The effect of long-term thermal exposure on the tensile behavior of a high W content nickel-based superalloy K416B was investigated.The microstructure and the deformation characteristics were observed by scanning electron microscopy and transmission electron microscopy,and the phase transformation of the alloy during long-term thermal exposure was analyzed by X-ray diffraction patterns and differential thermal analysis.Results showed that after thermal exposure at 1000℃,the MC carbides in the K416B alloy decomposed into M6C.During tensile deformation,dislocations slipping in y matrix crossed over the M6C by Orowan bowing mechanism.With the increase of thermal exposure time,the secondary M6C reduced greatly the yield strength of the alloy at room temperature.Meanwhile,the continuous distribution of the secondary M6C with great brittleness in the grain boundary could become the main source of crack,which might change the fracture characteristic of the alloy from trans-granular to intergranular.