Thermophysical properties of solid and liquid Inconel 718 alloy

Transcription

1 High Temperatures ^ High Pressures, 2001, volume 33, pages 405 ^ ECTP Proceedings pages 973 ^ 972 DOI: /htwu340 Thermophysical properties of solid and liquid Inconel 718 alloy Herwig Hosaeus }, Achim Seifter }, Erhard Kaschnitz #, Gernot Pottlacher } } Institut fu«r Experimentalphysik, Technische Universita«t Graz, Petersgasse 16, A-8010 Graz, Austria; fax: ; pottl@iep.tu-graz.ac.at # Oë sterreichisches GieÞerei-Institut, ParkstraÞe 21, A-8700 Leoben, Austria Presented at the 15th European Conference on Thermophysical Properties, Wu«rzburg, Germany, 5 ^ 9 September 1999 Abstract. Wire-shaped alloy samples are resistively volume heated as part of a fast capacitor discharge circuit. Time resolved measurements with submicrosecond resolution allow the calculation of specific heat and the mutual dependences between enthalpy, electrical resistivity, temperature, and density of the alloy in the solid and liquid phase. Thermal conductivity is estimated from resistivity data by the Wiedemann ^ Franz law at the end of the solid phase and at the beginning of the liquid phase. These high-speed measurements are compared to results of quasistatic measurements of specific heat obtained by differential scanning calorimetry and expansion measurements by dilatometry. The results are presented and compared with results of other research groups. 1 Introduction Numerical simulation of fluid flow, heat transfer, solidification, or thermal induced stresses have gained tremendous significance in different branches of industry. With the advent of adequate computing power, full three-dimensional calculation of the determining physical equations has become possible. A major drawback of these simulation techniques is the lack of accurate thermophysical properties. Important input parameters for the heat transfer equation are heat capacity, heat of fusion, density, and thermal conductivity. Because direct measurements of thermal conductivity of alloys in the liquid state are very complex, its estimation from the electrical conductivity by the Wiedemann ^ Franz law is very useful. Inconel 718 is a nickel-based superalloy having a combination of high strength at moderate temperatures, corrosion and oxidation resistance, and a good creep and fatigue resistance (Bouse 1989; Knorovsky et al 1989). It has been one of the most highly utilised nickel-based superalloys in elevated-temperature gas-turbine applications now for over thirty years, for example in the aerospace industry, for rocket motors, and in spacecraft. Inconel 718 basically consists of a face-centred cubic solid solution of nickel, chromium, and iron, precipitation-hardened by g 00 [Ni 3 (Al, Ti, Nb)] and g 0 [Ni 3 (Al, Ti)] (Radharkrishnan and Thompson 1989; Dahotre et al 1993). Solidification begins with the formation of austenitic dendrites. Interdendritic solidification involves niobium carbides and a Laves phase occurs (Cieslak et al 1989). Inconel 718 alloy belongs to the age-hardenable alloys, in which the thermophysical properties are strongly dependent on the heat treatment. McElroy et al (1978) report for such a commercial heat treatment: C solution anneal plus duplex ageing [1258 8C (8h)and 11688C (8 h)]. Enthalpy, electrical resistivity, and density as a function of temperature of Inconel 718 alloy were measured by a fast resistive pulse-heating technique in the solid and liquid phase. These high-speed measurements are compared to results of quasistatic heat capacity measurements by differential scanning calorimetry (DSC) and static thermal expansion measurements by pushrod dilatometry.

2 406 H Hosaeus, A Seifter, E Kaschnitz, G Pottlacher 15 ECTP Proceedings page Experimental and data reduction Wire-shaped Inconel 718 specimens (0.8 mm diameter, 50 mm length) were resistively self-heated by a nearly rectangular current pulse of 10 ka supplied by a high-voltage capacitor discharge circuit. Melting is reached in less than 30 ms and heating continues up into the liquid phase. Because of the short timescale, gravitational forces do not distort the geometry of the specimen even in the liquid phase. Time resolved, with submicrosecond resolution, the following quantities have been measured: voltage drop along a defined inner part of the specimen as well as current through it, and the surface temperature radiation. Additionally, pictures of the specimen have been taken at various instants by a high-speed CCD framing camera. These measurements allow the direct determination of enthalpy, electrical resistivity, temperature, and density of Inconel 718 in the high-temperature solid and in the liquid phase and their mutual dependences. The linear slope of the graph of enthalpy versus temperature (figure 1) allows the determination of the specific heat capacity for the solid and the liquid phase. Thermal conductivity can be estimated from resistivity data by the Wiedemann ^ Franz law. For more experimental details see work by Seifter et al (1998) and Kaschnitz et al (1992). Within the temperature range of room temperature to C, heat capacity and thermal expansion were measured by a high-temperature DSC (Netzsch DSC 404, Netzsch-Gera«tebau GmbH, PO-Box 1460, D-P50 88, Selb, Germany) and a pushrod dilatometer (Netzsch DIL 402E) H kj kg Temperature 8C Figure 1. Enthalpy as a function of temperature (*, measured values from pulse heating; ö, fit to pulse-heating values; ~, DSC measurements). Table 1. Analysis of Inconel 718, as used for pulse-heating and for quasistatic measurements. Element Content/wt% Inco-alloys (1999) pulse-heating measurements quasistatic measurements Nickel 50.0 ± 55.0 balance balance Chromium 17.0 ± Iron balance Niobium 4.75 ± Molybdenum 2.80 ± Titanium 0.65 ± Aluminium 0.20 ± Manganese 0.35 maximum Silicon 0.35 maximum Carbon 0.08 maximum

