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Ti-20mol%Al (Ti-20Al, i.e. Ti-12.3mass%Al) alloy was diffusion-bonded to eutectoid steel at 1273 K for 3.6 ks in a vacuum. The joint had a space of a few micrometers in thickness between the Ti-20Al alloy and the steel, and several specimens separated near the interface promptly after the bonding treatment. This phenomenon, which is referred to as "interface separation", is known to depend on alloy composition, bonding temperature and holding time. This paper describes the influence of carbon content in iron materials on the interface separation. Four kinds of carbon steels, an alloy steel and a cast iron were used as a mating material for the Ti-20Al alloy. Diffusion bonding was carried out at 1273K for 3.6 ks in a vacuum. The diffusion of constituent elements into each parent material and the formation of reaction phases were observed in the vicinity of the interface in all cases. The interface separation was clearly seen in the joints with the carbon steels and the cast iron containing more than 0.82 mass% C. It was also confirmed that the space formed near the interface spread to the iron material side. On the other hand, the bonding of the Ti-20Al alloy to the alloy steel with high carbon content was achieved without incident. In this joint, the diffusion of Fe into the Ti-20Al side was inhibited by TiC layer formed at the interface. The mechanism of the interface separation is discussed from the viewpoint of thermal stress, reaction phase and diffusion behavior.

Ti-20 mol% Al (Ti-12.3 mass% Al) alloy was diffusion-bonded to high carbon steel (0.82 mass% C) at 1273 K for 3.6 ks in a vacuum. The joint had a space of a few micrometers in thickness between the Ti-20 mol% Al alloy and the steel, and several specimens separated near the interface promptly after the bonding treatment. This phenomenon is referred to as 'interface separation'. This paper describes the influence of heating temperature on the interface separation. Diffusion bonding of the Ti-20 mol% Al alloy to the high carbon steel was carried out at 1173 to 1423 K for 0.9 to 3.6 ks in a vacuum, and then several joints were heated at a given temperature for up to 176.4 ks in an evacuated silica tube. At 1173 K, the separation phenomenon could not be confirmed even after prolonged heat treatment. This joint had four kinds of reaction regions in the vicinity of the interface, and their thicknesses increased in proportion to square root of holding time. On the other hand, the joint bonded at more than 1273 K showed the separation at the interface. As the heating temperature increased, the holding time required to induce the phenomenon became shorter. To clarify a time when the separation occurs in the diffusion bonding process, the joint with a special shape was prepared and quenched into the water from 1273 K. The generation of voids was recognized at the interface. These results suggest that the occurrence of the interface separation is associated with interdiffusion between the Ti-20 mol% Al alloy and the steel.

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