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Corrosion behaviors, mechanical properties and microstructure of the steel matrix composites fabricated by HP–HT method
Materials Science & Engineering A 639 (2015) 671–680
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Corrosion behaviors, mechanical properties and microstructure of the steel matrix composites fabricated by HP–HT method Iwona Sulima a,n, Remigiusz Kowalik b a b
Institute of Technology, Pedagogical University of Cracow, Podchorazych 2 Str., 30-084 Krakow, Poland Faculty of Non-Ferrous Metals, AGH University of Science and Technology Mickiewicza 30 Av., 30-059 Krakow, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 19 April 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 27 May 2015
Steel-based composites with varying content of TiB2 were manufactured by HP–HT process. The main objective of the study was to determine the effect of sintering conditions and TiB2 content on the mechanical properties and corrosion behavior of steel–TiB2 composites. The corrosion resistance of the composites was examined in H2SO4 solution using two measurement techniques. The mechanical properties were determined in a compression test allowing also for the effect of elevated temperatures (600 °C and 800 °C). The results showed that both TiB2 content and sintering temperature affected the properties tested. The highest compressive strength was obtained in the composites with 8 vol% TiB2. The corrosion properties of composites showed only insignificant variations. The microstructure was characterized by a homogeneous distribution of the TiB2 reinforcing phase. & 2015 Elsevier B.V. All rights reserved.
Keywords: Titanium diboride Steel matrix composite Compressive strength Corrosion behavior
1. Introduction Austenitic steels are widely used in automotive, aerospace, marine, biomedical, and food industries. This wide range of applications is certainly due to their excellent properties, such as the high resistance to corrosion and abrasion, high resistance to oxidation at elevated temperatures and satisfactory strength [1–3]. Powder metallurgy creates opportunities for the manufacture of small parts with complex shapes. This brings savings in both materials and energy, and gives products characterized by high dimensional accuracy [4,5]. Sintering of steel by conventional methods is usually done under the conditions of relatively low temperature and pressure [6–8]. The resultant properties of sintered austenitic steel depend on the density and on the presence or absence of porosity. Therefore, the main problem in the conventional methods of sintering is open porosity, which considerably deteriorates the corrosion resistance. Open porosity increases the surface area of material exposed to the effect of corrosive environment. To improve the corrosion resistance of austenitic steel, the effect of various additives, including copper, tin, and other elements was investigated [9,10]. Another way to improve the corrosion resistance of steel is by adding the ceramic particles [11–13]. Steel-based composites reinforced with ceramic particles were tested not only for the corrosion resistance but also n
Corresponding author. E-mail address: [email protected] (I. Sulima).
http://dx.doi.org/10.1016/j.msea.2015.05.077 0921-5093/& 2015 Elsevier B.V. All rights reserved.
for the mechanical properties. One of the positive effects of the addition of ceramic particles to stainless steel was an obvious improvement of the wear resistance. Corrosion resistance was also improved [14–17]. Steel-based composites reinforced with ceramic particles form a group of materials characterized by attractive physical and mechanical properties. A lot of work was devoted to the sintering process of these materials [18–20]. As a reinforcing phase mostly oxides [21,22], carbides [23–25] or borides [26,27] were used. Studies were mainly related with the optimization of a sintering process and with the effect of sintering conditions on physical properties and microstructure. Tribological and mechanical properties of steel-based composites reinforced with ceramic particles were also taken into consideration [21,24,27,28]. For example, Patankar and Tan [25] examined the role of SiC in the sintering process of composites based on the 316L stainless steel. It was found that the addition of SiC significantly improved the density of the composites. This fact was attributed to the reaction of SiC with a steel matrix, resulting in the formation of a low-melting Fe–SiC phase. In contrast, Mukherjee and Upadhyaya [22] investigated the effect of Al2O3 particles (added in an amount of up to 8%) on the sintering behavior and properties of ferritic steel. The highest mechanical properties were obtained in materials containing 4– 6 vol% Al2O3. Tjong and Lau [27] studied composites with matrix based on the 304 L steel reinforced with TiB2 particles (5–20 vol%). They proved that the addition of TiB2 particles was very effective in improving the wear resistance, hardness and toughness of the composite.
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Fig. 1. SEM microstructure of composites with: (a) 2 vol% TiB2, (b) 4 vol% TiB2, (c) 6 vol% TiB2 and (d) 8 vol% TiB2.
Fig. 2. XRD diffraction patterns of the composites with 8 vol% TiB2 (sintered at 1300 °C–5 GPa and 1300 °C–7 GPa).
Available literature on the electrochemical behavior of composites based on an austenitic steel matrix is very limited. Corrosion behavior is a very important parameter determining the potential use of these materials. Introduction of a ceramic phase to the steel matrix improves hardness, abrasion resistance and strength, but problems of corrosion resistance are often neglected. There are no published studies on the corrosion resistance of stainless steel composites reinforced with borides. Ceramic particles can interact electrochemically, chemically or physically with the steel matrix, leading to rapid corrosion. In addition, voids at the reinforcement/matrix interface can play the role of potential
corrosion sites. The appearance of new phases as a result of the ceramic reinforcing phase–matrix interaction can also lead to fast penetration into the large interfacial areas of composites [19,29]. In most cases, all these phenomena promote and accelerate the corrosion of composite materials. Another important factor influencing the corrosion behavior is type of the sintering process. The use of conventional methods often gives the steel composites characterized by low density and high porosity reducing the corrosion resistance of these materials [30,31]. The sintering methods based on the application of high-pressure produce materials with a very high density and minimum porosity. Therefore, the main aim of this study was to evaluate the mechanical properties and corrosion resistance of steel–TiB2 composites. The AISI 316L steel matrix composites were sintered by a high-pressure method. The next step was determination of the effect of TiB2 volume fraction (2–8 vol%) and sintering conditions (pressure and temperature) on corrosion resistance, mechanical properties and microstructure of the composites.
