The structure and mechanical properties of Fe-15%Co-25%cr hard magnetic alloy subjected... Rev.Adv.Mater.Sci. 11 (2006) 109-115
109
THE STRUCTURE AND MECHANICAL PROPERTIES OF Fe-15%Co-25%Cr HARD MAGNETIC ALLOY SUBJECTED TO TORSION UNDER HIGH PRESSURE G. F. Korznikova Institute for Metals Superplasticity Problems RAS, Ufa, st. Khalturina 39, 450001, Russia Received: July 20, 2005 Abstract. The evolution of the structure and mechanical properties of a Fe-15%Co-25%Cr hard magnetic alloy during shear deformation at various angles of rotation in the Bridgman anvils was studied. Severe plastic deformation of the high-coercivity (α1 + α2) state was shown to result in the dissolution of the α1 phase in the early deformation stage. A further increase in the deformation leads to the formation of a single-phase nanocrystalline structure with a grain size of about 50 nm. The dissolution of the α1 phase in the α2 matrix during severe plastic deformation was found to cause an increase in the strength and plasticity characteristics of the Fe-15%Co-25%Cr alloy at all degrees of deformation studied. The maximum plasticity was detected in the alloy with a mixed structure consisting of regions of submicrocrystalline and cellular types, and the formation of nanocrystalline grains led to a certain decrease in the plasticity.
1. INTRODUCTION Unltrafine-grained materials attract great attention of investigators because of their specific physical and mechanical properties [1,2]. Severe plastic deformation (SPD) is one of the efficient techniques for manufacturing bulk submicrocrystalline materials. In particular, torsion under quasi-hydrostatic pressure [3,4] provides deformation without failure of both pure metals and brittle intermetallics [5]. In ductile and hardly hardenable metals and alloys a structure with a grain size of 100-200 nm is formed [5], whereas the size of grains that are formed during plastic deformation in intermetallics, alloys with insoluble elements, and metal-metalloid alloys is 10-20 nm [2,5,6].
Severe plastic deformation also strongly influences the phase composition of alloys [2,5]. It was shown [5,7] that the second phases often dissolve during severe plastic deformation to form solute concentrations in the solid solutions significantly exceeding the solubility limits of these elements at low temperatures. The Fe-15%Co-25%Cr alloy belongs to hard magnetic materials of the precipitation-hardening class [8]. Magnets are produced from this alloy by both casting and plastic working methods. The formation of a high-coercivity (α1 + α2) state (HCS) during spinodal decomposition leads to a sharp decrease in the strength and plasticity characteristics due to a modulated structure consisting of coherent ordered precipitates of the α1 phase in the α2 ma-
Corresponding author: G.F. Korznikova, e-mail:
[email protected] © 2006 Advanced Study Center Co. Ltd.
110
G. F. Korznikova
trix. It is known that the plasticity characteristics of industrial alloys can be significantly increased by changing the size and morphology of ordered phases [5,9]. The purpose of this work was to study the evolution of the structure and mechanical properties of the Fe-15%Co-25%Cr alloy during severe plastic deformation by torsion at a high pressure.
2. EXPERIMENTAL The Fe-15%Co-25%Cr alloy consists of 25 wt.% Cr, 15% Co, 1% Ti, 1% V, 0.4% Si, 1% Al, 1% Nb, and Fe the balance. After hot forging in a temperature range of 1000-800 °C corresponding to the twophase alloy state, a workpiece was water-quenched from 1200 °C. Cylindrical samples 8 mm in diameter were cut from the workpiece quenched to obtain a single-phase α solid solution and then subjected to standard thermomagnetic treatment and stepwise tempering in a range of 640-540 °C to obtain a two-phase (α1+α2) state. To study the effect of torsional severe plastic deformation on the structure and microhardness of the Fe-15%Co-25%Cr alloy with the initial (α1 + α2) state, the samples were deformed using angles of the die rotation of (ϕ = 180, 360, 1080, 1800, and 3600°). The samples for deformation were spark-cut as plates 0.3 mm thick. Deformation was conducted on a setup described in [4] using anvils of a VK6 alloy under a pressure exceeding the mean microhardness of the alloy under investigation after all treatments, which allowed us to eliminate slip between a sample and the anvils. The deformation time at ϕ = 3600° was 8 minutes. The degree of deformation was estimated by the technique described in [10] by the formula
F F ϕ* R IJ I + lnFG h IJ , e = lnG 1 + G H H h K JK H h K 1/ 2
0
2
iR
(1)
iR
where ho is the sample thickness before deformation, ϕ is the angle of rotation of the moving anvil, R is the distance from the rotation axis, and hiR is the sample thickness after deformation at a distance R from the center. X-ray diffraction analysis was performed on a DRON-3M diffractometer using Cu Kα radiation. The microstructure of the samples was examined in a JEM 2000 EX transmission electron microscope at an accelerating voltage of 150 kV. The coercive force was measured in a vibrating-sample magnetometer in a field of 1600 kA/m.
