Y-phase crystal

The titanium aluminide TiAl - often designated as y phase - crystallizes with the tetragonal LIq structure (CuAu-type) which is shown in Fig. 1. The LI o structure results from ordering in the f.c.c. lattice (Al), i.e. it is basically a cubic structure which is tetragonally distorted because of the particular stacking of the atom planes, as is seen in Fig. 1. The ratio of the lattice parameters c and a is cja = 1.015 at the stoichiometric composition and the density is 3.76 g/cm (Kim and Dimiduk, 1991), whereas for TiAl-base alloys the range 3.7-3.9 g/cm is given (see Table 2). This density is still lower than that of TijAl and has made the titanium aluminides most attractive for materials developments. 

The nickel aluminide NijAl - known as the y phase - crystallizes with the cubic LI2 structure (CujAu-type) which results from the fc.c. structure by ordering (see Fig. 1). Deviations from stoichiometry are accommodated primarily by antisite defects (Lin and Sun, 1993). The density of NijAl is 7.50 g/cm (see Liu et al., 1990) and thus is only slightly lower than that of the superalloys (see Table 2) which, however, is still of interest. The elastic constants have been studied experimentally and theoretically by various authors (e.g. Davies and Stoloff, 1965 Dickson et al., 1969 Kayser and Stassis, 1969 Foiles and Daw, 1987 Wallow et al., 1987 Yoo and Fu, 1991, 1993 Yasuda et al., 1991a, 1992). Young s modulus of cast polycrystalline NijAl at room temperature is about the same as that of pure Ni with a weaker temperature dependence (Stoloff, 1989),... 

A major y-phase crystal coexists with p- and a-phases at high crystallization temperature (=140-150°C), because the dispersed layer titanate particles acting as a nucleating agent. The overall crystallization rate and crystalline structirre of prrre PVDF were strongly influenced by the presence of the layered titanate particles. - ... 

Figure 19 (a) Percentage of y phase vs. crystallization temperature,Tc, for metallocene... 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

At Ty, the Gibbs free energy of phase a (i.e., melt) at all compositions is lower than that of mechanical mixture y + y" phase a is then stable over the whole compositional range. At T2, the chemical potential of component 1 in a is identical to the chemical potential of the same component in y . Moreover, the equahty condition is reached at the standard state condition of the pure component T2 is thus the temperature of incipient crystallization of y. At T, the Gibbs free energy of a intersects mechanical mixture y + y" on the component 1-rich side of the diagram and touches it at the condition of pure component 2. Applying the prin-... 

Figure 3.14. An HRTEM lattice image of the y-phase in (100) showing a layered structure, with an ED image and the computed structure shown in the insets. Inset (c) shows an SEM image of the catalytic crystals (after Gai 1983).

The y phase of bismuth molybdate underwent a reversible transformation to the metastable tetragonal y" modification. This metastable modification was observed in the temperature range of 520° to 550°C and underwent an irreversible transformation to the y modification which readily formed at 700°C. The results indicated that the y modification corresponds to that reported by Blasse (83). However, refinement of the crystal data utilizing a single crystal revealed that this y modification was orthorhombic with lattice parameters a = 15.99 A, b = 15.92 A, and c = 17.43 A. An additional observation was the reversible transformation of the y modification to y at 900°C. 

Another example of a dispersion of SWCNTs in a multi-component antiferro-electric smectic-C liquid crystal mixture was shown by Lagerwall and Dabrowski et al. [497]. In this study, SWCNTs caused the appearance of a single-layer SmC phase between the SmA phase and the crystalline state in comparison to the non-doped sample exhibiting an SmA and two specific intermediate phases, an SmC p and an SmC Y phase. 

Several papers report [4] that liquid alumina solidifies not in the thermodynamically most stable phase of (X-AI2O3, but rather in the form of Y-AI2O3. This is attributed to the fact that the solidified phase structure is basically determined by the relative critical free enthalpies of nucleation of alternative crystal structures. Consequently, not surprising, that considerable part of spheroidized particles composed of y-AbOs and other metastable phases (such as 8, 0) of alumina (Fig. 7). The latter were formed from the y phase according to the usual route of phase transformation on cal-... 

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