ISSN (Print): 2321-5747, Volume-1, Issue-1, 2013
108
Measurement of Thermo Physical Properties of Nickel Based Superalloys
Lavakumar Avala1, Mamatha Bheema2, Prince Kr Singh3, Rabindra Kr Rai4 & Sanjay Srivastava5
1,3,4&5Department of Materials Science and Metallurgical Engineering, Maulana Azad National Institute of Technology: Bhopal - 462051, India
2QA/QC Department, API Division, Bhushan Steel Limited, Khopoli-410203, India
Abstract - Nickel based superalloys are used primarily in turbine of aircraft engines, marine and power industry.
Thermal conductivity is an important physical property of materials which enable to evaluate the usefulness of a metallic material to high temperature structural applications. In present study, thermal conductivity of commercial nickel based superalloys Supercast 247A and Superni 263A was measured. The thermal conductivity was calculated as a function of density, specific heat and thermal diffusivity. Analysis of these measurements showed that the γ' phase (Ni3Al) affected the properties of thermal diffusivity and thermal conductivity and relationships have been identified between these properties and the γ' phase content.
Keywords - Superalloy, Thermal conductivity, Density, Specific heat.
I. INTRODUCTION
Nickel base superalloys used for modern gas turbines are continually being developed to increase thrust, operating efficiency and durability. Ni base superalloys such as Supercast 247A (equivalent to CM 247LC) and Superni 263A (equivalent to Nimonic 263LC) alloys have been used in gas turbines as blades at high temperature because of its excellent high temperature mechanical properties.1-5 Generally, the Ni- base superalloys with complex and multi-phase microstructures are stable at high temperatures and this characteristic is the main reason for using them in critical and severe service conditions.6-9 The high- temperature strength of superalloys is based on the principle of a stable austenitic matrix (γ phase) combined with solid solution hardening and/or precipitation strengthening. Thermal stability of γ phase, the possibility of solid solution hardening and precipitation strengthening, high elastic modulus of the matrix are main factors which define the application of superalloys. High solubility of many elements such as
cobalt, iron, chromium, molybdenum and tungsten gives the possibility of strength of austenitic matrix. The addition of aluminum and titanium causes precipitation of an ordered compound based on formula Ni3(Al,Ti) which is coherent with austenitic γ matrix. This phase is required for high - temperature strength and creep resistance.10-18 Disadvantageous property of superalloys is low thermal conductivity which is due to the high percentage of alloying elements. Thermal conductivity is an important physical property of materials which enable to evaluate the usefulness of a metallic material to high temperature structural applications. Rapid heat transfer afforded by high thermal conductivity enables efficient cooling which moderates the appearance of life limiting heat attacked spot. High thermal conductivity assures also a uniform temperature distribution, which reduces thermally induced stresses and thereby improves fatigue properties.18-19
The aim of the present work was to evaluate the thermal properties of selected nickel based superalloys and the dependence of these properties on the temperature.
II. EXPERIMENTALPROCEDURE
The thermal diffusivities of the alloys were measured using a NETZSCH 427 Laser flash apparatus (Fig 1).20-21 Disc shaped specimen 1–3 mm thick (L) were maintained in an atmosphere of purified argon. For measurements on solids the front face of the specimen was sprayed with graphite and a pulse of energy was focused on the front face and the temperature of the back face of the sample was monitored continuously with an InSb infra red detectoralong with the time. The thermal diffusivity of the sample at a specified temperature was determined from the temperature transient which yielded a value for the specimen to
ISSN (Print): 2321-5747, Volume-1, Issue-1, 2013
109 reach half of the maximum temperature rise (t). The thermal diffusivity (a) was then determined from the following relation 22
Thermal conductivity (k) values were calculated from the thermal diffusivity, specific heat (Cp) and density (ρ) values, and the relation is as follows.
𝐾 = 𝜌. 𝐶𝑝. 𝑎
The thermal diffusivity measurements were conducted under argon between room temperature to 1400K. Special sample holders with additional adapter rings were used due to different dimensions of samples.
The samples were coated with graphite on the front and back surfaces in order to increase absorption of the flash light on the sample’s front surface and to increase the emissivity on the sample’s back surface. The presented thermal diffusivity results are the average values of five individual tests. The amount of γ' formed in the alloy was calculated using an image analyzer software.
.
Table 1. Chemical composition of investigated alloys
Superalloy
Element content, % weight
Cr Co Mo Al Ti C B W Zr Hf Mn Fe Ni
Supercast
247A 8.2 9.3 0.5 5.6 0.81 0.074 0.015 9.5 0.014 1.51 - - Bal.
Superni
263A Bal 20 19.4 5.9 0.44 2.1 - - - 0.38 0.10
.
Fig. 1: General view of LFA 427 device: a) laser system connected via fiber optics, b) measuring unit with
furnace, a sample carrier and In-Sb detector, c) controller for measuring unit
Table 2. Specific heat, density and thermal diffusivity of superalloy Supercast 247A as a function of temperature
Nickel base superalloy : Supercast 247A Temperat
ure (K)
Specific heat, J/(g.K)
Density g/cm3
Thermal diffusivity
mm2/s
Thermal conducti
vity W/(m.K)
298 0.428 8.643 2.781 10.283
400 0.434 8.611 3.126 11.682
600 0.462 8.599 3.349 13.304
800 0.491 8.556 3.861 16.220
1000 0.542 8.434 4.344 19.857
1200 0.590 8.396 4.762 23.589
1400 0.620 8.209 5.159 26.257
Fig. 2(a) : Specific heat and Density of nickel base superalloy Supercast 247A as a function of temperature
200 400 600 800 1000 1200 1400
0.40 0.45 0.50 0.55 0.60 0.65
Specific heat Density
Temperature, K
Specific heat, J/(g.K)
8.2 8.3 8.4 8.5 8.6 8.7
Density, g/cm3
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110 Fig. 2(b) : Thermal diffusivity and Thermal conductivity
of nickel base superalloy Supercast 247A as a function of temperature
III. RESULTS AND DISCUSSION
The thermal properties of nickel based superalloys Supercast 247A and Superni 263A were investigated.
