In Situ Synchrotron Studies of Reversible and Irreversible Non-elastic Strain in a Two-Phase TiAl Alloy FRANCISCO ALFREDO GARCI ´ A-PASTOR, HUI JIANG, DAVID HU, XINHUA WU, PHILIP J. WITHERS, and MICHAEL PREUSS This paper contrasts the cyclic tensile loading of two-phase titanium aluminide Ti-44Al-8Nb-1B microstructures, namely fully lamellar (FL) and duplex (DP). The former, in contrast to the latter, shows premature yielding and hysteresis loops on cycling. These phenomena were studied by in situ cyclic loading of these specimens using high energy synchrotron X-ray diffraction. The results show significant differences in the micromechanics of deformation. Load transfer between the c and a 2 phases has been identified at stresses below the macroscopic yield point in the FL microstructure, while the DP microstructure showed no signs of such load transfer before its well-defined macroscopic yield point. A partially reversible pseudo-plastic deforma- tion mechanism seems to be operating at relatively low stresses in the FL specimen. This mechanism is believed to be twinning/partial reversible twinning of the c phase. The presence of twins at relatively low applied stresses has been confirmed for the FL microstructure by electron channeling contrast imaging. Further support of twinning/partial reversible twinning in FL but not DP microstructures below the macroscopic yield point was obtained by following the evolution of the integrated intensity of particular diffraction peaks measured during the in situ synchrotron X-ray experiments. DOI: 10.1007/s11661-013-2024-0 Ó The Minerals, Metals & Materials Society and ASM International 2013 I. INTRODUCTION TWO-PHASE (a 2 + 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) micro- structures are relatively brittle at room temperature while retaining their strength at high temperature. [2] 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 a 2 phase as described in. [7] This semi- coherent interface between the c and a 2 follows the crystallographic relationship described by Blackburn [8] ð0001Þa 2 111 f gc and ½11  20     ½1  10c ½1 giving six crystallographic variants of the c phase from a single a 2 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 hard. [9–11] This orientation dependence originates from the a 2 phase because slip of dislocations with non-basal Burgers vector requires a far higher stress than slip of dislocations with b parallel with h11  20i. The c phase deforms mainly by glide of ordinary dislocations on the {111} planes, with Burgers vector b = 1/2h110i and superdislocations with Burgers vectors b = h101i and b = 1/2h11  2i. Plastic deformation of this phase can also be realized by mechanical twinning with b = 1/6 h11  2i on the {111} planes. [9,11] 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: francisco.garcia@cinvestav.edu.mx HUI JIANG, formerly Post-Doctor- al 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, Aus- tralia. 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 VOLUME 45A, FEBRUARY 2014—607