2. Methodology The study used composites based on the AISI 316L austenitic steel matrix reinforced with 2–8 vol% TiB2. Additionally, to the composite matrix containing 8 vol% TiB2, boron was added in an amount of 1 vol%. The composites were fabricated by the technique of High Pressure–High Temperature (HP–HT) sintering, applying the pressure of 5 70.2 GPa and 77 0.2 GPa at a temperature of 1000 °C and 1300 °C. The time of sintering was 60 s. Details of the sintering technology applied in the case of the tested
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Fig. 3. (a) SEM microstructure of composite with 8 vol% TiB2 þ1 vol% B and (b–g) mapping of different elements: Ti, B, Cr, Mo, Fe and Ni.
composites are described in the literature [20,32,33]. The apparent density and open porosity of the sintered materials were determined by hydrostatic method (Archimedes). Microstructure was examined by scanning electron microscopy (SEM, JEOL JSM 6610LV) and analysis of the chemical composition (EDS). The phase compositions were analyzed by X-ray diffraction (XRD) using Cu Kα radiation with a scintillation detector. After corrosion tests, the sample surface was also examined. Compression tests were performed on an INSTRON TT-DM testing machine with a crosshead speed of 1*10 4 mm/s. Cylindrical test samples had the following dimensions: diameter – 3 mm, height – 4.5 mm. Compression tests were carried out at room temperature and at elevated temperatures of 600 °C and 800 °C. Testing at elevated temperatures was carried out in a protective argon atmosphere to prevent oxidation of samples and compression anvils. For each composite three compression tests were made in an appropriate regime of temperature (Tpok, T600, T800,). Studies of corrosion resistance were carried out in a solution of H2SO4 (0.1 M). Standard three-electrode electrochemical cell was used. The reference electrode was Saturated Calomel Electrode; the counter electrode was platinum plate with an area of 12 cm2. Tests were performed at room temperature using an Autolab 302N potentiostat. Two measurement techniques were applied in the corrosion resistance tests. Potentiodynamic measurements were carried out in the measuring range from 0.25 to 1.6 V referred to an open circuit potential (OCP). The scan rate was 0.001 V/s. Following the electrochemical impedance spectroscopy tests were performed in the range of corrosion potential. As a corrosion potential, the measured OCP value was adopted. The applied frequency range was 100,000–0.1 Hz at an amplitude of 0.01 V. The value of the OCP potential was determined by measuring the tested sample potential in absence of the current flow for 60 s. Before each measurement samples were polished with emery paper of 600 and 1000 grit, and then rinsed with deionized water.
3. Results 3.1. Microstructural characteristics of steel–TiB2 composites Fig. 1 shows microstructures of composites containing 2–8 vol% TiB2. The microstructures of the examined composites are very similar. The reinforcing phase shows a relatively homogeneous distribution in the steel matrix. The particles of TiB2 (black precipitates) tend to occupy locations along the grain boundaries in a
composite matrix. In the process of composite sintering, TiB2 powder with an average grain size of 2.5–3.5 μm was used [20,31]. In contrast, the size of the TiB2 reinforcing phase present in the matrix was in the range from 2 to several microns. Local formation of the TiB2 agglomerates above 10 μm in size (Fig. 1d) was also observed. X-ray microstructure analysis (Fig. 2) confirmed the presence of titanium diboride in a steel matrix. Fig. 3 shows an example of the microstructure and surface distribution of elements in composites reinforced with 8 vol% TiB2, modified next with 1 vol% boron. Boron-rich places were observed to occur on the borders of matrix next to the TiB2 phase (Fig. 3d). This suggests that, because of a very short time of sintering (60 s), boron might not be able to completely diffuse into the steel matrix. 3.2. Mechanical and physical properties of steel–TiB2 composites In all the composites sintered for a short time (60 s) by HP–HT, a very high degree of compaction was reached. The apparent density of sintered composites was in the range of 98–100% of the theoretical density, regardless of the sintering conditions applied (temperature and pressure). The examined composites with different TiB2 content were characterized by a very low porosity in the range of 0.004–0.0%. Figs 4–6 show the compressive stress–strain curves obtained at room temperature and at elevated temperatures (600 °C and 800 °C). The addition of a TiB2 reinforcing phase significantly improved the mechanical properties of the examined composites compared with the steel without reinforcement (Fig. 4). The AISI 316 L austenitic steel has reached a compressive strength of 600– 800 MPa (Fig. 4a). Depending on the sintering conditions, the addition of even 2 vol% TiB2 introduced to a steel matrix has raised the compressive strength by approximately 40–65%, i.e. to 1000– 1160 MPa (Fig. 4b), respectively. The increasing content of TiB2 in the matrix caused gradual increase of the composite strength without any significant deterioration of plastic properties (Fig. 4b– e). In composites with 8 vol% TiB2, the strength increase of about 10–15% was obtained. For these materials, the compressive strength assumed the highest values in the range of 1200– 1300 MPa (Fig. 4e). Analysis of the results showed that 1 vol% boron addition had no significant impact on the mechanical properties of the examined composites (Fig. 4f). In the composite material, the reinforcing effect was obtained by introducing the ceramic phase to a matrix, and then by either inhibiting further growth of the grains or by refining the grains already existing in
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Fig. 4. Stress–strain curves of (a) the austenitic AISI 316L stainless steel and (b–f) and steel–TiB2 composites under compressive loading at room temperature.
