Fracture toughness of gas borided Nimonic 80A alloy

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1 Inżynieria Materiałowa 1 (209) (2016) DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Fracture toughness of gas borided Nimonic 80A alloy Natalia Makuch *, Michał Kulka, Piotr Dziarski Instytut Inżynierii Materiałowej, Politechnika Poznańska, * natalia.makuch@put.poznan.pl Ni-based superalloys are often used in many industrial applications, for example in chemical, petrochemical, aeronautics, nuclear or space industries. These alloys are characterized by a unique combination of low thermal expansion coefficient, high temperature strength, high resistance to oxidation and high corrosion resistance. However, due to their low microhardness and sensitivity to abrasive, erosive and adhesive wear, their application is limited. The boriding process is the appropriate treatment, which will provide high hardness and high wear resistance of Ni-based alloys. Unfortunately, the use of boride layers is limited by their sensibility to cracking under mechanical stresses. Therefore, in this paper the microstructure, microhardness and fracture toughness of gas-borided layer produced on Nimonic 80A alloy were studied. Gas boriding in atmosphere was proposed to produce the hard boride layer on Nimonic 80A alloy. This process was carried out at 920 C (1193 K) for 2 hours. The carrier gas consisted of 75 vol.% and 25 vol.% H 2. Proposed gas boriding accelerated the diffusion of boron into the surface in comparison with other acceptable diffusion methods. The comparable thickness of boride layer was obtained after considerably shorter duration. Key words: gas boriding, Nimonic 80A alloy, microstructure, hardness, fracture toughness. 1. INTRODUCTION Nickel-base superalloys are generally used for application under conditions of high stresses in high temperature, because of their excellent combination of oxidation resistance, corrosion resistance, good stress relaxation resistance and good mechanical properties even at high temperature [1 3]. However, because of poor wear resistance, the Ni-base alloys under condition of mechanical wear (abrasive or adhesive), require suitable and effective protection. Boronizing is a surface treatment that increases the hardness and wear resistance of metals and their alloys. Generally, the borided layers are characterized by many advantageous properties: high hardness, high resistance to mechanical wear, low friction coefficient, high heat resistance, high corrosion resistance, high resistance to influence of liquid metals and alloys and high hardness at increased temperatures [4 9]. However, the well-known diffusion boriding methods require high temperature (up to 1000 C) and long duration (up to 12 hours). Nickel and its alloys can be effectively boriding using various methods: powder [10 16], paste [17], electrochemical [18] or fluidized bed technology [19]. However, the commercial boriding agents (containing SiC), widely used to boriding of iron and steels, cannot be applied in case of nickel and its alloys due to the simultaneous siliconizing of the surface. The occurrence of porous nickel silicides caused a decrease in hardness of the borided layers [10 12]. Recently, a special powder agent without SiC (e.g. Ekabor-Ni ) had been developed for boriding of Ni-base alloys [13 16]. The powder-pack boriding of pure nickel using this agent caused the formation of thick nickel borides layers (up to 100 μm), which were characterized by high hardness (1300 HK) and higher wear resistance in comparison with untreated Ni sample [13]. In case of Nickel 201 alloy, borided in Ekabor-II powder at a temperature of 1173 K for 4 h, the thickness of the borided layer was approximately 220 µm [14]. The hardness of the boride compounds formed on the surface of the Nickel 201 alloy ranged from 1153 to 1778 HV0.05. Ni-base alloys can be also easily borided using powder-pack method without SiC. The thickness, phase composition and hardness of borided layers, produced on Ni-base alloys, depended on the type of treated material. Boriding of Incoloy alloy caused an increase in microhardness up to 2200 HV [15]. The produced layers were characterized by thickness µm, however, the obtaining of thick layers