In an attempt to clarify the role of helium in promoting high-temperature intergranular failure in irradiated materials, in situ tension experiments have been conducted in a high-voltage electron microscope (HVEM) on 40-μm-thick foil microtensile specimensof Type 316 stainless steel irradiated with 80 keV helium ions to a fluence of about 1 × 10²¹ ions/m². It is found for irradiated austenitic stainless steel that when tension testing is performed above 700°C, intergranular failure occurs; but if the tension test is performed below 550°C, transgranular failure occurs. In order to better examine the role of temperature in causing intergranular failure, in situ tension testing was initiated at 700°C, briefly halted while the temperature was lowered to just below 550°C, and resumed at the lower temperature.

Specimens tested under these dual temperature tension test conditions exhibited intergranular failure at 700°C and, after the tension test was resumed at 550°C, changed to a transgranular failure mode. This result agrees with a growing body of data that supports the idea that when helium atoms are trapped in bubbles they are benign but if the helium atoms are released from their traps and can migrate to the grain boundary and reside there as lattice helium, that is, dissolved helium, then the role of helium is to reduce ductility and promote intergranular failure.