Atomic Redistribution in a Fe-Cr System in the Course of Mechanical Alloying and Subsequent Annealing
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NTRODUCTION
IT is known that the Fe-Cr binary alloy system forms the basis of a very important class of engineering materials, ferritic stainless steels, which combine good mechanical properties, corrosion resistance and resistance to nuclear radiation environments.[1] However, high-chromium steels are unstable at the elevated temperatures at which they are frequently used (see, e.g., References 1 and 2). The instability phenomenon manifests as either phase decomposition into two isostructural bcc phases, namely the Fe-rich (a) phase and Cr-rich (a¢) phase (miscibility gap), or precipitation of r-FeCr phase. Both phenomena lead to deterioration of the mechanical properties of these materials, which substantially limits their practical use. In general, the complexity of the Fe-Cr system study is determined by the non-monotonic behavior of many properties depending on the chromium content.[3] In particular, the so-called short-range order (SRO) inversion was observed experimentally. The SRO inversion,
VITALY E. PORSEV, ALEXANDER L. ULYANOV, and GENNADY A. DOROFEEV are with the Udmurt Federal Research Center UB RAS, 34, Baramzinoy Street, Izhevsk, Russia 426067. Contact e-mail: [email protected] Manuscript submitted April 12, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
namely changing the sign of the Cowley–Warren SRO parameter from negative to positive with increasing Cr concentration, was found by Mirebeau et al.[4,5] by neutron diffraction, Froideval et al.[6] by EXAFS-spectroscopy and Filippova et al.[7] by Mo¨ssbauer spectroscopy in the course of annealing at approximately 700 K. According to these studies, the SRO parameter was found to change its sign at 9 to 12 at. pct Cr critical content, i.e., when approaching the solubility limit. This points to the fact that the dissolved Cr atoms tend to surround themselves with Fe atoms provided that the Cr content is < 9 to 12 at. pct. On the contrary, if the Cr content is> 9 to 12 at. pct, these atoms tend to surround themselves with other Cr atoms, i.e., Cr atoms form clusters, segregate or precipitate as Cr-rich a¢-phase. Only 9 to 12 at. pct Cr content is suggested to correspond to the ideal solid solution. Many of these complex phenomena in Fe-Cr alloys have been understood on the basis of theoretical approaches. Using different thermodynamic calculations, References 8–11 show that the mixing enthalpy of random or quasi-random Fe-Cr solid solutions is negative below the critical concentration and becomes positive above it. A physical explanation of this effect was found in terms of the electron band and the magnetic properties of Fe and Cr. Another important result is that the standard symmetric Fe-Cr phase diagram is incorrect at low temperature and low Cr content. A new, more truthful a–a¢ miscibility gap in the Fe-rich area begins at the higher Cr
content with respect to the standard phase diagram. These theoretical concepts do not contradict with r-phase (FeCr) appearance at concentrations close to 50 at. pct Cr because the a–a¢ miscibility gap becomes meta
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