required higher temperature (1223 K) and longer time duration (up to 6 h). In case of Inconel alloys the thick layers (66 76 μm) of high hardness ( HK) were produced [16]. Paste boronizing caused the increase in hardness from 100 HV to 1100 HV in case of borided pure nickel, and from about 400 HV to 2500 HV in case of boriding of Nimonic 90 alloy [17]. The corrosion resistance of the produced layers was examined. Paste boriding caused the increase of corrosion resistance of pure nickel, due to the formation of a single phase (Ni 2 B) outer layer. However, the corrosion resistance of borided Nimonic 90 alloy was slightly lower in comparison to the unborided alloy. The advantageous effects were also shown after boronizing using the fluidized bed technology [19] as well as in case of electrochemical boriding [18]. In general, all the nickel-based alloys, such as Hastelloy, Incoloy, Inconel, Nimonic or Haynes, can be successfully boronized. However, some of the boriding methods (e.g. powder-pack boriding) require high temperature (up to 1273 K), long time duration (up to 8 hours) and extensive manual work. The alternative method is the gas boriding, which accelerated the diffusion of boron into the surface in comparison with other acceptable diffusion boriding methods. The comparable thickness of boride layer was obtained after considerably shorter duration [20, 21]. In this study, the gas boriding in atmosphere was used in order to produce the hard borided layer on Nimonic 80A alloy. The microstructure, hardness and fracture toughness (as a main disadvantage of borided layers) were studied and compared to the results obtained by other diffusion methods. 2. EXPERIMENTAL PROCEDURE The nominal composition of the Nimonic 80A alloy, used in this study, was shown in Table 1. The ring-shaped specimens (external diameter ca. 20 mm, internal diameter 12 mm and height 12 mm) were used. Table 1. Chemical composition of material used, wt pct Tabela 1. Skład chemiczny stosowanego materiału, % mas. Material C Si Mn S Al Ti Cr Cu Fe Ni Nimonic 80A < balance 10 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

2 The gas boriding in atmosphere used in this study was previously described [20, 21]. The devices used for gas boriding were shown in Figure 1. They consist of the following basic systems: heating system with an electric furnace, gas-supply system, system of regulation and control of temperature. The heating system included electric furnace LT 75/750/13 (1) with a quartz retort (2), which during the process was cooled by water in the sample loading zone. The power-supply system with a control system of the temperature (3) consisted of three independent heating zones, which provided a constant temperature throughout the cross-section of the furnace retort. The heating process was controlled by a personal computer (16) with HTMonit software. Simultaneously, the temperature near the treated samples was monitored with the separate measuring system (4). To control the temperature in the sample loading zone the NiCr NiAl thermocouple was used. Control thermocouple was placed in a quartz tube, centered in the retort. The gas supply system consisted of gases in a high purity: mixture of 75 vol. % and 25 vol. % H 2 (8), (9) with a purity of 6.0 and boron trichloride BCl 3 (10) with a purity of 4.0. The cylinder with boron trichloride was inserted into the cryostat (11) and cooled by liquid nitrogen put in Dewar flask (13). First the cleaned specimens were put into the quartz retort (2) and the system was checked by vacuummeter (7) in order to ensure that the air had been removed by the vacuum pump (5). Then, the flow of nitrogen (9) was activated, and the heating process was started. After the furnace (1) had reached a temperature of 920 C (1193 K), the boriding process was started. During boriding the gas mixture + H 2 (8) flowed through the cylinder with BCl 3 with a flow rate of 100 l/h. The content of boron trichloride during boriding was very important, because too high content could cause the obtaining porous layer of low quality. Simultaneously, too low BCl 3 content caused producing the thin layer. The concentration of BCl 3 in boriding atmosphere ( ) resulted from its vapour pressure, dependent on the temperature, which