Tensile ductility and rupture behaviour of Si3N4/SiC composites were studie
d in the 1595 and 1640 degrees C temperature range under strain rates of 5x
10(-5) s(-1) and 10(-5) s(-1). The behaviour was compared to that of Si3N4
monoliths containing the same initial alp Si3N4 ratios with reference to an
alpha-phase rich superplastic silicon nitride. The ductility of the compos
ites was much smaller than that of the corresponding monoliths. It decrease
d when the SiC content increased and increased when the temperature was rai
sed. Under a strain rate of 5x10(-5) s(-1), ductility remained limited (eps
ilon<25%) whatever the composition or the temperature, and failure occurred
by slow crack growth. Contrary to the case of the alpha-phase rich superpl
astic silicon nitride for which the hardening was strain-enhanced and essen
tially time-independent, a dramatic change in hardening appeared and the du
ctility was markedly enhanced when the strain rate was lowered to 10(-5) s(
-1). It was concluded that, under the higher strain rate, deformation was c
ontrolled by grain boundary sliding of the micro-sized Si3N4 grains. In thi
s case, the SiC nanoparticles located along the grain boundaries hampered s
liding of the Si3N4 grains, leading to a poor ductility. On the contrary, u
nder the lower strain rate, the SiC nanoprecipitates contributed to the def
ormation and the ductility is improved. (C) 1998 Elsevier Science Limited.
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