63
W
Fig. 6.10. TransmissionelcctronmicrographshowingthinMGtypccarbideplatclctsmalloy452aftcrannealing I hrat l177째Cand 160 hr at 76OOC.
44
Unfortunately, both alloys contained carbide
show that the memory effect must have been present
stringers after fabrication was complete, as demonstrated by the microstructure ofalloy451 (0.017 wt % carbon) in the as-swaged condition (Fig. 6.1 la). However, when a sample of alloy45 1 was given a I-hr solution anneal at 1177O C, almost all of the carbides were dissolved (Fig. 6.1 Ib). Moreover, aging for 223 hr at 760' produced no carbide stringers (Fig. 6.1 IC). Instead, tiny carbide particles precipitated out on grain boundaries, twin boundaries, and preferentially on crystallographic planes. These details were more apparent with the electron microscope. Figure 6.12 shows MC-type carbides in a grain boundary of the aged sample of alloy 45 1. Figure 6.13 shows small MC platelets located near the center of a grain in the sample. Traceanalysis showed that the platelets all lie on (111) planes. Note that one of the thin MC platelets also lies in the 0 1 1) foil plane. These results
during fabrication of alloy 451 but was eliminated when small samples of the same heat were solutionannealed for 1 hr at 1 177" C after swaging. Assuming that the fabrication furnaces were at the correct temperatures, the solution annealing time necessary to dissolve the carbides must be sensitive to the mass of material being treated. Intuitively, this is reasonable and has actually been observed repeatedly throughout the investigation. The results for alloy 453 were similar to those for alloy 451 except that a final 1 hrat 1177OCdid not remove the memoryeffect from that alloy. Higher annealing temperatures or longer times at I 177OC would be required to produce a uniform carbide distribution in this 0.035-wt % carbon alloy. The rate of carbide dissolution was determined quantitatively for both alloys 452and 45 1 by solution