In Situ Synchrotron Studies of Reversible and Irreversible Non-elastic Strain in a Two-Phase TiAl Alloy
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TWO-PHASE (a2 + c) titanium aluminides are promising intermetallic alloys for aerospace applications because of their excellent mechanical properties at high temperature.[1,2] However, they exhibit extremely low ductility at room temperature, which has been a factor in limiting their use.[3] Current engineering alloys can display a wide range of microstructures depending on the thermo-mechanical processing route followed. As expected, the mechanical properties are very different depending on the microstructure, with duplex (DP) microstructures showing an increased ductility but reduced strength at high temperatures,[4,5] while fully lamellar (FL) and nearly fully lamellar (NFL) microstructures are relatively brittle at room temperature while retaining their strength at high temperature.[2] FRANCISCO ALFREDO GARCI´A-PASTOR, formerly Ph.D. Student with the School of Materials, The University of Manchester, Grosvenor Street, Manchester, M1 7HS, U.K., is now Lecturer with the Cinvestav Unidad Saltillo, Av. Industria Metalurgica No. 1062, Parque Industrial, 25900 Ramos Arizpe, COAH, Mexico. Contact e-mail: [email protected] HUI JIANG, formerly Post-Doctoral Associate with the IRC in Materials, The University of Birmingham, Edgbaston, B15 2TT, U.K., is now Scientist with the Oxford Instruments plc, High Wycombe, Bucks, HP12 3SE, U.K. DAVID HU, Post-Doctoral Associate, is with the IRC in Materials, The University of Birmingham. XINHUA WU, formerly Professor with the IRC in Materials, The University of Birmingham, is now Professor with the Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. PHILIP J. WITHERS and MICHAEL PREUSS, Professors, are with the School of Materials, The University of Manchester. Manuscript submitted June 1, 2012. Article published online October 2, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
DP microstructures (generated by slow cooling from the a transus) comprise an even distribution of lamellar colonies and equiaxed grains. In contrast, lamellar microstructures (generated at higher cooling rates than those for DP microstructures) contain at least 80 pct of these colonies in the NFL form, and approach 100 pct of lamellar colonies in the FL microstructure.[6] Lamellar colonies are platelets of twinned tetragonal c phase and of the hexagonal a2 phase as described in.[7] This semicoherent interface between the c and a2 follows the crystallographic relationship described by Blackburn[8]
ð0001Þa2 f111gc and ½1120 ½1 10c ½1 giving six crystallographic variants of the c phase from a single a2 grain. These variants can be distinguished by their stacking order into three matrix-variants and three twin-variants. The presence of these variants gives rise to three different c/c interfaces.[7] It is well known that the mechanical response of lamellar colonies is highly orientation-dependant, with colonies oriented with the (0001) planes at 45 deg to the stress axis considered to be soft while colonies oriented at 0 and 90 deg are considered to be h
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