Thermophysical Properties of L12 Intermetallic Compounds of Iridium
Thermophysical Properties of L12 Intermetallic Compounds of Iridium
THERMAL CONDUCTIVITY AND THERMAL EXPANSION OF IR3X FOR ULTRA HIGH-TEMPERATURE APPLICATIONS
Thermal conductivity and thermal expansion for the intermetallic compounds Ir3X (X = Ti, Zr, Hf, V, Nb or Ta) were measured in the temperature range between 300 and 1100 K. The thermal conductivities of Ir3X are distributed in the range from 41 to 99 W m−1 K−1 at 300 K, while the difference of thermal conductivities becomes less emphasised at higher temperatures. The coefficient of thermal expansion (CTE) values of Ir3X are insensitive to temperature, and fall around 8 × 10−6 K−1 at 800 K. The Ir3X intermetallic compounds with X = Ti, Zr, Hf, Nb or Ta are suitable for ultra high-temperature structural applications due to their higher thermal conductivities and smaller CTE values.
The L12 intermetallic compounds based on iridium (Ir3X) have been pursued as the next generation of high-temperature structural materials (1–6). The advantages of Ir3X are summarised as follows. Firstly, the melting points are between 600 and 1000 K higher than those of nickel-based superalloys (7). Secondly, an L12 crystal structure offers the possibility of enhanced ductility as a result of the large number of possible slip systems. Finally, the two-phase γ/γ′ type microstructure formed in Ni-based superalloys can also be produced in Ir-based alloys (8–10).
Thermal conductivity and thermal expansion are key parameters to evaluate the suitability of metallic materials for high-temperature structural applications (11, 12). Rapid heat transfer afforded by high thermal conductivity enables efficient cooling, which suppresses the appearance of life-limiting heat-attacked spots (6). A smaller thermal expansion is desirable to avoid thermal fatigue by cyclic thermal conditions, since thermal stress depends directly on the magnitude of the thermal expansion. However, no data on the thermal properties of Ir3X are available in the literature.
The Ir-based compounds Ir3X form an L12 crystal structure when the partner component X belongs to Group 4 or 5 of the Periodic Table (13–15). The present study was conducted to provide the data for thermal conductivity and thermal expansion of Ir3X (X = Ti, Zr, Hf, V, Nb or Ta) which serve to evaluate the suitability of the compounds for high-temperature structural applications. The alloy compositions prepared in this study are given in Table I, together with the compositional range of the L12 phase at the homogenised temperature (1573 K) (7). The stoichiometric composition was chosen for each compound except Ir3Hf. Note that the composition close to stoichiometry with L12 single phase was selected for Ir3Hf, since an L12 single phase is not achieved at the stoichiometric composition.
|Compound||Nominal composition, at.%||Composition range of L12 phase at 1573 K, at.%|
Thermal conductivity measurements were performed by the laser flash method in vacuum in the temperature range between 300 and 1100 K, using a disc specimen of diameter 10 mm and thickness 2 mm (16). A short duration laser pulse is emitted from a ruby rod onto the surface of the disc specimen. The temperature change on the other side of the specimen was measured over time by both an infrared detector and a type R thermocouple. From the temperature-time profile, thermal conductivity was obtained (17). Thermal expansion measurements were made using a dilatometer which consists of an alumina pushrod driving a linear voltage differential transformer (LVDT) (18). Dilatometer specimens were normally 3 mm square and 8 mm long. Thermal expansion tests were conducted over the temperature range from 300 to 1100 K at a heating rate of 10 K min−1 in an argon atmosphere.
Figure 1 shows the thermal conductivities of Ir3X compounds as a function of temperature. The thermal conductivity tends to decrease with increasing temperature for Ir3Nb and Ir3Ta, which have thermal conductivities above 80 W m−1 K−1 at 300 K. Conversely, a continuous increase in thermal conductivity with increasing temperature is observed for Ir3V, which has a smaller thermal conductivity at 300 K. The thermal conductivities of Ir3Ti, Ir3Zr and Ir3Hf are rather insensitive to temperature. The thermal conductivities of Ir3X at 300 K are widely distributed in the range from 41 to 99 W m−1 K−1, while the difference becomes less emphasised at higher temperatures.
The temperature coefficient of thermal conductivity, k, in the temperature range between 300 and 1100 K can be estimated from Equation (i):
where λ300 K and λ1100 K are the thermal conductivities at the temperature indicated by the subscript. The temperature coefficients of Ir3X are plotted against the thermal conductivity at 300 K in Figure 2, together with the plots for pure metals (19–21) and intermetallic compounds (22–24).
