Materials Science Forum Vols. 706-709 (2012) pp 2393-2399 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.706-709.2393
Characterization of precipitation kinetics of Inconel 718 superalloy by the stress relaxation technique Jessica Calvo1,2, a, Sheyu Shu1 and José María Cabrera1,2, b 1
Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat Politècnica de Catalunya, Barcelona, Spain 2 a
CTM Centre Tecnològic, Barcelona, Spain b
[email protected],
[email protected]
Keywords: Inconel 718, stress relaxation test, precipitation kinetics, hot deformation.
Abstract. Inconel 718 is a nickel-chromium-iron superalloy which presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These characteristics can be optimized with an appropriate control of microstructural features such as grain size and precipitation. Precipitates of different nature can form in these alloys, i.e. γ’’ (a metastable metallic compound Ni3Nb), γ’ (Ni3(Ti, Al), carbides and/or δ phase (intermetallic Ni3Nb). Aging treatments are usually designed to obtain the precipitation required in order to optimize mechanical properties. However, precipitation can also appear induced by deformation and therefore interfere with hot forming operations, such as forging. Under these conditions, precipitation may lead to an increase of the loads required to carry out the process. The aim of the work was the characterization of precipitation kinetics for Inconel 718. With this purpose, stress relaxation tests were carried out at temperatures ranging from 950°C to 800°C. Moreover, different amounts of deformation were applied to the samples, prior to stress relaxation, to evaluate the effect of this variable on inducing precipitation. Some samples were quenched at different relaxation times for metallographic evaluation. The results obtained through mechanical testing, together with a proper characterization of precipitation by Scanning Electron Microscopy, were the basis for obtaining precipitation-timetemperature (PTT) diagrams after different deformation conditions. Introduction Inconel 718 superalloy is commonly used for its outstanding corrosion and mechanical properties at high temperatures, i.e. it can be used at temperatures up to 650ºC. These properties rely on the formation of γ’’ (Ni3Nb) phase which is the primary strengthening precipitate in Nb containing nickel-iron superalloys [1]. These precipitates have a body centered tetragonal structure, coherent with the matrix, and appear as disc-shaped particles in the microstructure. Other precipitates, such as γ’ phase Ni3(Ti, Al) can form simultaneously with γ’’, contributing to strengthening, but to a lesser degree [2]. In particular, γ’ appears as a fine dispersion of quasi-spherical particles and they are coherent with the matrix. Moreover, Inconel 718 superalloy also contains Fe which makes the material susceptible to the formation of δ phase (Ni3Nb) with an orthorhombic structure. This phase is incoherent with the matrix and does not contribute significantly to strengthening. Usually, it is considered that δ phase can form at temperatures between 650ºC and 980ºC and, although its transformation kinetics is slow under isothermal conditions, it can appear when the material is forged [1]. Even thought precipitation kinetics is usually considered from the point of strengthening, and are used for heat treatment assessment [3, 4], this phenomenon can also take place concurrently with deformation in forming processes [5, 6]. When precipitation takes place induced by deformation, the formation of new precipitates in the microstructure can lead to an increase of the operational loads. Moreover, the concentration of alloying elements in solid solution in the matrix would be reduced, and so would the strengthening capability of the material during the subsequent heat treatments. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 83.37.44.61-24/11/11,23:08:27)
2394
THERMEC 2011
In this work, the precipitation kinetics of Inconel 718 will be evaluated from the point of view of its effect on processing. With this purpose, stress relaxation tests were performed at different temperatures [7]. It is known that deformation can accelerate precipitation kinetics. The effect of deformation on precipitation kinetics was also evaluated by applying different levels of strain prior to relaxation. Precipitation-temperature-time (PTT) curves were derived from stress relaxation curves, and an accurate metallographic observation work was realized in order to identify the species forming at different testing conditions. Experimental method The material used for the experimentation was an Inconel 718 superalloy with the composition listed in table 1. This material belongs to the nickel-chromium-iron age-hardenable austenitic superalloys family, and it contains other elements such as Fe, Nb, Mo, Al and Ti which combine to offer high corrosion resistance, high strength and excellent creep resistance up to 650ºC. The asreceived material had been subjected to a mill thermal treatment at 980ºC for 1h and then water quenched. The hardness associated with this condition was 292HV. Table 1 Chemical composition (mass%)
Ni 54
Cr 18.21
Fe 17.29
Nb+Ta 5.35
Mo 2.98
Ti 0.91
Al 0.66
Co 0.24
C 0.03
Cu 0.07
In order to homogenize the microstructure and composition, the material was subjected to reheating treatments at 1100ºC for 1h in a tubular furnace. After heat treatment, cylindrical samples 10mm in length and 5mm in diameter were machined for mechanical testing in the dilatometer. The thermomechanical schedule followed by the samples is shown in Fig. 1. During testing the samples were heated up to 1100ºC and they remained at this temperature for 5’ until the temperature was homogeneous throughout the sample. After reheating, the samples were cooled down to the testing temperature which ranged from 775ºC to 950ºC. After a short stabilization period of 10s, the samples were deformed to different strain levels (0.005, 0.1 and 0.2) and then relaxed, i.e. deformation was kept constant and the variation of the stress with the time was recorded. The different deformations levels were selected to evaluate the effect of deformation on precipitation kinetics.