the matrix. The short sintering time (60 s) applied in the HP–HT method inhibited the grain growth. The same phenomena were also observed in composites based on aluminum alloys [34,35]. Additionally, the compressive stresses occurring in this material in the first stage of compression contributed to the closure of pores in the sintered product, thus delaying the onset of cracks at the matrix/reinforcement interface [36]. The results of compression tests show certain regularities. The mechanical properties of the examined composites depend on the sintering conditions applied in the HP–HT method. Specifically, the temperature of sintering has a significant impact on both ductility and compressive strength. For most of the composites reinforced with 2–8 vol% TiB2, two different characteristics of the
true stress–strain curves were obtained. Regardless of the pressure applied, the compression curves obtained for the composites sintered at a lower temperature of 1000 °C were characterized by high yield strength and small range of plastic strain (Fig. 4b–f). In contrast, the true stress–strain curves of composites sintered at 1300 °C were characterized by a large range of plastic strain combined with double reduction in the elastic strain range. Figs. 5 and 6 show the effect of high temperature (600 °C and 800 °C) during compression test on the mechanical properties of the examined composites. High temperature applied during the test has resulted in a definite deterioration of both the composite strength and ductility. The deterioration of ductility was most pronounced in the composites sintered at 1000 °C. The lowest
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Fig. 5. (a–e) Stress–strain curves of steel–TiB2 composites sintered at different conditions (compression tests at T ¼600 °C).
mechanical properties were obtained in the tests carried out at a temperature of 800 °C. The compressive strength of the composites sintered at 1000 °C was three times lower (270–420 MPa) compared with the strength obtained in tests carried out at room temperature (1000–1300 MPa). It should be noted that in the composites subjected to compression at 600 °C (Fig. 5), different characteristics of the true stress–strain curves were also obtained, as in the results of tests carried out at room temperature (Fig. 5). Detailed analysis of the results of high temperature compression tests also showed an improvement of the mechanical properties obtained with the TiB2 content increasing in a steel matrix. During compression tests at a temperature of 600 °C, the compressive strength obtained for the composites with 2 vol% and
8 vol% TiB2 was comprised in the range of 580–810 MPa and 650– 860 MPa (Figs. 5 and 6a and d), respectively. In contrast, in tests carried out at 800 °C, the strength of the same composites was 270–350 MPa and 380–420 MPa (Figs. 5 and 6a and d), respectively. 3.3. Corrosion behavior of steel–TiB2 composites Figs. 7–11 present the results of corrosion tests carried out on composites with different content of TiB2. The plotted potentiodynamic curves are very complex and in all cases indicate the occurrence of a passivation process. From these curves, the corrosion potential can be determined (Table 1). The more positive is
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Fig. 6. (a–e) Stress–strain curves of steel–TiB2 composites sintered at different conditions (compression tests at T ¼800 °C).
the value of the corrosion potential, the higher is the corrosion resistance under the conditions tested. The anode part shows the range of active dissolution of steel, passive state and transpassive state. The shape of polarization curves in the range of active dissolution is characterized by high corrosion currents. In passive range, the value of corrosion currents decreases. The value of corrosion currents in this range also indicates the corrosion resistance of tested samples. The results of impedance tests are shown in the Nyquist plots. The results are arranged in characteristic flattened semicircles. The largest diameter of the semicircle indicates the highest corrosion resistance. The impedance spectra deviate from a model semicircle.
This phenomenon may be due to a heterogeneity of the composite microstructure or variations in local chemical composition, e.g. at the matrix–reinforcing phase interface. To determine the corrosion resistance, potentiodynamic curves and Nyquist plots were examined for all the sintered composites (Figs. 7–11). From the potentiodynamic curves it follows that the values of corrosion potential are very similar to each other, but curves are changing their shape in the anode part, especially in the activation control range. The results obtained reflect differences in the mechanism of steel dissolution, due to variations in the composition, or in the phase condition or crystalline state of the samples tested. In the case under discussion, the factors responsible for the above mentioned
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Fig. 7. (a) The polarization curves and (b) Nyquist plots for composites with 2 vol% TiB2 sintered at different conditions.
Fig. 8. (a) The polarization curves and (b) Nyquist plots for composites with 4 vol% TiB2 sintered at different conditions.
