VLIV VODÍKU NA MECHANICKÉ VLASTNOSTI SUPERSLITINY IN738LC HYDROGEN EFFECT ON MECHANICAL PROPERTIES OF IN738LC SUPERALLOY

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1 VLIV VODÍKU NA MECHANICKÉ VLASTNOSTI SUPERSLITINY IN738LC HYDROGEN EFFECT ON MECHANICAL PROPERTIES OF IN738LC SUPERALLOY Monika LOSERTOVÁ, Kateřina KONEČNÁ, Jan JUŘICA, Petr JONŠTA VŠB-TU Ostrava, 17. listopadu 15, , Ostrava-Poruba, Česká republika, Abstrakt Studium vlivu vodíku na mechanické vlastnosti bylo provedeno na vzorcích superslitiny IN738LC. Z dodaného stavu po tepelném zpracování byly připraveny dva soubory tahových zkoušek: sycené a nesycené vodíkem. Vodíkování proběhlo po dobu 4 hodin při 800 C v atmosféře vodíku. Tahová zkouška byla provedena na obou souborech vzorků při rychlosti deformace 5,8 x 10-5 s -1. Slitina ve stavu nesyceném vodíkem dosahovala střední hodnoty meze kluzu MPa a meze pevnosti MPa. Tepelné zpracování ve vodíku vedlo ke zvýšení střední hodnoty jak meze kluzu, tak meze pevnosti na MPa, respektive MPa. U dvou vybraných vzorků sycených vodíkem byl po tahové zkoušce stanoven obsah vodíku pomocí analyzátoru LECO RH600 a byly naměřeny hodnoty 47,6 a 71,4 ppm vodíku. Bylo provedeno metalografické, fraktografické studium, jakož i mikroanalýza vybraných vzorků po tahové zkoušce. V mikrostruktuře byly pozorovány pomocí optické mikroskopie licí vady, které ovlivnily rozptyl meze pevnosti u nevodíkovaných vzorků. Strukturně fázová analýza jednotlivých fází byla provedena pomocí EDS. Charakter lomových ploch vzorků po tahové zkoušce byl sledován na vodíkovaných i nevodíkovaných vzorcích pomocí SEM JEOL JSM-6490LV. Vliv tepelného zpracování ve vodíku vedl ke změně charakteru lomového porušení u vzorků superslitiny. Bylo pozorováno, že lomové plochy tahových zkoušek sycených vodíkem vykazují vyšší zastoupení interkrystalického lomu a transkrystalické lomy vykazovaly přechod od kvazištěpného ke štěpnému charakteru s mnohem četnějšími a řádově většími sekundárními trhlinami. Abstract The study of hydrogen effect on mechanical properties was carried out on samples of IN738LC superalloy. Two ensembles of tensile specimens in the heat treated state were prepared: hydrogen-charged and without hydrogen treatment. The hydrogenation was performed at 800 C for 4 hours in flowing hydrogen gas. Tensile tests were realized on both ensembles of samples with a strain rate of 5.8 x 10-5 s -1. The yield strength and tensile stress of the superalloy samples non-hydrogenated reached the average values of MPa and MPa, respectively. The hydrogen effect resulted in a rise of the average values of yield strength as well as of tensile stress to MPa and MPa, respectively. The hydrogen content in two selected specimens was measured using LECO RH600 analyzer and the values of 47.6 and 71.4 ppm of hydrogen were determined. Metallographic and fractographic studies as well as microanalysis of selected specimens were carried out after tensile testing. Casting defects were observed in the microstructure using light microscopy. These imperfections influenced the fluctuation of tensile ultimate stress values of nonhydrogenated samples. An EDS phase analysis of superalloy microstructure was performed. Feature of fracture surfaces of the non-hydrogenated as well as hydrogenated tensile test specimens was investigated by means of SEM JEOL JSM-6490LV. Hydrogen was found to affect the mode of fracture of superalloy samples. The fracture surfaces exhibited an increased amount of intergranular fracture due to the dissolved hydrogen, in case of transgranular cracking it was possible to observe a change on the fracture pattern from quasi-cleavage to cleavage one with more secondary cracks that were greater in order. Keywords: hydrogen embrittlement, superalloys, yield strength.

