Peterhof, St. Petersburg, , Russian Federation. Nan Gang District, Harbin, , China. a

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1 Applied Mechanics and Materials Online: ISSN: , Vol. 566, pp doi: / Trans Tech Publications, Switzerland Investigation of High-Speed Loading Effects on the Properties of Ferromagnetic Alloys Processed in an External Magnetic Field Yuri Sudenkov 1, Svetlana Atroshenko 1, a, Ivan Smirnov, Natalya Naumova 1 and Sun Xueyin 2 1 St. Petersburg State University, Universitetsky prospect, 28, Peterhof, St. Petersburg, , Russian Federation 2 Harbin Institute of Technology, 92 West Dazhi Street, Nan Gang District, Harbin, , China a satroshe@mail.ru Keywords: high-speed deformation, pulse loading, ferromagnetic alloys, Fe-Cr-Co, foil explosion, spall strength, Hugoniot elastic limit. Abstract. The behavior of the ferromagnetic alloy based on Fe-Cr-Co under high-speed loading is presented. Three types of samples were prepared with different pre-treatments: quenching only; quenching and ageing; and quenching and ageing under an intense external magnetic field. The submicrosecond impact load was created by the installation for the electrical explosion of foils. The developed method of loading allows a pressure pulse to be registered before impact on a flat sample and after its exit to a free surface of this sample. Changes in the mechanical properties of the ferromagnetic alloy with various technologies of preliminary processing before and after shock loading are discussed. Introduction Ferromagnetic materials are substances which under the influence of an external (magnetizing) magnetic field are capable of being magnetized. At the same time they create magnetic field in the surrounding space. Ferromagnetic materials are widely used in various engineering fields [1]. The application of a ferromagnetic material depends on the physical/chemical parameters. For example, such materials can be used for parts and mechanisms operating with high dynamic loads: rotors of hysteresis motors, parts of gyroscopes, lifting devices, and so on. One of these magnetic materials based on alloys of Fe-Cr-Co was proposed by H. Kaneko [2]. Fe-Cr-Co alloys have a successful and very rare combination of high magnetic properties with corrosion resistance, durability, plasticity, and rather low cost because of the low content of expensive cobalt and lack of nickel. Therefore the Fe- Cr-Co materials have not lose practical relevance, and active research on them continues. A solution to many technical problems is connected with understanding of the processes of initiation and propagation of shock loads, and also with knowledge of the physical and mechanical parameters defining the behaviour of materials under high-speed deformation. A study of shockwave processes in materials makes it possible to receive information about the state equations of solids for a wide range of durations and intensities of loading, including during phase transformations. This paper presents a study of the deformation and fracture of the ferromagnetic alloy based on Fe-Co-Cr at the micro- and submicrosecond range of pressure pulse durations. We consider the dynamic strength and change of the material microstructure after high-speed loading depending on the pretreatment of samples in an external magnetic field. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-17/09/16,06:26:15)

2 Applied Mechanics and Materials Vol Method of research Hard magnetic materials show the highest values of magnetic parameters in a single-domain nonequilibrium form of crystals with non-ferromagnetic inclusions of large sizes, as well as when there are a large number of different kinds of distortions in the crystal lattice (the structure of the supersaturated interstitial solid solution) and high residual stresses. The listed conditions complicate the turning of magnetization vectors; that is, they complicate demagnetization. Alloys of this system usually pass through thermomagnetic processing, which consists of heating to quenching temperatures, holding at this temperature to achieve a single-phase state, and fast cooling to 900 C with subsequent slow cooling in a magnetic field. In the course of cooling in a magnetic field, the separated particle phase (anisotropic in form) with high magnetic saturation is arranged in the long axis direction parallel to the magnetic field vector, improving its characteristics. In this work, three types of samples based on Fe-Co-Cr (see Table 1) were investigated. The magnetic alloy samples were in the form of plates of Ø40 mm with a thickness of 4-9 mm, which underwent different pretreatments. Table 1. Heat treatment of the samples. Parameters of heat treatment Comments Cx1h (quenched) Cx1h (under 0,4T magnetic field) + aged (620 0 Cx1h Cx1h Cx2h Cx4h Cx6h Cx8h Cx10h) Cx1h (quenched) Cx1h (without magnetic field) + aged (620 0 Cx1h Cx1h Cx2h Cx4h Cx6h Cx8h Cx10h) Quenching, aging and treatment in magnetic field Quenching, aging Cx1h (quenched) Quenching The phase state was controlled by an Axio-Observer-Z1-M metallographic microscope after appropriate etching. The study of the fracture surfaces after the test was carried out on an optical microscope in a dark field. The investigation of the cross-sectional structure was carried out in bright field or polarized light. The quantitative ratio of phase decomposition of α-solid solution was determined by a metallographic analysis (a random linear intercept method). The microhardness was measured on a PMT-3 at a load of 100g. Fig. 1. Shock wave loading by foil explosion: 1 aluminum foil mm, 2 electrodes, 3 the frame from dielectric, 4 waveguide (steel), 5 optical window (PMMA, quartz glass, sapphire).