Fig. 9. (a) The polarization curves and (b) Nyquist plots for composites with 6 vol% TiB2 sintered at different conditions.
differences are variations in the content of the reinforcing phase and different processes of the heat treatment of samples. The results show that both the sintering conditions and the content of the ceramic reinforcing phase have some influence on the corrosion properties of composites. With the TiB2 content increasing in the composite matrix, the corrosion potential of samples changes and tends towards more positive values, thus indicating an improvement of the corrosion resistance, although differences in the values of the corrosion potential are small
(Table 1). The resistance determined from the calculated values of the corrosion potential is changing. It has also been observed that corrosion potential depends on the sintering conditions and changes slightly pointing to an improvement of corrosion resistance in the sequence given below: 1000 °C–5 GPa, 1000 °C– 7 GPa, 1300 °C–5 GPa, 1300 °C–7 GPa. For composites with 2–8 vol% TiB2, the shape of polarization curves changes in the anode part in the range of the activation control. The same dependence was also observed in the boron-
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Fig. 10. (a) The polarization curves and (b) Nyquist plots for composites with 8 vol% TiB2 sintered at different conditions.
Fig. 11. (a) The polarization curves and (b) Nyquist plots for composites with 8 vol% TiB2 þ 1 vol%. B sintered at different conditions. Table 1 Effect the sintering conditions on the corrosion potential of composites. Steelþ2 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ4 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ6 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ8 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ8 vol% TiB2 þ1 vol% B Sintering conditions Corrosion potential [V]
1000 °C–5 GPa 0.372
1000 °C–7 GPa 0.352
1300 °C–5 GPa 0.346
1300 °C–7 GPa 0.328
1000 °C–5 GPa 0.366
1000 °C–7 GPa 0.342
1300 °C–5 GPa 0.325
1300 °C–7 GPa 0.319
1000 °C–5 GPa 0.347
1000 °C–7 GPa 0.342
1000 °C–5 GPa 0.327
1300 °C–7 GPa 0.317
1000 °C–5 GPa 0.345
1000 °C–7 GPa 0.340
1300 °C–5 GPa 0.320
1300 °C–7 GPa 0.317
1000 °C–5 GPa 0.347
1000 °C–7 GPa 0.330
1500 °C–5 GPa 0.322
1300 °C–7 GPa 0.315
modified composites with 8 vol% TiB2 (Fig. 11). This indicates that different mechanisms govern the dissolution of the steel matrix. Figs. 7a–11a show that for all the sintered composites, the corrosion current values change in the anode part of polarization curves in the range of sample passivation. The corrosion current assumes the highest value in the composite samples sintered at 1000 °C– 5 GPa, which indicates the lowest corrosion resistance. In contrast, the corrosion current assumes the lowest value for the composite samples sintered at 1300 °C–7 GPa. This indicates the best corrosion resistance of these composites. Nyquist plots of impedance spectra confirm the examined results of corrosion resistance, obtained from the polarization curves and referred to the sintering
conditions (Figs. 7b–11b). It can be concluded that the best corrosion resistance show composites with varied content of TiB2 (2– 8 vol%), sintered by HP–HT at 1300 °C–7 GPa. The impedance measurements also allowed determining a relationship between the TiB2 content in steel matrix and corrosion resistance. The results of these tests showed in the form of Nyquist plots are arranged in characteristic flattened semicircles. For composites with 2 vol% TiB2 (Fig. 7a), the largest diameters of the semicircles were obtained, which indicates the highest resistance to corrosion in sulfuric acid solution (0.1 mol/dm3). It was observed that with an increasing amount of TiB2, the corrosion resistance was decreasing. The smallest diameters of the semicircles
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Fig. 12. Microstructure of composites with: (a) 8 vol% TiB2 (1300 °C–7 GPa), (b) 8 vol% TiB2 þ 1 vol% B (1300 °C–7 GPa) after the corrosion tests in H2SO4 solution (0.1 mol/ dm3).
Fig. 13. Microstructure SEM of composites with: (a) 8 vol% TiB2 (1300 °C–7 GPa), (b) 8 vol% TiB2 þ1 vol% B (1300 °C–7 GPa) after the corrosion tests in H2SO4 solution (0.1 mol/dm3).
in Nyquist plots were obtained for composites with 8 vol% TiB2 modified with 1 vol% boron. This demonstrates the lowest resistance to corrosion in the examined environment of sulfuric acid solution. Figs. 12 and 13 show surface changes in the sample composites after corrosion testing. More distinct outlines of the grain boundaries are observed. In the area of the reinforcing TiB2 phase, pits and losses of the matrix occur. A probable cause of this phenomenon is the corrosive environment acting on the steel–TiB2 phase boundary. This can result in rapid penetration of the large interfacial areas of the composite and corrosion spreading along the grain boundaries. Changes in the surface condition were most severe in the composites with 8 vol% TiB2 modified with 1 vol% boron.
4. Conclusions The main results of this study can be summarized in the following way: 1. Irrespective of the sintering temperature or pressure, the short time of the HP–HT sintering produced composites characterized by an apparent density comprised in the range of 98–100% of the theoretical density. 2. Different sintering conditions and different TiB2 content exert strong effect on the composite strength and ductility, leaving
the corrosion properties practically unaffected. 3. The addition of the TiB2 reinforcing phase enhances the mechanical properties of composites compared with the steel without reinforcement. 4. High temperature during compression test has an adverse impact on the composite strength and ductility. The lowest mechanical properties were obtained in the compression tests conducted at 800 °C.
