Fatigue-induced nanoscale patterns and microstructures

Technical Report · Wed Jan 01 04:00:00 EST 1986

OSTI ID:7152932

Gerberich, W W; Sheth, J K; Kaczorowski, M

Dislocation patterns as produced by monotonic loading of polycrystalline aluminum or fatigue loading of a low-alloy ferritic steel have been evaluated. A proposed theoretical model involves an accounting procedure based upon dislocations emanated at grain boundary sources being stored in dislocation patterns as a series of subcells. Near equilibrium, the scale parameters are roughly 100 ..mu..m grains being divided into 1..mu.. subcells which contain dislocations spaced at about 10 nm intervals within the cell walls. Pattern generation and dislocation spacings are dictated by the quasi-static state of equilibrium during the work-hardening phase of the loading. Experimental nanometer-scale probes, used in the fatigue study, consisted of three forms of electron microscopy. The first involved direct imaging of intersecting slip at the free surface of electropolished samples, using channeling contrast from back-scattered electrons (BSE). The second involved the use of selected-area electron channeling patterns (SACP's) to evaluate crystallography and defect densities, while the third directly evaluated cell sizes and dislocation spacings with transmission electron microscopy of thin films (TEM). BSE, SACP and TEM probes determined dislocations to be packed into cell walls with a wave length of 0.78 ..mu..m and an interwall spacing of 33 nm, consistent with the theoretical model. BSE channeling contrast revealed elliptical patterns to develop within single grains and SACP's verified that these were sharp valleys with a crystallographic misorientation of 17/sup 0/ across the valley. One of those was shown to lead to fatigue crack nucleation in a transgranular mode.

• Research Organization: Minnesota Univ., Minneapolis (USA). Dept. of Chemical Engineering and Materials Science
• DOE Contract Number: FG02-84ER45141
• OSTI ID: 7152932
• Report Number(s): DOE/ER/45141-T3; ON: DE87001877
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360102* -- Metals & Alloys-- Structure & Phase Studies
360103 -- Metals & Alloys-- Mechanical Properties
ALLOYS
ALUMINIUM
CHANNELING
CRACKS
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
DISLOCATIONS
ELECTRON CHANNELING
ELECTRON MICROSCOPY
ELEMENTS
FATIGUE
FERRITIC STEELS
GRAIN BOUNDARIES
GRAIN SIZE
IRON ALLOYS
IRON BASE ALLOYS
LINE DEFECTS
MECHANICAL PROPERTIES
METALS
MICROSCOPY
MICROSTRUCTURE
NUCLEATION
SIZE
SLIP
STATIC LOADS
STEELS
TRANSMISSION ELECTRON MICROSCOPY