Interlocking grains

Finally both spiral and interlocked grain present diffieulties in finishing. The grain runs out of the surface in opposite directions on either faee of the board (spiral... 

FIGURE 1.9 Microstructures of tungsten wires (A) fibrous structure of the as-drawn tungsten wire, (B) equiaxed grain structure of recrystallized pure wire, (C) interlocking grain structure of recrystallized NS-doped wire. By courtesy of Philips Lighting B.V, Eindhoven, The Netherlands. 

Before a mass of tightly packed particles can flow, it must increase in volume to permit interlocking grains to move past one another. Without such dilation flow is not possible. 

The ultimate microstructure of a solid will depend on how quickly different crystal faces develop. This controls the overall shape of the crystallites, which may be needle-like, blocky or one of many other shapes. The shapes will also be subject to the constraint of other nearby crystals. The product will be a solid consisting of a set of interlocking grains. The size distribution of the crystallites will reflect the rate of cooling of the solid. Liquid in contact with the cold outer wall of a mould may cool quickly and give rise to many small crystals. Liquid within the centre of the sample may crystallise slowly and produce large crystals. Finally, it is important to mention that the microstructure will depend sensitively on impurities present. This aspect is discussed in Chapters 4 and 8. 

Figure 77. Interlocking grain boundaries bridged by WB2 particles. Note the thin TiWB2 stacks in the host crystals.

See types of gain, i.e., cross grain, interlocked grain, coarse grain, close train, diagonal grain, and dip grain. 

Interlocked grain n. Wood in which the fibers incline in one direction in a number of rings of annual growth, then gradually reverse and incline in an opposite direction in succeeding rings and then reverse again. 

It is well known that crack propagation properties are related to bridging effect and this effect is more pronounced in coarse-grained materials [24 27]. Coarse-grain alumina shows an R-curve behavior which is characterized by an increase in crack resistance with crack extension, due to crack surface interaction in the crack wake [28 29]. This R-curve behavior is due to the stresses required to overcome the traction of interlocking grains and pullout of unbroken ligaments in the wake of the crack. 

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