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Site Occupancy Determination by ALCHEMI of Nb and Cr in γ-TiAl and Their Effects on the a to γ Massive Phase Transformation

Published online by Cambridge University Press:  10 February 2011

T.M. Miller ,
L. Wang ,
W.H. Hofmeister ,
J.E. Wittig  and
I.M. Anderson
T.M. Miller
Affiliation:
EECE Dept Vanderbilt University, Nashville, TN 37212
L. Wang
Affiliation:
EECE Dept Vanderbilt University, Nashville, TN 37212
W.H. Hofmeister
Affiliation:
ChE Dept., Vanderbilt University, Nashville, TN 37212
J.E. Wittig
Affiliation:
EECE Dept Vanderbilt University, Nashville, TN 37212
I.M. Anderson
Affiliation:
Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376
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Abstract

Atom location by channeling enhanced microanalysis (ALCHEMI) has been used to characterize the site distributions of Nb and Cr alloying additions in the L10-ordered γ phase of ternary titanium aluminides. Two alloys, Ti50Al48Cr2 and Ti50A148Nb2, were processed by furnace cooling from 1300°C (within the α-γ two phase field) as well as by rapid solidification using twin-anvil splat quenching of electromagnetically levitated and undercooled samples. ALCHEMI studies of furnace cooled samples yield results generally consistent with those in the published literature. Nb alloying additions are found to partition exclusively to the ‘Ti’ sublattice, while Cr alloying additions exhibit an ‘Al‘ sublattice preference. However, a higher degree of disorder can be achieved with rapid solidification and high solid state cooling rates (105-106 K/s). Significant distribution of the ternary elements between the ‘Ti’ and ‘Al‘ sublattices has been measured in the splat quenched samples, with up to 12% of the Nb atoms occupying the ‘Al‘ sublattice and the fraction of Cr atoms on the ‘Ti’ sublattice doubling to ∼30%. Rapid solidification of TiAl produces an equiaxed hexagonal α phase solidification structure that transforms in a massive fashion to the tetragonal γ phase. Although the amount of massively transformed γ is dependent upon the solid state cooling rate, ternary alloying additions can more strongly influence the transformation kinetics. The Nb-modified alloy exhibits significant amounts of the massively tranformed γ, similar to the Ti52Al48 binary alloy, whereas little massively transformed γ is observed in the Cr-modified alloy. These results can be correlated with the relative atomic size, lattice distortion, and sublattice site occupancy of Nb and Cr in the L10 unit cell.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 589 , 1999 , 123
Copyright
Copyright © Materials Research Society 2001

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