Smart Oxygen Diffusion Barrier Based on IrAl Alloy
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KK8.33.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
CONCEPT The process of oxidation can be estimated using phase diagrams. Fig. 1 (a) shows the estimated Ir-Al-O ternary phase equilibrium at high temperature above 1400K. First, consider the oxidation of Al-rich Ir-Al intermetallics such as A151r 2 (other than IrAl): Als5 r 2 is dissolved into A120 3 and IrAl, but If (fr sold solution) is not formed as shown in Fig. 1 (b). This estimation is in good agreement with Lee and Worrell's result [3]. Second, consider the oxidation of IrAl: IrAl is dissolved into Ir and A1203 as shown in Fig. I (c). Thus, IrAl has a high potential to be ODB due to formed Ir, if Ir forms continuously and Al20 3 protects the Ir layer as PO. It should be mentioned that the Ir layer formed through oxidation might exhibit low oxygen diffusivity, even though the Ir layer contains solute Al atoms.
(a)
(c)
(b)
Ir IAr
A120
3
A 0
A1l5 r2
5Ir2
OA
SA10
0
IrAl
Ir 5 2 Ir
A13lrOA A120
0
A1203
Al
Figure 1. The estimated lr-A-O phase equilibrium and oxidation of Ir-AI intermetallics at high temperature above 1400K: (a) the phase equilibrium, (b) oxidation of A15lr2 and (c) oxidation of IrAl. By oxidation, alloy composition moves into the triangles of Al5lr2-lrAl-A 2 03 for A 51.r2, and IrAl-lr-A120 3 for IrAl. Only IrAl forms Ir by oxidation.
EXPERIMENTAL PROCEDURE An IrAl alloy was made by Ar arc-melting, where starting materials were 99.98%Ir and 99.99%A1. The ingot was mechanically crushed into smaller powder than 50jtm in diameter. Hot pressing at 2073K for 14.4ks in Ar and homogenization at 2173K for l8ks in vacuum followed by furnace-cooling (1K/s) were carried out. Chemical composition was determined to be Ir-48.6mol%A1 by inductively coupled plasma spectrometry - optical emission spectrometry (ICP-OES) method. Optical microscopy and X-ray diffractometry (XRD) were used for characterizing microstructure. Two kinds of oxidation tests, dynamic and isothermal oxidation, were carried out in 02 environment: (1) the former is under continuous heating up to 1773K where the heating rate was 0.167K/s and (2) the latter is under isotherms at 1273K, 1473K and 1673K. Mass change and reaction heat were measured using thermogravimetry (TG) and differential thermal analysis (DTA). Oxidation behavior of arc-melted pure-Ir was also investigated. In continuous heating, samples for dynamic oxidation tests were powders (about 50gtm) for IrAl and small pieces (about Imm) for Ir. For the isothermal oxidation, cubic specimens of 2mm x 2mm x 2mm were prepared by an electro-discharge machine (EDM), and the damaged surface was mechanically polished. Scanning electron microscopy (SEM) equipped with energy/wavelength dispersive X-ray spectroscopy (EDX/WDX) was carried out for identifying oxidized surface structure. Compressive rectangular specimens of 5mm x 2mm x 2ram were shaped by EDM and polished to remove damaged surface. Compression tests were carried out using an Instrontype machine from room temperature (RT) to 1873K in A
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