1.5 Measurement of the Internal Strain of Structural Materials Using a Neutron Beam

Fig. 1-9 (a) A part of a superconducting coil (coil radius: 500 mm). The Incoloy 908 jacket is a pipe with a square cross section of 45*45 mm2 and an inside diameter of about 35 mm, in which a bundle of copper wires is contained. Thin multi-wires of Nb3Sn superconductor are embedded in the copper wires which act as a stabilizer of the superconductor. For typical regions such as A, B, C and D the strain distribution was measured at several points along the radial direction by using the neutron diffraction method.
(b) Schematic drawing of neutron beam paths and sample orientation. Incident and diffracted beams are selected by two slits to survey the local strain at the measurement locations indicated by red points.

Fig. 1-10 Evaluated residual stresses, sigma x, sigma y and sigma z, for the three principal orientations in the regions A, B and C of the superconducting coil jacket as a function of the depth from the jacket surface. Tensile residual stresses above 200 MPa have been detected in the region C; the result has pointed out the importance of avoiding the SAGBO cracking condition by controlling the oxygen concentration.

A nickel-iron-based superalloy, Incoloy 908, is scheduled to be used as a jacket material for the central solenoid (superconducting coil) of the International Thermonuclear Experimental Reactor (ITER).
The ITER superconducting coil has to be heat-treated at 650 degrees for 240 h after having been mechanically formed into a coil shape in order to establish the Nb3Sn superconductor. The superalloy, Incoloy 908, is a metallic material that can be highly strengthened by this high-temperature and long-term heat treatment. On the other hand, this alloy may crack inside the material due to stress-accelerated grain boundary oxidation (SAGBO) under the combination of tensile residual stress above 200 MPa, in the temperature region of 450-850 degrees and an oxygen concentration above 0.1 ppm. Therefore, there is a risk of fracture of the jacket material due to SAGBO during the heat treatment.
We were asked to measure the internal residual stress distribution nondestructively in a large structural material such as the jacket material. This task can not be accomplished by conventional methods using strain gauges, X-ray diffraction, etc.
We performed a diffraction experiment using a neutron beam which can penetrate deep into the material. In this experiment we measured the strain of the crystal lattice as a function of the position inside the jacket material (with a square cross section of 45*45 mm2) of the superconducting coil after mechanical forming. From these strain data we succeeded in evaluating the three principal-orientation components of internal residual stress as a function of position.
The strain we are concerned with is not the macroscopic plastic deformation of the material as a whole, but the microscopic elastic deformation observed as a change in crystal lattice spacing. For example, the spacing of the (111) crystal lattice plane changed only from 0.2072 nm to 0.2084 nm. Thus, the microscopic strain measurement at atomic-size level, utilizing a neutron beam, has played an important role in evaluating the integrity of large structural materials.

Reference
Y. Tsuchiya et al., Residual Stress of a Jacket Material for ITER Superconducting Coil, Physica B, 241-243, 1264 (1998).

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