was measured by (12) and recorded with DLM Simple software. The average boron trichloride addition was equal 5.23 vol. % in relation to H 2, and 1.36 vol. % in relation to the entire atmosphere used ( ). The boriding process continued for 2 hours. After the process finished, the specimens were cooled in a nitrogen atmosphere. After the boriding process, the specimens were cut perpendicular to the treated surface. The specimens were metallographically prepared and polished with abrasive paper of different grain size, and in the end with the use of Al 2 O 3. The etching solution, consisting of CuSO 4, HCl and H 2 O, was used in order to reveal the microstructure. The microstructures of the polished and etched cross-sections of the specimens were observed using an light microscope (LM). The apparatus ZWICK 3212 B equipped with a Vickers indenter was used for hardness measurements. The load of 50 gf (0.49 N) was used to measure the microhardness (HV0.05) of the borided layer and substrate. The microhardness measurements of the specimen were taken in a cross-section from the borided layer surface to the substrate centre. The fracture toughness of borided layers was measured by indentation technique using a Vicker s diamond indenter. This method uses a model of elastic plastic behaviour of the material under the impression a system of the central and radial cracks is a result of the tensile stresses caused without load [22]. The tests were performed in cross-sections of the specimens using the load 100 gf (0.981 N). In this paper, the fracture toughness of borided layer produced on Nimonic 80A alloy has been determined on the basis of measurements of K C factor by most often used equation [23]: K C = A P c 3 2, MPa m 1/2 (1) Fig. 1. The devices used for gas boriding in atmosphere; 1 furnace, 2 quartz retort, 3 system of power supply and temperature regulation, 4 temperature measuring system, 5 vacuum pump BL8P, 6 pressure pickup, 7 vacuummeter, 8 cylinder with mixture, 9 cylinder with nitrogen, 10 cylinder with boron trichloride, 11 cryostat, 12 control system of the BCl 3 temperature, 13 Dewar flask, 14 rotameter, 15 gas scrubber, 16 personal computer with HTMonit and DLM Simple software, 17 mass-flowmeter Rys. 1. Urządzenia stosowane do borowania gazowego w atmosferze ; 1 piec, 2 kwarcowa retorta, 3 system regulacji temperatury, 4 system pomiaru temperatury, 5 pompa próżniowa BL8P, 6 czujnik ciśnienia, 7 próżniomierz, 8 butla z mieszaniną gazów, 9 butla z, 10 butla z BCl 3, 11 kriostat, 12 system kontroli temperatury BCl 3, 13 zbiornik Dewara, 14 rotametr, 15 płuczka gazów, 16 komputer z oprogramowaniem HTMonit i DLM Simple, 17 przepływomierz masowy 1 E 2 A = (2) H where: P load, N, c the radial crack length, m, A residual indentation coefficient, E Young s modulus, MPa, H hardness, MPa. Schematic views of the obtained indentation marks and cracks are presented in Figure 2. Crack lengths were immediately measured using optical microscope. 3. RESULTS AND DISCUSSION The cross-sectional morphology of borided layer produced on Nimonic 80A alloy was shown in Figure 3. The borided layer was characterized by compact, homogeneous microstructure. The diffusion zone consisted of a compact boride layer (1) and an area with the borides at grain boundaries (2). Beneath the diffusion zone, the base material (substrate) was visible (3). The microcracks and pores were not detected. The total depth of compact boride zone was NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 11

3 Fig. 2. Schematic views of obtained indentation marks and cracks Rys. 2. Schematy uzyskanych odcisków i pęknięć Fig. 4. Microhardness profile of gas borided layer formed on Nimonic 80A alloy Rys. 4. Profil mikrotwardości borowanej gazowo warstwy wytworzonej na stopie Nimonic 80A Fig. 3. Microstructure of gas-borided layer formed on Nimonic 80A alloy; 1 compact zone of borides, 2 zone with borides at the grain boundaries, 3 substrate (base material); LM Rys. 3. Mikrostuktura warstwy borowanej gazowo wytworzonej na stopie Nimonic 80A; 1 zwarta strefa borków, 2 strefa borków występujących na granicach ziaren, 3 podłoże; mikroskop świetlny equal about 75 µm. The compact borides layer was about 3 times thicker than that-obtained by the paste boriding