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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.09.008
Open Access
Structural and phase changes under electropulse treatment of fatigue-loaded titanium alloy VT1-0
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Sergey Konovalova,b,
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ksv@ssau.ru
konovserg@qq.com

Corresponding author.
, Irina Komissarovab,c, Yurii Ivanovd, Victor Gromovc, Dmitry Kosinovc
a Institute of Laser and Optoelectronic Intelligent Manufacturing, Wenzhou University,Wenzhou, Zhejiang, China
b Samara National Research University, Samara 443086, Russia
c Siberian State Industrial University, Novokuznetsk 654007, Russia
d Institute of High Current Electronics of the Siberian Branch of the RAS, Tomsk 643055, Russia
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Abstract

The effect of electropulse treatment (EPT) of titanium alloy VT1-0 on the change of its fatigue life, structure and phase composition has been investigated. The study has shown that EPT improves the fatigue life by 1.3 times. Methods of TEM were used to analyze changes of the structure and phase composition of samples subjected to EPT and endurance testing. In the process of fatigue tests involving cantilever bending, a grain and sub-grain structure is formed in the surface of commercially pure titanium. The study has disclosed that the surface layer of the material has a multi-phase structure. Nano-crystal grains are formed by α-titanium. Oxide phase of titanium has been revealed, which is located along the crystallite boundaries of α-titanium. Particles of oxide phase have been identified exclusively in the top nano-structured layer. In the rest of the sample there are no particles of this phase. EPT causes dimensional changes of oxide phase particles and many-fold dimensional changes of crystallites in the surface layer. Owing to this treatment there are less internal stress concentrators in the surface layer of the material, which results, therefore, in cut down on probable spots of crack origination.

Keywords:
Titanium alloy
Electropulse treatment
Microstructure
Phase composition
Fatigue strength
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1Introduction

To date, improvement of strength, resource and fatigue life capability is a top-priority issue in the field of utilizing machines and constructions. The most responsible and unique products, machines and constructions are operated in conditions of cyclic deformations, which can cause a failure even under light loads. Fatigue failure is one of the most frequent reasons for breakdowns of equipment, mechanisms, machines and facilities. This fact is caused by the specific character of multi-cycled fatigue, which involves, in the first place, origination and propagation of cracks even if stressed insignificantly, secondly, a fast response of fatigue life to various structural, engineering and operational factors, thirdly, many-fold dispersion of fatigue characteristics as against those of static strength, in the fourth place, the local and selective character of origination of cracks and their propagation without visible residual displacement until an emergency situation takes place. Therefore, it is important to prevent fatigue failures of critical parts (enhance their operational life), especially, in industries where emergency situations might lead to a disastrous effect [1–3].

Both research and design-experimental and engineering developments are focused on the issues of fatigue and strength. Fatigue strength and life are significant criteria when assessing operating capacity and resource of machine parts and structures. They are essential to up to date highly-loaded critical machine parts subjected to cyclic loads in conditions of high- and low-cycle fatigue. It is a challenging issue to estimate cyclic strength of constructional materials, since a fatigue failure depends upon numerous factors (structure, state of the surface layer, temperature and environment when testing, frequency of loading, concentration of stresses, cycle asymmetry, scaling factor, etc.). In general, fatigue involves gradual accumulation, reciprocal action of defects (vacancies, interstitial atoms, dislocations, and disclinations, twins, grain and block boundaries) in a crystal lattice, resulting, as a consequence, in fatigue failures like origination and propagation of micro- and macro-cracks [1–5].

Different methods of strengthening treatment are used to improve the fatigue life of metallic materials. Recent work by researchers is aimed at development, study, improvement and introduction into practical use of surficial strengthening technologies based on highly-concentrated energy sources, including ion, plasma, laser and electron beams to form high-strength nanostructures in the surface layer. Basic advantages provided by these methods of modification include reciprocal action of dispersive inclusions with the substrate during the reaction, dispersion of material structure, synthesis of nano-scale metastable phases, as well as of nano-composites and intermetallic compounds with unique physical and chemical characteristics [6–12].