As a general rule, the thermal conductivity and the temperature coefficient are inversely correlated in pure metals and intermetallic compounds. All the Ir3X compounds other than Ir3V are characterised by larger thermal conductivities and smaller temperature coefficients. In particular, the thermal conductivities of Ir3Nb and Ir3Ta are nearly equal to that of NiAl, which is widely recognised as a high thermal conductivity compound (17, 25).
The thermal conductivity of an intermetallic compound is quantitatively correlated with those of the constituents of the compound though Nordheim's relation (26). The high thermal conductivities of Ir3X may be partly due to the high thermal conductivity of pure Ir, whose thermal conductivity at 300 K is 147 W m−1 K−1.
Results of the thermal expansion measurements (ΔL/L) are shown in Figure 3. The dilatation curves for all the Ir3X compounds are smooth functions of temperature exhibiting no sudden changes in slope. The curves in Figure 3 reveal that the thermal expansion of Ir3Ta is slightly smaller than that of either Ir3V or Ir3Nb over the temperature range between 300 and 1100 K. Also, the data indicate the smaller thermal expansion of Ir3Ti in comparison with those of Ir3Zr and Ir3Hf.
The slope of the curve of ΔL/L vs. temperature is the CTE. The relatively flat dilatation curve for each compound indicates that the CTE of Ir3X are insensitive to temperature in the range 300 to 1100 K. The CTE of Ir3X compounds at 800 K are summarised in Table II. All the values of CTE are concentrated around 8 × 10−6 K−1. The largest CTE is found in Ir3V with 8.4 × 10−6 K−1, while Ir3Ti shows the smallest at 7.5 × 10−6 K−1.
|Compound||Coefficient of thermal expansion at 800 K, K−1|
|Ir3Ti||7.5 × 10−6|
|Ir3Zr||8.2 × 10−6|
|Ir3Hf*||8.2 × 10−6|
|Ir3V||8.4 × 10−6|
|Ir3Nb||8.0 × 10−6|
|Ir3Ta||7.6 × 10−6|
Figure 4 shows the correlation between the CTE at 800 K and the melting point for Ir3X, together with the plots for pure metals (21, 27) and intermetallic compounds (22, 28). It is found that all the plots of pure metals and intermetallic compounds including Ir3X are arranged by a universal curve, irrespective of crystal structure. The CTE of Ir3X are approximately equal to that of pure Ir and one half those of conventional intermetallic compounds such as Ni3Al and NiAl. The smaller CTE values of Ir3X correlate well with the higher melting points of the compounds.
The interatomic force in metallic materials is characterised by cohesive energy, Ecoh, defined as the difference between the potential energy of atoms in the gas state and that in a crystal of the material. The cohesive energy in intermetallic compounds is expressed as the sum of the sublimation energy of the alloy, Esub, and the heat of formation of ordered structure, ΔH (29), Equation (ii):
Table III summarises the Ecoh, Esub and ΔH values for the Ir3X compounds, where Esub was obtained from the data source (30) and ΔH was calculated from Miedema's formula (31, 32). The data for Ni3Al and NiAl are also indicated in Table III. It can be seen that the cohesive energy of intermetallic compounds originates mostly from the sublimation energy rather than the heat of formation of ordered structure. The cohesive energy for Ir3X is located around 700 kJ mol−1, which is 1.7 times larger than that of Ni3Al and NiAl. The larger cohesive energy of Ir3X would result in the higher melting point and in the smaller CTE of the compounds.
|Compound||Cohesive energy, Ecoh, kJ mol−1||Sublimation energy*, Esub, kJ mol−1||Heat of formation**, ΔH, kJ mol−1|
Thermal conductivity and thermal expansion of Ir3X (X = Ti, Zr, Hf, V, Nb or Ta) were surveyed in the temperature range between 300 and 1100 K. The thermal conductivity and the temperature coefficient are inversely correlated for Ir3X. All the Ir3X compounds other than Ir3V have larger thermal conductivities and smaller temperature coefficients. The CTE of Ir3X compounds are insensitive to temperature, and fall around 8 × 10−6 K−1 at 800 K. The smaller CTE of Ir3X are well correlated with the higher melting points of the compounds. The L12 intermetallic compounds Ir3X with X = Ti, Zr, Hf, Nb and Ta are characterised by larger thermal conductivity and smaller thermal expansion.
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Yoshihiro Terada is an Associate Professor in the Department of Materials, Physics and Energy Engineering, Nagoya University, Japan. His main activities are in the thermal and mechanical properties in metallic materials for high-temperature applications. His major field of present interest is the creep mechanisms of heat resistant magnesium alloys.