Alloys metastable

Microscopic theory of nucleation in metastable alloy states... 

Martensitic traasfonnation Master ec[uations Mean field crossover to Ising Mechanical properties Metallic alloys Metallic glasses Metastable alloys Microhardness test Microscopic theory of nucleation... 

Quenching (front the solid state). Metastable alloys have been very familiar to metallurgists for a long time now. Several alloys employed in everyday applications contain metastable phases. Typical examples are quenched steels and precipitation hardened aluminium alloys. Until the 1960s, metastable alloys were always obtained by quenching (rapid cooling) from the solid state. 

It should be emphasised that it is the rule rather than the exception for p to change markedly with crystal structure (Table 8.2). It is therefore unwise to assume that various metastable allotropes can be given the same value of P for the stable structure. In some cases values of p can be extrapolated from stable or metastable alloys with the requisite crystal structure, but in others this is not possible. A significant development is that it is now possible to include spin polarisation in electron energy calculations (Moruzzi and Marcus 1988, 1990a,b, Asada and Terakura 1995). This allows a calculation of the equilibrium value of to be made in any desired crystal structure. More importantly, such values are in good accord with known values for equilibrium phases (Table 8.2). It has also been shown that magnetic orbital contributions play a relatively minor role (Eriksson et al. 1990), so calculated values of P for metastable phases should be reasonably reliable. 

Pettifor showed how density functional theory (DFT, see above) may strengthen the CALPHAD conclusions for complex alloy systems by providing information on compositionally nearby metastable alloy phases that may affect alloy properties. 

Whereas the solubility of Cu in aluminum metal is ca. 5 wt% at temperatures in excess of 500°C, the solubility drops to ca. 0.1 wt% at room temperature. Hence, a metastable alloy is present when the high temperature alloy is rapidly quenched. Subsequent annealing will result in further strengthening similar to what we discussed for martensite. The strengthening effect is thought to occm due to the formation of Cu-rich discs (approx, diameter of 100 atoms, and thickness of ca. 4 atoms) that align themselves preferentially with selected planes of the host A1 lattice, causing coherency strains within the solid-state structure. 

Advances in electrochemical systems rest in large measure with the evolution of new materials that exhibit chemical stability in severe environments, high electrocatalytic activity, rapid ion conductivity, etc. Examples include RuOx-TiOy-Ti electrocatalysts, the polymer Nafion, yttrium-stabilized zirconate and beta-alumina electrolytes, and metastable alloys produced by rapid solidification processing. 

Metastable alloys, formed by rapid quenching, may be produced as metastable or nonequilibrium solids that are either glassy or microcrystalline. Because of the required cooling rates (106 K/sec)... 

Of particular interest are austenitic stainless steels, which are the most commonly used in cryogenic engineering. These steels are subdivided into structurally stable and metastable, depending on their nickel concentration. The metastable alloys have a martensitic phase transition at low temperatures, which significantly affects practically all their physical properties. The low-temperature mechanical properties of typical austenitic stainless steels are given in Table II. 

Kamasa P and Myslinski P (1999) Study of the thermal effects during microstmctural changes of metastable alloys by temperature modulated thermomagnetometry. Thermochimica Acta 337 51-54. 

Col] Colinet, C., Applications of the Cluster Variation Method to Empirical Phase Diagram Calculations , Calphad, 25(4), 607-623 (2001) (Assessment, Phase Relations, Review, 96) [2001Nas] Nascimento, V.P., Passamani, E.C., Takeuchi, A.Y., Larica, C., Nunes, E., Single Magnetic Domain Precipitates of Fe/Co and Fe and Co in Cu Matrix Produced from (Co-Fe)ZCu Metastable Alloys , J. Phys. Condens. Matter, 13(4), 665-682 (2001) (Crys. Structure, Experimental, Magn. Prop., Phase Relations, 17)... 

KOZ] Microhardness measurement Microhardness of metastable alloys with modulated structure... 

It is a process for adhering two multilayers, especially a substrate and deposited smface layer. The process involves bombarding layered samples with doses of ion radiation in order to promote mixing at the interface, and generally serves as a means of preparing electrical jrmctions, especially between non-equilibrium or metastable alloys and intermetallic compoimds. Ion implantation equipment can be used to achieve ion beam mixing. 

Ion Implantation. The introduction of foreign ions into solids, using accelerating energies in the range 20-500 keV. It is a non-equilibrium process, useful for preparing metastable alloys or for modifying suface properties such as friction, wear and corrosion. It can be used to modify superconductive behaviour. 

Instead, the transversal mode is quite possible in this case and must take place as a simultaneous decomposition of inhomogeneous metastable alloys within the concentration ranges (c ,( ) and (C , C ) on each side of the interface. Some aspects of the problem, regarding the shape of the nudeus, are considered in Refs [18, 19, 23]. The shape of the nucleus, being an important factor of nudeation... 

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