Three-dimensional characterization of fatigue cracks in Ti-6246 using X-ray tomography and electron backscatter diffraction

Abstract In the present study, fatigue crack propagation was imaged non-destructively in three dimensions during in situ fatigue loading of Ti-6246 using X-ray microtomography on beam line ID19 at the European Synchrotron Radiation Facility. Phase contrast enabled the visualization of the two-phase microstructure and in combination with absorption contrast the crack chronology was recorded in situ during the fatigue experiment. In order to obtain the crystallographic orientation of individual grains along the crack path, a 3D electron backscatter diffraction volume was subsequently recorded. By combining both techniques it was possible to identify the role of prior β grain boundaries and the crystallographic orientation of the α lamellae on the crack path. It is shown that a near-surface crack that cuts across a prior β grain boundary is often diverted and crack bifurcation takes place. This results in local retardation of crack propagation and a first-order undulation of the crack front. In addition, the lamellar grain orientation and morphology causes a second-order crack front undulation with the tendency of large misorientations between α lamellae preferably orientated for prismatic 〈a〉 slip or pyramidal 〈c + a〉 slip deflecting the crack path.

[1]  H. Fraser,et al.  The role of crystallographic and geometrical relationships between α and β phases in an α/β titanium alloy , 2003 .

[2]  B. N. Cox,et al.  Inductions from Monte Carlo simulations of small fatigue cracks , 1989 .

[3]  Henry Proudhon,et al.  Three-dimensional visualisation of fatigue cracks in metals using high resolution synchrotron X-ray micro-tomography , 2006 .

[4]  Anthony Gravouil,et al.  Fatigue crack propagation: In situ visualization using X-ray microtomography and 3D simulation using the extended finite element method , 2006 .

[5]  A. K. Vasudevan,et al.  Fatigue crack growth behavior of titanium alloys , 2005 .

[6]  M. Donachie Titanium: A Technical Guide , 1988 .

[7]  Peter Cloetens,et al.  Study of the interaction of a short fatigue crack with grain boundaries in a cast Al alloy using X-ray microtomography , 2003 .

[8]  P. A. Blenkinsop,et al.  Crystallographic features of intralamellar fracture in a fully lamellar TiAl based alloy , 1998 .

[9]  G. Lütjering,et al.  Titanium : Engineering Materials and Processes , 2007 .

[10]  W. Ludwig,et al.  High resolution X-ray tomography of micromechanisms of fatigue crack closure , 2006 .

[11]  M. Benedetti,et al.  The effect of notch plasticity on the behaviour of fatigue cracks emanating from edge-notches in high-strength β-titanium alloys , 2008 .

[12]  James C. Newman,et al.  An empirical stress-intensity factor equation for the surface crack , 1981 .

[13]  B. N. Cox,et al.  Monte Carlo simulations of the growth of small fatigue cracks , 1988 .

[14]  J. Mendez,et al.  Analysis of the different slip systems activated by tension in a α/β titanium alloy in relation with local crystallographic orientation , 2005 .

[15]  J. Petit,et al.  Electron backscattering pattern identification of surface morphology of fatigue cracks in TA6V , 1994 .

[16]  A. Gourgues Electron backscatter diffraction and cracking , 2002 .

[17]  C. Sarrazin-Baudoux Abnormal near-threshold fatigue crack propagation of Ti alloys: role of the microstructure , 2005 .

[18]  T. Hayakawa,et al.  Near‐field optical microscope observation of dye‐containing nano‐domains , 2004, Journal of microscopy.

[19]  Françoise Peyrin,et al.  Observation of microstructure and damage in materials by phase sensitive radiography and tomography , 1997 .

[20]  T. C. Lindley,et al.  Effect of microtexture on fatigue cracking in Ti–6Al–4V , 2007 .

[21]  K. Ravichandran THREE‐DIMENSIONAL CRACK‐SHAPE EFFECTS DURING THE GROWTH OF SMALL SURFACE FATIGUE CRACKS IN A TITANIUM‐BASE ALLOY , 1997 .

[22]  P. J. Hurley,et al.  Prediction of fatigue initiation lives in notched Ti 6246 specimens , 2008 .

[23]  W. Evans,et al.  The effects of texture in titanium alloys for engineering components under fatigue , 2001 .