Fatigue crack propagation mechanisms in an aluminium lithium alloy

Environmental Fatigue-Crack Surface Crystallography for Al-Zn-Cu-Mg-Mn/Zr

“…Computer-controlled crack-growth experiments were [20] Ranganatan et al [14] Gingell and King [27] Garrett and Knott [21] Brotzu et al [13] Bowles and Broek [25] etch pit Piascik and Gangloff [6] Gao et al [28] Pelloux [26] stereology/EBSD Pelloux [26] * stereology/EBSD Ro et al [17] Laue X-ray diffraction Ro et al [17] Forsyth et al [18] Lynch [24] {110} stereology/EBSD stereology/EBSD etch pit Ro et al [17] Ro et al [17] Nix and Flower [29] Pelloux [26] * TEM diffraction Nix and Flower [29] Bowles and Broek [25] {111}…”

“…[13][14][15][18][19][20][21][22][23][24][25][26][27][28][29]32] However, according to recent EBSD-based studies of Al-Cu-based alloys, there is a complete absence of facets parallel to {111} for moistenvironment fatigue but rather a complex distribution of facetlike features oriented across the range from {001} to {011} planes. [8,17] Explanation of the distinction between EBSD/stereology and historical results includes (1) the tendency for prior studies (i.e., X-ray based [21] ) to be performed on large-grain polycrystals, (2) the fact that few measurements were performed and perhaps sampling a method-biased area (i.e., by TEM-based electron diffraction [25,29] ), and (3) the questionable precision (i.e., etch pit analysis [6,14,28] ).…”

“…[5] For example, large crack facets observed for Al-Li and under-aged Al-Cu-Mg alloys fatigue stressed in vacuum are attributed to propagation along and through localized dislocation bands that form on {111} planes due to shearable precipitates. [6][7][8][9][10][11][12][13][14][15][16][17] Moist-aggressive environment enhances da/dN in aluminum alloys and produces a strong transition in fatigue crack-surface features. [1,3] Morphologies continue to exhibit complex-faceted features but over a much smaller length scale relative to grain size, particularly for lower DK loading.…”

“…[1][2][3] Many approaches have been employed to quantitatively characterize crack-path crystallography and morphology for Al alloys. Techniques include X-ray Laue back reflection, [18][19][20][21][22][23][24] etch-pit shape, [6,[13][14][15][25][26][27][28] selected area electron diffraction in the transmission electron microscope (TEM), [25,29] stereology [9] combined with diffraction techniques including X-ray diffraction, [30] and electron backscattered diffraction (EBSD). [8,9,17,31] The majority of these methods are semiquantitative, restricted to large grain-size model microstructures or limited to very small areas and sparse results.…”

Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li

“…[2][3][4] Such tortuous faceted cracking in vacuum is attributed to intense slip localization along {111} planes, due to shearable d¢ (Al 3 Li) precipitates. [3,4,[11][12][13] Verification of crystallographic cracking due to intense slip localization was based on several experimental approaches, including etch pit shape, [2,[14][15][16] X-ray texture with singlesurface facet trace angle, [17] and electron backscatter diffraction (EBSD) with stereology. [4,5] The results from these techniques consistently indicate exact {111} or near-{111} crack facet planes, and thus slip plane or deformation band cracking is the likely fatigue damage mechanism.…”