In order to clarify the high temperature strengthening mechanisms of boron and zirconium in a Ni-Cr alloy, high-purity Ni-20Cr alloys have been studied from the view point of creep behavior and microstructure. With the boron or zirconium addition, creep resistance, rupture life and rupture elongation of Ni-20Cr alloy were improved, and it was assumed that the improvement of creep resistance is due to solid solution strengthening effect by zirconium, and the effect of boron is very little. Under high stress levels, the formation of subgrains and the dynamic recrystallization during creep were promoted by the boron or zirconium addition, and these microstructural changes seem to reduce the creep resistance. The longer rupture life and larger rupture elongation with the addition of boron or zirconium would be also attributed to the formation of subgrains and the dynamic recrystallization which retard the initiation and propagation of grain boundary cracks. Finally, it was suggested that these effects would be caused by the boron and zirconium dissolved in the γ-matrix rather than by the grain boundary segregation or sulfer-getting effects of these elements.
