2010 - 19th Annual NC State Undergraduate Research Spring Symposium

Session Time : 4/22/10 10:30 AM - 4/22/10 11:45 AM

Content Area : Materials Science & Engineering

Student Presenters :
Jeanelle R Todd Materials Science & Engineering

Joshua Nunn Materials Science & Engineering;
Justin Greene Materials Science & Engineering;
Kevin Aycock Materials Science & Engineering

Mentors and/or Co-Authors :
Carl Koch Material Science Engineering

Abstract Title : Omega Phase in Cold Rolled ATI 425 Titanium Alloy

Abstract :
Ti-6% Al-4% V is currently the most commonly used titanium alloy; however, due to its reduced ductility at low temperatures, an alternate grade has been developed by ATI Allvac. ATI 425 has reduced amounts of Al and V, which are replaced with Fe and O2 to increase cold-working ability at low temperatures. Samples of ATI 425 sheet coil experienced failure after production, causing metallurgists at Allvac to suspect the presence of an embrittling omega phase. The focus of this research has been on proving either the presence or absence of omega phase and designing a processing sequence to prevent future failures. Omega phase forms both athermally and isothermally. Athermally, the (111) plane collapse model for beta to omega transformation is a displacive transition. These transitions do not require any thermal activation and proceed with the velocity of elastic disturbances in the crystals. Isothermally the transition is similar, but also includes a diffusion controlled step. Both theories were considered when testing for the presence of omega phase. Testing included Rockwell C Hardness, XRD analysis, optical microscopy of microstructure, and SEM fractography. The results from every test supported the absence of omega phase. Two separate processing sequences were used for the failed and unfailed samples. These sequences were analyzed against a TiAlV Ternary diagram as well as Ti6-4 TTT diagrams to determine the best processing sequence. It is believed that this new processing sequence will eliminate future failures of the ATI 425 alloy.