FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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directions around the circumference is the reverse of the longitudinal strain
and about 25% of its value. It appears that the relationship between the
strains,
given by
EL, ER, £H in the longitudinal, radial and hoop directions is
£ R
V V
where v is Poisson's ratio, which for Incoloy-800 lies between 0.28 and 0.34
[6] . If we assume that the strains are uniaxial the residual stress may be
obtained by making use of Young's moduli derived from X--ray measurements. The
residual stresses measured with neutrons are roughly 1.5 times those derived
from X-ray diffraction on the same tube. This may indicate major gradients
through the wall or surface relaxation effects. It is clear from these
measurements that neutrons are an excellent diagnostic tool for measuring the
residual elastic strain in engineering components, capable of giving Important
insights Into the mechanical behaviour of deformed metals.
3.3 Texture measurements
Measurements of crystallographic texture by neutron diffraction are not particularly
new. For a recent review, see Bunge [7] . However, they have major
advantages since the number of grains sampled by a neutron beam Is much larger
than with typical X-ray beams. There are few geometrical constraints for most
materials and it is often possible to place an entire component in the neutron
beam.
3.4 Measurement of second phases in materials
The existence of second phases often controls the strength and fracture
behaviour of materials. The sensitivity of neutrons to second phase material
is often higher than X-rays partly because the signal is an average over many
grains, and partly because the light element constituents of many precipitates,
such as carbon, nitrogen and oxygen, have scattering cross sections that are
just as large as most metals or other materials of interest to the engineer.
Detection of volume fractions less than 1% is fairly routine. As an example of
a recent application of neutron diffraction, Windsor et al. [8] have studied
the development of fee second phase material in bec maraging steels.
3.5 Small angle scattering of neutrons
Small angle scattering relies on an effective contrast in scattering between
the matrix and the embedded "small particles" that are the objects of interest.
The sizes of small particles in the range 10-1000 Â may be readily determined;
they may be lattice distortions around impurity atoms, nucleating phases barely
detectable by other means, small precipitates etc. There is already a large
literature on the technological applications of small angle scattering (see for
example ref. [9]).