Alloy interface velocity

The phase distribution observed in the alloys deposited from AlCb-NaCl is very similar to that of Mn-Al alloys electrodeposited from the same chloroaluminate melt [126 129], Such similarity may also be found between the phase structure of Cr-Al and Mn-Al alloys produced by rapid solidification from the liquid [7, 124], These observations are coincident with the resemblance of the phase diagrams for Cr-Al and Mn-Al, which contain several intermetallic compounds with narrow compositional ranges [20], inhibition of the nucleation and growth of ordered, often low symmetry, intermetallic structures is commonly observed in non-equilibrium processing. Phase evolution is the result of a balance between the interface velocity and... 

Most of the work has been done in silicon, in a comparison [ 23 ] with metals one must note that usually interface velocities are higher due to the greater thermal conductivity. Supersaturated alloys have been obtained by ion implantation in combination with electron or laser beam pulsed heating. In some cases the coupling between thermal and matter transport, i.e. the Soret effect, has been evidenced [24] Precipitation in the liquid phase has been shown in Sb-implanted A1 system together with the measurement of submicrosecond nucleation times. Amorphous phase formation requires usually alloys. 

At intermediate stress intensity levels (stage 2) the crack propagation rate shows a plateau velocity Vpiateau that is virtually independent of the mechanical stress, but depends on the alloy/environment interface and the the rate-limiting environmental processes such as mass transport of the aggressive species to the crack tip. The plateau in a quenched and tempered low-alloy steel of 1700 MPa yield strength in deaerated water at 100°C... 

Example 18.1 Suppose a slab of 2024 aluminum alloy flying through the air at 1.8 km/s (5900 ft/s) strikes a slab of 304 stainless steel. What particle velocity would be generated in the two materials at the impact interface What shock pressure would be generated How fast would the shock be traveling into each material ... 

Examination of Eqs. (35) to (41) does not reveal any parameter that is obviously dependent on flow rate, provided that the applied voltage is maintained constant and vacancy condensation dominates the induction time. Thus, neither the thermodynamics of absorption of X into a surface oxygen vacancy nor the ejection of a cation from the film is expected to depend on flow velocity, nor are the events (e.g., vacancy condensation) that occur at the metal/film interface expected to be sensitive to fluid motion. Thus, the PDM predicts that the breakdown ( pitting ) potential for passive alloys that are of interest to the thermal power industry should not be sensitive to flow rate. The PDM also predicts that the induction time should be insensitive to fluid flow velocity, provided that the induction period is dominated by vacancy condensation at the metal/film interface. 

The classical linear stability theory for a planar interface was formulated in 1964 by Mullins and Sekerka. The theory predicts, under what growth conditions a binary alloy solidifying unidirectionally at constant velocity may become morphologically unstable. Its basic result is a dispersion relation for those perturbation wave lengths that are able to grow, rendering a planar interface unstable. Two approximations of the theory are of practical relevance for the present work. In the thermal steady state, which is approached at large ratios of thermal to solutal diffusivity, and for concentrations close to the onset of instability the characteristic equation of the problem... 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

First, we examine the behavior of the velocity of the front moving from Cq using the approximation (Equation 12.48) for shD. Lines 1 and 2 in Figure 12.10c show the dependence of the transformation front velocity taking into account the energy dissipation at the ao/a interface (only the term 4>pz(/ solj jjjjg 1, the sum of the terms dz(Az °, L ° ) and do(Az °, 1 ° ) - line 2). Here, the values of the velocity determined from the optimization procedure (i) are overestimated by an order for alloys with small supersaturation, (ii) coincide... 

In catalytic channels, the flat plate surface temperature in Eq. (3.32) is attained at the channel entry (x O). As the catalytic channel is not amenable to analytical solutions, simulations are provided next for the channel geometry shown in Fig. 3.3. A planar channel is considered in Fig. 3.3, with a length L = 75 mm, height 21) = 1.2 mm, and a wall thickness 5s = 50 pm. A 2D steady model for the gas and solid (described in Section 3.3) is used. The sohd thermal conductivity is k = 6W/m/K referring to FeCr alloy, a common material for catalytic honeycomb reactors in power generation (Carroni et al., 2003). Surface radiation heat transfer was accounted for, with an emissivity = 0.6 for each discretized catalytic surface element, while the inlet and outlet sections were treated as black bodies ( = 1.0). To illustrate differences between the surface temperatures of fuel-lean and fuel-rich hydrogen/air catalytic combustion, computed axial temperature profiles at the gas—wall interface y=h in Fig. 3.3) are shown in Fig. 3.4 for a lean (cp = 0.3) and a rich cp = 6.9) equivalence ratio, p = 1 bar, inlet temperature, and velocity Tj = 300 K and Uin = 10 m/s, respectively. The two selected equivalence ratios have the same adiabatic equilibrium temperature, T d=1189 K. 

Fig. 9. (from Ref. 21) Schematic illustration of a steady state reaction front (A) (T > 215 C) moving at velocity V to form crystalline ZrH2 and a crystalline Rh-alloy from crystalline Zr Rh with hydrogen. the minimm grain size required by the interface energy Bh-Zr H... 

A continuous increase in the solidification velocity of a liquid diluted alloy gives rise thus to an oscillating behaviour in the interface stability. It is interesting also to note that the width of the four regions depends upon dopant concentration. An... 

---