Acknowledgments The study was performed under Research Project no. N N507 222840. Project was financed by the National Science Centre.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Corrosion behaviors, mechanical properties and microstructure of the steel matrix composites fabricated by HP–HT method Iwona Sulima a,n, Remigiusz Kowalik b a b
Institute of Technology, Pedagogical University of Cracow, Podchorazych 2 Str., 30-084 Krakow, Poland Faculty of Non-Ferrous Metals, AGH University of Science and Technology Mickiewicza 30 Av., 30-059 Krakow, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 19 April 2015 Received in revised form 14 May 2015 Accepted 23 May 2015 Available online 27 May 2015
Steel-based composites with varying content of TiB2 were manufactured by HP–HT process. The main objective of the study was to determine the effect of sintering conditions and TiB2 content on the mechanical properties and corrosion behavior of steel–TiB2 composites. The corrosion resistance of the composites was examined in H2SO4 solution using two measurement techniques. The mechanical properties were determined in a compression test allowing also for the effect of elevated temperatures (600 °C and 800 °C). The results showed that both TiB2 content and sintering temperature affected the properties tested. The highest compressive strength was obtained in the composites with 8 vol% TiB2. The corrosion properties of composites showed only insignificant variations. The microstructure was characterized by a homogeneous distribution of the TiB2 reinforcing phase. & 2015 Elsevier B.V. All rights reserved.
Keywords: Titanium diboride Steel matrix composite Compressive strength Corrosion behavior
1. Introduction Austenitic steels are widely used in automotive, aerospace, marine, biomedical, and food industries. This wide range of applications is certainly due to their excellent properties, such as the high resistance to corrosion and abrasion, high resistance to oxidation at elevated temperatures and satisfactory strength [1–3]. Powder metallurgy creates opportunities for the manufacture of small parts with complex shapes. This brings savings in both materials and energy, and gives products characterized by high dimensional accuracy [4,5]. Sintering of steel by conventional methods is usually done under the conditions of relatively low temperature and pressure [6–8]. The resultant properties of sintered austenitic steel depend on the density and on the presence or absence of porosity. Therefore, the main problem in the conventional methods of sintering is open porosity, which considerably deteriorates the corrosion resistance. Open porosity increases the surface area of material exposed to the effect of corrosive environment. To improve the corrosion resistance of austenitic steel, the effect of various additives, including copper, tin, and other elements was investigated [9,10]. Another way to improve the corrosion resistance of steel is by adding the ceramic particles [11–13]. Steel-based composites reinforced with ceramic particles were tested not only for the corrosion resistance but also n
Corresponding author. E-mail address: [email protected] (I. Sulima).
http://dx.doi.org/10.1016/j.msea.2015.05.077 0921-5093/& 2015 Elsevier B.V. All rights reserved.
for the mechanical properties. One of the positive effects of the addition of ceramic particles to stainless steel was an obvious improvement of the wear resistance. Corrosion resistance was also improved [14–17]. Steel-based composites reinforced with ceramic particles form a group of materials characterized by attractive physical and mechanical properties. A lot of work was devoted to the sintering process of these materials [18–20]. As a reinforcing phase mostly oxides [21,22], carbides [23–25] or borides [26,27] were used. Studies were mainly related with the optimization of a sintering process and with the effect of sintering conditions on physical properties and microstructure. Tribological and mechanical properties of steel-based composites reinforced with ceramic particles were also taken into consideration [21,24,27,28]. For example, Patankar and Tan [25] examined the role of SiC in the sintering process of composites based on the 316L stainless steel. It was found that the addition of SiC significantly improved the density of the composites. This fact was attributed to the reaction of SiC with a steel matrix, resulting in the formation of a low-melting Fe–SiC phase. In contrast, Mukherjee and Upadhyaya [22] investigated the effect of Al2O3 particles (added in an amount of up to 8%) on the sintering behavior and properties of ferritic steel. The highest mechanical properties were obtained in materials containing 4– 6 vol% Al2O3. Tjong and Lau [27] studied composites with matrix based on the 304 L steel reinforced with TiB2 particles (5–20 vol%). They proved that the addition of TiB2 particles was very effective in improving the wear resistance, hardness and toughness of the composite.
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Fig. 1. SEM microstructure of composites with: (a) 2 vol% TiB2, (b) 4 vol% TiB2, (c) 6 vol% TiB2 and (d) 8 vol% TiB2.
Fig. 2. XRD diffraction patterns of the composites with 8 vol% TiB2 (sintered at 1300 °C–5 GPa and 1300 °C–7 GPa).
Available literature on the electrochemical behavior of composites based on an austenitic steel matrix is very limited. Corrosion behavior is a very important parameter determining the potential use of these materials. Introduction of a ceramic phase to the steel matrix improves hardness, abrasion resistance and strength, but problems of corrosion resistance are often neglected. There are no published studies on the corrosion resistance of stainless steel composites reinforced with borides. Ceramic particles can interact electrochemically, chemically or physically with the steel matrix, leading to rapid corrosion. In addition, voids at the reinforcement/matrix interface can play the role of potential
corrosion sites. The appearance of new phases as a result of the ceramic reinforcing phase–matrix interaction can also lead to fast penetration into the large interfacial areas of composites [19,29]. In most cases, all these phenomena promote and accelerate the corrosion of composite materials. Another important factor influencing the corrosion behavior is type of the sintering process. The use of conventional methods often gives the steel composites characterized by low density and high porosity reducing the corrosion resistance of these materials [30,31]. The sintering methods based on the application of high-pressure produce materials with a very high density and minimum porosity. Therefore, the main aim of this study was to evaluate the mechanical properties and corrosion resistance of steel–TiB2 composites. The AISI 316L steel matrix composites were sintered by a high-pressure method. The next step was determination of the effect of TiB2 volume fraction (2–8 vol%) and sintering conditions (pressure and temperature) on corrosion resistance, mechanical properties and microstructure of the composites.
