Effect of Helium on the Fatigue Behavior of the Molybdenum-Base Alloy TZM at 900°C

The effect of cyclotron-implanted helium on the fatigue behavior of the molybdenum-base alloy TZM was investigated at 900°C. The results show that the helium-implanted TZM alloy fatigue specimens exhibit almost double the fatigue life of the unimplanted fatigue specimens. Optical and transmission electron microscope examination of specimen sections adjacent to the fracture revealed no evidence of significant differences between the helium-implanted and unimplanted specimens. However, transmission electron microscope examination of helium-implanted fatigue specimen sections annealed at 1310°C (0.55 T_m), following testing, revealed the presence of cavities within the grain matrix as well as cavities associated with dislocations. By comparison, the unimplanted specimens exhibited a structure characteristic of a well-annealed material. Based on the results of this study and on other available experimental evidence, it was concluded that the enhanced fatigue life of helium-implanted TZM alloy at 900°C resulted from the presence of substitutional helium-defect clusters, possibly associated with dislocations and/or precipitates. However, additional experiments will be necessary to firmly establish the details of the strengthening mechanism indicated by the present results.