at comparable temperature and time [1]. Therefore, it was assumed that gas boriding method accelerated the saturation by boron and its diffusion into the nickel-based supperalloy. The analysis of literature data [15 17] indicated, that, probably, in borided layer the mixture of nickel and chromium borides occurred. The results of microhardness measurements, after gas boriding of Nimonic 80A alloy were presented in Figure 4. The hardness of the base material (Nimonic 80A alloy) was HV. The gasboriding process caused an increase in hardness up to 1790 HV. As a consequence of the presence of different type of borides (chromium and nickel borides) the obtained microhardness values were characterized by some fluctuation within the range from 1070 to 1790 HV. Probably, the lower microhardness values corresponded to the nickel borides, while the higher hardness was characteristic of chromium borides. The LM images of Vickers indentation marks with cracks generated in cross-section of gas-borided layer produced on Nimonic 80A alloy were presented in Figure 5. It was clearly visible, that near the top surface the crack lengths were greater than those-obtained at higher depths from the surface. The fracture toughness of borides depended strongly on chemical composition of treated material. However, the type of boride formed on the surface was more important than the effect of the alloying elements of substrate material [24]. Some literature data [24, 25] reported that Cr has a negative effect on the fracture toughness of borides formed on the chromium-based low alloy steel. Probably, the same effect can be observed in case of nickel-chromium superalloys. The boride layers formed on Nimonic superalloys were characterized by composite microstructure, in respect of phase composition. These layers consisted of chromium borides and nickel borides. The lower hardness of nickel borides can be accompanied by higher fracture toughness. Simultaneously, the harder chromium borides caused the decrease in fracture toughness. Moreover, the Young s modulus of chromium and nickel borides differ greatly. In earlier paper [26], the Young s modulus of chromium borides and nickel borides were evaluated using a CSM Instruments nanoindenter with a Berkovich diamond tip. The following average values of E modulus were determined: for chromium borides E = 355 GPa and for nickel borides E = 287 GPa. The gas-borided layer produced on Nimonic 80A substrate consisted of mixture of chromium and nickel borides. Therefore to K C calculation the average Young s modulus were taken (E = 321 GPa). The fracture toughness of gas-borided layer produced on Nimonic 80A alloy strongly depends on the distance measured from the surface. In Figure 6 and Table 2 the influence of distance from the surface on K C values was presented. According to these results, the fracture toughness of the gas-borided layer increased as a function of the indentation distance. These results suggest that the layer was more brittle near the surface (up to depth 30 μm from the top-surface). The minimal fracture toughness (maximum brittleness) value K C = 0.6 MPa m 1/2 for the gas-borided layer was established on 13 μm from the surface (Figure 5, indentation marks 1 and 3). The maximal fracture toughness value K C = 4.5 MPa m 1/2 was measured at μm from the surface (Figure 5, indentation marks 8 and 9). Probably, the differences in phase volume fraction were the reason for this situation. Near the top surface, the higher content of chromium borides was expected. This hypothesis should be confirmed using e.g. point X-ray microanalysis. 12 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

