The mechanism of precipitation strengthening in Fe–Ni austenitic alloy electron beam weldment

Precipitates and their effects on deformation and mechanical performance of Fe–Ni alloy electron beam weldment after post-weld aging treatment were investigated by transmission electron microscopy and microhardness tests. Results show that precipitates in the base material and the weld are both γ′ (or Ni3(Al, Ti) ) phase. However, precipitates in the weld are much larger in size and more dispersed than that of the base metal. Atomic resolution observation revealed lots of vacancy clusters inside the γ′ precipitates and the γ′/γ interface in the weldment. In contrast to the dislocation planar slip mode in the base metal, dislocations in the weldment tended to entangle around the γ′ precipitates forming dislocation loops. Microhardness of the weldment was about 15% lower than that of the base metal. The precipitate size, distribution and vacancy clusters are correlated with the observed dislocation structure and microhardness. It is concluded that for the weld, the resistance to fracture was determined by the size and distribution of precipitates.