Interface Separation Phenomenon in Ti-20mol%Al Alloy/Iron Material Joints

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.