3 Thermophysical properties of Inconel 718 alloy ECTP Proceedings page 975 The composition of the investigated Inconel 718 specimens was analysed by means of inductive coupled plasma (ICP) spectroscopy and of glow discharge ^ optical emission spectrometer (LECOó MI, USA) (LECO) and is listed in table 1. Solidus and liquidus temperatures (1255 8C and C, respectively) were determined by differential thermal analysis (DTA) (Netzsch STA 509). 3 Results The variation of enthalpy (H H 25, where the subscript refers to 25 8C) of Inconel 718 as a function of temperature in the range 850 8C upto18008c is shown in figure 1. For the entire temperature range a spectral emissivity, which is assumed to be equal to the emissivity at the liquidus point, is used so that the emissivity ratio in Planck's law cancels out and the actual emissivity value does not have to be known. This is a general assumption that has to be made for the emissivity values of the liquid phase because of the lack of measured values. Enthalpy values in the solid range obtained by pulse heating are approximately 4% lower than the (partially extrapolated) results of the DSC measurements. The data in the solid and liquid regions are fitted by linear functions by the leastsquares method. The values at the solidus and liquidus points are considered to be the enthalpies at the beginning and end of melting; the difference is equal to the heat of fusion. From the slope of the enthalpy versus temperature function, a heat capacity in the liquid state of 775 J kg 1 K 1 is derived. It is notable that melting in figure 1 occurs almost with a melting plateau, an effect which is probably due to the high heating rate, which clearly leads to nonequilibrium conditions at the phase transition. The variations of electrical resistivity of Inconel 718 with and without correction for thermal expansion as a function of temperature in the range 850 8C up to C are shown in figure 2. The volume change between room temperature and solidus point is approximately 7%, and it is another 3% between solidus and liquidus. In the solid region, electrical resistivity is increasing from 1.39 mo mto1.43mo m at the onset of melting; in the liquid, the resistivity versus temperature function shows a slight positive slope. The difference between the values at the solidus and liquidus points is, even for the results corrected for thermal expansion, rather small. Table 2 summarises the experimental results of enthalpy and electrical resistivity obtained by pulse-heating and DSC measurements r mo m Temperature 8C Figure 2. Electrical resistivity as a function of temperature (*). Electrical resistivity without correction for thermal expansion (&). Values from the literature: R, McElroy et al (1978);!, Tye et al (1972).

4 408 H Hosaeus, A Seifter, E Kaschnitz, G Pottlacher 15 ECTP Proceedings page 976 Table 2. Thermophysical properties of Inconel 718 at different temperatures. Values obtained by pulse heating and by DSC measurement: enthalpy H, specific heat c p, specific electrical resistivity without volume correction r 0, corrected specific electrical resistivity r, densityd. Densityat room temperature (Inco-alloys 1999): 8190 kg m 3. Temperature 8C Pulse heating DSC r 0 mo m H H 25 c p H H 25 c p kj kg 1 Jkg 1 K 1 kj kg 1 Jkg 1 K 1 r mo m d kg m (635) (649) (704) (653) (s) 679 (659) (1) Discussion Measurements of enthalpy and specific heat capacity in the vicinity of melting reported in the literature were performed by DSC (Henderson and Strobel 1995; Quested et al 1998), the results show good agreement. An exception is the heat capacity in the liquid range, which is approximately 10% lower for measurements obtained by DSC (Henderson and Strobel 1995; Quested et al 1998), but within the combined uncertainties. The values of electrical resistivity (not corrected for thermal expansion) are in good agreement with the values of McElroy et al (1978). The values of Tye et al (1972) are somewhat lower than those of this work, but still within the stated uncertainty. Expansion-corrected values for comparison are not available. Thermal conductivity as a function of temperature of Inconel 718 has been reported by Henderson and Strobel (1995) and Quested et al (1998). The values were calculated from thermal diffusivity, heat capacity, and density. A second approach to estimate the electronic part of thermal conductivity is the application of the Wiedemann ^ Franz law with the assumption of a constant Lorentz number (L ˆ 2: V 2 K 2 ). In addition, the lattice component of the thermal conductivity makes a significant contribution to the overall thermal conductivity of complex superalloys. Figure 3 shows thermal conductivity as a function of temperature calculated from electrical resistivity results. Because the lattice component is supposed to be a weak function of temperature, a value of 4:8 Wm 1 K 1, estimated in a temperature range of 127 8C to7278c (Klemens and Williams 1986), is added as lattice component in the solid phase to the electronic part. Tye et al (1972) derived Lorentz numbers (L ˆ 4: V 2 K 2 at 300 K decreasing to 2: V 2 K 2 at 1200 K). In the liquid phase, with the collapse of the lattice, the thermal conductivity estimate from electrical resistivity can be considered to be even more accurate than in the solid phase. Thermal conductivity values in the liquid phase, reported by Quested et al (1998), are significantly lower than the values derived from electrical resistivity. This might be due to the high volume expansion reported there. The values of McElroy et al (1978) and of Tye et al (1972) show very good agreement to lattice-corrected values of this work.

