Columnar grains

Why is it undesirable to have a columnar grain structure in castings Why is a fine equiaxed grain structure the most desirable option What factors determine the extent to which the grain structure is columnar or equiaxed ... 

Many of these problems can be solved by using continuous casting (Fig. 14.3). Contraction cavities do not form because the mould is continuously topped up with liquid metal. Segregation is reduced because the columnar grains grow over smaller distances. And, because the product has a small cross-section, little work is needed to roll it to a finished section. 

The structure of a CVD material can be classified into three major types which are shown schematically in Fig. 2.13. In Zone (A), the structure consists of columnar grains which are capped by a domelike top. In Zone (B), the structure is also columnar but more faceted and angular. In Zone (C), it consists of fine accost grains. Examples of these structures are shown in Fig. 2.14. " This is the CVD equivalent of the structural model for vacuum-evaporated films first introduced by Movchan and Demshishin.P l... 

Figure 2.13. Schematic of stractures obtained by CVD (a) columnar grains with domed tops, (b) faceted columnar grains, (c) equiaxed fine grains.

At higher temperatures, deposits tend to be columnar (Fig. 2.13 a and b) as a result of uninterrupted grain growth toward the reactant source. The structure is also often dependent on the thickness of the deposit. For instance, grain size will increase as the thickness increases. A columnar-grain structure develops, which becomes more pronounced as the film becomes thicker. 

Magnicol [Magnetic columnar] A process for making Alnico (an iron-based magnetic alloy containing Al, Ni, Co, and Cu) crystallize with a columnar grain structure in order to optimize its magnetic properties. Successive additions of silicon, carbon, and sulfur are made to the initial melt. 

Ni-YSZ cermets deposited by RF sputtering (230 nm) were found to have micro-structural features consisting of columnar grains 13 to 75 nm long and 9 to 22 nm wide, and showed good adhesion to the YSZ layer on which they were deposited [128], In a three-layer Ni-YSZ-Ni film deposited on NiO by RF sputtering in another study, the YSZ layer exhibited a columnar structure with some pinholes [129], Microstructural and electrochemical features of Pt electrodes patterned by lithography on YSZ have also been studied [130,131]. 

Here, d is the average grain size of the columnar grains, JB is the diffusional flux along the grain boundaries, dcB/dx = [cs(0) —cB(l)] /l, where cs(0) and cB(l) are the diffusant concentrations in the boundaries at the source surface and accumulation surface, respectively, and l is the specimen thickness. In the early stages, cB(l) 0 and, therefore, to a good approximation,... 

Fig. 17. Structural diagram (51) for sputtered layers. Zone 1 is a porous structure consisting of tapered crystallites separated by voids. Zone 2 shows columnar grains, and Zone 3 has a recrystallized grain structure. Zone T is a transition structure consisting of densely packed fibrous grains.

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