To date, it is a burning issue of solid-state physics to reveal physical mechanisms describing formation and evolution of structural-phase states and a dislocation substructure in metals and alloys under outside energy deposition. Experimental studies on phase composition and structures formed in products as a result of this treatment are of great importance for understanding the physical nature of transformations because they make it possible to change structure and operational parameters of products with a definite purpose. In this case, it is necessary to understand physical mechanisms and the nature of structural and phase transformations on macro- to nano-levels for achieving required properties of strength and plasticity [13–15].

The use of high-power pulse currents is perspective to modify structural and phase state of metallic materials with a certain purpose. This method provides a lot of possibilities to control and regulate an amount of supplied energy and to create a big area where concentrated energy flows contact with the material to be treated; furthermore, this treatment is characterized by low coefficients of energy reflection, higher concentration of energy in a volume unit of material, as a consequence, more possibilities of transforming material into a highly non-equilibrium state are available. Pulse electric currents are distinguished by high energy efficiency, better homogeneity of energy density throughout the flow and higher frequency of these flows in comparison to other energy deposition methods if used for technological aims [16–19].

Recently, researches of different countries (China, Russia, UK, USA, France, etc. (listed in descending order of publications in Scopus)) have shown an increased interest to the effect of current pulses on metallic materials. To date, materials Mg–3Al–1Zn, Ti–6Al–4V, Al–Mg–Si, Mg–9.3Li–1.79Al–1.61Zn, Al–5%Cu, Al–Si, Al–Mg, TiAl3, NiTi, Ni, Cr17Ni6Mn3, AISI 304, Ti3SiC2, ZnO have been the subject of investigations. The effect of current pulses has been closely examined for different deformation types of samples: cold and hot rolling; cold drawing; tensioning; stamping, etc. The studies have also been focused on different forms of samples: stripe; wire; bar; cylindrical rod; foil samples, etc. As a variable parameter density and amplitude of current, exposure time, frequency of applying current pulses, etc. have been used by investigators. A correlation has been found between ECP treatment and transformation of grain structure, dislocation substructure, mechanical and technical properties. Electric current treatment facilitates various technological operations, e.g. sheet rolling [16–21].

Titanium-based alloys subjected to EPT are in the focus of researchers because these alloys are broadly used as a construction material in space and aircraft industries due to their thermal stability, high specific strength and corrosion resistance [16,18,20].

Therefore, the aim of this work is to examine the effect of EPT of titanium alloy VT1-0 on its fatigue life and to carry out a comparative analysis of structure and phase composition.

2Materials and methods

Samples of commercially pure titanium VT1-0 (up to 0.18 Fe; up to 0.07 C; up to 0.04 N; up to 0.1 Si; up to 0.12 O; up to 0.004 H; 0.3 other impurities; remainder Ti, mass.%) were used as a test material. Fatigue test were carried out using a special unit in accordance with the procedure of cantilever bending (2 point bending) (as in [22]). The geometry and the dimensions of the fatigue test samples are given in Fig. 1. The specimens were cycled sinusoidally at 10Hz frequency and at a stress ratio of R=0.1 (the ratio of the minimum stress to the maximum stress in one cycle). Tests were conducted at temperature of 300K. The test was carried out at a cycle stress of the ultimate strength of the alloys determined in pure bending tests. At least 10 samples were used. The samples were machined and polished with emery papers (200–2000 grit silicon carbide). Prior to fatigue testing all samples were carefully electrolytically polished in order to minimize the surface effect on the fatigue properties.

Fig. 1.

Geometry of cycle fatigue tests of samples (dimensions in mm). (a) Side view; (b) top view.

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Fatigue tests were conducted in conventional conditions and in the process of EPT. Our previous research [23–25] has revealed that in the mid-stage of fatigue loading current pulses (a severe drop of speed of ultrasonic waves in the samples is registered on this stage) are the reason for the increasing number of cycles to destruction. Therefore, the same sequence is used in this work. For the purpose of proceeded investigations two groups of samples were examined: samples deformed without energy deposition and ones destroyed after a mid-stage EPT.

