Prototype design of an SEM capable of detecting helium desorption during high temperature tensile test

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A large amount of helium is generated in fusion reactor materials by 14MeV neutron irradiation leading to so called high temperature helium embrittlement. The mechanism of helium embrittlement is believed to be due to the accumulation of helium to grain boundaries where grain boundary cohesion is severely impaired. The details of the mechanism, however, are still not known yet. In the present project, a commercially available scanning electron microscope (SEM) is customized in such a way that helium desorbed from deforming specimen at high temperature can be detected along with in situ observation. Thereby the relation between helium desorption and slip band formation and crack opening is examined in order to investigate the mechanism of helium embrittlement.Main functions of this apparatus are : (1) magnified observation of specimen surface, (2) in situ tensile test capability, (3) specimen heating, (4) helium desorption detection, etc. A lead screw type tensile mechanism has been ado … More pted driven by a stepping motor. Specimen heating was done by direct electrical heating. This method lacks in uniformity in temperature while temperature response is superior and the volume of the heated zone can be minimized. Temperature measurement is done using a thermocouple attached to the specimen grip. Actual temperature of the specimen is obtained by calibrating using a dummy specimen on which thermocouple is attached in the central part of the gage portion. A quadrupole mass spectrometer is used to measure the helium desorption.Several iteration cycles were necessary to optimize the shielding of thermal electrons without obstructing the view area. The apparatus with intended functions has been successfully constructed. Vanadium alloy specimens doped with tritium trick have been tensile-tested in situ in this apparatus. Stress-strain curves and helium desorption curves were recorded simultaneously with video-images of SEM images on a VTR.Some of the important findings from these experiments are that helium desorption occurred in synchronous with crack opening at the specimen surface. Less