Crystal plasticity, fatigue crack initiation and fatigue performance of advanced titanium alloys

The fatigue behaviour of a novel large grained variant of the near α titanium alloy Ti 685 is described. Load controlled low cycle fatigue fractures in plain cylindrical specimens demonstrate the highly crystallographic nature of the failure process. When compared to similar data for conventional grain size Ti 685 variants the LG685 material clearly offers reduced fatigue and static strength. An alternative flat plate specimen design was employed together with electronic speckle pattern interferometry and strain gauges to monitor the inhomogeneous strain accumulation in this large grained variant under load control conditions. Dwell fatigue tests indicated that the eventual location for crack initiation and subsequent failure could be identified as early as the first loading cycle. The precise crystallographic orientations of the surrounding microstructure were defined using electron back scattered diffraction. In contrast to previous models to describe facet formation and early fracture in this class of alloy, basal plane slip systems were not implicated. The use of EBSD to identify “effective structural units” of common orientation, which do not necessarily relate to colony size, will be demonstrated for both the large grained material and a conventional compressor disc alloy Timetal 834 in typical forged condition.