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Formation of new phases, dark spots and slide bands in low carbon steel under the effect of cyclic deformation.

1. Introduction

Cyclic loading of certain metals in the equilibrium condition (carbon steels, copper, lead polycrystals [1), etc.) is accompanied by the formation of dark spots in the microstructure. In the initial stage, these spots appear in the form of separate dark areas which in further stages, especially at the start of fracture, may extend to the entire grain and occupy a large part of the deformed section.

Figure 1 shows the temperature-kinetic curve of fatigue indicating the observation points and microstructures, corresponding to these points: broken lines and Roman numerals on the curve--classification on the basis of the periods of fatigue failure.

In [2] it has been proposed to investigate the process of fatigue failure as consisting of the following periods: 1) incubation, 2) the period of active formation of slip bands; 3) the period of local buildup of damage and changes, formed during the first and second period; 4) the period of propagation and growth of the main crack; 5) the period of failure of the specimen (the duration of this period for the specimens with a small cross-section is very short).


2. Experimental results and discussion

The data shown in Fig. 1 indicate that no new bands formed after the completion of the stage of active formation of slip bands (examination point 2), but the relief and density of dark formations rapidly gradually increase, reaching the maximum at the end of the third period (examination point 3).

This phenomenon has been studied in a large number of investigations, for example [3], but the reasons and nature of these formations have not been completely explained.

Previously, a preferential interpretation of the reasons for the formation and propagation of these spots was proposed in [4]: the grains and sections of the grains, subjected to the effect of the highest cyclic stresses, causing active cyclic deformation, causing heating which may lead to the oxidation of these microvolumes. Evidently, the nature of these formations is the same as that of disintegration taking place along the slip bands. They are highly intensive in rimming steel whose special feature is the high plasticity and oxygen content. Therefore, the intensity of these formations may be associated with gas saturation of metal, also taking into account the fact that these spots also form in tests in vacuum [5].

In this study, the microstructure of ductile specimens of 08kp steel (C = 0.05-0.12%, Mn = 0.25-0.50%, Si = 0.03%) with a working cross-section of 1 * 10 mm was analysed. Cyclic loading was carried out by alternating bending with a loading frequency of 2800 cycle/min in equipment with a constant oscillation amplitude [6].


Auger spectroscopy

Figure 2 shows the results of Auger spectroscopy examination in the form of graphs of the concentration of elements in the thickness of the initial and deformed specimens.

Comparative analysis of these graphs shows that after cyclic deformation the iron content on the surface decreases whereas the carbon content increases. This should indicate that diffusion processes resulted in the displacement of carbon to the subsurface layer heated to high temperatures in which this element could react with the components of the alloy--oxygen and iron.

Figure 3 shows the results of scanning the surface of the initial (left) and deformed specimens with the beam with a diameter of 1 urn, and a scanning section (the photograph was taken from the monitor of the Auger spectrometer, at a modification of approximately 60).

On the basis of the data in Figure 3 it may be concluded: the absolute symmetry (mirror nature) of the distribution of the graphs of carbon and iron may indicate that the various high-intensity chemical reaction between these two elements; oxygen was also activated in the deformed specimen, and the maximum peaks on the carbon and iron graohs correspond to 'surges' of oxygen activity; with the approach of the scanning beam to the fatigue crack where the deformation reaches the highest value, the difference between the lines of iron (concentration values) and carbon increases; the form of the carbon peak indicates that the carbon is in the bonded condition (as carbides or oxycarbides) [12, 13] (Figure 4).


Consequently, it may be assumed that cyclic loading of the metal is accompanied by complicated physical-chemical processes with the possible formation of new compounds.

Figure 4 shows the Auger spectra from the surfaces of the initial and deformed specimens. Comparison of the spectra shows that the amplitude of the Auger lines of carbon and oxygen in the deformed specimen increased which also indicates and confirms the high content of these elements in the subsurface region in comparison with the initial specimen.

Accurate measurements of the amplitude of the Auger line of iron show that the amplitude slightly decreased, indicating the reduction of the iron concentration in the subsurface region of the deformed section.

If it is assumed that some of the iron atoms have entered into interaction with the oxygen and carbon atoms, this reduction should be difficult to notice--the carbon content of 08kp steel does not exceed 0.08%, and the oxygen content is even lower. Nevertheless, the data in Fig. 1 shows that the spots occupy a small part of the deformed surface.

Results of examination by the x-ray fluorescence method

One of the parameters of x-ray fluorescence analysis is the recording of the variation of the ratio of the intensity of the emission lines of a single series [7].


