Effect of Environment on Fatigue Crack Wake Dislocation Structure in Al-Cu-Mg

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 43; no. 7; pp. 2275 - 2292
• Main Authors: Ro, Yunjo, Agnew, Sean R., Gangloff, Richard P.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01-07-2012; Springer; Springer Nature B.V
• Subjects: Alloys; Applied sciences; Characterization and Evaluation of Materials; Chemistry and Materials Science; Crack propagation; Crystallography; Exact sciences and technology; Fatigue; Hydrogen; Materials fatigue; Materials Science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metal fatigue; Metallic Materials; Metals. Metallurgy; Microscopy; Nanotechnology; Structural Materials; Surfaces and Interfaces; Thin Films; Dislocation Substructure; Fatigue Crack; Slip Band; Fatigue Crack Growth Rate; Dislocation Cell; Aluminium base alloys; Medium effect; Magnesium alloy; Mechanical properties; Dislocation structure; Fatigue; Dislocation; Fatigue crack
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-012-1089-5

Fatigue-induced dislocation structure was imaged at the crack surface using transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of naturally aged Al-4Cu-1.4Mg stressed at a constant stress intensity range (7 MPa√m) concurrent with either ultralow (~10
−8
 Pa s) or high-purity (50 Pa s) water vapor exposure at 296 K (23 °C). A 200-to-600-nm-thick recovered-dislocation cell structure formed adjacent to the crack surface from planar slip bands in the plastic zone with the thickness of the cell structure and slip bands decreasing with increasing water vapor exposure. This result suggested lowered plastic strain accumulation in the moist environment relative to the vacuum. The previously reported fatigue crack surface crystallography is explained by the underlying dislocation substructure. For a vacuum,
facets dominate the crack path from localized slip band cracking without resolvable dislocation cells, but cell formation causes some off-
features. With water vapor present, the high level of hydrogen trapped within the developed dislocation structure could promote decohesion manifest as either low-index
or
facets, as well as high-index cracking through the fatigue-formed subgrain structure. These features and damage scenario provide a physical basis for modeling discontinuous environmental fatigue crack growth governed by both cyclic strain range and maximum tensile stress.