High strength in high Nb containing TiAl alloy sheet with fine duplex microstructure produced by hot pack rolling

The Ti-44Al-8Nb-(W, B, Y) alloy sheet with a large thickness reduction of 85% was successfully obtained by hot pack rolling with 25% reduction per pass. The original coarse nearly lamellar structure with mean grain sizes of 120 mu m was converted into a fine duplex microstructure with mean grain sizes of 5.3 mu m. The as-worked alloy contains more than 70% vol. % of disordered alpha phases before the hot pack rolling. After the repeated deformation, these disordered a laths transform into interrupted fine recrystallization and recovery sub-grains. Then both alpha ->alpha 2 ordering transformation and gamma laths precipitation from the interrupted alpha phases take place during the cooling. Furthermore, the gamma grains and gamma laths also carry large strains and produce a large number of dislocations. These dislocations tangle and form dislocations networks which promote the discontinuous dynamic recrystallization nucleation and growth. As deformation proceeds during the subsequent hot pack rolling, the recrystallization, recovery and phase transformations of the previously formed recrystallization and recovery regions take place at triboundaries and inside the lamellar, resulting in the formation of fine duplex microstructure. At room temperature, the as-rolled alloy exhibits yield strength, ultimate tensile strength and ductility of 962 MPa, 1174 MPa and 1.01%, respectively. When temperature becomes as high as 850 degrees C, the failure strength still remains higher than 630 MPa with a good elongation value of above 45%. (C) 2016 Published by Elsevier B. V.