4 Table 2. Results of fracture toughness measurements (cracks length L 1, L 2, hardness and K C ) for different distance from the surface Tabela 2. Wyniki badania odporności na kruche pękanie (długość pęknięć L 1, L 2, twardość oraz K C ) Indentation mark Distance from the surface µm L 1 µm L 2 µm HV K C MPa m ½ ,64 30, , , , ,39 23, , ,88 17, , , , ,59 12, , ,54 8, , , , , ,11 Fig. 6. Dependence between fracture toughness of borided layer and the distance of indentation mark from the surface Rys. 6. Zależność pomiędzy odpornością na kruche pękanie a odległością od powierzchni odcisku Vickersa layer, the fracture toughness values were determinate for the Vickers indentation marks and cracks obtained in the parallel direction to the surface. 4. CONCLUSIONS Fig. 5. LM images of Vickers indentation marks with cracks Rys. 5. Obrazy odcisków Vickersa wraz z pęknięciami It should be noted, that cracks appeared only in direction parallel to the surface (Fig. 5). Due to the fact that the fracture toughness is higher in the direction normal to the surface, it was assumed that the compressive stresses were higher in the normal direction. Therefore, the growth and propagation of cracks, produced by microindentation in normal direction, were difficult. Probably, this situation was a consequence of high compressive residual stresses normal to the surface region. Due to the anisotropy of the borided Gas boriding in atmosphere was applied to produce the boride layer on Nimonic 80A alloy. In this method instead of H 2 atmosphere, safer gas mixture, consisting of of limited hydrogen content (about 25 vol. %), was used. Microstructure of the entire diffusion layer consisted of two zones: a compact boride layer, and a zone with borides at grain boundaries occurring below the first one. The obtained depth of borided layer was significantly increased in comparison with those-formed on Nimonic alloys using e.g. paste boronizing method. Although the temperature was lower and process duration was shorter, produced gas-borided layer was even 3 times thicker. The thickness of gas borided layer (75 µm) was similar to those-measured in case of other acceptable diffusion method (pack-boriding, paste boriding). The gas boriding caused the significant increase in hardness of the Nimonic 80A alloy. The gas-boriding process caused the increase in hardness up to 1790 HV. The fracture toughness of gasborided layer produced on Nimonic 80A alloy strongly depends on the distance measured from the surface. The highest brittleness (minimal fracture toughness) was established at 13 μm from NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 13

5 the surface (K C = 0.6 MPa m 1/2 ). The maximal fracture toughness value K C = 4.5 MPa m 1/2 was measured at μm from the surface. Probably, the differences in the phase volume fraction (especially chromium borides content) was the reason for such a situation. ACKNOWLEDGMENTS This work has been financially supported by the National Science Centre in Poland as a part of the UMO-2012/07/N/ST8/03744 project. REFERENCES [1] Srinivasan N., Prasad Y. V. R. K.: Hot working characteristics of Nimonic 75, 80A and 90 superalloys: a comparison using processing maps. Journal of Materials Processing Technology 51 (1995) [2] Kim D. K., Kim D. Y., Ryu S. H., Kim D. J.: Application of nimonic 80A to the hot forging of an exhaust valve head. Journal of Materials Processing Technology 113 (2001) [3] Xu Y., Yang C., Ran Q., Hu P., Xiao X., Cao X., Jia G.: Microstructure evolution and stress-rupture properties of Nimonic 80A after various heat treatments. Materials and Design 47 (2013) [4] Xu C. H., Xi J. K., Gao W.: Improving the mechanical properties of boronized layers by superplastic boronizing. J. Mater. Process. Technol. 65 (1997) [5] Wierzchoń T., Bieliński P., Sikorski K.: Formation and properties of multicomponent and composite borided layers on steel. Surf. Coat. Technol. 73 (1995) [6] Wierzchoń T.: The role of glow discharge in the formation of a boride layer on steel in the plasma boriding process. In: Boenig H. V., editor. Advances in Low-temperature Plasma Chemistry, Technology, Applications, vol. 2. Lancaster-Besel: Technomic Publishing Co. Inc. (1988) [7] Graf von Matuschka A.: Borieren. Carl Hanser Verlag, München, Wien (1977). [8] Hunger H. J., Löbig G.: Generation of boride layers on steel and nickel alloys by plasma activation of boron trifluoride. Thin Solid Films 310 (1997) [9] Pertek A.: The structure formation and the properties of boronized layers obtained in gaseous boriding process. Dissertation No Publishing house of Poznan University of Technology, Poznan (2001). [10] Ozbek I., Akbulut H., Zeytin S., Bindal C., Ucisik A. H.: The characterization of borided 99.5% purity nickel. Surface and Coatings Technology 126 (2000) [11] Lou D. C., Akselsen O. M., Solberg J. K., Onsoien M. I., Berget