2 1. INTRODUCTION Alloy IN738LC is a low carbon nickel-based superalloy used for land-based turbine blades and nozzle vanes that derives its strength from precipitation of the ' - Ni 3 Al (L1 2 structure). Nickel-based superalloys like most of structural materials are susceptible to hydrogen embrittlement. However, the effect of hydrogen on the plastic properties of superalloys as well as in case of other alloys depends on the manner in which the hydrogen atom is introduced into the system, the concentration of hydrogen, the strain rate, the temperature, the formation of hydrides or other secondary phases and the effective diffusivity of hydrogen [1]. Hydrogen embrittlement of materials at room temperature is caused by the presence of hydrogen in the structure resulting from gaseous as well as cathodic hydrogen charge. Further, it is important to distinguish between the internal hydrogen, which has entered the alloy prior to deformation, and external hydrogen that penetrates into the material during deformation. So far, it is not clear whether or not the hydrogen embrittlement is related to the same basic mechanism for internal as well as for external hydrogen. In most cases, an increase of the yield strength with increasing content of hydrogen in materials has been reported [1, 2]. The aim of this paper is to estimate the hydrogen effect on mechanical properties of superalloy IN738LC and determine the relationship between the obtained values and observed microstructure feature in dependence on the presence of hydrogen in the structure. 2. EXPERIMENTAL PROCEDURE The experiment material of the IN738LC alloy, which nominal chemical composition is given in Table 1, was supplied by PBS Velka Bíteš, a.s. in heat treated condition. The effect of hydrogen on the yield strength and ultimate tensile stress was investigated for comparison purposes on the hydrogenated and nonhydrogenated tensile specimens with 25 mm gage length and 5 mm diameter. Hydrogen was charged into the specimens prior to tensile tests via a heat treatment at 800 C for 4 hours in gaseous hydrogen with slight over-pressure (700 Pa). The hydrogenated and non-hydrogenated specimens were deformed at room temperature in air and at strain rates of 5.8x10-5 s -1 using an Instron-type machine. For a microstructure observation after the tensile test, the specimens were polished and chemically etched by the solution of 40 ml HCl + 2g CuCl ml C 2 H 5 OH. The chemical composition of phases observed in the microstructure was determined by scanning electron microscope JEOL JSM-6490LV equipped with microprobe EDS INCA x-act. The analysis of total hydrogen content in tested alloy was performed using analyser LECO RH600. Fracture surfaces of selected samples were subjected to fractographic evaluation on JEOL JSM-6490LV. A particular attention was paid to the change of failure feature in hydrogenated specimens. Tab 1 Nominální chemické složení superslitiny IN738LC (hmotn.%) Table 1 Nominal chemical composition of IN738LC (mass %) Ni Cr Co Al Ti W Mo Ta Nb balance Zr B Fe Si Mn Cu P C S max 0.3 max 0.2 max 0.1 max max max

3 Stress [ MPa ] Stress [ MPa ] hydrogenated nonhydrogenated , Brno, Czech Republic, EU 3. RESULTS AND DISCUSSION The values of mechanical properties determined from the tensile test are summarized in Table 2. Fig. 1 shows the stress strain curves corresponding to hydrogen-precharged and hydrogen-free specimens strained at rates of 5.8 x 10-5 s -1. It can be seen that the internal hydrogen increased the yield strength 0.2 by 11.6 % and decreased the fracture toughness f by 75 %. The values of ultimate tensile stress (UTS) were at all cases below the specification according to TLV and the hydrogen presence led surprisingly to higher UTS of about 5.4 %. Tab 2 Mechanické hodnoty v tahu u vybraných vzorků superslitiny IN738LC Table 2 Tensile properties of selected samples of IN738LC alloy specimen 0.2 UTS f mean value standard deviation H H H mean value standard deviation specimen 1 specimen 5 specimen specimen 5H 5H specimen 3H 3H specimen 2H specimen specimen 2H 300 specimen specimen 3H 200 specimen specimen 5H Strain [ % ] Strain [ % ] Obr. 1 Mechanické vlastnosti vybraných vzorků IN738LC: a) nesycené vodíkem a b) sycené vodíkem. Fig. 1 Mechanical properties of selected samples of IN738LC: a) non-hydrogenated and b) hydrogenated. The metallographic observation of the dendritic microstructure revealed great amount of casting defects in interdendritic regions (Fig. 2). These imperfections caused lower values of UTS and their fluctuation for nonhydrogenated samples (Table 2). The alloy matrix is strengthened by the precipitates of irregular cubic morphology with a nonuniform distribution. At the grain boundaries and in the interdendritic domains the coarse precipitates (Fig. 3a) and + eutectic were observed.