The microhardness was measured on a PMT-3 device at a load of 0.2 kg using more than 15 measurements per point. The measurement error for various samples deformed at the same degree of deformation was less than 5%. Mechanical three-point bending tests were conducted on an Instron dynamometer equipped with a strain-gage transducer with a maximum load of 50 N. The samples for the bending tests were sparkcut and finished to dimensions of 1.5x0.15x8 mm by grinding and polishing. The bending stresses σ were calculated by the formula [11]:
σ = 15 .
PL bh
2
,
(2)
where P is the load, L is the distance between the supports, b is the sample width, and h is the sample thickness. Based on calculated ultimate strength values, we drew conclusions about the mechanisms of strain hardening. We used ultimate bending flexure to estimate the plasticity of samples. To a first approximation, this correlates with the ultimate strain [11].
3. RESULTS AND DISCUSSION 3.1. Structure and coercivity of the Fe-15%Co-25%Cr alloy We consider the evolution of the structure during torsional severe plastic deformation for the angles of anvil rotation ϕ = 180, 360, 1080, 1800, and 3600°. X-ray diffraction analysis of this state showed that in the initial undeformed state all α-phase lines are split into two peaks, which is due to the decomposition of the α-solid solution into isomorphic α1 and α2 phases coherently related to one another. As a result of SPD at ϕ = 360°, the peaks of the α1 and α2 phases partially merge, the relative maximum intensities of the higher-order peaks decrease, and the line profiles change. After deformation at ϕ = 1080°, the peaks of the α1 and α2 phases completely merge, which evidences the transformation of these phases into a single a phase as a result of severe plastic deformation. A further increase in the deformation to ϕ = 3600° leads to a certain broadening of the peaks of the a phase. The fine structure of the initial state is shown in an electron-microscopic micrograph in Fig. 1a. It is seen that the structure is two-phase and consists of α1 precipitates in the α2 matrix. The mean grain size of the matrix determined metallographically is 150 mm. In the early deformation stage (ϕ = 360°),
The structure and mechanical properties of Fe-15%Co-25%cr hard magnetic alloy subjected...
111
a)
c)
b)
d)
Fig. 1. Fine structure (a, b, c) and selected area electron diffraction pattern (d) of the Fe-15%Co-25%Cr alloy: (a) without deformation, (b) after deformation at ϕ= 360° and (c, d) at ϕ= 3600°.
some deformation bands with a high dislocation density appear in the structure, and the α1 phase begins to dissolve in these bands (Fig. 1b). An increase in the degree of deformation to ϕ = 3600° leads to the complete dissolution of α1 particles and the formation of a nanocrystalline fcc structure with a mean grain size of about 50 nm (Fig. 1c). A ring
electron diffraction pattern taken from a 0.5 µm2 region exhibits high-angle misorientations between crystallites (Fig. 1d). After deformation at ϕ = 3600°, coercive force is equal to 13.5 kA/m, which is one third of that in the initial state (40 kA/m). This also supports the X-ray diffraction data on the formation of the α solid solu-
112
G. F. Korznikova a)
b)
Fig. 2. Microhardness (a) and deformation distribution (b) as function on the distance from the sample after shear under pressure at various angles of anvil rotation: ♦-ϕ = 180°; n-ϕ = 360°; -ϕ = 1080°; l-ϕ = 1800°; o-ϕ = 3600°; ¡- without deformation.
tion after deformation. It should be noted that this value is 2.6 times as great as that in the state of the α solid solution obtained by high-temperature quenching (the coercive force 5 kA/m), which is due to a high defect density and a small size of crystallites in a sample after shearing under pressure.
