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

Abstract

A combined electron backscatter diffraction (EBSD)/stereology method successfully quantifies the orientation of fatigue crack surfaces for Al-Li-Cu and Al-Cu-Mg alloys stressed at low DK, in which deformation is localized in slip bands and cracking is highly faceted. The method orients features as small as~1 lm in complex microstructures. Vacuum fatigue facets align within 15 deg of up to four variants of {111} slip planes, governed by the distribution of crack tip resolved shear stress. The small fraction of precisely oriented {111} facets suggests that cracking involves complex intraband and multiple-band interface paths. Water vapor and NaCl solution affect a similar dramatic change in the crack path; near-{111} facets are never observed, at odds with mechanisms for H-enhanced slip localization and associated slip band cracking. Rather, two environmental crack facet morphologies, broad flat and repeating step, exhibit a wide range of orientations between {001} and {110}, as governed by crack tip resolved normal stresses. The repetitive stepped facets appear to contain areas parallel to {100}/{110} on thẽ 1-lm scale, coupled with surface curvature consistent with a mechanism of discontinuous fatigue crack growth involving H-enhanced {100}/{110} cleavage and intermingled crack tip plasticity. Broad-flat faceted regions are parallel to a variety of planes, consistent with a mechanism combining high crack tip tensile stresses and H trapped at the dislocation structure from cyclic deformation, within 1 lm of the crack tip.

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

“…[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.…”

“…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. Scanning electron microscope (SEM)-based stereology combined with EBSD was employed in the present study to best probe micron-scale features that are relevant to technologically important alloy systems.…”

“…[31] This method has produced substantial data for environment-sensitive fatigue-crack facet crystallography for Al-Cu-based alloys. [8,9,17,31] Published determinations of fatigue-facet crystallography are summarized in Table I for precipitationhardened Al-Cu-Mg, Al-Cu-Li, and Al-Zn-Cu-Mg alloys stressed in water vapor or aqueous-NaCl solution.…”

“…[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] ).…”

Influence de l'environnement sur la propagation des fissures de fatigue

“…Par ailleurs, les similitudes observées dans de nombreux cas, tant en terme d'accélération de la propagation que de mécanismes de rupture, entre les solutions aqueuses et les atmosphères gazeuses, pour lesquelles il n'a pas d'électrolyte présent en pointe, suggèrent que ce sont les processus liés à l'hydrogène généré par les réactions électrochimiques qui sont prédominants suivant des mécanismes (HELP ou HEDE) qui restent là encore à préciser. Ainsi Ro et al [71,72] montrent, dans le cas d'alliages Al-Cu-Li-Mg/Li ayant une propension à développer des modes de propagation fortement cristallographiques sous environnement inerte conduisant à la formation de facettes proches de {111}, que l'air ambiant comme une solution 3.5% NaCl ne conduisent plus à la formation de ce type de facettes mais engendre un changement marqué des surfaces de rupture, ce qu'un modèle deLa chimie et composition de la solution au voisinage d'une fissure constituent donc un nouveau domaine d'étude à part entière [74,76,77]. Deux points essentiels ressortent: le milieu est généralement plus acide en pointe de fissure car les conditions électrochimiques sont très productrices d'hydrogène, et la solution peut se désaérer du fait d'un apport d'oxygène limité.…”