4 Tab 3 Výsledky EDS mikroanalýzy vybraných vzorků IN738LC (hmotn.%) Table 3 Results of EDS microanalysis of selected specimens of IN738LC (mass %) Phase/Element Al Ti Ta Nb Cr W Zr Ni Co Mo Si (block) (small particle) Specimen , Carbide M 23 C Matrix with Specimen 1H (block) (small particle) Carbide M x C y The primary carbides MC are precipitated at the grain boundaries and in the interdendritic regions as block shapes and rod shapes as shown in Fig. 3a. The results of EDS microanalysis (Table 3) proved that the unevenly distributed MC carbides contain variable amounts of carbide formers, like Ta, Nb and Ti. The grain boundaries of the hydrogen-charged samples are bordered by fine network of both carbides MC and M 23 C 6 (Fig. 3b), and seems to be similar to the microstructure after a long-term high temperature treatment. The precipitation of secondary chromium-reach M 23 C 6 carbides is closely related to the decomposition of the primary carbides during the prior solution treatment. The changes in chemical content in the M 23 C 6 carbides after heat treatment in hydrogen, mainly increasing Mo and decreasing Cr contents (Table 3), could be due to their dissociation and formation of M 6 C carbides. The chemical composition of MC particles was also found changed in Mo content. It is not known what role hydrogen atoms could play in dissociation of M 23 C 6 carbide into M 6 C, but on the other side it is widely accepted [3-6] that carbide particles, as carbide/matrix interfaces are irreversible hydrogen trapping sites. Fracture surfaces in non-hydrogenated IN738LC show generally more ductile feature with dimples or quasicleavage areas (Fig.4a). Intergranular or dendritic fractures were less abundant. The blocky type MC carbides are considered as crack initiators at their interface with the matrix [7, 8] and failure propagated along this interface has cleavage feature (Fig. 4a). The feature of fracture surfaces of hydrogenated tensile specimens have been changed to a brittle cleavage mode due to the dissolved hydrogen. The failure exhibited an increased amount of intergranular fracture, in case of transgranular cracking it was possible to observe a change on the fracture pattern from quasicleavage to cleavage one (Fig. 4b) with more secondary cracks that were greater-order than in nonhydrogenated specimens. Generally, hydrogen embrittlement is considered to be a consequence of interaction between the absorbed hydrogen and the bulk material, when hydrogen accelerates crack nucleation and propagation. Moreover, an important role in the hydrogen embrittlement sensitivity of the alloy would play the carbide particles beeing irreversible hydrogen traps and affecting the hydrogen content in the alloy.

5 a) b) Obr.2 Mikrostruktura vzorků IN738LC: a) nevodíkovaný a b) vodíkovaný. Fig.2 Microstructure of IN738LC samples: a) non-hydrogenated and b) hydrogenated. a) b) Obr. 3 SEM snímky karbidických částic MC a M 23 C 6 a hranic zrn v mikrostruktuře vzorku IN738LC a) nevodíkovaného a b) vodíkovaného. Fig. 3 SEM microstructure of MC and M 23 C 6 carbide particles and grain boundaries in IN738LC samples a) non-hydrogenated and b) hydrogenated. a) b) Obr. 4 SEM snímky lomových ploch po tahové zkoušce: charakter lomu matrice a částic MC a M 23 C 6 u vzorku IN738LC a) nevodíkovaného a b) vodíkovaného. Fig. 4 SEM fractography of IN738LC: failure feature of alloy matrix and MC a M 23 C 6 carbides in tensile sample a) non-hydrogenated and b) hydrogenated.

6 The content of hydrogen was measured in two selected tensile specimens and reached the values of 41.6 and 71.4 ppm. However, it is necessary to take into account that the distribution of hydrogen atoms would not be uniform and will be related with occurance and abundance of the phases in the microstructure of hydrogenated specimens. 4. CONCLUSION Based on the fractographic study of IN738LC alloy the following conclusions have been drawn: 1. The casting defects in the microstructure influenced the UTS values of non-hydrogenated specimens. 2. An important increase in yield strength and decrease in fracture strain due to the presence of hydrogen in the structure were determined. 3. The fracture feature correlates well with the occurence of hydrogen in the structure of tensile specimens, higher ductility and ductile dimple mode of cracks were found in non-hydrogenated specimens, while more brittleness with cleavage cracking was observed in hydrogen-precharged tensile specimens. 4. The hydrogen determined in selected samples reached the amount of 41.6 and 71.4 ppm. Acknowledgement The experimental works were supported by the project Processes of preparation and properties of highpurity and structurally defined special materials, No. MSM , financed by the Ministry of Education, Youth and Sports of the Czech Republic and the project of Operational Programme Research and Development for innovation entitled "Regional Materials Science and Technology Centre" No. CZ.1.05/2.1.00/ , co-financed by EU funds - European Regional Development Fund and Ministry of Education, Youth and Sports of Czech Republic. LITERATURA [1.] BIRNBAUM, H.K., ROBERTSON, I.M., SOFRONIS P. Hydrogen Effects on Plasticity. In: Multiscale Phen. in Plasticity. Kluwer Academic Publishers, 2000, p [2.] BOND G.M., ROBERTSON I.M., BIRNBAUM H.K. Acta Metall., 1989, 37, 5, p [3.] POUND, B.G. Acta Metall. Mater., 1990, 38, 12, p [4.] POUND, B.G. Corrosion Science, 2000, 42, p [5.] PRESSOUYRE, G.M., BERNSTEIN, I.M. Acta Metall., 1979, 27, p. 89. [6.] YOUNG, G.A. SCULLY J.R. Scripta Mater., 1997, 36, 6, p [7.] LIU, L. et al. Scripta Mater., 1994, 30, p [8.] BALIKCI, E., MIRSHAMS, R.A., RAMAN, A. Mat. Sci. Eng., 1999, A265, p