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.

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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.

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The correlation between the microstructures and tensile properties of strain-based American Petroleum Institute (API) X60 pipeline steels was investigated. Eight types of strain-based API X60 pipeline steels were fabricated by varying the chemical compositions, such as C, Ni, Cr, and Mo, and the finish cooling temperatures, such as single-phase and dual-phase regions. In the 4N and 5C steels, the volume fractions of bainitic ferrite (BF) and the secondary phases increased with the increasing C and adding Cr instead of Ni. In the 5C and 6NC steels, the volume fractions of acicular ferrite (AF) and BF decreased with increasing C and adding Ni, whereas the volume fractions of polygonal ferrite (PF) and the secondary phases increased. In the 6NC and 6NM steels, the volume fraction of BF was increased by adding Mo instead of Cr, whereas the volume fractions of PF and the secondary phases decreased. In the steels rolled in the single-phase region, the volume fraction of polygonal ferrite ranged from 40 to 60 pct and the volume fraction of AF ranged from 20 to 40 pct. In the steels rolled in the dual-phase region, however, the volume fraction of PF was more than 70 pct and the volume fraction of AF was below 20 pct. The strength of the steels with a high volume fraction of AF was higher than those of the steels with a high volume fraction of PF, whereas the yield point elongation and the strain hardening exponent were opposite. The uniform elongation after the thermal aging process decreased with increasing volume fraction of PF, whereas the uniform elongation increased with increasing volume fraction of AF. The strain hardening exponent increased with increasing volume fraction of PF, but decreased with increasing volume fraction of AF and effective grain size.