Fig. 1 Thermomechanical cycle of stress relaxation tests After stress relaxation, samples were quenched to retain the microstructure for further metallographic observation. Moreover, additional samples were quenched at intermediate relaxation times to assess the progress of precipitation. A Field Emission Scanning Electron Microscope (FESEM) JEOL JSM- 7001 was used for microstructure characterization.
Materials Science Forum Vols. 706-709
2395
Results and discussion As-received vs. heat treated material. The as-received material presented a big amount of precipitation as shown in Fig. 2a and 2b. There are two different families of precipitates, i.e. globular precipitates (white arrows in Figure 2b) and plate-like precipitates (black arrows in Figure 2b). EDX analysis was performed to characterize the precipitates and it was found that globular precipitates contain high Nb contents and some Ti, whereas plate-like precipitates contain some Nb. Therefore, globular precipitates can be identified as Nb,Ti carbides and plate-like are δ precipitates. In order to evaluate precipitation kinetics it is important to dissolve these precipitates and put the elements back into solid solution. The heat treatment selected to solubilize the samples consisted on a reheating at 1100ºC for 1h. Fig. 2c and 2d shows that δ precipitates have disappeared, however, there are still some Nb,Ti carbides in the microstructure. In fact, it would be very difficult to completely dissolve these precipitates because they form at high temperatures and further increasing the reheating temperature would result in an excessive grain growth.
a)
b)
c) d) Fig. 2 Microstructure of a) and b) the as-received material and c) and d) the material after a heat treatment at 1100ºC for 1h Stress relaxation curves. The stress relaxation curves for the three levels of initial strain can be observed in Fig. 3. Side by side of the stress relaxation graphs, the inflection points for each curve have been plotted in a temperature vs. time graph. The black arrows in the stress relaxation graphs indicate relaxation conditions at which samples were quenched for metallographic observation. In general, it could be observed that the samples relaxed after a deformation of 0.1 or 0.2 present stress levels which are higher for decreasing temperatures. This behavior is not totally consistent when relaxation is applied after a deformation of 0.005 because this strain is within the elastic deformation regime, i.e. the effect of temperature is not as noticeable and small variations in the control of the initial strain can lead to big variations in the initial stress before relaxation.
2396
THERMEC 2011
1000
150
800°C 825°C 850°C 875°C 900°C 925°C 950°C
100 50
ε =0.005
Temperature (°C)
Stress (MPa)
200 950 900 850 800
0 1
10
100
1000
10
Time (s)
100
1000
Time (s)
a)
1000
Stress (MPa)
250 200
800°C 825°C 850°C 875°C 900°C 925°C 950°C
150 100 50
ε =0.1
0 1
10
100
Temperature (°C)
300 950 900 850 800
1000
10
Time (s)
100
1000
Time (s)
b)
1000 400 300
775°C 800°C 825°C 850°C 875°C 900°C 925°C 950°C
200 100
ε = 0.2 0 1
10
100
Time (s)
1000
Temperature (°C)
Stress (MPa)
950 900 850 800 750 10
100
1000
Time (s)
c) Fig. 3 Stress relaxation curves and precipitation-temperature-time curves after a) 0.005, b) 0.1 and c) 0.2 strain In general, the shape of the curves change between the relaxations carried out at high temperatures and at low temperatures. For relaxations after 0.005 strain, the curves for temperatures below 875ºC exhibit an increase of stress when relaxation starts. This could be related to the appearance of a strengthening mechanism, i.e. precipitation, which would be active since the first stages of relaxation. Above 875ºC there is no strengthening at the beginning of the relaxation, but some strengthening, which starts around 200s after the beginning of relaxation, can be observed. The relaxation curves after 0.1 and 0.2 strain also present differences depending on the testing temperature range. In this case, the temperature which limits the two different behaviours is 850ºC; above 850ºC curves present only one change in relaxation rate whereas below 850ºC curves show two changes in relaxation rates. These two changes in relaxation rates could be indicating the precipitation of two different species with different kinetics. Given the shape of the curves at 850ºC, which exhibit a pronounced change in the relaxation slope, it could be argued that the precipitates forming at this temperature have the strongest strengthening effect, probably because they are small, coherent with the matrix and/or they are forming in a higher volume fraction.
