Cellular precipitation
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the cellular precipitation a supersaturated phase, a ' , decomposes Into a new phase, 13, and a structurally Identical phase, a, that is partially depleted of its t h e r modynamic excess of solute by the growth of colonies or cells of the ~ + 13 mixture into the ~ ' phase as lllust.rated In Fig. 1. These cells a r e composed, most c o m monly, of parallel plates of alternating a and/3 phases lying parallel to the growth direction. The p r o c e s s is also called "discontinuous" or " a u t o c a t a l y t i c " precipitation. It is difficult to distinguish metallographically or even ktnetlcally from cooperative eutectoid decomposition, e . g . , pearlite growth in steels. Both proc e s s e s are characterized by a fairly constant interlamellar spacing (if c a r r i e d out isothermally), the growth velocities show typical " C - c u r v e " dependence on temperature, growth occurs in the form of colonies that originate on grain boundaries of the parent phase and grow radially, and a well-defined interface (grain boundary) separates the ~ and ~' phases. Although not as common as "continuous" precipitation, cellular p r e cipitation phenomena are not at all r a r e and have been studied In numerous alloy systems including some of considerable c o m m e r c i a l importance. As with all p r e cipitation p r o c e s s e s , the effect of cellular precipitation on mechanical, physical, and chemical properties is usually v e r y marked. There is abundant evident t-9 that the n e c e s s a r y mass transport for the growth p r o c e s s in cellular precipitation occurs by tnterfactel diffusion in the or-or' grain boundary and in the advancing ot'-~ interface. What work there is that suggests a volume diffusion mechanism 19'x~ can probably be reanalyzed tn a more thorough fashion to show that interface diffusion is the dominant mechanism. ~2,xs The origin of the cellular mode of precipitation has been puzzled over for some time. Experimentally, it is observed that cellular precipitation always starts out at a grain boundary; that is, starts out at isolated points on this boundary with new/3 precipitates f o r m Ing near previously formed 13 plates, forming a " c e l l , " BRUCE E. SUNDQUIST, formerly with the U.S. Steel Corp., E. C. Bain Laboratory for Fundamental Research, is now with Westinghouse Advanced Reactor Development, Madison, Pa. 15663. Manuscript submitted December 18, 1972. METALLURGICAL TRANSACTIONS
that each cell advances into only one of the adjacent grains; that the c~ lamella of the cell have the crystal orientation of the other grain, and that the interlamellar spacing is fairly constant for isothermal growth (varying by no more than a factor of 2 or 3). Most of these experimental observations have been explained v e r y well by the work of Turnbull and Tu. x4'15 The e s sential features of their explanation of the origin of cellular precipitation are given in Fig. 2. The f i r s t step in the p r o c e s s is the nucleation of an isolated precipitate on the grain boundary (Fig. 2(a)). Because of th
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