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

Abstract

The scanning electron microscope (SEM)-based electron backscattered diffraction (EBSD)/ stereology technique quantitatively establishes distributions of the crystallographic characteristics of environmental-fatigue crack features for slightly overaged Al-Zn-Cu-Mg-X (X = Zr or Mn) alloys stressed in the low-growth-rate regime. Results for these homogeneous slip alloys conform to a substantial companion study of planar slip-prone Al-Cu-Mg/Li. Transgranular-crack characteristics are similar for the Mn and Zr variants, independent of grain size and recrystallization. Two morphologies of facetlike features exhibit a wide range of crystallographic orientations, change character at grain boundaries indicating an important role of grain orientation, and form in highly tensile-stressed spatial orientations about a crack tip. Similar characteristics for Al-Zn and Al-Cu suggest a common damage mechanism, speculatively attributed to hydrogen-environment embrittlement by decohesion. Slip-deformation band cracking resulting in facets near {111}, stimulated by H-enhanced localized plasticity, is not a viable mechanism for environmental fatigue. Repetitively stepped facets with surface curvature may involve H-enhanced cleavage along {100} or {110} planes subsequently distorted by plasticity. Broad-flat facets speculatively result from tensile stress-based cracking through dislocation cell structure, evolved by cyclic plasticity and containing trapped H.

Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy

“…9, crack nucleation occurred toward the center of a grain on a near-{0 0 1} crystallographic plane with the crack-surface normal inclined 28°from the global loading direction. The observed crack nucleation on a near-{0 0 1} plane rather than a {1 1 1}-slip plane seems to be consistent with observations of environmental hydrogenassisted fatigue cracking [9,[15][16][17]57], though the precise mechanism cannot be substantiated from the current study.…”

“…For many aluminum alloys under certain conditions, crack nucleation and early propagation have been associated with a {1 1 1} slip-based mechanism identified as Stage I cracking [52][53][54][55][56]. Alternatively, a number of studies have revealed non-slip-based mechanisms in aluminum alloys, often with crack paths near parallel to {0 0 1}, {1 0 1}, or high-index planes [9,[15][16][17]57]. These non-slipbased observations have been attributed to environmental hydrogen-assisted cracking.…”

“…For example, rather than relying on 2-D trace analyses to determine possible crystallographic planes on which MSFCs evolve, some have used fractography with diffraction-based methods to determine quantitatively the precise crystallography of individual facets on fatigue-crack surfaces [11][12][13][14][15][16][17][18].…”

“…plastic deformation). Indirect methods, on the other hand, rely on grainorientation information collected on a prepared surface adjacent to the faceted fatigue-crack surface [11,[13][14][15][16][17][18]. For the indirect methods, quantitative facet crystallography involves first determining the facet normal,n, in the spatial reference frame using scanning electron microscopy (SEM)-based stereology [20].…”

Effect of water vapor pressure on fatigue crack growth in Al–Zn–Cu–Mg over wide-range stress intensity factor loading

“…SEM fractography of UHV K-shed fracture surfaces ( Figure 8) for both T-L and L-T oriented specimens demonstrate faceted features (Figure 8a and 8c, respectively) at low ΔK. Prior work on this lot of material [61] and other precipitation hardened Al alloys [62,63] cracked in inert environments used quantitative crack-surface stereology coupled with EBSD to establish that such facets are characteristic of slip band cracking (SBC) parallel to {111} slip planes. Consistent with prior work, this SBC morphology is not observed at higher ΔK ( Figure 8b and 8d), likely due to elimination of discretely localized planar slip banding through activation of multiple slip systems as the stress intensity range and crack tip plasticity increase.…”