Materials Science Forum Vols. 706-709
2397
Regarding the effect of strain on precipitation kinetics, precipitation-temperature-time curves do not show a strong effect of this variable for the studied range of strains, although slight acceleration of precipitation with strain can be detected. Metallographic observation by FE-SEM. In order to identify the nature of the precipitates some samples were quenched after different relaxation times. Fig. 4 shows the images of precipitation at 900ºC and different initial strains and relaxation times. In general, precipitation taking place at this temperature can be identified as δ phase given the location of precipitates at grain and twin boundaries and their plate-like shape. According to Fig. 4a, Nb,Ti carbides also act as precipitation nucleation sites. After 0.005 strain, precipitation seems to be very sluggish because the degree of precipitation after 600s relaxation is lower than the degree of precipitation after 600s when 0.2 deformation had been applied to the samples (Fig. 4b). For longer relaxation times (Fig. 4c and 4d), δ phase has extensively precipitated throughout the microstructure at grain and twin boundaries. Moreover, other disc-like precipitates can also be observed within the matrix, which are probably γ‘‘. This precipitation might have started short before 3600s and longer relaxation times would be required in order to detect the precipitation start of this phase in the relaxation curves.
a)
b)
c)
d)
Fig. 4 Samples tested at 900ºC and quenched after a) 0.005 strain and 900s relaxation time b) 0.2 strain and 600s relaxation time and c) and d) deformed 0.2 and quenched after 3600s At 800ºC, precipitation-temperature-time curves in Fig. 3 show two separated regions of precipitation. After 0.005 deformation, the samples quenched after short relaxation times did not show any precipitation, i.e. no evidence could be obtained of the real phenomena which could be causing the increase of the stress at the beginning of the relaxation for the samples deformed 0.005 at temperatures below 850ºC. After 0.2 deformation, even after short relaxation times some precipitation could be observed at grain boundaries. For longer relaxation times (Fig. 5) precipitation is apparent both at grain boundaries and within the matrix. At 800ºC, precipitates at grain boundaries are no longer plate-shaped, instead, they look disc- or spherical-shaped (Fig. 5a and 5c for 0.005 and 0.2 respectively). Given their location and shape, this precipitates could either be γ’’ or δ and they have formed before precipitates in the matrix because they were observed in deformed samples quenched after short relaxation times. In fact, it is assumed that they are γ’’ because it has been reported that this phase precedes the formation of δ phase at temperatures up to 900ºC [4].
2398
THERMEC 2011
Precipitation within the matrix is a combination of disc- and spherical-shaped particles which can be identified as γ’’ and γ’ respectively. These precipitates can form concurrently, and, according to Fig. 5b and 5c it looks like deformation promotes the formation of γ’ and prevents the formation of γ’’.
a)
b)
c)
d)
Fig. 5 Samples tested at 800ºC and quenched after a) and b) 0.005 strain and 3600s relaxation time and c) and d) 0.2 strain and 900s relaxation time
Summary A stress relaxation based technique has been used to characterize precipitation kinetics of Inconel 718 at temperatures within the hot deformation range. It has been shown that δ precipitation is relatively sluggish and it takes places at temperatures ranging from 850ºC and 950ºC under deformation conditions. At lower temperatures, γ’’ and γ’ phases can appear. The former one, appear first located at grain boundaries and its transformation kinetics are fast, which means that this precipitate is more likely to appear concurrently with deformation.
Materials Science Forum Vols. 706-709
2399
References [1] R. C. Reed, The superalloys, Fundamentals and Applications, Cambridge University Press, Cambridge, 2006. [2] C. Slama, M. Abdellaoui, Structural characterization of the aged Inconel 718, J. Alloys Compd., 306 (2000) 277-284. [3] C.-M. Kuo, Y.-T. Yang, H.-Y. Bor, C.-N. Wei and C.-C. Tai, Aging effect on the microstructure and creep behavior of Inconel 718 superalloy, Mat. Scie. Eng. A 510-511 (2009) 289-294. [4] S. Azadian, L.-Y. Wei and R. Warren, Delta phase precipitation in Inconel 718, Mater. Charact. 53 (2004) 7-16. [5] A. Thomas, M. El-Wahabi, J.M. Cabrera and J.M. Prado, High temperature deformation of Inconel 718, J. Mater. Process. Technol. 177 (2006) 469-472. [6] H. Yuan and W.C. Liu, Effect of δ phase on the hot deformation behavior of Inconel 718, Mater. Scie. Eng. A 408 (2005) 281-289. [7] H. Monajati, F. Zarandi, M. Jahazi and S. Yue, Strain induced γ’ precipitation in nickel base superalloy Udimet 720 using a stress relaxation based technique, Scripta Mater. 52 (2005) 771-776.
