![]() One could claim that it was the development of directional dendritic growth of superalloys that allows today's cost-effective, safe, efficient, high-speed air travel. Giamei, both research metallurgists at Pratt & Whitney Aircraft Corporation. The process of directional dendritic growth was developed in the 1960s by F. The dendrites maintain strict crystallographic integrity and high structural perfection throughout the entire crystal, even after many meters of continuous ribbon growth.Ī commercially successful example of controlled dendritic crystal growth is manufacturing near-net-shape superalloy single crystals for jet engines-currently a multi-billion dollar world-wide technology for aircraft engines and their refurbishment. Web silicon crystal growth employs pairs of widely-spaced growing dendrites, between which is stretched a thin film of molten Si that is withdrawn from the bulk melt and steadily crystallized as a continuous single crystal ribbon or sheet. Dendritic ribbons were grown for the commercial purpose of producing high-efficiency, inexpensive solar cells. Known as “web-silicon,” this crystal growth process produced electronic-grade single-crystal ribbons of Si directly from the melt. Seidensticker and his associates at Westinghouse Semiconductor Division. On the positive side, for example, dendrites proved essential in an industrial crystal growth process developed in the early to mid-1960s by R. Such practical considerations dictate the need and added expense for downstream processing to reduce chemical segregation, lower residual stresses, and improve as-cast mechanical properties to meet engineering requirements. Dendrites also determine a material's as-cast solidification texture, porosity, and grain size, all of which collectively influence the mechanical and chemical properties of cast materials. For example, in casting metallurgy, dendrites determine the difficulty of melt flow to “feed” remote areas within a solidifying casting, and often dictate the distance scales over which chemical microsegregation occurs as well as the time-scales to eradicate chemical segregation by annealing. In the industrial important sectors of primary metals production, alloy casting, and welding, dendritic growth modes during solidification are unavoidable, and their potentially negative impact on the properties of cast and fusion-welded materials must be dealt with by foundry and welding engineers. As mentioned above, avoiding cellular breakdown and dendritic growth at a growing crystal interface is far more preferable than attempting amelioration of their detrimental effects on single crystal quality and performance by resorting to post-growth processing. The “cost” of avoiding dendrites and other forms of interfacial instability accompanying bulk crystal growth involves either reducing rates of crystal growth-i.e., diminished productivity-and/or accepting the application of higher, more technically difficult, and potentially defect-inducing, temperature gradients. This can be accomplished in practice by adopting specific precautions and strategies to be discussed later in this chapter. The challenge to contemporary crystal growers is the necessity to avoid, at acceptable cost, the appearance during crystal growth of dendrites, or the onset of other unstable growth forms, such as interfacial cells that can cause similar problems. Such deleterious aspects during bulk crystal growth degrade chemical and structural uniformity, and are harmful in crystals for which inhomogeneities in chemical, electrical, magnetic, or optical properties would diminish their quality and possibly compromise device performance. It is their geometrically complicated, indeed, nearly fractal character, that is responsible for the onset of chemical microsegregation, sub-boundary defects, non-equilibrium phase distributions, void formation, and inclusions in dendritically ‘contaminated’ single crystals. The appearance of dendrites in these important products must be avoided wherever possible. Adapted from Ref.Ĭomplex dendritic morphologies, in fact, are clearly undesirable in most crystal growth processes, especially those developed to produce nearly perfect, homogeneous, stress-free, bulk crystals. Steady-state crystallization fronts observed in a dilute organic alloy, showing three major interfacial morphologies as the growth speed normal to the interface is increased step-wise from slowest, panel (A) Plane front (B) Periodically rippled (C) Deep cells to fastest, panel (D) Aligned three-dimensional dendrites. ![]()
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