Electron Microscopic Study on the Suction Cast In Situ Ti-Fe-Sn Ultrafine Composites

The current investigation reports detailed study on the microstructural evolution in the suction cast hypereutectic Ti71Fe29-xSnx alloys during Sn addition with x = 0, 2, 2.5, 3, 3.85, 4.5, 6, and 10 at. pct and the solidification of these ternary alloys using SEM and TEM. These alloys have been prepared by melting high-purity elements using vacuum arc melting furnace under high-purity argon atmosphere. This was followed by suction casting these alloys in the water-cooled split Cu molds of diameters, phi = 1 and 3 mm, under argon atmosphere. The results indicate the formation of binary eutectic between bcc solid solution beta-Ti and B2 FeTi in all alloys. beta-Ti undergoes eutectoid transformation, beta-Ti --> alpha-Ti + FeTi, during subsequent solid-state cooling, leading to formation of hcp alpha-Ti and FeTi. For alloys x < 2, the primary FeTi forms from the liquid before the formation of eutectic with minute scale Ti3Sn phase. For alloys with 2 >= x <= 10, the liquid is found to undergo ternary quasi-peritectic reaction with primary Ti3Sn, L + Ti3Sn --> beta-Ti + FeTi, leading to formation of another kind of FeTi. In all the other alloy compositions (3.85 >= x <= 10), Ti3Sn and FeTi dendrites are observed in the suction cast alloys with profuse amount of Ti3Sn being formed for alloys with x >= 4.5. The current study conclusively proves that the liquid undergoes ternary quasi-peritectic reaction involving four phases, L + Ti3Sn --> beta-Ti + FeTi, which lies at the invariant point Ti69.2+0.8Fe27.4+0.7Sn3.4+0.2 (denoted by P). Below P, there is one univariant reaction, i.e., L --> beta-Ti + FeTi for all alloy compositions, whereas above P, liquid undergoes one of the univariant reactions, i.e., L + beta-Ti --> Ti3Sn (Sn = 2, 2.5, 3, and 4.5 at. pct) or L + FeTi --> Ti3Sn for alloys (Sn = 6, 10 at. pct). For alloy with Sn = 3.85 at. pct, the ternary quasi-peritectic reaction is co-operated by two monovariant eutectic reactions, i.e., L --> beta-Ti + FeTi below P and L --> FeTi + Ti3Sn above P. Detailed microstructural information allows us to construct liquidus projection of the investigated alloys. The results are critically discussed in the light of available literature data. DOI: 10.1007/s11661-012-1404-1 (C) The Minerals, Metals & Materials Society and ASM International 2012