Phase relations and diffusion layer formation in the systems Cu-Nb-Sn and Cu-Nb-Ge
- PDF / 2,657,244 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 84 Downloads / 214 Views
re the phases that develop d u r i n g the diffusion process can be understood in t e r m s of the phase relations in the appropriate t h r e e component system, Cu-Nb-Sn. The intimate connection between ternary phase equilibria and diffusion layer formation was elegantly demonstrated by C l a r k and Rhines.S We believe the concepts they developed a r e critical to understanding compound formation by the bronze technique. Consider Fig. 1, part of an isotherm in the hypothetical ternary system C-D-E which contains t h r e e stable binary solid solution phases, ~, 6, and E. A diffusion couple in such a system can be represented by any two t e r m i n a l compositions, for example, D and Co, w h e r e the latter composition lies within the solid solution field. A sequence of phases that might be observed in the couple following diffusion is ~-E-5. This phase sequence represents a composition or diffusion path on the isotherm as shown by the dashed line on the d i a g r a m . The p a t h can be defined quantitatively from the elemental profiles in the diffusion zone, as shown in Fig. l(b). The importance of the diffusion path is that it provides an immediate picture of the phases and phase compositions formed from any particular diffusion couple g i v e n an appropriate length of reaction time.S Only when the diffusion r a t e s of the t h r e e components in the ternary system are equal is the diffusion path a straight line connecting the t e r m i n a l compositions through the system, s,6 In the more g e n e r a l case the diffusion path c r o s s e s the system by a more irr e g u l a r route through the various phase fields present in the d i a g r a m . Frequently the diffusion path closely follows two phase tielines and a knowledge of the orientation of such tielines, a l o n g with the relative diffusion r a t e s of the components, allows predictions as t o the g e n e r a l route the path will follow. An important point is that diffusion paths are apparently unique, given the temperature and t e r m i n a l compositions, and it is thus possible t o form any phase in the ternary system if the appropriate t e r m i n a l compositions of the couple are chosen,s Clearly to do this r e q u i r e s a VOLUME 8A, JANUARY 1977-91
3.2. Diffusion Studies ~0) Comoosilion Profile for EJemeni E in Diffusion Couple C o / D
CEy
Co
cYE
I
C6c y
E
5
X
Fig. 1--(a) The D rich corner of an isotherm, T, in the hypothetical ternary system C - D - E , . The dashed line traces a hypothetical diffusion path established during annealing of a diffusion couple formed from the terminal compositions D and Co. (b) The composition profile of element E in the diffusion couple corresponding to the diffusion path illustrated in (a)°
knowledge of the pertinent phase d i a g r a m s . We thus set out t o e x a m i n e the association between phase relations and diffusion l a y e r formation in the Cu-Nb-Sn and Cu-Nb-Ge ternary systems. T h e s e systems illust r a t e respectively the successful and unsuccessful application of the bronz
Data Loading...
