Microstructure and Tensile Properties of Neutron-Irradiated (Fe_0.61Ni_0.39)₃V Ordered Alloy

Small tensile specimens of the (Fe_0.61Ni_0.39)₃V long-range-ordered alloy were irradiated in the ORR to 4 dpa at 523, 623, and 823 K and subsequently tested at the same respective temperatures. The alloy remained ordered after irradiation at all three temperatures. Irradiation at 523 and 623 K increased the yield strength of the material by producing Frank loops in the microstructure and reduced the total elongation. The low strain hardening observed was attributed to planar slip. Irradiation at 823 K embrittled the alloy. Premature failure was apparently initiated by helium bubbles on sigma phase boundaries which grew rapidly during the test to form microcracks. Fracture occurred after a microcrack propagated across grain boundaries that were weakened by helium and possibly sulfur. Modifying the composition of LRO alloys to eliminate sigma phase should lead to improved performance under neutron irradiation.