Metastable phases

It is frequently observed that precursor and metastable phase form before the formation of stable phase. For example, Schoonen and Barnes (1991) experimentally clarified that precursor, FeS , is requisite for the formations of marcasite and pyrite (FeS2). The changes from precursor through metastable phase to stable phase are commonly recognized. This process is called Ostwald ripening. Therefore, precipitation kinetics of precursor and metastable phase has to be elucidated. 

Nielsen and Toft (1984) have experimentally obtained the stable field of precurser of calcite (CaC03-6H20) on PA (PA = —log A) — PB (PB = —log B) diagram (Fig. 3.5). Degree of supersaturation for the solution from which the precursor forms is very high. 

Although FeS formed by the experiments of Schoonen and Barnes (1991), its stability field is not determined. 

Grain size of precursor and metastable phase is generally very small. Grain size (r ) relates to surface energy (o) and degree of supersaturatirMi (m/nieq) as follows (Steefel and CappeUen 1990). 

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A homogeneous metastable phase is always stable with respect to the fonnation of infinitesimal droplets, provided the surface tension a is positive. Between this extreme and the other thennodynamic equilibrium state, which is inhomogeneous and consists of two coexisting phases, a critical size droplet state exists, which is in unstable equilibrium. In the classical theory, one makes the capillarity approxunation the critical droplet is assumed homogeneous up to the boundary separating it from the metastable background and is assumed to be the same as the new phase in the bulk. Then the work of fonnation W R) of such a droplet of arbitrary radius R is the sum of the... 

Since zeolites are metastable crystallization products tliey are subject to Ostwald s mle which states tliat metastable phases are initially foniied and gradually transfonii into tlie tlieniiodynaniically most stable product. The least stable zeolitic phase (tliat witli tlie lowest framework density) is tlierefore foniied first and consumed with furtlier syntliesis time at tlie expense of a more stable phase due to a continuous crystallization/redissolution equilibrium. 

Fig. 9. The MgSO H20 system where the dashed lines represent metastable phases.

Zeohtes are formed under hydrothermal conditions, defined here in a broad sense to include 2eoHte crystalli2ation from aqueous systems containing various types of reactants. Most synthetic 2eoHtes are produced under nonequilihrium conditions, and must be considered as metastable phases in a thermodynamic sense. 

Al—Li [12042-37-4] 5. The nature of the phase relationships involving 5 has been the subject of much discussion. Portions of the metastable phase boundaries have not yet been agreed upon. 

The phases present in products can differ from those predicted from equilibrium diagrams. Nonequilibrium metastable phases form at solidification rates experienced in commercial ingots. Because of the low rate of diffusion of iron in alurninum, equilibrium conditions can only be established by long heat treatments and are very slowly approached at temperatures below about 550 °C. Small additions of other elements, particularly manganese, can also modify the phase relations. 

Precipitation Heat Treatment. The supersaturated solution produced by the quench from the solution temperature is unstable, and the alloys tend to approach equiUbrium by precipitation of solute. Because the activation energies required to form equiUbrium precipitate phases are higher than those to form metastable phases, the soHd solution decomposes to form G-P zones at room temperature (natural aging). Metastable precursors to the equihbrium phases are formed at the temperatures employed for commercial precipitation heat treatments (artificial aging). 

Under equiUbrium vapor pressure of water, the crystalline tfihydroxides, Al(OH)2 convert to oxide—hydroxides at above 100°C (9,10). Below 280°—300°C, boehmite is the prevailing phase, unless diaspore seed is present. Although spontaneous nucleation of diaspore requires temperatures in excess of 300 °C and 20 MPa (200 bar) pressure, growth on seed crystals occurs at temperatures as low as 180 °C. For this reason it has been suggested that boehmite is the metastable phase although its formation is kinetically favored at lower temperatures and pressures. The ultimate conversion of the hydroxides to comndum [1302-74-5] AI2O2, the final oxide form, occurs above 360°C and 20 MPa. 

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

Tridymite. Tridymite is reported to be the siUca form stable from 870—1470°C at atmospheric pressure (44). Owing to the sluggishness of the reconstmctive tridymite—quart2 conversion, which requites minerali2ers such as sodium tungstate, alkah metal oxide, or the action of water under pressure, tridymite may persist as a metastable phase below 870°C. It occurs in volcanic rocks and stony meteorites. 

Fig. 5. Metastable Fe—Ni—Cr "temary"-pliase diagram where C content is 0.1 wt % and for alloys cooled rapidly from 1000°C showing the locations of austenitic, duplex, ferritic, and martensitic stainless steels with respect to the metastable-phase boundaries. For carbon contents higher than 0.1 wt %, martensite lines occur at lower ahoy contents (43). A is duplex stainless steel, eg. Type 329, 327 B, ferritic stainless steels, eg. Type 446 C, 5 ferrite + martensite D, martensitic stainless steels, eg. Type 410 E, ferrite + martensite F, ferrite + pearlite G, high nickel ahoys, eg, ahoy 800 H,...

The interpretation of metastable phases in terms of Gibbsian thermodynamics is set out simply in a paper by van den Broek and Dirks (1987). 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

FIG. 16 Negative normal stress T.. as a funetion of substrate separation from grand eanonial ensemble Monte Carlo simulations at T = 1.00, ii = —11.0, and = 0.0 ( )(Pbuik = 0.486) the solid line represents a eubie spline fit to the dis-erete data to guide the eye. Also shown are three isobars —T% = 0.000, 0.598, and 1.196, indieated by horizontal lines. Interseetions between the isobars and the eurve Tzzi z) correspond to stable and metastable phases of the eonfined fluid (see text) (from Ref. 66). 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

Figure 1 Disorder-Llo-Ll2 phase diagram [9]. The broken line indicates the (100) Spin-odal ordering locus and dotted lines e metastable phase boundaries. The temperature axis is normalized with respect to the neaiest neighbor pair interaction energy. The ordering transition temperatures of L q and LI2 phases are 1.89 and 1.92, respectively.

CuZn. This noble-metal alloy has a very rich phase diagram Many different metastable phases can be found depending on method of measuring and history of... 

R. Yang, S.V. Parker, J.A. Leake, R.W. Cahn, New metastable phases in nickel -rich Ni-Al-Ti alloys, in Alloy Modelling and Design, ed.G.M.Stocks and P.A. Turchi, The Minerals, Metals Materials Society (1994), p.303. 

The a — 0 transformation has a large hysteresis in hydrogenated titanium alloys, and different thermal treatments change their phase content. Various degrees of metastability due to hysteresis are implicit for the alloys after different thermal treatments. Metastable phases undergo transformation to a more equilibrium state during deformation, which can effect the flow of the alloy. Below we consider the effect of the thermal pre-strain treatment on ductility on the strength of the Ti-6A1-2Zr-1.5V-lMo-rH alloys. ... 

Preliminary pre-strain thermal treatment of hydrogenated alloys increases content of the metastable phases and thus markedly increases the alloy ductility. 

The fact that oxides can exist as metastable phases is illustrated by the Ni-HjO diagram (Fig. 1.18) in which the curves for the various oxides of nickel have been extrapolated into the acid region of Ni stability, and this diagram emphasises the fact that nickel can be passivated outside the region of thermodynamic stability of the oxides". 

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