2. Methodology The study used composites based on the AISI 316L austenitic steel matrix reinforced with 2–8 vol% TiB2. Additionally, to the composite matrix containing 8 vol% TiB2, boron was added in an amount of 1 vol%. The composites were fabricated by the technique of High Pressure–High Temperature (HP–HT) sintering, applying the pressure of 5 70.2 GPa and 77 0.2 GPa at a temperature of 1000 °C and 1300 °C. The time of sintering was 60 s. Details of the sintering technology applied in the case of the tested
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Fig. 3. (a) SEM microstructure of composite with 8 vol% TiB2 þ1 vol% B and (b–g) mapping of different elements: Ti, B, Cr, Mo, Fe and Ni.
composites are described in the literature [20,32,33]. The apparent density and open porosity of the sintered materials were determined by hydrostatic method (Archimedes). Microstructure was examined by scanning electron microscopy (SEM, JEOL JSM 6610LV) and analysis of the chemical composition (EDS). The phase compositions were analyzed by X-ray diffraction (XRD) using Cu Kα radiation with a scintillation detector. After corrosion tests, the sample surface was also examined. Compression tests were performed on an INSTRON TT-DM testing machine with a crosshead speed of 1*10 4 mm/s. Cylindrical test samples had the following dimensions: diameter – 3 mm, height – 4.5 mm. Compression tests were carried out at room temperature and at elevated temperatures of 600 °C and 800 °C. Testing at elevated temperatures was carried out in a protective argon atmosphere to prevent oxidation of samples and compression anvils. For each composite three compression tests were made in an appropriate regime of temperature (Tpok, T600, T800,). Studies of corrosion resistance were carried out in a solution of H2SO4 (0.1 M). Standard three-electrode electrochemical cell was used. The reference electrode was Saturated Calomel Electrode; the counter electrode was platinum plate with an area of 12 cm2. Tests were performed at room temperature using an Autolab 302N potentiostat. Two measurement techniques were applied in the corrosion resistance tests. Potentiodynamic measurements were carried out in the measuring range from 0.25 to 1.6 V referred to an open circuit potential (OCP). The scan rate was 0.001 V/s. Following the electrochemical impedance spectroscopy tests were performed in the range of corrosion potential. As a corrosion potential, the measured OCP value was adopted. The applied frequency range was 100,000–0.1 Hz at an amplitude of 0.01 V. The value of the OCP potential was determined by measuring the tested sample potential in absence of the current flow for 60 s. Before each measurement samples were polished with emery paper of 600 and 1000 grit, and then rinsed with deionized water.
3. Results 3.1. Microstructural characteristics of steel–TiB2 composites Fig. 1 shows microstructures of composites containing 2–8 vol% TiB2. The microstructures of the examined composites are very similar. The reinforcing phase shows a relatively homogeneous distribution in the steel matrix. The particles of TiB2 (black precipitates) tend to occupy locations along the grain boundaries in a
composite matrix. In the process of composite sintering, TiB2 powder with an average grain size of 2.5–3.5 μm was used [20,31]. In contrast, the size of the TiB2 reinforcing phase present in the matrix was in the range from 2 to several microns. Local formation of the TiB2 agglomerates above 10 μm in size (Fig. 1d) was also observed. X-ray microstructure analysis (Fig. 2) confirmed the presence of titanium diboride in a steel matrix. Fig. 3 shows an example of the microstructure and surface distribution of elements in composites reinforced with 8 vol% TiB2, modified next with 1 vol% boron. Boron-rich places were observed to occur on the borders of matrix next to the TiB2 phase (Fig. 3d). This suggests that, because of a very short time of sintering (60 s), boron might not be able to completely diffuse into the steel matrix. 3.2. Mechanical and physical properties of steel–TiB2 composites In all the composites sintered for a short time (60 s) by HP–HT, a very high degree of compaction was reached. The apparent density of sintered composites was in the range of 98–100% of the theoretical density, regardless of the sintering conditions applied (temperature and pressure). The examined composites with different TiB2 content were characterized by a very low porosity in the range of 0.004–0.0%. Figs 4–6 show the compressive stress–strain curves obtained at room temperature and at elevated temperatures (600 °C and 800 °C). The addition of a TiB2 reinforcing phase significantly improved the mechanical properties of the examined composites compared with the steel without reinforcement (Fig. 4). The AISI 316 L austenitic steel has reached a compressive strength of 600– 800 MPa (Fig. 4a). Depending on the sintering conditions, the addition of even 2 vol% TiB2 introduced to a steel matrix has raised the compressive strength by approximately 40–65%, i.e. to 1000– 1160 MPa (Fig. 4b), respectively. The increasing content of TiB2 in the matrix caused gradual increase of the composite strength without any significant deterioration of plastic properties (Fig. 4b– e). In composites with 8 vol% TiB2, the strength increase of about 10–15% was obtained. For these materials, the compressive strength assumed the highest values in the range of 1200– 1300 MPa (Fig. 4e). Analysis of the results showed that 1 vol% boron addition had no significant impact on the mechanical properties of the examined composites (Fig. 4f). In the composite material, the reinforcing effect was obtained by introducing the ceramic phase to a matrix, and then by either inhibiting further growth of the grains or by refining the grains already existing in
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Fig. 4. Stress–strain curves of (a) the austenitic AISI 316L stainless steel and (b–f) and steel–TiB2 composites under compressive loading at room temperature.
