Interdiffusion studies between a Mo-based alloy and Ti
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DUCTION
IN a tokamak fusion reactor, the diverter plates are made of carbon-fiber composites and tungsten and Mo-based alloy tubes are used as coolant tubes.[1] Several designs use He-cooled structures with a nominal coolant temperature of 1173 to 1273 K.[2] Operation of these reactors at such high temperatures requires refractory alloys with excellent high-temperature strength along with high thermal conductivity. Although Mo-based alloys show such properties, fabrication of structural components would require joining of these alloys. Due to the high thermal stresses generated during operation, coupled with the limited ductility of refractory alloys in general, joining by fusion welding is a poor choice. Solid-state diffusion bonding, on the other hand, is known for producing strong metallurgical bonds with less defects and better strength. Since the recrytallization temperature of refractory metals and alloys is very high, direct bonding would require very high temperatures throughout the furnace under vacuum, which is both difficult to achieve and uneconomical. The bonding temperature can be lowered to a large extent by using interlayers of reactive metals that have high solubility and do not form any brittle intermediate phase. Among the Mo-based alloys used commercially, the most important and widely used alloy is TZM (Mo-0.5 pct Ti-0.08 pct Zr-0.05 pct C). This alloy is designated as molybdenum alloy 363 and molybdenum alloy 364 (ASTM standard ASTM B386-91). Due to its constituent elements, titanium (T), zirconium (Z), and molybdenum (M), the alloy is popularly known as TZM. The alloy will be referred to as TZM henceforth in this article. The interdiffusion studies on the TZM-Ti system are not reported in the literature. However, some studies on the binary system of Ti-Mo are reported,[3,4] but there seems to be some inconsistency in the values of the diffusion coefficients evaluated. Whereas the diffusion studies were performed in the temperature range 1199 to 1518 K, by Kale and Patil,[3] those reported A. LAIK, Scientific Officer, G.B. KALE and K. BHANUMURTHY, Group Leaders, Diffusion and EPMA Group, are with the Materials Science Division, Bhabha Atomic Research Center, Mumbai - 400 085, India. Contact e-mail: [email protected] Manuscript submitted February 17, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
by Thibon et al.[4] were performed at a higher temperature range of 1573 to 1873 K. The interdiffusion coefficients evaluated by Kale et al.[3] at 1518 K and by Thibon et al.[4] at 1523 K vary by three orders of magnitude at 90 at. pct Mo composition. However, the composition dependence of the interdiffusion coefficient showed a similar trend in both these studies. Kale and Patil[3] have shown that the activation energy Q and the pre-exponential factor D0 increase with the increase in Mo concentration, whereas in the study by Thibon et al.,[4] both Q and D0 decrease with the increase in Mo concentration, with a local maxima at 30 at. pct Mo. The present work deals with the study of the diffusion behavior of
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