Abstract:
This thesis is concerned with the study of cavitation
behaviour of two-phase superplastic alloys, using mainly a hot
microhardness technique. Cavitation is a serious problem in
copper and iron base alloys, it drastically limits the elongation
to failure and may affect the service properties of superplastically
formed components. There is some empirical evidence to
suggest that, for cavitation to occur, the two phases involved
must be 'incompatible' that is, they show marked differences in
certain properties. Ideally, this phenomenon would be investigated
directly by an in-situ study of the deformation behaviour
of each phase separately in the microduplex alloy at the superplastic
temperature and hence pinpointing the phase incompatibilities,
but existing techniques do not allow for this. Here,
a hot microhardness tester was commissioned and used to determine
the hardness and activation energy of each phase in-situ in a
duplex alloy in thc superplastic temperature range.
Zn/Al eutectoid and 60/40 brass were here used as models
to represent a non-cavitating and cavitating class of superplastic
alloys. In addition, a series of alloys were produced of compositions
corresponding to those of the individual phases in these
systems over a supcrplastic temperature range. These alloys were
hot tensile. tested to find the ductility and ten'fle strength of
each phase of superplastic alloy. The hot hardness data was
correlated with the hot tensile data for the alloys over the whole
temperature range from 0.4 to 0.84 Tm. Variation of Young's
modulus with temperature was determined by a vibration technique
and activation energy calculated form LnH versus Tm plots for all
the alloys. In 60; 40 brass, the amount E of -T cavitation
decreased with an increase in temperatur% and this alloy gave a
maximum elongation of ti 110 pct at 250°C 1,0.45 Trrj, but it still
exhibited extensive cavitation. In the Zn/Al eutectoid, no
cavitation was observed apart from that associated with a few
large inclusions. The hardness values of the two phases in a/ß
brass were very different at the superplastic temperature, as were
the load extension curves from the tensile tests and the calculated
activation energies. In. the Zn/Al eutectoid however, the
mechanical properties of the two phases were similar, though the
activation energies were different.
Finally, an attempt was made to explain the occurrence
of cavitation in Supral 150 and certain steels, in some cases
experimentally and in others from data available in the literature.
