Evolution of Nanostructure of FeNi(Ti/Cr)N Alloys During Phase Cycling
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Evolution of Nanostructure of FeNi(Ti/Cr)N Alloys During Phase Cycling N. G. Chechenin, P. M. Bronsveld, A. R. Chezan, C. B. Craus, D. O. Boerma, J. Th.M.de Hosson and L. Niesen Materials Science Center, University of Groningen, Nijenborgh 4, NL 9747 AG Groningen, The Netherlands ABSTRACT A possibility to manipulate the microstructure of cold rolled (down to less than 1% of the initial thickness) Fe94Ni4Ti2 and Fe93 Ni4 Cr3 foils via inter-phase cycling by nitriding and de-nitriding is investigated. The grains in the as-rolled material had a dominating (001)[110] texture and contained a complicated internal nanostructure due to a dislocation network. The interphase cycling α↔γ’-Fe4N and α↔ε-FexN (x~3) was investigated as a tool to change or destroy the texture. We observed that the α↔γ’-Fe4N phase transformations weaken but do not erase the texture. A stronger reduction of the texture was obtained in the α↔ε-cycling, where grains with a new orientation appear in XRD scans. During the α→γ’ phase transformation a lamella structure is formed which is coarsening with the number of cycles. It was found that the rates of the α↔γ’ and α↔ε interphase transitions were slowing down with the number of cycles. The observation is explained by an increase of kinetic barriers while removing/transforming the rolling-induced defects during the cycling. INTRODUCTION Films or layers of nanocrystalline nitrided iron-based dilute alloys are very promising in applications as ultra-soft magnetic materials for high frequency applications. However, their magnetic properties, as well as a number of other important practical materials properties, are very sensitive to the microstructure: size distribution and orientation of grains, texture, presence and type of impurities and inclusions, etc. Hot and cold rolling of iron alloys are basic technology steps in the industry of soft magnetic materials [1,2]. However, there are various shortcomings, like rolling-induced defects, strong texture and large grain size which often prevent to obtain good magnetic properties at high frequencies. Dislocations, induced in the plastic deformation steps, can be (partially) removed by thermal treatments via a recovery or a recrystallization of the material. Still, after annealing, there are residual defects, the grain size stays large and the texture remains. Here we study the possibility of manipulating the microstructure via phase transformations in the Fe-N system from the α-phase (bcc) to the γ’-Fe4N (fcc) and to the ε-FexN (hcp) (x≈3.0) by thermochemical nitriding in a NH3/H2-gas mixture and by subsequent back transformation to the α-phase by de-nitriding (reduction) in a pure hydrogen atmosphere. We found that the strong texture in as-rolled films can be reduced by the α↔ε phase transformation cycling.
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EXPERIMENTAL DETAILS Slices (0.5mm thick) of bulk Fe+Ni(4at%)+Ti(2at%) (Ti-samples) and Fe+Ni(4at%)+Cr(3at%) (Cr-samples) materials were cold-rolled in two perpendicular directions to 2-4 µm thick foils. Such a severe plastic deformation leads to
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