Effect of thermomechanical microstructural modification and resulting crystallographic texture on the crack initiation mechanism and fatigue behaviour of PM Ti-6Al-4V

The crack initiation mechanism and fatigue behaviour of thermomechanically processed PM Ti-6Al-4V was systematically studied as a function of microstructural modifications and the associated crystallographic texture. Uniaxial fatigue tests, fractographic analysis and thorough EBSD analysis were performed on an extruded blended elemental PM Ti-6Al-4V alloy to reveal the relationship between crystallographic texture and fatigue properties. We demonstrate that the fatigue crack initiation mechanism is related to the microstructural features of the alloy (colonies in the lamellar microstructure, colony-like primary a plates in the acicular microstructure, and strongly textured primary a grains in the bimodal microstructure) and not to porosity. Through in-depth crystallographic analysis, we demonstrate that the highest fatigue strength achieved with the bimodal microstructure is due to the sharp [10 (1) over bar0]//extrusion direction crystal texture of the primary alpha grains which require higher applied stresses in order to induce deformation along basal systems as well as crack opening along basal planes. The crystallographic texture of the alloy thermomechanically processed in the beta field is not favourable for fatigue and the resulting lamellar microstructure has the lowest fatigue strength as grains are easily deformed along basal systems. The grain refinement typical of the solution and aged acicular microstructure increases the fatigue resistance with respect to the lamellar microstructure as fatigue strength increases with the reduction of the slip length.