This geometry is illustrated in Fig

This geometry is illustrated in Figure 5.8(b) for three of the six edges. Each new atomic layer initiated at the 141° re-entrant corner( type I) results in the growth of a layer that covers the two adjacent {111} planes as shown in Figure5.8(c). When these layers reach a twin plane at a 219° ridge , a 109-5° re-entrant corner (type II) is created. This is an equally attractive site for atomic additions and growth from these corners preserves the original re-entrant system and permits continuous, unlimited propagation in the direction of the original 141° re-entrant grooves. Thus the TPRE system allows growth to proceed preferentially in any or all of the six < 211 > directions depending on the heat-flow direction. This change in growth mechanism in region C defines a new eutectic morphology. The TPRE mechanism allows the system to maintain the co-operative coupled growth front characteristics of region Β, at least at the lowest velocities. As the growth velocity increases, growth at the twinned Si interface becomes too slow to maintain the coupled growth front and the Si will grow at a lower temperature in an attempt to maintain coupled growth. This, in turn, leads to overgrowth of the Si phase by the non-faceting phase. Complete occlusion is avoided because the undercooling is sufficient for the Si to respond with bursts of twin formation that allows the Si to branch at angles to the parent flake and so maintain the Si phase continuity. This explains how an apparently random orientation is compatible with a continuous aggregate of Si. The liquid-solid interface is non-isothermal and its profile fluctuates both laterally and in the local growth direction. This means that the Si phase diverges and converges as the flakes branch and terminate. This situation is a consequence of the growth anisotropy of the Si phase, which is restrictive, not only in the local growth direction, but also prevents spontaneous lateral adjustments of the Si phase that would allow optimization of the diffusion conditions. Eutectic growth in region C occurs with a fluctuating diffusion process that is inefficient and therefore requires a large driving force. Simple arguments have been used 7 to show that approximately 9 5 % of the interface undercooling is used to drive the diffusion process. It is not surprising that larger undercoolings and spacings have been recorded for this structure than for lamellar structures grown under similar conditions. As discussed in the previous chapter, the temperature gradient in the liquid at the interface must influence this type of interface process and, as Figures 5.5 and 5.9 show , the spacing and undercooling depend on the temperature gradient . The flake structure is the irregular anomalous structure defined by region D in Croker ' s classification and described on page 133. The growth conditions in region C in Figure 5.1 are those encountered in normal foundry practice a nd the flake structure is the typical cast structure of unmodified A l casting alloys based on the A l - S i system.