Influência da adição de cério na microestrutura e comportamento de oxidação de aços inoxidáveis Fe-Mn-Si-Cr-Ni com efeito de memória de forma

Abstract

In this study, the influence of small cerium additions on the microstructure and oxidation behavior of Fe-Mn-Si-Cr-Ni shape memory stainless steels (SMSSs) was investigated. The results show that the Ce addition in the alloy promotes a decrease in grain size and the formation of both Ce-containing oxide inclusions as well as secondary phase particles. The fraction of thermal martensite-ε decreases with increasing Ce content in the alloy. On the other hand, the MI temperature increases with the Ce content to values above the room temperature. The shape memory effect (SME) enhances with Ce additions in the alloy of 0.18 and 0.42 wt.%. However, the Ce addition of 0.96 wt.% degrades SME. The oxidation resistance of the tested alloy increases significantly with increasing Ce content. Nevertheless, the Ce addition of 0.96 wt.% decreases relatively to the oxidation resistance. The restrict parabolic law is only observed for alloys containing 0.18 and 0.42 wt.% of Ce. The oxide layer formed after 120 h of oxidation is composed of Mn oxides, promoting the depletion of this element below the oxide and phase transformation from austenite to ferrite. The obtained results show that the Ce addition of 0.18 and 0.42 wt.% in the alloy decreases the MnS inclusions leading to the formation of a more compact oxide layer and an increase on the resistance to oxidation. However, the Ce addition of 0.96 wt.% increases the amount of Ce-containing oxide inclusions that accelerate the Mn diffusion toward the oxide layer, leading to the formation of a less protective oxide layer.