Abstract
TIMETAL® 575, developed by Titanium Metals Corporation (TIMET), is a high strength forgeable α+β titanium alloy with comparable density, beta transus temperature and processing characteristics to Ti-6Al-4V but with enhanced static and fatigue strength primarily aimed at aero-engine disc or blade applications. Recent research on this alloy has focussed on microstructure evolution as a means to optimise mechanical behaviour and it has been concluded that a solution heat treatment followed by an ageing step yields a resulting “tri-modal” microstructure, consisting of equiaxed primary α and bi-lamellar transformation product containing nano-scale “tertiary alpha” laths, which appear to provide an excellent balance of strength and ductility. The key objective of the work presented here is to characterise this complex nanoscale microstructure in detail at various stages of alloy processing. For that purpose various advanced and recently developed transmission electron microscopy (TEM) techniques have been used. These include alpha and beta phase mapping Precession Electron Diffraction (PED), overall microstructure imaging with conventional BF and DF TEM, distinction of fine phase detail with high angle annular dark field (HAADF) scanning TEM (STEM), and correlation of the nanostructure to the elemental distribution using scanned Electron Energy Loss Spectroscopy (EELS).
Highlights
Titanium alloys can be engineered to have a range of desirable properties and as such they are widely used in several areas such as medicine, chemistry, architecture and transportation [1]
TIMETAL® 575 (Ti-575), developed by Titanium Metals Corporation (TIMET), is a high strength forgeable α+β titanium alloy with comparable density, beta transus temperature and processing characteristics to Ti-6Al-4V but with enhanced static and fatigue strength primarily aimed at aero-engine disc or blade applications
Research into the microstructural evolution of this alloy has been performed with the aim of improving and optimising the mechanical behaviour and it has been determined that a solution heat treatment followed by an ageing step resulted in an optimised ‘tri-modal’ microstructure consistent with those previously described by Lutjering, which appear to provide an excellent balance of mechanical properties [7][8][9][10]
Summary
Titanium alloys can be engineered to have a range of desirable properties and as such they are widely used in several areas such as medicine, chemistry, architecture and transportation [1]. In relation to the thermo-mechanical processing, some authors have reported methods by which a lamellar microstructure could be processed into a bi-lamellar microstructure consisting of nano-scale α plates within the β lamellae. The same method was proposed for modifying alloys with bi-modal starting microstructure in order to produce a ‘tri-modal’ microstructure which would consist of equiaxed primary α grains (αp) and secondary α laths (αs) as well as nano-scale. Research into the microstructural evolution of this alloy has been performed with the aim of improving and optimising the mechanical behaviour and it has been determined that a solution heat treatment followed by an ageing step resulted in an optimised ‘tri-modal’ microstructure consistent with those previously described by Lutjering, which appear to provide an excellent balance of mechanical properties [7][8][9][10]. The main objective of the work presented here is to characterise nano-scale microstructure of Ti575 after various stages of processing in order to investigate the development and corroborate the presence of a tri-modal structure
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