Transmission electron microscopy of deformed Ti-6Al-4V micro-cantilevers

Single alpha-beta colony micro-cantilevers were machined from a polycrystalline commercial Ti-6Al-4V sample using a focussed ion beam. Each cantilever contained several alpha lamellae separated by thin fillets of beta. A nanoindenter was used to perform micro-bending tests. The a(3) prismatic slip system was selectively activated in the cantilevers by controlling the crystal orientation along the micro-cantilever. Specimens for transmission electron microscopy (TEM) were prepared using a dual-beam focussed ion beam from a series of micro-cantilevers deformed to various extents. Bright field scanning transmission electron microscopy (BF-STEM) was used to investigate the processes of slip nucleation, propagation and transmission through the alpha/beta interface. The cantilevers had an equilateral triangular cross-section with the bar at the top and the apex at the bottom. The compressive stresses developed near the apex were thus twice the tensile stresses near the top. Dislocations initiate first from the bottom and then from the top and move toward the neutral line. Even in the sample with a small deflection, i.e. 0.5 mu m, dislocations were observed at the bottom of the cantilever, but dislocations were not observed at the top until the deflection reached 3 mu m. Pile-ups pushed the dislocations past the neutral line when the micro-cantilevers were deflected to more than 4 mu m.