The method makes it possible to determine the form in which the atoms are present in the solid on the basis of the ratio of the integral intensity of the [K.sub.[beta]]-lines of the x-ray spectrum to the integral intensity [K.sub.[alpha]].

The energy spectra were also measured, Fig. 5, and the ratio Fe[K.sub.[alpha]1,2]/Fe[K.sub.[beta]1,2] were calculated.

The following results were obtained. The ratio for the initial specimen is 7.0677624 [+ or -] 0.0005, and in the deformed specimen it is 7.025773 [+ or -] 0.0005. The difference in the results is greater than the measurement error and indicates the change of the chemical state (valence) of the iron atoms in the deformed specimen.

This may indicate that some of the iron atoms have entered into the physical-chemical interaction with the components of the material. The small difference in the peaks of the deformed (broken line in Figure 5) and the initial specimen is explained by the fact that the concentration of iron cannot be compared with the concentration of the other components--carbon and oxygen.

As indicated by the results of atomic force microscopy analysis (AFM), Fig. 6, the cyclic deformation of the steel specimens is accompanied by a large increase of the surface roughness, at least by a factor of 2.5-3.

X-ray diffraction analysis results

To improve the accuracy of the results presented previously, DRON-3 diffractometer was used for x-ray diffraction analysis of the initial and deformed surface of the specimen. Analysis of the diffraction diagram shows that in addition to the high-intensity reflections of [alpha]-Fe, the diffraction patterns also contain new low intensity peaks. The phase composition of these lines determined on the basis of the ASTM library shows that the compound [C.sub.2]Fe[O.sub.4] is closest as regards the planar spacing (iron oxylate).


At taking into account the importance of this conclusion in the interpretation of the fatigue kinetics processes,--the formation of the compound identified in a DRON-3 diffractometer, requires a temperature of the order of 600-800[degrees]C, whereas heating of the specimen prior to failure did not exceed 1.5%--it was decided that it would be necessary to verify the results in a diffractometer of the latest modification X'Pert PRO PANanalitical (Holland).


Examination parameters: the range of the diffraction angle 42-140[degrees], the size of the step 0.05[degrees], speed 0.05[degrees]/s, U = 40 kV, I = 40 mA; emitter --copper.

Figure 7 shows the diffraction diagram of the initial specimen. The graphs indicate that it has the formal characteristic of [alpha]-Fe: lines [alpha]-Fe-110), [alpha]-Fe (200), [alpha]-Fe (211), [alpha]-Fe (220), [alpha]-Fe (310).

The diffraction diagram of the deformed area of the specimen (Fig. 8) shows, in addition to the lines characteristic of [alpha]-Fe, two new lines (two 'forks' at the top of the diffraction diagram). This indicates and confirms the formation of new phases. Identification of these phases on the basis of the data of International Centre for Data Diffraction (Copyright 2007) shows that they contain the following compounds: iron carbonate (FeC[O.sub.3], characteristic lines at 78.3036 and 112.7 988[degrees]) and iron oxylate ([C.sub.2]Fe[O.sub.4], characteristic line at 78.0749[degrees]).

Results of electron microscopic analysis

These formations have been studied quite extensively by optical microscopy and, consequently, it is interesting to investigate them under high magnifications. Figure 9a-c shows photographs of the structures, produced in a JSM-6510 scanning electron microscope.



According to the GOST 5640-59 standard, steel 08kp can contain structurally free cementite [Fe.sub.3]C. In Fig. 1, position 1, this phase is present in the form of individual inclusions (in the centre in Fig. 1. 1, position 1) and in the form of colonies--clusters at the grain boundaries (in the lower part of the photograph). Figure 9a shows the form of these compounds and the distribution under a high magnification.

The detected formations are distributed in the slip bands and this also explains the increase of the width of the band during cyclic loading (Fig. 9b in the centre of the photograph, surrounded by residual cementite inclusions).

In dark spots (Fig. 9c) they form a large area with cavities. In measurements of hardness in a NHN-SAX-000X nanohardness meter at a load of [P.sub.m] =10 mN the indentor appears to 'collapse' (Fig. 9d)--the area in the diagram in Fig. 8d. This means that a porous fine-dispersion mixture of these formations is identified. Hardness is half the hardness of the initial grain (HV of 163 and 303, respectively).

On the basis of these results it may be assumed that the process of cyclic deformation is accompanied by the separation of the metal microparticles in the slip planes under the effect of cyclic friction, and the effect of high-temperatures results in the formation of suitable conditions, in submicrovolumes, for the occurrence of chemical reaction with the formation of the detected new phases.