the matrix. The short sintering time (60 s) applied in the HP–HT method inhibited the grain growth. The same phenomena were also observed in composites based on aluminum alloys [34,35]. Additionally, the compressive stresses occurring in this material in the first stage of compression contributed to the closure of pores in the sintered product, thus delaying the onset of cracks at the matrix/reinforcement interface [36]. The results of compression tests show certain regularities. The mechanical properties of the examined composites depend on the sintering conditions applied in the HP–HT method. Specifically, the temperature of sintering has a significant impact on both ductility and compressive strength. For most of the composites reinforced with 2–8 vol% TiB2, two different characteristics of the
true stress–strain curves were obtained. Regardless of the pressure applied, the compression curves obtained for the composites sintered at a lower temperature of 1000 °C were characterized by high yield strength and small range of plastic strain (Fig. 4b–f). In contrast, the true stress–strain curves of composites sintered at 1300 °C were characterized by a large range of plastic strain combined with double reduction in the elastic strain range. Figs. 5 and 6 show the effect of high temperature (600 °C and 800 °C) during compression test on the mechanical properties of the examined composites. High temperature applied during the test has resulted in a definite deterioration of both the composite strength and ductility. The deterioration of ductility was most pronounced in the composites sintered at 1000 °C. The lowest
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Fig. 5. (a–e) Stress–strain curves of steel–TiB2 composites sintered at different conditions (compression tests at T ¼600 °C).
mechanical properties were obtained in the tests carried out at a temperature of 800 °C. The compressive strength of the composites sintered at 1000 °C was three times lower (270–420 MPa) compared with the strength obtained in tests carried out at room temperature (1000–1300 MPa). It should be noted that in the composites subjected to compression at 600 °C (Fig. 5), different characteristics of the true stress–strain curves were also obtained, as in the results of tests carried out at room temperature (Fig. 5). Detailed analysis of the results of high temperature compression tests also showed an improvement of the mechanical properties obtained with the TiB2 content increasing in a steel matrix. During compression tests at a temperature of 600 °C, the compressive strength obtained for the composites with 2 vol% and
8 vol% TiB2 was comprised in the range of 580–810 MPa and 650– 860 MPa (Figs. 5 and 6a and d), respectively. In contrast, in tests carried out at 800 °C, the strength of the same composites was 270–350 MPa and 380–420 MPa (Figs. 5 and 6a and d), respectively. 3.3. Corrosion behavior of steel–TiB2 composites Figs. 7–11 present the results of corrosion tests carried out on composites with different content of TiB2. The plotted potentiodynamic curves are very complex and in all cases indicate the occurrence of a passivation process. From these curves, the corrosion potential can be determined (Table 1). The more positive is
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Fig. 6. (a–e) Stress–strain curves of steel–TiB2 composites sintered at different conditions (compression tests at T ¼800 °C).
the value of the corrosion potential, the higher is the corrosion resistance under the conditions tested. The anode part shows the range of active dissolution of steel, passive state and transpassive state. The shape of polarization curves in the range of active dissolution is characterized by high corrosion currents. In passive range, the value of corrosion currents decreases. The value of corrosion currents in this range also indicates the corrosion resistance of tested samples. The results of impedance tests are shown in the Nyquist plots. The results are arranged in characteristic flattened semicircles. The largest diameter of the semicircle indicates the highest corrosion resistance. The impedance spectra deviate from a model semicircle.
This phenomenon may be due to a heterogeneity of the composite microstructure or variations in local chemical composition, e.g. at the matrix–reinforcing phase interface. To determine the corrosion resistance, potentiodynamic curves and Nyquist plots were examined for all the sintered composites (Figs. 7–11). From the potentiodynamic curves it follows that the values of corrosion potential are very similar to each other, but curves are changing their shape in the anode part, especially in the activation control range. The results obtained reflect differences in the mechanism of steel dissolution, due to variations in the composition, or in the phase condition or crystalline state of the samples tested. In the case under discussion, the factors responsible for the above mentioned
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Fig. 7. (a) The polarization curves and (b) Nyquist plots for composites with 2 vol% TiB2 sintered at different conditions.
Fig. 8. (a) The polarization curves and (b) Nyquist plots for composites with 4 vol% TiB2 sintered at different conditions.
