Due to its good creep strength, molybdenum (Mo) single crystal is used for space thermionic reactor fuel element. However, stray grain formation during Mo single crystal welding leads to the degradation of mechanical properties. In this paper, Mo single crystal is welded by electron beam welding and the joint properties are evaluated by means of small punch test (SPT). The load-displacement curves of Mo single crystal matrix and fusion area are measured. The experimental data is fit to establish the relationship between yield load and standard test yield strength. The results show that the yield strength of the fusion is much lower than the other material. The micro hardness of the both materials is almost equal. Simultaneously the displacement value of the fusion area is the smallest which means the joint brittleness increases. SEM observation of rupture fracture demonstrates the embrittlement of the weld.

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