5 Thermophysical properties of Inconel 718 alloy ECTP Proceedings page l WK 1 m Temperature 8C Figure 3. Thermal conductivity as a function of temperature: *, derived from electrical resistivity by the Wiedemann ^ Franz law; öö, lattice thermal conductivity of 4:8 Wm 1 K 1 added to the electronic part; ^, Quested et al (1998); R, McElroy et al (1978);!, Tye et al (1972). 5 Estimation of uncertainties Estimates of the uncertainties in measured current, voltage, and specimen parameters such as length, diameter, and mass have been discussed elsewhere (Kaschnitz et al 1992). The uncertainty of the enthalpy measurement is estimated to be 4% at the beginning of the liquid state, increasing to 6% at the highest temperature. In the solid range, enthalpy has an uncertainty of 3% for the DSC measurement, and 7% for the pulse-heating, because of the assumption of constant emissivity. The uncertainty of the uncorrected electrical resistivity is estimated to be 4%, increasing to 6% for the corrected electrical resistivity. 6 Conclusions Thermophysical properties of Inconel 718 in the solid and liquid states were determined by a fast pulse-heating technique. High-speed measurements have been compared to results obtained by quasistatic methods. Basically, the analysis of this very complex alloy system by means of pulse heating is possible, but regions with phase transitions have to be avoided. A careful comparison with quasistatic methods is necessary. As can be seen from the comparison to static data, melting does establish thermodynamic equilibrium even at very high speeds of heating. Thermal conductivity was estimated from the electrical resistivity by the Wiedemann ^ Franz law. Taking into account the lattice component, the comparison to static data shows good agreement in the solid phase. In the liquid, where direct measurements of thermal conductivity are almost impossible, the calculation from electrical resistivity is one of the rare methods of an indirect approximation. Acknowledgements. We would like to thank Dr P Quested (NPL, London, UK) for donating the Inconel 718 samples. This work was supported by the ``Fonds zur Fo«rderung der wissenschaftlichen Forschung, Austria'' under contact No. P12804-PHY and P12775-PHY. References Bouse G K, 1989, in Superalloy 718öMetallurgy and Applications Ed. E A Loria (Metals Park, OH: The Minerals, Metals, and Materials Society) p. 69 Cieslak M J, Knorovsky G A, Headley T J, Roming A D, 1989, in Superalloy 718öMetallurgy and Applications Ed. E A Loria (Metals Park, OH: The Minerals, Metals, and Materials Society) p. 59 Dahotre B N, McCay M H, McCay T D, Hubbard C R, Porter W D, Cavin O B, 1993 Scr. Metall. Mater ^ 1364

6 410 H Hosaeus, A Seifter, E Kaschnitz, G Pottlacher 15 ECTP Proceedings page 978 Henderson J B, Strobel A, 1995, ``Measurements of the thermophysical properties of molten metal alloys'', Netzsch report, TPS No. 1-1E, Netzsch-Gera«tebau GmbH, PO-Box 1460, D , Selb, Germany Inco-alloys, 1999, Special Metals Wiggin Ltd, Holmer Road, Hereford HR4 9SL, UK Kaschnitz E, Pottlacher G, Ja«ger H, 1992 Int. J. Thermophys ^ 710 Klemens P G, Williams R K, 1986 Int. Met. Rev ^ 215 Knorovsky G A, Cieslak M J, Headley T J, Roming A D, Hammetter W F, 1989 Metall. Trans. A, Phys. Metall. Mater. Sci ^ 2158 McElroy D L, Williams R K, Moore J P, Graves R S, Weaver F J, 1978, in Thermal Conductivity 15 Ed. V V Mirkovich (New York: Plenum) p. 149 Quested P N, Mills K C, Brooks R F, Day A P, Szelagowski H, 1998, personal communication Radharkrishnan B, Thompson R G, 1989 Metall. Trans. A, Phys. Metall. Mater. Sci Seifter A, Pottlacher G, Ja«ger H, Groboth G, Kaschnitz E, 1998 Ber. Bunsenges. Phys. Chem ^ 1271 Tye R P, Hayden R W, Spinney S C, 1972 High Temp. ^ High Press ^ 511 ß 2001 a Pion publication printed in Great Britain