Deformation mechanism at large strains in a high-Nb-containing TiAL at room temperature

This paper investigates the deformation behavior and nano-scale structural characteristics after large deformation (29%) at room temperature of a Ti-45Al-9Nb-2.5Mn (at.%) alloy with duplex microstructure. Dislocation glide is the main deformation mode at low strains (such as 2%). The interaction of moving dislocations reduces the mobility of dislocations by pinning, and when deformation strain reachs 5%, both dislocation glide and mechanical twining are active. With further increase in strain, deformation twining becomes the dominant deformation mode, dividing the original gamma grains and lamellar structure into nano-scale. The characteristic feature of the deformation twin in gamma grains after large deformation at room temperature is the bent deformation twin (DT or DT'). There are many sub-structures in the DT, which makes the (111)(T) not exactly straight. High resolution TEM observations showed that (111)(DL) plane of the DT twins exhibits a large misorientation angle with the (111)(M) plane of the gamma matrix. The orientation relationship between the matrix and the DT or DT' is actually not the expected true twin relationship. The existence of numerous 1/3[111] Frank partial dislocations was regarded as accommodating the bending of the boundaries of DT. Local zigzag bending of the barrier twin boundaries can be observed due to twin intersections. Intersection induced lattice distortion in the intersection area for type I twin intersection is more severe than that for type II intersection. As results of the relaxation of the heavy lattice distortion, nano-scale subgrains boundaries may form together with a rectangular region near the intersection area. Nanotwins could be observed in the rectangular region for type I intersection. The nanotwins are formed with a homogeneous 1/6[11 (2) over bar] twinning dislocation glide mechanism. For type II twin intersection, the intersection induces the formation of secondary twinning ST in the barrier twin. No region with nanotwins can be observed near the intersection area. The local stress concentration in type II twin intersection may be relaxed by the emission of dislocations from the intersection area. (C) 2002 Elsevier Science B.V. All rights reserved.