J., Dahl N.: Silicon-boronising of Nimonic 90 superalloy. Surface & Coatings Technology 200 (2006) [12] Mu D., Shen B. I., Yang C., Zhao X.: Microstructure analysis of boronized pure nickel using boronizing powders with SiC as diluents. Vacuum 83 (2009) [13] Ueda N., Mizukoshi T., Demizu K., Sone T., Ikenaga A., Kawamoto M.: Boriding of nickel by the powder-pack method. Surface and Coatings Technology 126 (2000) [14] Gunes I., Kayali Y.: Investigation of mechanical properties of borided Nickel 201 alloy. Materials and Design 53 (2014) [15] Aytekin H., Akcin Y.: Characterization of borided Incoloy 825 alloy. Materials and Design 50 (2013) [16] Petrova R. S., Suwattananont N., Samardzic V.: The effect of boronizing on metallic alloys for automotive applications. J. Mater. Eng. Perform. 17 (3) (2008) [17] Lou D. C., Solberg J. K., Akselsen O. M., Dahl N.: Microstructure and property investigation of paste boronized pure nickel and Nimonic 90 superalloy. Materials Chemistry and Physics 115 (2009) [18] Sista V., Kahvecioglu O., Kartal G., Zeng Q. Z., Kim J. H., Eryilmaz O. L., Erdemir A.: Evaluation of electrochemical boriding of Inconel 600. Surface & Coatings Technology 215 (2013) [19] Anthymidis K. G., Zinoviadis P., Roussos D., Tsipas D. N.: Boriding of nickel in a fluidized bed reactor. Materials Research Bulletin 37 (2002) [20] Makuch N., Kulka M., Dziarski P.: Gas boriding of Inconel 600 alloy. Inżynieria Materiałowa 6 (2013) [21] Kulka M., Makuch N., Popławski M.: Two-stage gas boriding of Nisil in atmosphere. Surf. Coat. Technol. 244 (2014) [22] Lawn B. R., Evans A. G., Marshall D. B.: Elastic/plastic indentation damage in ceramics: The median/radial crack system. Journal of the American Ceramic Society 63 (9-10) (1980) [23] Üçisik A. H., Bindal C.: Fracture toughness of boride formed on low-alloy steels. Surface & Coatings Technology (1997) [24] Taktak S., Tasgetiren S.: Identification of delamination failure of boride layer on common Cr-based steels. Journal of Materials Engineering and Performance 15 (2006) [25] Bindal C., Ucisik A. H.: Characterization of borides formed on impuritycontrolled chromium-based low alloy steels. Surf. Coat. Technol. 122 (1999) [26] Kulka M., Makuch N., Dziarski P., Piasecki A.: A study of nanoindentation for mechanical characterization of chromium and nickel borides mixtures formed by laser boriding. Ceramics International 40 (2014) INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVII

6 Inżynieria Materiałowa 1 (209) (2016) DOI / Copyright SIGMA-NOT MATERIALS ENGINEERING Odporność na kruche pękanie borowanego gazowo stopu Nimonic 80A Natalia Makuch *, Michał Kulka, Piotr Dziarski Instytut Inżynierii Materiałowej, Politechnika Poznańska, * natalia.makuch@put.poznan.pl Słowa kluczowe: borowanie gazowe, Nimonic 80A, mikrostruktura, twardość, odporność na kruche pękanie. 1. CEL PRACY Stopy na bazie Ni są często stosowane w przemyśle ze względu na unikatowe połączenie właściwości: dużej żarowytrzymałości, dobrej odporności na utlenianie i korozję. Jednakże ze względu na ich małą twardość i wrażliwość na zużycie przez tarcie ich zastosowanie jest ograniczone. Proces borowania jest obróbką zapewniającą dużą twardość i odporność na zużycie stopów na bazie Ni. Niestety, zastosowanie warstw borowanych jest ograniczone z powodu ich wrażliwości na pękanie. Celem pracy było określenie odporności na kruche pękanie warstw borowanych gazowo wytworzonych na stopie Nimonic 80A. Ze względu na skład fazowy tych warstw (borki niklu i borki chromu) pomiary odporności na kruche pękanie przeprowadzono na prostopadłym przekroju warstwy, w różnych odległościach od powierzchni. Ponadto przeprowadzono badania mikrostruktury i wyznaczono profil mikrotwardości. 