Structure, phase composition, and microhardness of carbon steels after high-pressure torsion
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The phase composition, mechanical properties, and microstructure of binary Fe–C alloys with various carbon concentrations (0.25, 0.45, 0.6, 1.3, 1.5, and 1.7 wt.%) were studied by transmission electron microscopy, X-ray diffraction analysis, and microhardness measurements. The investigations were carried out for three states of the material, namely for as-cast, annealed (725 °C) and deformed by high-pressure torsion (HPT) samples. The grain size after HPT is in the nanometer range. Only Fe3C (cementite) and α-Fe remain in the alloys after HPT. The residual austenite disappears and phase composition closely approaches the equilibrium corresponding to the temperature and pressure of HPT. Analysis of the microhardness behavior revealed that hardening of the deformed alloys takes place due to the grain refinement and dispersoid mechanism.
KeywordsFerrite Austenite Cementite Electron Diffraction Pattern Severe Plastic Deformation
Manufacturing materials with a very small grain size in the nanometer range is an important way of improving their mechanical properties . Such nanograined alloys are considerably stronger than their coarse-grained counterparts and at the same time they retain reasonable ductility. Different variants of severe plastic deformation (SPD) are very promising techniques for obtaining nanograined metals. Such SPD techniques like equal channel angular pressing (ECAP) and high-pressure torsion (HPT) do not induce changes in the material geometry compared to the conventional deformation processes like rolling or wire drawing. Apparently the SPD grain refinement from millimeter or micrometer to the nanometer range automatically leads to the strengthening of a material. However, it has been demonstrated recently that SPD along with the grain refinement may simultaneously result in the material softening . This means that the processes of structural changes during SPD are very complicated and have not yet been fully understood. Comprehensive investigation of nanograined polycrystals is important especially for two- and multicomponent systems, which are of the most importance for the technological use. The Fe–C system belongs to the alloys of crucial importance for structural and functional applications. The aim of this work is to study the structure and mechanical properties of Fe–C alloys in the broad interval of a carbon concentration in an as-cast state, after HPT deformation and after long annealing in the α + Fe3C region of the Fe–C phase diagram.
Iron–carbon alloys with carbon concentrations of 0.25, 0.45, 0.60, 1.3, 1.5, and 1.7 wt.% were prepared from high-purity 5N Fe and C by vacuum induction melting in the form of cylindrical 12 mm diameter ingots. The 2 mm thick discs were cut from the cast ingots in order to investigate the as-cast state. Discs of 0.4 mm thickness and 12 mm diameter were subjected to HPT in a Bridgman anvil-type unit at room temperature and pressure at 5 GPa (5 torsions, shear strain was about 6). Samples for structural investigations were cut from the HPT-deformed discs at a distance of about 3 mm from the sample center (i.e., in the middle of the HPT disc radius). Some of the cast alloys with 0.25, 0.60, 1.3, and 1.7 wt.% C were additionally annealed during 950 h at 725 °C (i.e., slightly below the temperature of eutectoid transformation) in order to achieve a structure corresponding to the equilibrium Fe–C phase diagram. Light microscopy (LM), transmission electron microscopy (TEM), and X-ray diffraction analysis (XRDA) were engaged to perform structural investigations of the alloys. Mechanical properties of the alloys were characterized by microhardness measurements on a standard facility with a load of 200 g. The value of microhardness was evaluated as a result of 10 measurements for each experimental point.
Results and discussion
The phase state of the alloys after different treatment was characterized by the XRDA technique. It was established that ferrite and cementite were the main structural elements in all as-cast alloys. Also the residual austenite was detected with a volume fraction of about 10%. Both the HPT deformation and the long annealing at 725 °C (i.e., below the eutectoid temperature) resulted in the structure equilibration: the residual austenite disappeared and the phase composition of the alloys corresponded to the equilibrium phase diagram at room temperature . It was also reported  that the lattice parameter of the α-Fe solid solution in the samples after HPT corresponded to about 0.01 wt.% C, i.e., to the solubility at room temperature . The lattice parameter of the (Fe) solid solution in the as-cast alloys does not differ from that in the HPT state more than 10−4 nm. Magnetization measurements  also support the fact that the phase composition of the alloys is very close to the equilibrium one. In other words, HPT does not lead to the formation of supersaturated carbon solid solution in α-Fe. It looks that ball milling is quite different from this point of view: the formation of supersaturated carbon solid solution was reported by Ohsaki et al. . Most probably, the ball milling influences the free surfaces of milled particles and in certain point is similar to the ion implantation. In the early work, Korznikov et al.  observed the disappearance of cementite peaks from XRD spectra and supposed that, similar to the ball milling, SPD leads to the formation of supersaturated carbon solid solution in α-Fe. However, the later careful measurements demonstrated that the very fine cementite particles and carbon segregation layers in the ferrite GBs form during SPD, but not the oversaturated carbon solid solution in α-Fe [8, 9, 10]. Therefore, the early hypothesis that SPD can lead to the formation of supersaturated solid solution in steels  was not supported by the later experiments [8, 9, 10].
Microstructure investigations with LM and TEM
Alloys annealed at 725 °C
Alloys after HPT
In the annealed samples, the microhardness substantially decreases in respect to the as-cast state, which is explained by: (i) complete equilibration of the composition of all phases, (ii) decrease of the dislocation density, and (iii) drastic increase of the cementite grain size and disappearance of the cementite hardening precipitates. This can be easily observed in the LM images (Fig. 3), where the precipitates are rather spherical than needle-shaped.
Microhardness of HPT alloys increases with increasing carbon content, but this is not associated with the change in their lattice parameter. This means that the hardening takes place due to the dispersoid and Hall–Petch mechanisms [13, 14]. The Hall–Petch hardening is attributed to the difference in orientation of the slip systems in two neighboring grains. This causes difficulties in the slip transfer across the grain boundaries and the resulting dislocation pile-ups hinder further deformation.
The data obtained for iron–carbon alloys after the SPD by Korznikov et al.  and Ivanisenko et al.  have been also inserted in the Fig. 5. The nanograined state of the UIC 860V pearlitic steel , containing 0.6–0.8% C, 0.8–1.3% Mn, 0.1–0.5% Si, 0.04% S, and 0.04% P, was also produced by the HPT technique but the applied pressure was 7 GPa. The hardness steadily increases by increasing the degree of deformation, expressed in the number of anvil rotations. To acquire a deformation close to our samples with a microhardness value of about 9.5 GPa, which is somewhat higher than that presented in this article, a higher pressure was applied. In , the steel Fe–1.2 wt.% C–0.22 wt.% Mn–0.25 wt.% Zn–0.01 wt.% S–0.025 wt.% P–0.2 wt.% Mg–0.2 wt.% Cu was HPT deformed at 10 GPa with true deformation e = 7. Both deformation and pressure were higher than in our case. This is probably the reason why the microhardness was also higher. Another reason for the increased hardness could be the presence of additional alloying elements in the steels studied in [7, 8].
Severe plastic deformation leads to a drastic increase in hardness for both hypoeutectoid and hypereutectoid Fe–C alloys. The hardening of alloys after HPT can be attributed to the influence of numerous interphase and grain boundaries in the nanograined structure formed after HPT (Hall–Petch mechanism). The hardiness increase of as-cast and HPT alloys with increasing carbon content can be attributed to the dispersoid hardening (Mott–Nabarro mechanism).
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) for their financial support of this research.
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