3.2. Microhardness of the Fe-15%Co25%Cr alloy We plotted the dependence of microhardness HV on the distance from the center of a sample for all deformed samples (Fig. 2a). The results exhibit that at all angles of anvil rotations the microhardness smoothly increases with the distance from the center. Note that after deformation at an angle of rotation ϕ = 3600° the microhardness levels saturate. Moreover, we calculated by Eq. (1) and plotted the dependence of the degree of deformation on the radius of a sample e(r) taking into account its thickness (Fig. 2b). The calculation results show that the degree of deformation monotonically increases with the distance from the center at all angles of rotation, and the same degree can be achieved in samples deformed at different ϕ and different distances from the center. Using the measured HV(r) data and the calculated e(r) data, we can plot the strain-hardening curve HV(e).
Fig. 3 illustrates the HV(e) curves for samples deformed using various angles ϕ of anvil rotation. To compare samples deformed using various angles of anvil rotation, we analyzed the microhardness values obtained at the same distance (2 mm) from the sample center. It is seen that the points corresponding to various angles of rotation fall onto one curve only at the angles of rotation ϕ = 180 and 360°, whereas the points corresponding to the angles ϕ = 1080, 1800, and 3600° fall onto straight lines with gradually decreasing slopes; that is, the microhardness distribution over the radius of samples corresponds to the deformation distribution upon shear under pressure only at small angles of anvil rotation. This mainly can be due to the fact that no phase transformations occur in the early deformation stages, as is shown by X-ray diffraction studies. At greater angles of anvil rotation, the loading time substantially increases and a sample is inevitably heated. The calculations of the energy evolving in the bulk of samples during their plastic deformation as the energy of the flux of moving dislocations [12] demonstrate that the heat release in a sample upon shearing under pressure can achieve 40-180 J, depending on the sample thickness. The related heating can activate phase transformations. In our case,
The structure and mechanical properties of Fe-15%Co-25%cr hard magnetic alloy subjected...
113
Fig. 3. Microhardness of the deformed Fe-15%Co-25%Cr alloy: ¡- microhardness measured at the radius R = 2 mm on the samples deformed at various ϕ; microhardness measured measured along the radius R = 1, 1.5, 2 and 2.5 mm on the samples deformed at various ϕ: ♦-ϕ = 180°; n-ϕ = 360°; -ϕ = 1080°; l-ϕ = 1800°; o-ϕ = 3600°.
the deformation time was 8 min and the temperature of the sample increased by 80 K. Similar dependences were obtained in Ref. [10] dealing with the deformation of structural steels 20G2R, 30G2R, 30KhGSN2A, and armko iron. Degtyarev el al. [10] showed that points corresponding to the measurements of microhardness on samples deformed at various ϕ fell onto one curve. The strain-hardening curve for armko iron had a steeper slope at small degrees of deformation, whole in the steels, on the contrary, a small slope was observed up to e = 2-4 and it abruptly increased at large degrees of deformation; that is no microhardness saturation was detected in any of the materials studied. The difference from the results obtained in [10], where only single-phase materials were considered, can be due to dynamic softening related to the dissolution of α1 particles in the α2 matrix and the formation of the a solid solution (α1+α2→α).
3.3.Mechanical properties Fig. 4 illustrates the mechanical properties of the Fe-15%Co-25%Cr alloy after the bending tests, depending on the angle of rotation of the moving anvil. It is seen from Fig. 4a that the alloy in its initial state at ϕ = 0 after heat treatment for high-
coercivity state fails in the elastic regime of loading. A sample of the alloy subjected to SPD at α = 180° becomes plastic and its yield stress is σy = 1405 MPa. A further increase in the degree of deformation leads to a monotonic increase in σy to its maximum value σy = 2270 MPa at ϕ = 3600°. Fig. 4b shows that the dependence of the ultimate strength σu on the degree of deformation also has a two-stage character. At the first stage, up to ϕ = 180°, σu increases from 990 to 1620 MPa. At the second stage, σu smoothly increases to 2570 MPa at ϕ = 3600°. Fig. 5c illustrates the dependence of the ultimate bending flexure h on the degree of deformation. This dependence is seen to be nonmonotonic. An increase in h when the deformation increases to ϕ = 180° is likely due to partial dissolution of the α1+α2 phases. A further increase in the deformation to ϕ = 360° and 1080° results in a further increase in the plasticity, and samples do not fail at a bending flexure of more than 2 mm (shown by arrows in the curve). Electron-microscopic studies and X-ray diffraction analysis exhibited that the increase in the alloy plasticity is caused by the transformation of the α1+α2 phases into the a phase and the formation of a mixed structure consisting of regions with a cellular structure and regions with a submicrocrystalline structure. An increase in the
114
G. F. Korznikova a)
c) ↑
b)
deformation to ϕ = 1800° and 3600° leads to a smooth decrease in the ultimate bending flexure to 0.9 and 0.35 mm, respectively. Transmission electron microscopy shows that the decrease in the plasticity is caused by a decrease in the volume fraction of regions with a cellular structure and the formation of a homogeneous nanocrystalline structure in the a solid solution in the alloy. The effect of the structure on the mechanical properties of the alloy can be explained by considering the interrelation between plastic deformation and failure. Note that these processes are competing, and the predominance of one of them is determined by a number of factors. It should be noted that both plastic deformation and failure depend on many factors. The enhanced plasticity of the nanocrystalline alloy is likely to be due to the disappearance of the coherently related α1 and α2 phases as a result of the complete dissolution of the α1 phase during SPD and the formation of a nanocrystalline structure. In this case, the alloy can be considered as a conventional severely coldworked single-phase structure.
