| Summary: | The ability of metals and alloys to form and retain protective oxide
scales is crucial to their stability at elevated temperatures for
extended times. Hence the identification of factors that promote or
limit the integrity of oxides on high temperature materials has been the
subject of intensive investigations.
In the present study the mechanical properties of this chromiwm.-rkh
scale on 304 stainless steel foil has been investigated in relation to
the deformation rates in the substrate. It was shown that heavy cold
working (up to 90%) delays the onset of breakaway oxidation and results
in a very adherent scale.
The cracking behaviour of the scale was found to be strain rate and
temperature dependent under slow strain rate conditions when the
substrate deforms by creep. No strain rate dependence was observed over
the temperature range 700-900°C when faster strain rates (> 10-
S
sec -1)
were applied. The transition between these two responses was found to
vary only slightly with temperature between S.Ox10-
S
sec-
1
and 7.Sx10-
S
sec
-1
,increasing as the temperature is raised.
A new method has been described for determining the fracture behaviour
of oxide scale by estimating the composite defect size. From a
knowledge of the onset of scale cracking, determined in Sl(U usi ng (h~
acoustic emission technique, it was possible to correlate the measured
intercrack spacing with the fracture toughness from which the tensile
properties of the scale can be evaluated.
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