Metals polymorphic transformations

Boggs, J.L. Prentice, K.J. Kraeutle J.E. Crump, The Role of the Scanning Electron Microscope in the Study of Solid Propellant Combustion , inavwepsceiiu ir h/zo yiyoy) do) u,u, Graber, F.C. Rauch A.J. Fanelli, Observation of Solid-Solid Polymorphic Transformation in 2,4,6-Trinitro Toluene , JPhChem 73, (10), 3514—15 (1969) 39) J.E. Crump, J.L. Prentice K.J. Kraeutle Role of Scanning Electron Microscopy in the Study of Solid Propellant Combustion. Part 11—Behavior of Metal Additives , NavWepsCentr TP-5142-PT-2 (1969) 40) J.A. Markham A.R. Cox, Applications... 

The electrons in simple metals are well described by a free electron gas. Since the atomic volume is changed by only a few percent as a result of polymorphic transformations, the electronic contribution to a free energy does not show any significant structure dependence for these metals. For transition metals, the elec-... 

Elastic properties serve an obvious utility in mechanics of materials, e.g., stress-strain relations and dislocation characteristics (Fisher and Dever, 1967 Fisher and Alfred, 1968). Moreover, elastic properties and their temperature dependencies provide important information and understanding of such physical characteristics as magnetic behavior, polymorphic transformations, and other fundamental lattice phenomena. In this section the elastic properties and their temperature dependencies are presented for all the rare earth metals except promethium, for which there is no data. To the writer s knowledge this is the first one-source compilation of the temperature dependencies of the elastic properties of the rare earth metals. 

Here no chemical segregation occurs due to the polymorphic transformation into the amorphous state. This transformation is fast in time compared to the nucleation and growth of the two crystalline phases where diffusion of the metal atoms is necessary. The low diffusivity of the metal atoms compared to the high mobility of the hydrogen atoms connected with the attractive interaction between one metal species and the hydrogen gives the basic understanding of the SSR process. 

Investigations of phase equilibria in the erbium-yttrium system have been conducted by Markova et al. (1964) and by Spedding et al. (1973). The first group used distilled yttrium metal of 99.6(wt )% purity and erbium metal of about 98(wt )% purity. The principal impurities included Ca, Fe, Cu, Ta and other rare earth metals. The alloys were arc-melted under purified helium and studied in the annealed state. Microscopy, differential thermal analyses and X-ray methods were utilized and measurements of hardness and electrical resistivity were performed on the alloy specimens. The main difficulty experienced in their thermal analysis was the narrow temperature interval between the melting of their alloys and the polymorphic transformations. 

The semiconductor dependence o (T) for stoichiometric compositions obtains the most common electrical property of binary chalcogenides. Deviation fi-om stoichiometry influences the electrical conductivity value and even slight deviations from stoichiometry lead to degeneration of the electron gas. It also changes the semiconductor conductivity to the metal one. Conduction changes occur in some systems as a result of polymorphic transformations under temperature variations for fixed compositions [1]. 

Vesnin YI, Khirpin LA (1966) Polymorphic transformations of alkali metal bichromates. Zh Neorg Khim 11 2216-2221... 

Reshetnikov NA, Baranskaya EV (1967) Heats of fusion and polymorphic transformations of alkali metal hydroxides. Izv Vyssh Ucheb Zaved Khim Khim Tekh 10 496-499... 

The pressure in the chamber was determined by the chamber-pressure/ press-force calibration curves. The calibration curves for each pair of anvils were plotted by the changes of electrical resistance at polymorphic transformations in reference metals Bi (7.7 GPa), Sn (9.5 GPa), Ba (12.5 GPa), Pb (13 GPa), and ZnS (15 GPa) at room temperature (Figure 16.1b). 

We saw in Chapter 6 that diffusive transformations (like the growth of metal crystals from the liquid during solidification, or the growth of one solid phase at the expense of another during a polymorphic change) involve a mechanism in which atoms are attached to the surfaces of the growing crystals. This means that diffusive transformations can only take place if crystals of the new phase are already present. But how do these crystals - or nuclei - form in the first place ... 

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

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