2. MATERIAŁ I METODYKA BADAŃ Materiałem do badań był stop Nimonic 80A, którego skład chemiczny podano w tabeli 1. Proces borowania gazowego prowadzono w atmosferze w temperaturze 920 C (1193 K) przez 2 godziny. Gaz nośny zawierał 75% i 25% H 2. Stanowisko do borowania gazowego przedstawiono na rysunku 1. Oczyszczone próbki umieszczano w kwarcowej retorcie i za pomocą pompy próżniowej odpompowano z układu powietrze oraz ewentualne ślady innych gazów. Nagrzewanie prowadzono w atmosferze. Po osiągnięciu temperatury 920 C (1193 K) rozpoczęto proces borowania, podczas którego mieszanina gazów + H 2 przepływała przez butlę z BCl 3. Trójchlorek boru znajdował się w kriostacie i podczas borowania był chłodzony za pomocą ciekłego azotu w celu osiągnięcia jego odpowiedniej ilości w atmosferze. Podczas borowania średnia zawartość BCl 3 w porównaniu z wodorem wynosiła 5,23%, natomiast w odniesieniu do całkowitej atmosfery 1,36%. Proces prowadzono przez 2 godziny, a następnie próbki chłodzono razem z piecem w atmosferze azotu. Po borowaniu próbki przecięto prostopadle do obrobionej powierzchni. Wykonane zgłady metalograficzne wytrawiono odczynnikiem składającym się z CuSO 4, HCl i H 2 O. Obserwacje mikrostruktury przeprowadzono za pomocą mikroskopu świetlnego. Badania mikrotwardości prowadzono sposobem Vickersa, stosując obciążenie 50 G (0,49 N). Do wyznaczania odporności na kruche pękanie zastosowano metodę, która polega na otrzymaniu odcisku za pomocą wgłębnika Vickersa. W celu wyznaczenia odporności na kruche pękanie K C mierzy się przekątne odcisków oraz długości pęknięć wychodzących z naroży odcisków zgodnie z rysunkiem WYNIKI I ICH DYSKUSJA Mikrostrukturę warstwy borowanej gazowo wytworzonej na stopie Nimonic 80A przedstawiono na rysunku 3. Warstwa była wolna od pęknięć i porowatości i składała się z trzech stref: zwartej strefy borków (1), strefy, w której borki występują na granicach ziaren podłoża (2) oraz rdzenia (3). Grubość warstwy (75 µm) była trzykrotnie większa w porównaniu z warstwami borowanymi uzyskanymi za pomocą innych metod borowania w zbliżonej temperaturze i czasie procesu. Borowanie gazowe spowodowało zwiększenie twardości do 1790 HV (rys. 4). Na rysunku 5 przedstawiono mikrostrukturę warstwy borowanej gazowo wraz z widocznymi odciskami wykonanymi wgłębnikiem Vickersa. Tuż przy powierzchni pęknięcia generowane z naroży odcisku są znacznie dłuższe w porównaniu y tymi uzyskanymi przy większych odległościach od powierzchni. Jest to związane z różnicą w składzie fazowym na przekroju warstwy. Prawdopodobnie w obszarach, w których dominujący udział stanowią twarde borki chromu, pęknięcia generowane z naroży odcisków są dłuższe. Ponadto borki chromu i niklu różnią się modułem Younga: odpowiednio E = 355 GPa i E = 287 GPa. Wielofazowość warstwy borowanej gazowo wymagała zastosowania do obliczeń K C średniej wartości modułu Younga (E = 321 GPa). Stwierdzono, że odporność na kruche pękanie warstw borowanych na stopie Nimonic 80A silnie zależy od odległości od powierzchni. Na rysunku 6 i w tabeli 2 przedstawiono wpływ odległości od powierzchni na długość pęknięć L 1 i L 2 oraz wartości K C. Wyniki wskazują, że wraz ze wzrostem odległości od powierzchni odporność na kruche pękanie zwiększa się. Warstwa borowana jest najbardziej skłonna do pęknięć w obszarze do 30 μm od powierzchni. Minimalną odporność na kruche pękanie (K C = 0,6 MPa m 1/2 ) uzyskano dla odcisku wykonanego w odległości 13 µm od powierzchni. Najmniejszą kruchość (największą wartość K C = 4,5 MPa m 1/2 ) zmierzono dla odcisku wykonanego w odległości μm od powierzchni. Prawdopodobnie większa kruchość warstwy tuż przy powierzchni jest efektem większego udziału borków chromu w tym obszarze. Ponadto należy zauważyć, że pęknięcia zostały wygenerowane tylko w kierunku równoległym do powierzchni warstwy. Prawdopodobnie jest to spowodowane występowaniem silnych naprężeń ściskających w kierunku normalnym do powierzchni. 4. PODSUMOWANIE W pracy do wytworzenia warstw borowanych na stopie Nimonic 80A zastosowano borowanie gazowe w atmosferze w temperaturze 920 C (1193 K) przez 2 h. Grubość warstwy (75 µm) była trzykrotnie większa w porównaniu z warstwami borowanymi uzyskanymi za pomocą innych metod borowania w zbliżonej temperaturze i czasie procesu. Borowanie gazowe spowodowało zwiększenie twardości do 1790 HV. Wykazano, że odporność na kruche pękanie silnie zależy od odległości od powierzchni. Największą kruchość warstwa borowana gazowo wykazuje tuż przy powierzchni (K C = 0,6 MPa m 1/2 ). Największą wartość K C = 4,5 MPa m 1/2 zmierzono dla odcisku wykonanego w odległości μm od powierzchni. Prawdopodobnie większa kruchość warstwy tuż przy powierzchni jest efektem większego udziału borków chromu w tym obszarze. NR 1/2016 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING 15