Melting transition zone

The fact that the appearance of a wall slip at sufficiently high shear rates is a property inwardly inherent in filled polymers or an external manifestation of these properties may be discussed, but obviously, the role of this effect during the flow of compositions with a disperse filler is great. The wall slip, beginning in the region of high shear rates, was marked many times as the effect that must be taken into account in the analysis of rheological properties of filled polymer melts [24, 25], and the appearance of a slip is initiated in the entry (transitional) zone of the channel [26]. It is quite possible that in reality not a true wall slip takes place, but the formation of a low-viscosity wall layer depleted of a filler. This is most characteristic for the systems with low-viscosity binders. From the point of view of hydrodynamics, an exact mechanism of motion of a material near the wall is immaterial, since in any case it appears as a wall slip. 

As the polymer granules enter the melting zone (which we also refer to as the transition zone), their temperature rises and they begin to melt, The heat required to melt the polymer comes from two sources, external heaters and mechanical work from the action of the screw. When we extrude viscous polymers at high rates we may need to cool the barrel to remove some of the heat induced by working the molten polymer. 

It is well know that melting occurs more rapidly if there is a positive pressure at the end of the transition zone. If Film B is considered to be large and if the pressure in the melt Film C is essentially zero at the trailing flight, a first order approximation of this pressure effect can be achieved by adding a pressure dissipation term to the rate of material loss in the y direction of the solid surface adjacent to the Film C ... 

The melting zone. The melting or transition zone is the portion of the extruder were the material melts. The length of this zone is a function of material properties, screw geometry, and processing conditions. During melting, the size of the solid bed shrinks as a melt pool forms at its side, as depicted in Fig. 3.10 which shows the polymer unwrapped from the screw channel. 

Taking, for instance, Al, with a melting point of 660 °C and a web substrate temperature of 50 °C, zone I formations will be created (porous structure, pointed crystallites, large voids) and up to 250 °C, formations in the transitional area (densely packed fibers) will appear. Up to 450 °C zone II (pillar-shaped crystallites), and above this temperature zone III (conglomerate-type crystallites) formations will be seen. Because of the relatively low maximum thermal stress that may be applied to polymer webs, the growth in metallized layers on polymer webs mainly occurs in Zone I or in the transitional zone. The different growth is also evident from comparison of cooling drum and free-span coater methods. 

Two intermetallic layers were found to occur at the interface between the stainless steel and the saturated aluminium melt. A typical micrograph of the steel-aluminium transition zone is shown in Fig. 2.9. The layer (Layer I) adjacent to the steel base was compact, while that (Layer II) adjacent to the melt material was somewhat porous. The thickness of Layer II was a few times greater than that of Layer I. 

Fig. 2.9. Microstructure of the stainless steel-aluminium transition zone.197 Temperature 700°C, dipping time 3000 s, melt A1 + 2.5 mass % Fe and corresponding amounts of other elements from the steel. Microhardness indentations were made at a load of 0.196 N (20 g). 

Fig. 5.13. A micrograph of the transition zone between molybdenum and Mo-saturated liquid aluminium.309 Temperature 750°C, dipping time 1800 s. Inclusions in the aluminium matrix are crystals of MoA14 formed during cooling the aluminium melt.

Note that in the case of the aluminium melt saturated with molybdenum the thickness of the MoA14 layer at t = 300 s is 14><10 6 m. In welding dissimilar metals, the maximum permissible thickness of an intermetallic layer is known to be 2-5 pm. Thus, the dissolution conditions in question obviously ensure the formation of the Mo-Al transition zone with a MoAl 4 layer thickness not exceeding this value. 

Typical microstructures of the alloy-aluminium transition zones of the Al-(Fe+Ni) specimens obtained under the same conditions (temperature 700°C, dipping time 3600 sec, saturated aluminium melts) are shown in Fig. 5.16. The intermetallic layer, 80 10 pm thick, grown between a 90 mass % Fe-10 mass % Ni alloy and the aluminium melt saturated with the alloy constituents, seems to be one-phase (Fig. 5.16a). 

Two intermetallic layers are observed in the transition zone between a 50 mass % Fe-50 mass % Ni alloy and the aluminium melt saturated with the alloy constituents (Fig. 5.16c). The layer adjacent to the alloy base is only about 8 pm thick and consists of the Fe2Al7 compound. The much thicker layer bordering with the aluminium matrix tends to destroy. The ternary FeNiAl9 compound is dominant in this layer. Its microhardness is 6.2 GPa. The microhardness of the alloy base is 2.1 GPa and that of the aluminium matrix is 0.7 GPa. 

Fig. 5.17. Optical micrographs of the transition zone between a 25 mass % Fe-75 mass % Ni alloy and aluminium. Temperature 700°C, dipping time 900 s. Initial liquid phase (a) aluminium melt saturated with the alloy constituents, (b) pure aluminium (go = 24.0 rad sec ).

DTA experiments indicated a second transition zone 0 + a occurs between 1710 10 °C (X-phase) and 1575 15 °C (Y-phase). Finally a continuous liquidus-solidus relation characterizes the high-temperature a-solid solution series with melting temperatures of 1770 10 °C (X-phase) and 1700 35 °C (Y-phase). Melting experiments performed with various X—Y phase mixed crystals resulted in inhomogeneous quenched products. Since a small vapor loss could hardly be prevented, some exsolved molybdenum metal was always observed. 

In another example, Richard et al. (2002) simulated the transport of water in a two-dimensional mantle convection model. They found that mantle flow, not diffusion, was the primary control on water distribution, which led to a homogeneous distribution of water in the mantle. If this is the case, the transition zone may contain less water than could be dissolved into the nominally anhydrous phases present there. Because of the low solubility of water in lower-mantle nominally anhydrous phases (Bolfan-Casanova et al., 2000), Richard et al. proposed that there might be a water-rich fluid phase in the lower mantle. They did not, however, consider the possibility of water-induced partial melting, leading to a melt rather than a fluid. 

Herzberg C. and Zhang J. (1996) Melting experiments on anhydrous peridotite KLB-1 compositions of magmas in the upper mantle and transition zone. J. Geophys. Res. 101, 8271-8295. 

The investigations [59] reveal that the two-phase region may be conventionally divided into two zones solid-liquid and liquid-solid. The continuous front of solidification is the border between these zones. The analysis of the temperature within the liquid bath (see Figure 15) shows that the ultrasonic treatment and overheating of a melt narrow the transition zone from the liquid side, i.e. in the liquid-solid region. 

Temperature dependence of dynamics 8.7.2 Transition zone of polymer melts... 

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