The effects of minor alloying additions and microstructure on the creep behavior of titanium-47aluminum

This dissertation examined the effects of minor alloying additions and microstructure on the creep behavior of a Ti-47Al alloy (all compositions in at%). There were several objectives of this research. The first was to determine solid solution strengthening and precipitation strengthening effects of C and Si on Ti-47Al. Solid solution strengthening effects were studied at lower alloying levels (0.2C and 0.2Si), and precipitation strengthening effects were studied at higher alloying levels (0.3C and 0.5Si). Another objective was to devise heat treatments for each alloy composition producing a fully lamellar microstructure with similar grain sizes (prior a) across all compositions (500–700μm) and to establish two distinct lamellar lath spacings (0.15μm, 0.6μm) for each composition. This work showed that similar grain sizes could be achieved across several alloy compositions and that two distinct lamellar lath spacings could be achieved through control of the heat treatment cooling rate. Each alloy in the study had similar lath spacings. The addition of solid solution carbon (0.2C) was found to significantly improve the creep resistance. Carbon is thought to improve the creep resistance through a solute drag mechanism. Although solid solution silicon improves the creep resistance in several other high temperature alloy systems, the addition of 0.2Si had no effect on the creep resistance of the Ti-47Al alloy. Refinement of the lamellar structure improved the creep resistance in the Ti-47Al and Ti-47Al-0.2Si alloys. Only minimal improvement was noted in the Ti-47Al-0.2C alloy indicating that the solid solution strengthening effect overwhelmed any lath spacing effect. The Ti-47Al-0.2C alloy in both lath conditions had the best creep resistance of any of the alloys of this study indicating that alloy modification through minor alloying additions provides a more effective means of improving creep resistance than does microstructural modification within a fully lamellar microstructure. The precipitation strengthening effects could not be established unambiguously.