4. CONCLUSIONS (1) Severe plastic deformation of the Fe-15%Co25%Cr alloy in its high-coercivity (α1+α2) state was found to lead to the dissolution of the α1 and α2 phases and the formation of a supersaturated a solid solution with a grain size of about 50 nm. (2) The microhardness of the alloy deformed to ϕ= 3600° was shown to reach saturation; HV in-
↑
Fig. 4. Mechanical properties of the Fe-15%Co25%Cr alloy upon bending tests. The variation of (a) the yield strength σy, (b) ultimate strength σu and (c) ultimate bending flexure h as function of the angle ϕ of rotation of mobile anvil. Arrows (↑) correspond to samples that not fail upon bending.
creased from 3900 MPa in the initial state to 7420 MPa. (3) We revealed that the dependence of the mechanical properties of the Fe-15%Co-25%Cr alloy on the degree of deformation in the high-coercivity state is nonmonotonic. The maximum plasticity is characteristic of the alloy with a mixed submicrocrystalline and cellular structure. The formation of a nanocrystalline structure results in a decrease in the plasticity.
REFERENCES [1] H. Gleiter // Acta Mater. "& (2000) 1. [2] A.I.Gusev, Nanokristallicheskie materialy: metody polucheniya i svoistva (Ekaterinburg: URO RAN, 1998), In Russian. [3] P.W.Bridgman // Phys. Rev. "& (1935) 825. [4] R.I.Kuznetsov, V.I.Bykov, V.P.Chemyshev, et al., Plastic Deformation of Solids under Pressure (Preprint of the Inst. of Metal Physics, Ural Sci. Center, USSR Acad. Sci., Sverdlovsk, no. 4/85, 1985), In Russian. [5] A.V. Korznikov, Structure and Mechanical Properties of Metals and Alloys Subjected to Severe Plastic Deformation, (Doctoral (Tech.) Dissertation, Ufa: Inst. for Metals Superplasticity Problems, 2000), In Russian. [6] R.A.Andrievskii and A.M.Glezer // Phys. Met. Metallogr.&& (1999) 45. [7] V.V. Sagaradze, V.A.Shabashov, T.M.Lapina, N.L.Pecherkina and V.P Pilyugin // Phys. Met. Metallogr. %& (1994) 619.
The structure and mechanical properties of Fe-15%Co-25%cr hard magnetic alloy subjected... [8] V.V.Sergeev and T.I.Bulygina, Magnitotverdye materialy (Moscow: Energiya, 1980), In Russian. [9] O.A. Kaibyshev, Sverkhplastichnost promyshlennykh splavov (Moscow: Metallurgiya, 1984), In Russian. [10] M.V Degtyarev, T.I. Chashchukhina, L.M. Voronova, L.S.Davydova and V.P. Pilyugin // Phys. Met. Metallogr. ' (2000) 604.
[11] A.A.Popov, R.Z.Valiev, I.Yu.Pyshmintsev, S.L.Demakov and A.G. Illarionov // Phys. Met. Metallogr. &! (1997) 550. [12] M.Kh.Shorshorov and A.V Korznikov // Materialovedenie (2002) 37, In Russian.
115
