S. Nag, A. Devaraj, R. Banerjee, University of North Texas, Denton, TX; R. Williams, S. Rajagopalan, The Ohio State University, Columbus, OH; H. L. Fraser, Center for Accelerated Maturation of Materials, Columbus, OH
The omega phase is commonly observed in many commercial beta or near-beta titanium alloys on quenching from the solutionizing temperature in the single beta phase field. These omega precipitates typically have an embrittling effect on the alloy and are therefore considered detrimental for its mechanical properties. However, since omega precipitates are highly refined (nanometer scale) and homogeneously distributed, they can potentially act as heterogeneous nucleation sites for the precipitation of the equilibrium alpha phase. This leads to a homogeneous distribution of refined alpha precipitates that can substantially strengthen the alloy. Therefore, the detailed investigation of omega precipitation in the beta matrix of titanium alloys is rather important. The present study focuses on omega precipitation within the beta matrix of simple binary titanium alloys, such as Ti-Mo, as well as commercial alloys, such as Ti-5553. Quenching from beta solutionizing temperatures results in the formation of athermal omega precipitates that typically inherit the composition of the parent beta matrix. These athermal omega precipitates are formed by a displacive mechanism that transforms the structure from bcc to hexagonal. On subsequent isothermal annealing, coarsening of the omega precipitates is accompanied by the diffusional partitioning of the alloying elements. Advanced characterization techniques such as 3D atom probe (3DAP) tomography and high-resolution scanning transmission electron microscopy (HRSTEM) will be employed for determining the true atomic scale structure and chemistry changes associated with the precipitation of omega as a function of heat-treatments in these alloys.
Summary: Over the years, metastable omega phase in titanium alloys have intrigued the researchers, mainly due to its detrimental influence on their mechanical properties. Although some studies have been conducted on as-quenched (athermal) and annealed (isothermal) omega precipitates, the entire sequence of beta to omega shuffle transformation on quenching, followed by partitioning of elements across the beta/omega interface on aging have been poorly understood. This is particularly important as in some cases the omega phase subsequently act as heterogeneous nucleation sites for alpha precipitates, thus strengthening the material. Thus this study focuses on investigating the structure and chemistry changes of these omega particles in binary (Ti-Mo) as well as complex commercial (Ti5553) alloys, that were subjected to different heat-treatments. For this, state of the art nano-characterization tools like 3D atom probe and high resolution TEM were employed.
