Effects of Ce addition on mechanical properties and thermal expansion behavior of Fe-36Ni Invar alloy

To pursue superior yield strength and low thermal expansion characteristic of Fe-36Ni Invar alloy, Fe-36Ni Invar alloys containing various cerium (Ce) contents (0 wt%, 0.0050 wt%, 0.0170 wt%, and 0.0360 wt%) were prepared by vacuum induction melting, followed by the heat treatment process. With increasing Ce content, the evolution of the second-phase particles followed the sequence: MnO-MnS (0% Ce) -> MnO-MnS + Ce2O3 (0.0050% Ce) -> Ce2O3 + Ce2O2S (0.0170% Ce) -> Ce2O3 + Ce2O2S + Ni5Ce (0.0360% Ce). The results indicated that the grain size and the fraction of annealing twins (fAT) were fairly sensitive to the Ce content. The smallest average grain size (d) and the highest fAT were obtained by adding 0.0170% Ce. The small addition of Ce was beneficial to the improvement of strength and low thermal expansion behavior. In particular, 0.0170% Ce alloy achieved the optimal combination properties. However, the excessive addition of Ce (0.0360%) induced a sharp decrease in mechanical performance and an undesirable increase in the coefficient of thermal expansion (CTE). The Ni5Ce phase was unfavorable to the enhancement of mechanical properties and low thermal expansion behavior. Last but not least, 0.0170 wt% Ce content was advised for Fe-36Ni Invar alloy by considering the balance of strength and thermal expansion behavior.