Regardless of the fact that the effect of temperature on the fatigue test results is well-known, nevertheless the thermal effects, formed in the material under cyclic loading, have been studied insufficiently.

The form of the temperature-kinetic fatigue curves depends on the type of crystal lattice. Figure 10 shows the temperature-kinetic fatigue curves for different materials [8]. The data in the figure shows that the temperature-kinetic curves for the FCC metals are the mirror reflection of the curves for the BCC metals.

It should be mentioned that the curves, describing indirectly the fatigue process (temperature, internal friction, mechanical hysteresis loops, etc) are quantitatively identical.

The role of the thermal processes in cyclic loading can be illustrated as follows.

Figure 11 shows the results obtained for fatigue tests of the variation of the intensification of the specimens made of steel 50 with and without cooling, with other conditions being equal [9].

The comparative analysis of the curve shows that in the normal conditions (without cooling), the form of the kinetic curve is typical of the BCC metals, whereas in cooling it is typical of the FCC metals. It appears that this example is a highly convincing confirmation of the effect of internal thermal phenomena on the fatigue process. As regards the formation of the new phases, reported in this study, their formation may be caused by the effect of thermal fluctuations.




3. Conclusions

The formation in the investigated steel of new phases under cyclic deformation is not doubted. They may be caused by cyclic friction in the slip planes of grains deformed with high intensity with possible separation of the individual sub-microstructural particles of the metal, and the effect of high temperatures results in the formation of suitable conditions for chemical reactions in the submicrovolumes [14]. This fact provides more accurate information of the processes associated with fatigue.

In the equilibrium condition, under cyclic loading, dark spots were detected in a number of metals (carbon steel, copper, lead polycrystals, etc). In the initial phase, they appear in the form of individual fine dark areas which may subsequently propagate (especially at the start of failure) over the entire grain and occupy a large part of the deformed section. The diffraction patterns of the deformed specimens also contain new phases FeC[O.sub.3]--iron carbonate (78.3036 and 112.7988[degrees] ICDD) and [C.sub.2]Fe[O.sub.4]--iron oxylate (78.0749[degrees]). They may be caused by the process of cyclic friction in the slip planes of grains deformed at high intensity with the possible precipitation of the individual sub-microstructural metal particles, and the effect of high temperatures in the sub-microvolumes results in suitable conditions for chemical reactions.


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[2.] L. A. Gorbachev, et al., Zh. Prikl. Mekh. Tekh. Fiz., No. 5, 133 (1970).

[3.] A. V. Gur'ev and G. Yu. Stolyarov, Izv. An SSSR, Metally, No. 3, 133 (1967).

[4.] L. A. Gorbachev, Investigation of kinetics of fatigue failure by the temperature method Dissertation, Leningrad, 1971.

[5.] Lozinskii M. G., Strength of metals in cyclic loading, Nauka, Moscow, 1967.

[6.] L. A. Gorbachev, et al., Tr. Leningrad. Polit. Inst., No. 314, 128 (1970).

[7.] A. A. Verigin, Energy-dispersing x-ray spectrum. Application in industry, Tomsk State University, Tomsk, 2005.

[8.] L. A. Gorbachev, Zavod. Lab., No. 12, 1500 (1972).

[9.] N. I. Kharitonov, et al., Probl. Prochn., No. 9, 44 (1972).

[10.] L. A. Gorbachev and A. D. Pogrebnjak, Fiz. Inzh. Poverkh., 7, No. 1/2, (2009).

[11.] L. A. Gorbachev and A. D. Pogrebnjak, Phys. Surface Engineer, 7, No. 1/2, 22 (2009).

[12.] A. D. Pobregnjak and A. M. Tolopa, Nucl. Instrum. Methods, B, 52, No. 1, 25 (1990)

[13.] A. N. Didenko, et al., ibid, 27, No. 3, 421 (1987).

[14.] K. A. Gorbachve, Confirmation of observation of a new phenomenon, Kazakhstan, 2006.

L.A. Gorbachev and A.D. Pogrebnyak ***

East Kazakhstan State Technical University, Ust'-Kamenogorsk, Kazakhstan

* Sumy State University, ul. Rimskogo-Korsakova 2, Sumy, Ukraine

** Sumy Institute for Surface Modification, 40030 Sumy, Ukraine
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Author:Gorbachev, L.A.; Pogrebnyak, A.D.
Publication:Physics of Metals and Advanced Technologies
Article Type:Report
Geographic Code:4EXUR
Date:Jan 1, 2010
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