Abstract
The microstructure of Ti-64 strongly affects the forming and in-service behaviour of aerospace components. The development of microstructure, including crystallographic texture, during thermomechanical processing involves changes caused by deformation, annealing and phase transformation. Recently, it has been reported that deformation and transformation occur concurrently, in a dynamic α → β transformation process, driven by the net flow softening it produces. Here, we present the results of an in-situ synchrotron diffraction experiment that aimed to detect and measure the extent of dynamic phase transformation during hot deformation and its contribution to the crystallographic texture of the alloy. Forged Ti-64 samples were deformed in uniaxial tension at 950°C and a strain rate of about 0.02 s-1, using an electro-thermal mechanical tester (ETMT) mounted on the I12: JEEP beamline at Diamond Light Source. By using a high energy 89 keV synchrotron X-ray beam and a fast acquisition rate (10 Hz) we were able to measure, during deformation, the internal elastic lattice strains in the two phases, the crystallographic texture of both phases and the α → β phase fraction. The results show, for the first time, direct evidence of dynamic transformation during isothermal hot deformation.
Highlights
The properties and performance of dual-phase Ti alloys in-service are strongly influenced by their microstructure and crystallographic texture, which can be controlled by thermomechanical processing during manufacture
Dynamic transformation is missing from any efforts to model the hot deformation behaviour of dual-phase Ti alloys
During deformation, an α → β transformation occurs, with the volume fraction of β-phase increasing by ~5% up to 0.2 strain
Summary
The properties and performance of dual-phase Ti alloys in-service are strongly influenced by their microstructure and crystallographic texture, which can be controlled by thermomechanical processing during manufacture. Post-mortem microstructural observations of hot-deformed Ti-64 samples have shown evidence that this interaction involves dynamic α → β transformation, during deformation, which increases the volume fraction of β-phase by up to 12% [1,2]. Dynamic transformation is missing from any efforts to model the hot deformation behaviour of dual-phase Ti alloys. The aim of the work presented here was to use high energy synchrotron X-ray diffraction (SXRD) to observe dynamic transformation in Ti-64 directly, during isothermal deformation at 950°C, high in the α + β regime. At these temperatures, high strain rates of deformation are required to develop the conditions for dynamic phase transformation. Only synchrotron radiation can be used for in-situ observations [4,5], which has significantly faster measurement times than neutron diffraction [6]
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