Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1609744 | Journal of Alloys and Compounds | 2015 | 14 Pages |
Abstract
The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (<10 nm) and athermal Ï (â¼1-3 nm) formed during accelerated cooling from 800 °C and slow heating to an ageing temperature of 650 °C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the Ï phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or Ï nuclei. APT also failed to detect the Ï phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing.
Related Topics
Physical Sciences and Engineering
Materials Science
Metals and Alloys
Authors
Mansur Ahmed, Tong Li, Gilberto Casillas, Julie M. Cairney, David Wexler, Elena V. Pereloma,