Tensile behavior of rapidly solidified Al-Li-Zr and Al-Li-Cu-Mg-Zr alloys at 293 and 77 K
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properties of the alloy. Heat treatment, solution plus subsequent aging, did improve the distribution of the M23C6 precipitates in the alloy substantially. The more uniformly distributed M23C6 precipitation gives the alloy a higher strength, particularly, superior stress-rupture properties at high temperature. The stress-rupture lives of the alloy increased by several times. Although the aging treatments degraded alloy ductility severely, an adequate ductility was retained, which resulted from the columnar grained matrix with ^001& preferential orientation. In addition, it is noted that the stress-rupture properties of the specimen aged at 950 8C are inferior to those at 850 8C. This may be related to a lot of large rodlike M23C6 precipitates in the specimen aged at 950 8C. Their coarse morphology is detrimental to stress-rupture property. The present work indicates that the use of heat treatments is a promising way to enhance mechanical properties at room and high temperatures of the DZ40M alloy by improving distribution of the secondary M23C6 carbide. Obviously, a more uniform distribution of secondary precipitates would bring about a higher strength. Elimination of imperfections would increase their dispersiveness, which could possibly be achieved by controlling the cooling rate after solution treatment. REFERENCES
Fig. 5—X-ray diffraction patterns of the aged DZ40M alloy.
could have formed during cooling after the solution treatment.[9] Compared with the as-cast alloy, the heat-treated alloy has a higher room-temperature strength. This should be attributed to M23C6 precipitation hardening. The as-cast alloy derives its strength principally from the solid solution matrix due to its absence from fine secondary precipitates. At high temperature, for the as-cast DZ40M alloy, precipitation hardening is an important strengthening mechanism.[4] During high-temperature exposure, the primary carbides dissolve, and secondary M23C6 carbide precipitates heavily. The latter exerts a direct effect on obstructing dislocation movement. However, the precipitation hardening is less effective, because the M23C6 precipitates are rather unevenly distributed. Their precipitation occurs mainly around the primary carbides, because as a carbon reservoir, the primary carbides provide the precipitation reaction with carbon atoms. There is a dense precipitation around the primary carbides and a sparse distribution in the interior of grains. Obviously, such a nonuniform microstructure is unfavorable to mechanical 2254—VOLUME 30A, AUGUST 1999
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