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The partitioning of alloying elements between α and β phases was measured at different
steps of the isothermal transformation at 710 and 610°C in Ti 17 alloy using EDX analyzer in the
TEM. In addition, the transformation crystallography was determined. No differences in substitution
elements were observed for a same transformation temperature in the α phase, however the
composition varied with the transformation temperature. For the partial transformed specimens,
gradient in composition were obtained. Results are compared to calculated compositions using
ThermoCalc software and Saunders database.
Nowadays, β-metastable titanium alloys such as Ti17 that exhibit high specific properties up to
medium service temperatures (350-400°C) are largely used in the aeronautical industry for the
manufacturing of aircraft engine parts such as fan and shorts blades. These mechanical properties
are closely linked with the microstructure resulting from the decomposition of the β phase, as the
α phase can adopt a variety of morphologies according to the transformation conditions. In order to
predict the microstructure, namely in large parts, different modeling tools are developed, the more
recent ones based on nucleation and growth kinetics [1,2]. The mechanisms involved as well as
quantitative data to compare with modeling are thus necessary. For the higher temperatures about
100°C below the β transus it is commonly admitted that the growth kinetics is controlled by
diffusion since a high diffusion rate of the alloying elements is expected. However, for lower
temperatures (below T
0
), the nature of the transformation mechanism is still discussed since for this
range of temperature the lower diffusion rate and the higher driving force may lead to a partial
displacive mechanism. Indeed, two trends can be found in the literature. Some authors [3,4]
observed by TEM equipped with an EDX analyzer that the most part of partitioning of the alloying
elements occurred only after the transformation completion, which would be partially displacive at a
first stage, with limited partitioning. Other authors [5,6] showed that the α phase produced during
the transformation has a homogeneous chemical composition, close to the final equilibrium one
while in the β phase a concentration gradient is observed. Moreover, this was observed even for
transformation temperatures about one hundred degrees below T
0
, where diffusionless mechanism is
possible. In addition, time resolved XRD has shown for Ti 17 alloy that the relative cell parameter
of the parent phase varies at the beginning of the isothermal transformation for a large range of
temperature (until Tβ – (400°C-450°C)) highlighting a solute diffusion [7]. The present study
focuses on the characterization of the solute distribution within and near the α phase, which is
isothermally formed after quenching from the β region in β-metastable titanium alloy. TEM
examination coupled with EDX analysis provides complementary chemical, crystallographic and
morphological information, enabling a more detailed understanding of the transformation.
Solid State Phenomena Vols. 172-174 (2011) pp 396-401
Online available since 2011/Jun/30 at www.scientific.net
© (2011) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/SSP.172-174.396
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
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