Fracture Processes in Fully Crystallized Co 84 Nb 10 B 6 Metallic Glass
- PDF / 1,472,791 Bytes
- 4 Pages / 417.6 x 639 pts Page_size
- 14 Downloads / 182 Views
FRACTURE PROCESSES IN FULLY CRYSTALLIZED Co8 4 Nb1 0 B6 METALLIC GLASS JANEZ MEGUSAR, TINA VARGAS AND NICHOLAS J. GRANT Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA ABSTRACT Fracture processes have been studied in a fully crystallized Co9 4 Nb1 0 B6 glass, with the mean grain size ranging from 43 nm to 0.65 pm. Tensile tested specimens with a mean grain size• 0.3 um showed a ductile (dimple) fracture with microvoids initiated at the boride-cobalt matrix interfaces. As the mean grain size (more correctly, the corresponding mean free path of the deforming cobalt phase) was •0.1 pm, the fracture process may be similarly initiated by the microvoid formation at the boride-cobalt interface. However, one-to-one correspondence between dimples and boride particles may not be preserved. As a result, dimples become more shallow and may include several boride particles instead. Although the fracture surface of the brittle fine grained Co8 4 Nb1 0 B6 alloy gives an appearance of a cleavage fracture on a macroscale, it is distinctly different, by allowing for the microvoid formation on a microscale. INTRODUCTION Most metallic glasses are brittle when fully crystallized. This has been associated with a high volume fraction of brittle phases in the crystallized state. In order to develop a ductile crystalline material from the glassy state, it is therefore necessary to reduce the metalloid content. This was demonstrated for Fe-based metallic glasses where the boron content has been reduced to about 10 at% [1,2]. Recently, it has been shown [3] that tCo 4NbB6 glass with a metalloid content of only 6 at% can be similarly devitri led uto develop a ductile crystalline material. As shown in Fig. 1, the bend diameter of the melt spun ribbon reaches its maximum when the Co8 4 Nb1 0 B6 glass is annealed at 973 and 1073 K; it then becomes progressively more ductile with increasing temperature, becoming fully ductile and
E 2.4-
E
_
e-.
Fully crystalline-P
Increasing
43nm
"` E 1.6- _ ductility -03 0.8 -
,,
-,
--
O1- ,•0J4.L4rm ,,
0.65tFr
m0L 573 673 773 873 973 1073 1173 1273 1373 Annealing temperature, K(1h Anneal) Fig. 1: Bend ductility of a crystallized Co8 4 Nb 1 0 B6 glass as a function of annealing temperature (I h -1 anneal). Tx (onset) is 717 K for a heating rate of 20 K min .
Mat. Res. Soc. Symp. Proc. Vot. 58. 1986 Materials Research Society
446
s
Fig. 2: Fracture surfaces (SEM) of fully crystallized Co84 NblnB6 glass annealed for 1 h at temperatures of 1323 K (A), 1173 K (B) and 1073 K (C). Also see Figs. 3B and 4B, which are higher magnifications of Figs. 2A and 2C, respectively. bendable 180 degrees without fracturing after an anneal for lh at 1273 K. Crystallization of Co ANbo 0B6 glass has been systematically studied [3] in order to explain the p§es¢ of development of the ductile crystalline material from the amorphous state. X-ray diffraction showed that the Co84Nb1 0B6 glass crystallized into a cobalt-based solid solution phase and an unid
Data Loading...
