Coercivity and domain structure of nanograined Fe–C alloys after high-pressure torsion
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The microstructure and magnetic properties of binary hypo- and hyper-eutectoid Fe–C alloys were studied. The investigations have been carried out on the samples in the as-cast state, after a long annealing at 725 °C and on the specimens after the high-pressure torsion (HPT). The deformation was carried out at the ambient temperature and the pressure of 5 GPa. The grain size after HPT is in the nanometer range. Long annealing leads to a drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H c increases with increasing carbon content. The distance L between pinning points for domain walls decreases with increasing carbon content. Increase of the coercivity and decrease of L are more pronounced below the eutectoid concentration. The coercivity of the nanostructured samples is higher than that of the as-cast alloys. Due to the pinning of domain walls by the cementite particles, the hysteresis loop in the coarse-grained alloys both in as-cast and annealed states has a narrowing near the zero magnetization.
KeywordsFerrite Austenite Domain Wall Cementite Carbon Concentration
The study of nanocrystalline solids increased pronouncedly in the last decade. The interest is driven by the unique physical and mechanical properties of these materials. Among the most important techniques for nanograined polycrystal preparation, one can mention gas condensation, ball milling, crystallization from amorphous state, and severe plastic deformation (SPD) techniques. SPD techniques, such as high pressure torsion (HPT), do not cause changes in the material geometry, in the contrast to the conventional processes of high deformation like rolling or wire drawing [1, 2, 3]. The properties of newly developed nanostructured materials are defined in turn by their structural features. Therefore, the comprehensive investigation of nanograined polycrystals in the alloying systems, which are most important for the structural and functional applications, is crucial. The aim of this work is to study the influence of the microstructure on domain structure and coercivity in coarse-grained and nanograined Fe–C alloys within a broad interval of carbon concentrations.
Both hypo- and hyper-eutectoid Fe–C alloys with carbon concentration of 0.25, 0.45, 0.60, 1.3, 1.5, and 1.7 wt.% were prepared from high-purity 5 N iron and graphite by a vacuum induction melting in a form of cylindrical ingots. For HPT treatment the 0.4 mm thick discs were cut from the as-cast ingots, then ground and chemically etched. They were subjected to HPT at room temperature (5 torsions under a pressure of 5 GPa in a Bridgman anvil-type unit, shear strain was about 6). Samples for structural and magnetic investigations were cut from the HPT-deformed discs at a distance of 3 mm from the sample center. A set of the as-cast samples with 0.25, 0.60, 1.3, and 1.7 wt.% was additionally annealed during 950 h at 725 °C (i.e., below the eutectoid temperature) in order to achieve the equilibrium α + Fe3C structure. Light microscopy (LM) was performed on a Zeiss Axiophot microscope. For the metallographic investigations the samples were ground by SiC grinding paper, polished with 6, 3, and 1 μm diamond pastes and etched for 5–10 s with a 5 wt.% HNO3 solution in ethyl alcohol. Magnetic measurements were performed using a vibrating sample magnetometer (VSM). Magnetic field in the VSM was applied parallel to the sample plane. Transmission electron microscopy (TEM) investigations were carried out on a JEM–4000FX microscope with an accelerating voltage of 400 kV. Domain structure of the samples was visualized on a magnetic force microscopic facility.
Results and discussion
Samples after HPT
Coarse-grained as-cast alloys
Samples annealed at 725 °C and quenched in cold water
The resulted plot for the as-cast alloys is obviously composed of two parts: the first line with a large slope is for hypoeutectoid alloys and another almost horizontal one corresponds to the hypereutectoid alloys. Such a behavior can be understood if we compare the microstructure of the low carbon and high carbon alloys. At the low carbon concentrations, the amount of α-Fe is high (Fig. 2a), and the magnetization vector can easily change its direction with changing the external magnetic field. With increasing carbon content the amount of cementite increases and the distance between Fe3C platelets or particles decreases (Fig. 2b). At 0.6 wt.% C the easily magnetized primary ferrite grains almost disappear from the microstructure. The austenite grains are transformed into the fine-lamellar troostite.
In the microstructure of all studied alloys after HPT, no significant changes with the change of carbon concentration are observed. The cementite nanoparticles are finely dispersed among the nanometric ferrite grains. After long annealing below the eutectoid temperature, the differences between hypo- and hyper-eutectoid as-cast alloys also disappear (Figs. 3, 4).
The dependence of the coercivity on carbon concentration (Fig. 4) can be discussed based on the Kersten model [10, 11, 12]. In this model, it is assumed that the domain walls move by the magnetization through the array of pinning points. If the external magnetic field increases, the pinned domain wall bends first and jumps to the next couple of the pining points after that. Pores, dislocations, or particles of a second phase may act as pinning points. Coercivity H c in this case is defined by the equation
L is lower for all HPT alloys comparing with as-cast alloys. After HPT the microstructure of all studied alloys does not change much with carbon concentration. In other words, there is no such a drastic difference between the hypo- and hyper-eutectoid as-cast alloys. TEM revealed that the mean distance between the cementite particles slowly decreases with increasing carbon content. Long annealing below the eutectoid temperature also resulted in disappearance of structural differences for the alloys with different carbon concentrations (Fig. 5). L strongly decreases with increasing carbon concentration below the eutectoid concentration (0.8 wt.% C) and slowly decreases above it. The decrease of L value is especially pronounced for the as-cast alloys, because the distances between ferrite and cementite plates decrease with increasing carbon concentration stronger than for the samples after HPT and long annealing (Figs. 1–3). Therefore, the pinning of the magnetic domain walls is governed by the size and the arrangement of the cementite grains. These facts are perfectly reflected in the behavior of mean distance between the pinning points for the annealed alloys and alloys after HPT.
High-pressure torsion of the studied Fe–C alloys leads to the grain size refinement down to the nanometer range and increase of the coercivity. Long annealing leads to the drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H c and the distance L between pinning points for domain walls decreases with increasing carbon content. Increase of H c and decrease of L is more pronounced below the eutectoid concentration. Due to the pinning of the domain walls by the cementite particles, the hysteresis loop in the coarse-grained (as-cast and long-annealed) alloys has a narrowing near the zero magnetization.
The authors thank the Russian Foundation for Basic Research (contracts 06-03-32875 and 05-02-16528) and the INTELS foundation for science and education (contract G-35-06-01). They also greatly appreciate Mrs. T. Dragon’s (Max-Planck-Institut für Metallforschung, Stuttgart) help with the sample preparation for the domain structure investigations.
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