The change in thermal diffusivity, thermal conductivity, specific heat and density of Supercast 247A was showed in Fig. 2(a) & (b) and corresponding values are shown in Table 2. The thermal properties are dependent upon electron transport. Consequently, for the solid phase measured values are affected by the microstructure since electrons can be scattered by particles, grain boundaries, etc. Thus, there tends to be some variation in thermal diffusivity values for the solid phase since they are dependent upon the microstructure which is, in turn, dependent on the thermal history of the specimen.
Measured thermal diffusivity values derived in the temperature ranges where the γ' phase particles coarsen and then dissolve also tend to vary because these transformations are dependent upon both temperature and time.
Table 3. Specific heat, density and thermal diffusivity of superalloy Superni 263A as a function of temperature
Nickel base superalloy : Superni 263A Temper
ature (K)
Specific heat, J/(g.K)
Density g/cm3
Thermal diffusivity
mm2/s
Thermal conductivi
ty W/(m.K) 298 0.417 8.361 3.349 11.678 400 0.442 8.310 3.926 14.420
600 0.482 8.161 4.216 16.580 800 0.516 8.099 4.892 20.421 1000 0.576 7.862 5.122 23.189 1200 0.635 7.751 5.637 27.741 1400 0.652 7.700 5.826 29.248
Fig.3(a) : Specific heat and Density of nickel base superalloy Superni 263A as a function of temperature
Similar dependencies of thermal properties were detected for sample Superni 263A as functions of temperature were measured. The change in thermal diffusivity, thermal conductivity, specific heat and density of Supercast 247A was showed in Fig. 3(a) &
(b) and corresponding values are shown in Table 3.
Lower values of thermal conductivity are the results of higher alloying elements contamination in this superalloy (see Table 1). Slight steps were detected in thermal diffusivity, heat capacity and thermal conductivity for all samples above 800K (Figs. 2 & 3).
Much the same results was obtained by Przeliorz23 and other authors who investigated the heat capacity of superalloys by the DSC method. They observed the increase of heat capacity above the temperature of 800K and then the drop up to the temperature of 1000K, on the DSC curve. This phenomena is probably due to the distribution of γ' phase. Between temperature of 1100K and 1200K the thermal conductivities remains constant and then slightly increase (Figs. 2&3). Lower values of thermal diffusivity and conductivity were detected for samples of Supercast 247A which has the highest alloying elements content (Fig. 2, Table 2).
200 400 600 800 1000 1200 1400
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Thermal diffusivity Thermal conductivity
Temperature, K Thermal diffusivity, mm2/s
9 12 15 18 21 24 27
Thermal conductivity, W/(m.K)
200 400 600 800 1000 1200 1400
0.40 0.45 0.50 0.55 0.60
0.65 Specific heat
Density
Temperature, K
Specific heat, J/(g.K)
7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.3 8.4
Density, g/cm3
ISSN (Print): 2321-5747, Volume-1, Issue-1, 2013
111 Fig. 3(b) : Thermal diffusivity and Thermal conductivity
of nickel base superalloy Superni 263A as a function of temperature
Effect of γ' phase on properties
As mentioned above, the γ' phase precipitates formed in the γ phase matrix provide high temperature strength by hindering the movement of dislocations. The principal constituent of the γ' phase is Ni3Al but Ni3Fe and Ni3Cr also contribute but dissolve in the γ matrix at 793 and 823 K respectively. Consequently, these compounds do not contribute to the γ' phase above 823 K. The γ' phase content was calculated using image analyzer software. Since Ni3Al is the principal constituent of the γ' phase the amount of γ' phase was expressed as a function of the Al content of the alloy.
The following relation was found
γ' (at x%) = 16.1+10.6(wt x% Al)
Inspection of the results showed that the properties such as the density may be affected by the amount of γ' phase present in the alloy.
The Cp , ρ and T curves for Ni based superalloys show evidence of several transitions which occur with increasing temperature.
(i) around 870 K there is a step-like increase in the Cp in all samples which has been attributed to the rearrangement of atoms
(ii) between 1070K and 1270 K the γ' phase coarsens which results in an increase in the Cp
(iii) between 1270 and 1500 K dissolution of the γ' phase (i.e. γ'→γ) occurs and results in an increase in Cp culminating in a peak in the Cp–T curve (1(a) ,2(a)).
IV. CONCLUSIONS
Thermal conductivity of superalloys depends on the chemical composition of alloy and the temperature. It was confirmed that alloying elements decrease the thermal conductivity of superalloys. Thermal conductivity increases with the increase of the temperature from 10.2 Wm-1K-1 at room temperature to 26.2 Wm-1K-1 at 1400K for alloy Supercast 247A (equivalent to CM 247LC), and 11.6 Wm-1K-1 at room temperature to 29.24 Wm-1K-1 at 1400K for Superni 263A (equivalent to Nimonic 263C).
The γ' phase content of the alloy has a significant effect on the thermal properties. Slight steps were detected in thermal diffusivity and conductivity for samples above 500°C which is probably due to the distributing of γ' phase.
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