Metastable miscibility

Note Mixtures exhibiting metastable miscibility may remain unchanged or they may undergo phase separation, usually by nucleation or spinodal decomposition. 

Polymer blend that exhibits metastable miscibility. 

Note In polymers, because of the low mobility of polymer chains, particularly in a glassy state, metastable mixtures may exist for indefinite periods of time without phase separation. This has frequently led to confusion when metastable miscible polymer blends are erroneously claimed to be miscible. 

Fig. 4. Cr-Cu-Fe. Calculated isotherms of the cupola of stable and metastable miscibility gap of the liquid phase. Dashed lines are the tie lines at 1127°C.

Mur] Fourier transform IR, X-ray analysis 600-940°C, metastable miscibility gap in the corundum solid solution... 

A metastable miseibflity gap is observed in the liquid phase along the Cu-Fe binary edge. The addition of Mn deereases the critical temperature as shown in Fig. 3 reprodueed fiom the Calphad assessment of [2004Wan2]. The miscibility gap at 927°C was not included in the drawing since the aeeepted Cu-Fe binary system presents only a metastable miscibility gap above 1107°C. A metastable solid miseibility gap island of the y phase was predieted by Calphad method [2004Wan2] as depleted in Fig. 4. 

Wan] Electrolytic Co (99.99%), Fe (99.99%), Mo (99.5%). Melting in alumina crucibles in a high induction furnace under an Ag atmosphere. Hot-rolling at 800 °C. Optical microscopy, SEM/energy dispersive X-ray analysis. Thermodynamic calculations with the Redlich-Kister model and thermodynamic parameters evaluated with the PARROT software. The alloys of the compositions near Cu/ Fe 50/50 with Mo from 0 to 6 mass%. Annealing at 800 to 1300°C for 3 to 1680 h. Experimentally determined compositions of the phases in equilibrium at 1300, 1200, 1100, 1000, 900, 800°C. Calculated isothermal sections at 1500, 1300, 1100, 900°C vertical sections at 5 and 10 mass% Mo and up to 30 mass% Cu metastable miscibility gap of the liquid phase. 

Fig. 4. Cu-Fe-Mo. Calculated metastable miscibility gap. Solid thick lines isotherms of the miscibility gap in the liquid state. Dashed lines the tie lines between liquid Cu and the Fe-Mo alloys at different Fe/Mo ratios. Solid thin lines the tie lines between L and L" at 2727°C...

The calculated metastable miscibility gap (see above) in the liquid-phase region of the Cu-Fe-Nb system is shown in Fig. 4 [2000Wan]. A miscibihty gap island exists inside the ternary system. The tie-lines in the immiscible region he along the radial hnes fiom the Cu comer to die Fe-Nb side. In Fig. 4 they are shown by dashed lines for different temperatures at different Fe/Nb ratios. Sohd hnes in Fig. 4 show the tie-lines at 1827°C. 

The Cu-Fe, Cu-Ni and Fe-Ni phase diagrams are accepted from [2007Tur], [2002Leb] and [2007Kuz] respectively. In the Cu-Fe and Cu-Ni systems, metastable miscibility gaps in the liquid phase exist. However no liquid separation occurs in the Cu-Ni system. 

Fig. 18. Co-Cu-Fe. Calculated metastable miscibility gap in the liquid phase at several temperatures...

Fe—Cr. The Fe—Cr phase diagram. Fig. 3.1-106, is the prototype of the case of an iron-based system with an a-phase stabilizing component. Chromium is the most important alloying element of corrosion resistant, ferritic stainless steels and ferritic heat-resistant steels. If a-Fe—Cr alloys are quenched from above 1105 K and subsequently annealed, they decompose according to a metastable miscibility gap shown in Fig. 3.1-107. This decomposition reaction can cause severe embrittlement which is called 475 C-embrittlement in ferritic chromium steels. Embrittlement can also occur upon formation of the a phase. 

Partial miscibility Both miscible and immiscible polymer blends that also exhibit macroscopically uniform physical properties usually caused by sufficiently strong interactions between the components of the mixture. The partially miscible blends show either a homogeneous single phase or a heterogeneous phase, depending on certain conditions. This is called metastable miscibility. Sometimes, they may be only partially miscible yet may remain unchanged for an indefinite period. In some situations, however, phase separation of partially miscible systems may occur by activated nucleation. 

Figure 3.28 Metastable miscibility gap. In case that liquid-liquid demixing occurs only on supercooling, the miscibility gap is located below the liquidus curve. If the phase bounda of the miscibility gap is closely related to the metastable zone (here of A), the liquid-liquid phase separation may be favored before...

With regard to the latter, several examples have been reported where liquid-liquid phase separation was observed prior to crystallization [13]. A reasonable explanation is the existence of a metastable miscibility gap (located below the solubility curve of the solute) which is passed when nucleation starts. This situation is illustrated in Figure 3.28. Oiling-out, in particular when unexpected, that is, in metastable conditions, is undesired in industrial applications. The high supersaturation present at the onset of nucleation can lead to very small particles, subsequent agglomeration, low purity of the crystals obtained, and the formation of unwanted polymorphs. In consequence, the crystallization and subsequent downstream processes are difficult to control. 

Metastable miscibility gaps in (A) LijO-SiOj and (B) NajO-SiOj systems. Phase separation occurs below the solid curves the dashed curves are the spinodal boundaries. Between the dashed and solid curves, phase separation occurs by nucleation and growth of liquid droplets. Within thedashed curve, spinodal decomposition occurs. From Haller el al. [171. 

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