In this work, we have examined Fe-Mn-Si-Cr alloys possessing a good shape memory effect due to a high Mn content (28 wt pct). The addition of Cr (5 wt pct) was made in order to give fairly good corrosion resistance to the alloy. But we have verified that even in moderately corrosive environments, the presence of chromium does not bear any passivation. On the other hand, we have found that the alloy can acquire corrosion resistance by means of the formation of a Mn-depleted surface layer obtained by heating the alloy at high temperature (Ӎ1050 ЊC) in air. This modified layer forms since manganese is selectively oxidized with respect to the other components. The adhesion of this layer is maintained even under severe stress if the thickness of the modified region does not exceed 20 m. Under this limit, the shape memory characteristics of the alloy are not affected, and at the same time, the specimen acquires properties of passivity comparable with one of the most common austenitic stainless steels at the presence of the same environmental conditions.

Ž . when the hydrogen percentage attains the highest value. Hydrogen-free layers were deposited by magnetron sputtering in Ar a-C and N 2 Ž . a-CN : nitrogen fractions range from 8 to 28 at.% and increase when the substrate is closer to the graphite target during deposition. Film x hardness was evaluated by nanoindentation in order to avoid inflation of the assessed value consequent to the elastic recovery, which results to be quite large in the examined layers. The hardness measurements show values up to ; 30 GPa and point out that both Ž . nitrogenation of a-C:H films and deposition by magnetron of hydrogen-free a-C layers lead to softer materials ; 18 GPa . Main features of Raman spectra are broad 'G' and 'D' peaks, characteristic of DLC layers: correlation between parameters of these peaks is discussed. q 1998 Elsevier Science S.A.

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