EPT parameters were selected – frequency of current pulses: 70Hz; duration of treatment: 120s; amplitude of current pulses: 2kА; maximal current density (on the spot of a stress concentrator): 100А/mm2. After 180,000 loading cycles EPT was completed. In total, 10 samples were destroyed in each state. This action contributed to the increase of cycles to failure from 281,333±27,400 to 359,532±19,050. Consequently, fatigue life of samples was enhanced by ∼1.3 times. It should be noted that ECP treatment reduces the dispersion of data.

The structure and phase composition of sample surfaces destroyed when fatigue testing were examined by the method of transmission electron diffraction microscopy (TEM) of thin foils (JEM 2100F, JEOL). For preparation of foils plates were cut of destroyed samples parallel and close to the fracture surface. Using this procedure for preparation of samples to be examined, the structure of material was analyzed in relation to the distance to the sample surface. Grain size was measured using linear intercept method using TEM micrographs in ten individual fields.

3Results and discussion

In the structure of the surface layer of titanium destroyed in fatigue tests there is a thin top layer (maximal thickness is 4.5mm) with a nanocrystalline grain and sub-grain structure (Fig. 2). Average dimensions of grains in this layer are 58.6±21.5nm. The dispersion of grain sizes is similar to the log-normal one (Fig. 3). Grains form layers, which are disoriented relatively to each other and parallel to the sample surface (Fig. 3b). In most cases, the thickness of these layers is in line with the average size of grains.

Fig. 2.

Electron-microscopic image of the surface layer of the sample destroyed in fatigue tests. The arrows indicate the surface of samples. STEM was used to make an image of the material structure.

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Fig. 3.

Histogram (a) and electron-microscopic image (b) of grains formed in the surface layer of the sample destroyed in fatigue tests.

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The analysis has revealed a lamellar structure of the material close to the nano-structured surface layer and a lot of flexural extinction contours in it (Fig. 2a). This is a good illustration of high internal stress concentrators, which cause bending and torsion in the crystal lattice of the material. It should be noted that this nano-crystalline structure is found at the depth up to 7–8μm and formed in the conjunctions of titanium grains in the initial state.

Using diffraction electron microscopy and a dark-field method with further indicating of electron diffraction micro-patterns, phase composition of the surface nano-structured layer of samples destroyed in fatigue tests was investigated. Findings of this study are shown in Fig. 4.

Fig. 4.

The cross-section structure of samples destroyed in fatigue tests: (a) bright-field image of the surface layer (the bottom part of the figure is for the sample surface); (b) electron diffraction micro-pattern of the zone in the oval (a); (c) dark-field image of the foil section (a) obtained in [002]α-Ti (reflex 1 indicated by the arrow in b); (d) dark-field image of the foil section (a) obtained in [020]Ti3O5 (reflex 2 indicated by the arrow in b).

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From the electron diffraction micro-pattern (Fig. 4a) its circular structure is evident. This fact indicates, firstly, small dimensions of crystallites forming the electron diffraction micro-pattern, secondly, their high-angle disorientation [26,27], i.e. formation of a grain and sub-grain structure in the surface layer of commercially pure titanium subjected to fatigue testing.

Indicating of electron diffraction micro-pattern (Fig. 4b) revealed a multi-phase state of the surface layer in the material. The main phase is α-modification of titanium, reflexes of which form a circular structure. Therefore, nano-crystalline grains are formed by α-modification of titanium. The dark-field analysis of the surface layer in crystal lattice reflexes of α-titanium indicates presence of mottled contrast in crystallites (Fig. 4c). This fact can be evidence of presence of α-titanium with the defect substructure in the nano-scale crystals.

The electron diffraction micro-pattern shows reflexes of titanium oxide phase in addition to reflexes of α-titanium. Basically, titanium oxides are located along boundaries of α-titanium crystallites; their particles are round and their dimensions vary in the range of 10nm (Fig. 4d). It should be noted that oxide phase particles have been revealed exclusively in the top nano-structured layer of the sample. The particles have not been found in the rest of the sample. In our opinion, oxide phase particles are formed due to penetration of oxygen atoms along low- and high-angle boundaries of crystallites when materials are subjected to fatigue tests.

Samples subjected to fatigue tests under EPT were the second subject of electron microscopic micro-diffraction investigations.

