Metastable amorphous and crystalline (α,σ) phase in physical vapor deposited Fe–(Cr)–Ni–(C) deposits

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Abstract
Several microstructural features revealed by transmission electron microscopy in dc sputter deposits of various Fe–(Cr)–Ni–(C) alloys are discussed. The deposits are sputtered on metallic substrates with different Ar/CH4 gas mixtures at temperatures lower than 130 °C. The target’s source material are made of 304, 316, and 310 stainless steel or of Fe–Ni and 55Fe–45(Cr,Ni) alloys with various Cr/Ni ratios. The mean grain size d̄ of the crystalline deposits decreases as the carbon content x increases (x=0, d̄=15 nm) and, for a critical value xc, their structure is wholly amorphous (xc=3 wt. % C in a Fe–25Cr–20Ni–C deposit). All the crystalline deposits exhibit some bcc metastable ferrite coexisting with the fcc equilibrium γ austenite and a 〈111〉γ or 〈200〉γ texture essentially depending on the composition. Those that contain carbon lead to a very fine precipitation of M23C6 carbides upon heating above 500 °C. Mössbauer spectroscopy achieved on several two-phase Fe–Ni deposits has clearly shown that ferrite and austenite do not have the same composition. Moreover, it is surprising to identify sometimes, locally, the Frank–Kasper σ phase which reveals small twin domains rotated by 30° about the same c axis. In the amorphous deposits (with x<7.5 wt. % C), small austenitic grains first crystallize near 400–450 °C and, above 500 °C, M7C3 carbides nucleate and grow, embedding the primary austenitic grains.

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