The microstructures and the crack propagation of uranium-niobium alloys near hypoeutectoid composition aged at temperature 200℃ for 2 hours during a tension was investigated by means of in situ tension tests using TEM. The results show that the twinning planes inside and between the martensite laths move and merge

and then disintegrate in uranium-niobium alloys with monoclinic α′′ structure during the tension. The crack propagation can be described as follows. Under the tension

the thinning zone which is locally plastically deformed emerges in the front of the crack tip. After the process of nucleation

growth and conjunction

the microvoids connect with the main crack

which results in the fracture. Neither of emission

propagation and movement of dislocation was observed during the tension.