From the data of these investigations it is apparent that as a result of fatigue tests a sub-microcrystalline structure is formed in a relatively thin (maximum 4μm) surface layer of samples. Average dimensions of this structure are 422.7±400nm (Fig. 5). Therefore, when fatigue testing, EPT of samples results in a many-fold increase in crystallite dimensions of the surface layer.

Fig. 5.

Histogram (a) and electron-microscopic image (b) of grains formed in the surface layer of samples destroyed in fatigue tests. The tests were conducted in conditions of EPT.

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Another feature of the surface layer grain structure is a relatively high dispersion of grain dimensions in comparison to the surface layer structure formed in fatigue tests without EPT (Fig. 5a). From our point of view, it results from recrystallization of material taking place in conditions of EPT. A similar result has been reported when investigating austenite steel samples subjected to fatigue testing in conditions of EPT [23].

Furthermore, samples destroyed in fatigue tests in conditions of EPT have a relatively low level of bending and tension in the material crystal lattice. From micro-photographs of the structure (Fig. 6) it is apparent that there is a small number of flexural extinction contours in the surface layer of destroyed samples (if compared with microphotographs (Fig. 2)). Consequently, the number of internal stress concentrators in the surface layer of the material is reduced due to EPT of samples in the process of fatigue testing.

Fig. 6.

Electron-microscopic image of the surface layer structure in a sample destroyed in fatigue tests in conditions of EPT. The arrow indicates the surface of the sample. STEM method was used to obtain a representation of material structure.

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The phase analysis of the surface layer in samples destroyed in fatigue tests in conditions of EPT indicates a multiphase structure similar to that of samples considered above. In particular, the principal phase of the layer under consideration is α-titanium; the second phases are particles of titanium oxides. The outcomes of micro-diffraction analysis including a dark-field method are presented in Fig. 7.

Fig. 7.

The structure of the cross-section in a sample destroyed in fatigue tests in conditions of EPT; (a) bright-field image of the surface layer (the bottom part of the figure is for the sample surface); (b) electron.

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From the data in Fig. 7 it is seen that oxide phase particles are located along the boundaries of α-titanium grains and sub-grains. The dimensions of oxide phase particles vary in the range from 10nm to 35nm, and are significantly bigger than particles of the oxide phase formed in the titanium surface layer subjected to fatigue testing without EPT.

As stated before, fatigue testing of samples is associated with formation of a defect sub-structure in grains in the surface layer. A similar defect sub-structure is formed in α-titanium grains destroyed in fatigue tests in conditions of EPT. This sub-structure is apparent both in bright-field (Fig. 8a) and dark-field (Fig. 8b) images of the material structure (Fig. 8).

Fig. 8.

The structure of the cross-section of a sample destroyed in fatigue tests; (a) bright-field image of the surface layer; (b) electron diffraction micro-pattern; (c) dark-field image of the foil section (a) obtained in reflex [002]α-Ti (the reflex is indicated by the arrow in b).

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4Conclusions

Fatigue tests of commercially pure titanium VT1-0 have been conducted. The investigations have revealed that EPT of samples in the mid-stage of tests enhances the fatigue life of the material by ≈1.3 times in comparison to samples without this treatment. The phase composition and defect sub-structure of the surface layer in samples destroyed in fatigue tests have been examined in electron microscopic micro-diffraction studies; physical mechanisms improving the fatigue life of material have been disclosed for conditions of EPT. It has been indicated that energy treatment of samples, firstly, increases many-fold the dimensions of α-titanium crystallites in the surface layer due to recrystallization, secondly, reduces the number of internal stress concentrators in the top layer of the material, lowering, this way, the number of potential spots of crack origination; thirdly, leads to significant dimensional increase of oxide phase particles formed in the surface layer of commercially pure titanium when fatigue testing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was financially supported by the State task of Ministry of Education and Science of Russian Federation to perform research work (Grant number 3.1283.2017/4.6), Russian Fund for Basic Research (Grants numbers 16-32-60032 mol_a_dk and 16-58-00075 Bel_a).

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Journal of Materials Research and Technology

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