Surface Energy Driven Crystallization of Amorphous Ni 69 Cr 14 P 17 Alloy
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Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
RESULTS AND DISCUSSION Crystallization starts at 300oC with the nucleation of a metastable hexagonal (Ni,Cr)3P phase directly at the perforation edge with the c-axis perpendicular to the specimen surface. The micrograph shown in fig. 1 is taken near the perforation edge after thermal treatment at 3000C. The electron beam was perpendicular to the specimen surface. The hexagonal structure seen in this HREM image appears in a projection along [0001] as threefold point symmetry of dark dots separated by 0.597 nm equivalent to the separation of (1,0,-1,0) lattice planes.
Fig. 1.
HREM of the hexagonal phase taken near the perforation edge after annealing at 300 OC
The crystallization proceeds with time, the interface between the crystalline and the amorphous phase being practically parallel to the perforation edge. The width of the crystalline phase was measured as a function of time. Apart from a fast increase for short annealing times, the width was found to increase linearly with time in agreement with previously reported results [5]. In some specimens, dendrite formation was observed at higher annealing temperatures and longer annealing times. No measurements were performed in these cases. Figure 2a shows the specimen near the perforation edge after thermal treatment at 3500C. Figure 2b and 2c show parts of fig 2a with higher magnifications. The fringe spacing determined from fig. 2c corresponds to the separation of {1,0,-1,0) lattice planes of the hexagonal phase. Growth of the hexagonal phase stops at a specimen thickness of about 50 nm. In the present case the crystallized width was then about 500 nm.
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Fig. 2a-c. HREM micrograph of a hexagonal dendrite formed during thermal treatment at 350 oC. Figure 2b and Fig. 2c are higher magnifications of fig. 2a
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Thermal treatment above 530 0 C results in the transformation of the hexagonal (Ni,Cr) 3P phase into the stable b.c.t. (Ni,Cr)3P phase. Figure 3 shows the HREM micrograph of the b.c.t. (Ni,Cr)3 P phase transformed from the hexagonal phase. The (110) -lattice planes of the tetragonal phase are clearly visible. In addition, regions with Moiree pattern are seen which are probably caused by small crystallites twisted relative to their surroundings.
Fig. 3. HREM micrograph of the b.c.t. phase transformed at 530 0 C from the hexagonal phase. The results of this work suggest a strong influence of the specimen geometry on crystallization. The specimen geometry at the perforation edge might be approximately described by a wedge which is schematically shown in fig. 4. The two specimen surfaces include an angle a, and the volume of a wedge shaped crystal of length L and thickness d is given by V= x.L-d(x)/2, where x is the distance of the amorphous-to-crystalline grain boundary from the perforation edge at d/2. The driving force AF for crystallization, i.e. the net energy change per unit volume, for a wedge shaped crystal of volume V is then given by AF = -AGv - 2-b.L-{Fa-cr)/V + L-d.
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