Hot Salt Stress Corrosion Cracking Behavior of Ti-6242S Alloy
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THE key properties viz., high specific strength, toughness, and good corrosion resistance of titanium alloys find them widespread applications in gas turbines. Driven by the need to operate gas turbines at higher and higher temperatures, the applications of these alloys in gas turbine engines are also limited by their creep and oxidation resistance. The use of Ti-6Al-4V, the most commonly used a + b titanium alloy in gas turbine engines, is limited up to 588.15 K (315 °C) due to its poor creep and oxidation resistance.[1–3] To meet this demand, a near-a class of titanium alloys have been developed. Ti-8Al-1Mo-1V was the first near-a titanium alloy mainly developed for this purpose, but high aluminum content in this alloy led to stress corrosion cracking problems.[1] To overcome this, Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) in which the aluminum content was limited to 6 wt pct was developed. This alloy possesses improved creep and oxidation resistance and can be used up to about 723 K (450 °C).[1,2,4] The development of this alloy followed the following strategies. The high-temperature creep strength in this alloy was achieved by reducing the volume fraction of the beta phase, as this phase enables faster diffusion of vacancies due to its relatively open crystal structure than a phase. In addition, the element vanadium having high diffusivity has been replaced with a slow diffusing element namely molybdenum. Furthermore, the addition of tin promotes the formation of coherent MANGESH D. PUSTODE, Research Scholar, and V.S. RAJA, Institute Chair Professor, are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India Contact e-mail: [email protected] Manuscript submitted January 8, 2015. Article published online September 14, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates of Ti3Al, which act as a barrier for dislocation glide and climb. Later, it has been discovered that the addition of a small amount of silicon (about 0.1 wt pct) substantially improves the creep behavior Ti-6242 alloy. Silicon precipitates as silicides along a/b interfaces and grain boundaries hinder the dislocation motion improving the capability of the Ti-6242 alloy for use up to about 793.15 K (520 °C).[1,2] Moreover, the presence of aluminum, silicon, and molybdenum also decreases the diffusion rate of oxygen by forming thermally stable oxide film and enabling an increased high-temperature oxidation resistance.[2] This silicon-modified alloy was named as Ti-6242S alloy and employed for the initial stages of high-pressure compressor of aero gas turbine engines. However, it is known that titanium alloys are prone to hot salt stress corrosion cracking (HSSCC) in the operating temperature range from 473.15 K to 773.15 K (200 °C to 500 °C) when salts enter in gas turbines.[5–14] According to Jackson and Boyd,[15] a vast majority of titanium alloys are susceptible to HSSCC in the temperature range 561.15 K to 700.15 K (288 °C to 427 °C), though the more susceptible alloys such
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