Thermal Stability and Mechanical Behavior of Ti-base Nanocrystalline Alloys

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Non-technical Summary The thermal stability and mechanical behavior of selected titanium-base alloys with very small (nanocrystalline) grain sizes will be studied. The alloys will be prepared by the ball milling of powders that will then be consolidated by hot pressing. The structure and microstructures of the titanium alloys with appropriate additions of third elements that are predicted to stabilize the fine grain structure will be determined. Mechanical properties will be measured with techniques that evaluate the strength and ductility. Annealing studies will be carried out to quantitatively determine the thermal stability by measuring the growth of the fine grains. The results of these studies should have a substantial impact on both the processing and elevated temperature requirements of very fine grain size titanium alloys. Superior mechanical properties compared to conventional grain size titanium alloys can lead to breakthroughs in structural and bio-related (for example, implant materials) applications.Technical Summary Ti-base alloys processed to synthesize nanocrystalline microstructures with superior mechanical properties are of great interest for automotive and aerospace applications. These alloys are the focus of the proposed research. Powder compaction followed by elevated temperature consolidation is necessary to process the initial non-equilibrium alloys in most cases, and elevated temperatures are prerequisite for end-use. The research will investigate the kinetic and thermodynamic mechanisms that can provide high temperature thermal stability for nanocrystalline Ti-base alloys. Ti alloys with hcp alpha stabilizers and bcc beta stabilizers will be synthesized using mechanical alloying. A model will enable selection of optimum (oversize) solutes for thermodynamic stabilization. Kinetic stabilization by solute drag or Zener pinning will be achieved by in-situ (intermetallic precipitates) or ex-situ (oxide dopant) strategies. Thermal stabilization mechanism interactions and strengthening mechanism transitions (Hall Petch vs. Orowan strengthening) are highly relevant for achieving optimum microstructures. Microstructure characterization will be done using methods appropriate to the microstructural scale of interest. This will include structural and chemical investigation using state-of-the art instrumentation capable of atomic scale resolution. Mechanical properties will be measured with techniques that evaluate strength and ductility for the sample sizes produced by hot compaction of the processed powders. Isothermal annealing studies will be undertaken to establish grain-growth kinetics and activation energies. This will provide additional insight and modeling for stabilization mechanisms and interactions in selected temperature regimes.