and the additive-modified alloys. 

Figure 26 shows the surface of secondary crystallization of the Al comer after [1943Phi]. It should be noted that the data relate to slowly-cooled alloys in a non-equilibrium state. 

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