FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

- 251 -
absorption, the conversion of acoustic energy into heat. Scattering and
absorption are both caused by the presence of inhoraogeneities such as
precipitates, inclusions, voids, grain and interphase boundaries,
dislocations, interstitial impurities, etc. Since these same inhomogeneities
also play a primary role in determining mechanical properties, it is expected
that measurements of attenuation will reflect mechanical properties. The
sensitivity of attenuation to small changes in microstructure which affect
mechanical properties has been demonstrated for aluminum alloys »'' and
other materials (8)# in cases where grain boundary scattering is dominant,
various models (l>°,10) offering good agreement with experiment can be used
to calculate the grain size from the frequency dependence of attenuation. The
ultrasonically measured grain size, 6, can then be correlated with mechanical
properties via the use of the Hall-Petch relationship.
U = uo + kS-i (1)
where M is the mechanical property of interest, n, is a constant dependent
on composition, and k is a proportionality constant. This latter approach was
used by Klinman et al (11) to successfully predict yield strength, tensile
strength and DBTT's in carbon steels of known compositions. Another approach
was taken by A. Vary ("), who suggested an empirical relation relating
fracture toughness, K.ic, yield strength oy, and ultrasonic attenuation:
Klc 2 /Oy = 8.12 x 10 6 (Va&j) 0 ' 31 " 1 (2)
where V£ is the velocity of longitudinal waves in the material and ßg is
the slope of the attenuation versus frequency curve at a frequency, f =
Vg/6, where the wavelength is equal to the grain size. Since f is
generally outside the range of most normal attenuation measurements (eg. f =
0.5 GHz in steel with 6 = 10 Mm), the value of fJ$ is extrapolated from
measurements in the 10 to 50 MHz range. Two ma rag ing steels and a titanium
alloy, aged at various temperatures, were found to obey relation (2).
However, studies on Fe-C alloys (13) have shown that increasing the grain
size or increasing the cabon content, both cause an increase on the DBTT but
have opposite effects on ultrasonic attenuation. This is attributed either to
diminished absorption due to an increase in dislocation damping with
increasing C content, or to diminished scattering due to a decrease of elastic
anisotropy of the grains related to the presence of pearlite or cementite
around the ferritic grain boundaries. Thus it is difficult to isolate one
component of the ultrasonic attenuation, and in particular, to differentiate
between absorption and scattering. It is also difficult to vary one material
property such as fracture toughness while others remain constant.
In the present study, this is accomplished by correlating the ultrasonic
behavior and mechanical properties of a 403 stainless steel between -60 and
+40°C. In this temperature range, the plain strain fracture toughness, Kjc,
changes by =250% because of a smooth brittle to ductile transition, while
composition and microstructure remain unchanged.