3 544 Proceedings of the 8-th International Symposium on Impact Engineering The shock loading was created by means of electric explosion of a foil. The foil explosion was carried out on the installation with the following parameters: capacitor C = 6 mf, voltage U up to 50 kv, and stored energy E 7.5 kj. The developed technique made it possible to conduct laboratory studies of deformation and fracturing under pulse durations in the range of µs and pressures up to 20 GPa. The loading scheme is shown in Fig. 1. Due to the symmetry of the electric explosion of the foil, we are able to control both the pulse parameters of loading and the parameters of the stress pulse transmitted through the sample. High-strength steel cylinders with dimensions of Ø20 18 mm were used as waveguides. The measurement of the shock pulse parameters was carried out using differential laser interferometers aimed at the free surface of the sample and the waveguide. Results Initial state. Some mechanical properties of the material were determined before impact testing (Table 2). Table 2. Material parameters. Type of sample ρ [g/cm3] 7,683 7,695 7,663 C [m/s] E [GPa] HV [ ] The material structure before shock compression is presented in Fig. 2. The alloy structure after quenching only (sample 3) represents a homogeneous solid solution practically without precipitates (Fig. 2a). The sample structure after quenching and aging (sample 2) represents uniformly located disperse allocations of a second phase of white colour, and we only occasionally see a fragile nonmagnetic intermetallic σ-phase (Fig. 2b). After passing through the stages of quenching, ageing and processing in the magnetic field the material structure (sample 1) has precipitation of secondary phases and banding (anisotropy of structure) (Fig. 3). Fig. 2. Material structure before shock compression. a) sample 3; b) sample 2; c) sample 1.

4 Applied Mechanics and Materials Vol Dynamic Loading. Figure 3 shows the time profiles of the free surface speed of the sample and waveguide for corresponding types of samples. The values of the elastic precursor and the spall strength are presented in Тable 3. The fracture structures of the samples after corresponding types of pretreatments (Fig. 3) are shown in Figs. 4 and 5. a) v fs [m/s] ,0x10-6 6,0x10-6 7,0x10-6 time [s] waveguide, h=18.23 mm sample 3, h=4.62 mm U = 30 kv b) v fs [m/s] waveguide, h=18.2 mm sample 2, h=4.5 mm U=30 kv 4,0x10-6 5,0x10-6 6,0x10-6 7,0x10-6 time [s] c) v sf [m/s] sample 1, h=7.6 mm U=25 kv 0 6,0x10-6 7,0x10-6 8,0x10-6 time [s] Fig. 3. Typical time-velocity profiles of the free surface of the samples: a) in the state of a homogeneous solid solution (sample 3). b) in the state of a solid solution disintegration (sample 2). c) after quenching, aging and treatment in a magnetic field (sample 3). Table 3. Elastic precursor and spall strength. σ H [GPa] σ sp [GPa] Sample 1 2,36 7,95 Sample 2 1,16 11,2 Sample 3 9,15 12,4 In the type 3 sample, the fracture pattern shows a prevalence of fragile failure. Excess primary phases remaining in the alloy after quenching are observed here. The parts of a spall crack are located under different corners to magnetic domains in the direction of the stress wave distribution. In the type 2 sample, separate magnetic domains and a high density of a secondary phase are observed. Separate cracks are observed along the direction of stress wave distribution. This type of sample appeared to be the most plastic. Here, splitting of the sample into parts is absent, which correlates with the results of microhardness (Table 2). Plasticity of the sample can be explained by the smaller quantity of a fragile phase and the occurrence of twinning on loading.

5 546 Proceedings of the 8-th International Symposium on Impact Engineering In the type 1 sample, fragile fracture prevails (as for type 3). Cleavage is observed in a rupture, but sites of a viscous component are present too. Fracture of the sample consists of sites of longitudinal shear along the direction of the stress wave distribution and sites of rupture along the front of the stress wave, which are connected by zigzag cracks under different corners. Fig. 4. Free surface fracture of samples. a) sample 3; b) sample 2; c) sample 1. Fig. 5. Structure of metallographic cross-section of samples. a) sample 3; b) sample 2; c) sample 1.

6 Applied Mechanics and Materials Vol Summary Tests of a ferromagnetic alloy based on Fe-Cr-Co under high-speed deformation and destruction have been carried out. Depending on preliminary processing three types of samples were investigated: quenching only (type 3); quenching and ageing (type 2); and quenching and ageing under an intensive external magnetic field (type 1). The quenched sample (type 3) showed the highest spall strength; it also had the highest Hugoniot elastic limit. The quenched and aged sample (type 2) showed a significantly lower value of the dynamic elastic limit with a very high dynamic strength. This sample also had the highest ductility. The sample after treatment in a magnetic field (after quenching and ageing) showed the lowest spall strength. Samples of types 1 and 3 showed predominantly brittle fracture. References [1] R.A. McCurrie: Ferromagnetic materials: structure and properties. (London: Academic Press, 1994) [2] M. Kaneko, M. Homma, T. Minowa: IEEE Trans. Magn. Vol. 12, N. 6 (1976), p. 977.