Fig. 9. (a) The polarization curves and (b) Nyquist plots for composites with 6 vol% TiB2 sintered at different conditions.
differences are variations in the content of the reinforcing phase and different processes of the heat treatment of samples. The results show that both the sintering conditions and the content of the ceramic reinforcing phase have some influence on the corrosion properties of composites. With the TiB2 content increasing in the composite matrix, the corrosion potential of samples changes and tends towards more positive values, thus indicating an improvement of the corrosion resistance, although differences in the values of the corrosion potential are small
(Table 1). The resistance determined from the calculated values of the corrosion potential is changing. It has also been observed that corrosion potential depends on the sintering conditions and changes slightly pointing to an improvement of corrosion resistance in the sequence given below: 1000 °C–5 GPa, 1000 °C– 7 GPa, 1300 °C–5 GPa, 1300 °C–7 GPa. For composites with 2–8 vol% TiB2, the shape of polarization curves changes in the anode part in the range of the activation control. The same dependence was also observed in the boron-
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Fig. 10. (a) The polarization curves and (b) Nyquist plots for composites with 8 vol% TiB2 sintered at different conditions.
Fig. 11. (a) The polarization curves and (b) Nyquist plots for composites with 8 vol% TiB2 þ 1 vol%. B sintered at different conditions. Table 1 Effect the sintering conditions on the corrosion potential of composites. Steelþ2 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ4 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ6 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ8 vol% TiB2 Sintering conditions Corrosion potential [V] Steelþ8 vol% TiB2 þ1 vol% B Sintering conditions Corrosion potential [V]
1000 °C–5 GPa 0.372
1000 °C–7 GPa 0.352
1300 °C–5 GPa 0.346
1300 °C–7 GPa 0.328
1000 °C–5 GPa 0.366
1000 °C–7 GPa 0.342
1300 °C–5 GPa 0.325
1300 °C–7 GPa 0.319
1000 °C–5 GPa 0.347
1000 °C–7 GPa 0.342
1000 °C–5 GPa 0.327
1300 °C–7 GPa 0.317
1000 °C–5 GPa 0.345
1000 °C–7 GPa 0.340
1300 °C–5 GPa 0.320
1300 °C–7 GPa 0.317
1000 °C–5 GPa 0.347
1000 °C–7 GPa 0.330
1500 °C–5 GPa 0.322
1300 °C–7 GPa 0.315
modified composites with 8 vol% TiB2 (Fig. 11). This indicates that different mechanisms govern the dissolution of the steel matrix. Figs. 7a–11a show that for all the sintered composites, the corrosion current values change in the anode part of polarization curves in the range of sample passivation. The corrosion current assumes the highest value in the composite samples sintered at 1000 °C– 5 GPa, which indicates the lowest corrosion resistance. In contrast, the corrosion current assumes the lowest value for the composite samples sintered at 1300 °C–7 GPa. This indicates the best corrosion resistance of these composites. Nyquist plots of impedance spectra confirm the examined results of corrosion resistance, obtained from the polarization curves and referred to the sintering
conditions (Figs. 7b–11b). It can be concluded that the best corrosion resistance show composites with varied content of TiB2 (2– 8 vol%), sintered by HP–HT at 1300 °C–7 GPa. The impedance measurements also allowed determining a relationship between the TiB2 content in steel matrix and corrosion resistance. The results of these tests showed in the form of Nyquist plots are arranged in characteristic flattened semicircles. For composites with 2 vol% TiB2 (Fig. 7a), the largest diameters of the semicircles were obtained, which indicates the highest resistance to corrosion in sulfuric acid solution (0.1 mol/dm3). It was observed that with an increasing amount of TiB2, the corrosion resistance was decreasing. The smallest diameters of the semicircles
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Fig. 12. Microstructure of composites with: (a) 8 vol% TiB2 (1300 °C–7 GPa), (b) 8 vol% TiB2 þ 1 vol% B (1300 °C–7 GPa) after the corrosion tests in H2SO4 solution (0.1 mol/ dm3).
Fig. 13. Microstructure SEM of composites with: (a) 8 vol% TiB2 (1300 °C–7 GPa), (b) 8 vol% TiB2 þ1 vol% B (1300 °C–7 GPa) after the corrosion tests in H2SO4 solution (0.1 mol/dm3).
in Nyquist plots were obtained for composites with 8 vol% TiB2 modified with 1 vol% boron. This demonstrates the lowest resistance to corrosion in the examined environment of sulfuric acid solution. Figs. 12 and 13 show surface changes in the sample composites after corrosion testing. More distinct outlines of the grain boundaries are observed. In the area of the reinforcing TiB2 phase, pits and losses of the matrix occur. A probable cause of this phenomenon is the corrosive environment acting on the steel–TiB2 phase boundary. This can result in rapid penetration of the large interfacial areas of the composite and corrosion spreading along the grain boundaries. Changes in the surface condition were most severe in the composites with 8 vol% TiB2 modified with 1 vol% boron.
4. Conclusions The main results of this study can be summarized in the following way: 1. Irrespective of the sintering temperature or pressure, the short time of the HP–HT sintering produced composites characterized by an apparent density comprised in the range of 98–100% of the theoretical density. 2. Different sintering conditions and different TiB2 content exert strong effect on the composite strength and ductility, leaving
the corrosion properties practically unaffected. 3. The addition of the TiB2 reinforcing phase enhances the mechanical properties of composites compared with the steel without reinforcement. 4. High temperature during compression test has an adverse impact on the composite strength and ductility. The lowest mechanical properties were obtained in the compression tests conducted at 800 °C.
Acknowledgments The study was performed under Research Project no. N N507 222840. Project was financed by the National Science Centre.
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