Phase transformation temperature

One of the consequences of accepting the presence of multiple magnetic states is an additional contribution to the entropy and, therefore, several authors have considered the inclusion of multiple states in their description of low-temperature phase transformations in Fe and its alloys (Kaufman et al. 1963, Miodownik 1970, Bendick and Pepperhoff 1978). However, most authors have, in the end, preferred to describe the magnetic effects in Fe using more conventional temperature-independent values for the magnetic moments of the relevant phases. This is partly linked to the absence of any provision for the necessary formalism in current... 

One example of the least activation energy is when the structure of the new phase is closest to that of the existing phase(s) or when the structure is disordered so that it does not require elaborate and precise rearrangement. The Ostwald rule is especially applicable to low-temperature phase transformations because at these low temperatures it is difficult to overcome the high activation energies required to form a new phase. At high temperatures such as igneous temperatures, the Ostwald rule is less often encountered. 

Above 0 K all crystal constituents vibrate about their mean lattice positions. Quanta of thermal energy are referred to as phonons. On heating, amplitudes of atomic vibrations are increased, the crystal expands and, at appropriate temperatures, phase transformations, melting, sublimation or chemical reactions will occur. Enhanced mobility, in addition to increasing the tendency towards breaking bonds, either within or between lattice components (decomposition or phase changes, respectively), may also increase the ease of diffusion of all species present in the crystal. 

Fig. 9.8 The change in sample length observed when heating the LSCE from nematic to isotropic. Relative length A = L/Lisotropic of an LSCE versus reduced temperature Tred = (/.isotropic = length ofthe sample in the isotropic state, T = temperature, = phase transformation temperature).

At higher temperatures, phase transformation to B1 structure takes place. 

The light rare earth metals undergo a high temperature phase transformation... 

The phases of technical interest are the fee a phase and the bcc p phase. The high-temperature phase transforms to its ordered (CsCl type) variant at temperatures below 727 to 741 K depending on composition, as shown in Fig. 3.1-131. The intermetallic phases with higher Zn content are brittle and have no technical relevance. 

Taylor phases are based on binary AlsMn, the structure of which was firstly solved by Taylor [22]. The phase has ternary extensions into several systems (e.g., Pd, Ni, Fe) [61], all of which are referred to as Taylor- or T-phases. Binary Al3Mn is a high-temperature phase, transforming to a triclinic variant below about 900 °C [62]. Addition of the third element Pd, Fe, or Ni stabilizes the T-phase, and hence the existence ranges of the ternary extensions extend to much lower temperatures. 

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more compHcated and complex near a martensitic phase transformation. At this transformation, both crystal stmctural changes iaduced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

Eor the ferrite grades, it is necessary to have at least 12% chromium and only very small amounts of elements that stabilize austenite. Eor these materials, the stmcture is bcc from room temperature to the melting point. Some elements, such as Mo, Nb, Ti, and Al, which encourage the bcc stmcture, may also be in these steels. Because there are no phase transformations to refine the stmcture, brittieness from large grains is a drawback in these steels. They find considerable use in stmctures at high temperatures where the loads are small. 

Pure barium is a silvery-white metal, although contamination with nitrogen produces a yellowish color. The metal is relatively soft and ductile and may be worked readily. It is fairly volatile (though less so than magnesium), and this property is used to advantage in commercial production. Barium has a bcc crystal stmcture at atmospheric pressure, but undergoes soHd-state phase transformations at high pressures (2,3). Because of such transformations, barium exhibits pressure-induced superconductivity at sufftciendy low temperatures (4,5). 

---