Columnar grain structure

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 ... 

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. 

The microstructure of the tube taken in a direction normal to the growth direction is shown in Figure 6.24b. The multi-layer columnar grain structure in this figure is the result of cyclically interrupted deposition because of P-CVD. The deposition rate is calculated to be 30 pm-h 1. This is well below the deposition rate of 125 pm-h 1 normally achieved with the same parameters of temperature and... 

The indentation confidence depth was determined to be a function of the coating to substrate yield strength ratio for three different substrate configurations. Tilbrook et al. (2007) incorporated microstructural failure mechanisms into finite element simulations of nanoindentation of TiN thin films on elastic—plastic substrates. Intergranular sliding that occurs as a result of the columnar grain structure is incorporated into the model via anisotropic property definitions and nodal coupling. 

For columnar grain structure, the value of d t q)ically falls within the range 0.1 < d/hj < 10. Approximate solutions of the differential equation for stress history have been determined by numerical integration with initial stress (To = 2 X 10 N/m, film thickness of hf = 1/xm and grain sizes of d = 0.1 nm, 1/im and lOyum. The temperature history is again given by... 

Figure 8.34 (a) Polycrystalline turbine blade that was produced by a conventional casting technique. High-temperature creep resistance is unproved as a result of an oriented columnar grain structure (b) produced by a sophisticated directional soUdification technique. Creep resistance is further enhanced when single-crystal blades (c) are used. 

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