Microstructure metastable

The microstructure and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce structures 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 microstructure, metastable phases, incorporated impurities, and macro-and microporosity. All of these may affect the physical, chemical, and mechanical properties of the coating. 

Primary and secondary alpha generally are unaffected by aging, unless their alumintxm concentration exceeds the solubility limit. In the latter case, tt2-pre-dpitates may develop upon aging. The martensites readily decompose at aging temperatures into more stable a or a + p microstructures. Metastable beta alloys also form an unstable TVpe-1 a predpitate that decomposes into a Type-2 a during aging. 

The crystallization of glassy Pd-Ni-P and Pd-Cu-P alloys is complicated by the formation of metastable crystalline phaf s [26]. The final (stable) crystallization product consists of a mixture of a (Pd,Ni) or (Pd,Cu) fee solid solution and more than one kind of metal phosphide of low crystallographic symmetry. Donovan et al. [27] used transmission electron microscopy (TEM) and X-ray microanalysis to study the microstructure of slowly cooled crystalline Pd4oNi4oP2o- They identified the compositions of the metal phosphides to be Pd34Ni45P2j and Pdg8Ni[4Pjg. 

The use of chemically assembled multiclusters as precursors to solids with metastable porous microstructures constitutes a new approach to the preparation of heterogeneous hydrogenation... 

Figure 5.29. Fe-rich region of the Fe C phase diagram. Stable Fe-C (graphite) diagram solid lines metastable Fe-Fe3C diagram dashed lines. The following current names are used ferrite (solid solution in aFe), austenite (solid solution in 7Fe) and cementite (Fe3C compound). Pearlite is the name given to the two-phase microstructure which originates from the eutectoid reaction ...

The aim of this chapter is to introduce the reader to some of the ways in which the CALPHAD approach has been combined with kinetics to predict the formation of phases and/or microstructures under conditions which are not considered to be in equilibrium. Broadly speaking, the combination of thermodynamics and kinetics can be broken down into at least two separate approaches (1) the calculation of metastable equilibria and (2) the direct coupling of thermodynamic and kinetic modelling. 

Microstructures in cast irons are also dramatically influenced by cooling rates. If cooling is rapid, no graphite precipitates. Rather, the alloy solidifies in the metastable Fe-Fe3C state. In that state, the carbon is combined with iron as iron carbides. The fractured surface of carbidic cast iron is white. Such irons are hard and are not readily machined. Carbidic iron castings are used for some special applications, when abrasion resistance is important. 

In this paper, a few examples of Raney catalysts produced by metastable processes and their catalytic properties are discussed. Then, some examples of multi alloy systems, their microstructures and general properties will be shown. Finally, we will discuss the possibility of forming large particle materials with high specific surface area. 

Figure 4.4 Microstructure of a V-N diffusion couple illustrating the general appearance of the phase band structure. Top right corner precipitation of metastable V(N) phase(s).

Alternative processing methods also offer the potential to control the microstructure and final properties of nanocomposites. Both self-propagating high-temperature sintering and spark plasma sintering offer means to obtain metastable yet dense nanocomposites. Subsequent heat treatments can then be used to approach equilibrium microstructures, where the properties will be a function of the heat treatment temperature and time. In this way a variety of microstructures, and thus variations of the composite properties, can become available. 

It is instructive to consider the free-energy hierarchy and the metastable phase equilibria when crystallization of an amorphous material is discussed. Koster and Herold [56] discussed these aspects of crystallization and showed that crystallization reactions of amorphous alloys can be classified into the following three types polymorphic, primary and eutectic crystallization reactions. Among these three types, the slowest crystal growth process is expected for primary crystallization and thus, primary crystallization is ideal for tailoring fine microstructures upon decomposition of amorphous alloys. 

Other metastable centers appear to be associated with microstructural imperfections since light-induced effects are influenced by annealing treatments (Staebler and Pankove, 1980) and by variations in deposition conditions (Carlson, 1982b Hirabayashi et al., 1982). For a more thorough discussion of metastable effects see Chapter 11 by Schade of Volume 2 IB. 

With nonadsorbing polymer, rheological effects of similar magnitude accompany the phase transitions described earlier (Patel and Russel, 1989a,b). Since macroscopic phase separation takes weeks or months, rheological measurements performed within a few days on samples formulated within the two-phase region, with — Q>miJkT 2 - 20, detect a metastable structure that changes little over time. The systems respond as flocculated dispersions, but the microstructure recovers relatively quickly to a reproducible rest state after shear. Hence these weakly flocculated dispersions are quite tractable materials. 

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