Kinetic Competition in Undercooled Liquid Alloys
- PDF / 1,851,272 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 15 Downloads / 199 Views
for the synthesis of new materials and the attainment of new levels of material performance. Although a given microstructure may be synthesized through a variety of seemingly different processes routes, there is a common underlying thermodynamic criterion that is always applicable. Through the various states of a processing operation the initial state of a reacting material must be energized to an elevated free energy configuration and then retained in a kinetically arrested configuration. A metastable phase can be in internal equilibrium, but the free energy is above that of the same volume of material in the stable equilibrium state. The development of a stable phase is inhibited by kinetic barrier due to the creation of an interface between the stable and metastable phase. In the case of solidification operations, the energized state is achieved by undercooling the liquid below the equilibrium freezing temperature. All methods of rapid solidification permit access to the undercooled liquid [I]. Development of Undercooling Solidification of a large continuous bulk liquid metal will likely be catalyzed by the most potent nucleation site present so that a means of circumventing the effect of catalytic sites such as oxides or container walls is necessary in order to observe extensive undercooling. An effective approach to obtain large undercoolings is to disperse the bulk liquid into a collection of fine droplets as illustrated in Fig. 1a. As a stabilized liquid droplet emulsion is cooled those drops containing potent nucleants will freeze at low undercoolings, but the majority will
Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
not freeze until reaching a maximum nucleation undercooling level which can range from 0.30.4 Tm [2,3]. During the rapid freezing of melt streams, as in melt spinning as shown in Fig. lb, undercooled conditions will develop if the liquid exists for some time or distance in contact with the wheel without nucleation. This will develop if the thermal front associated with heat extraction moves more rapidly than the actual crystallization front. Alternately, during surface melting with a rapidly moving or pulsed heat source, an undercooled melt zone can develop in front of the resolidification interface if the rate of growth of the unmelted substrate is insufficient to keep pace with the rate of heat extraction to the substrate. The development of metastable crystalline phases, as well as amorphous products, during both melt spinning and surface melting operations is clear evidence for the existence of an undercooled alloy melt, [4]. Competitive Phase Selection Kinetics Some further characteristics related to the development of a highly undercooled solidification microstructure in terms of stable equilibrium phases or metastable phases depends on the relative nucleation and growth kinetics of the competing structures that are illustrated schematically in Fig. 2 [1]. The thermodynamic relationships for the molar free are given in Fig. 2a. energy G of a material as a liqui
Data Loading...
