Some observations on cyclic deformation structures in the high-strength commercial aluminum alloy AA 7150
- PDF / 1,562,024 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 57 Downloads / 165 Views
I.
INTRODUCTION
THE 7XXX aluminum alloys are used extensively in the aerospace industry. However, at peak strength, the fatigue ratio for these alloys (the ratio of the stress required to cause failure in 106 cycles to the tensile yield strength) may be as low as 0.3,[1] and therefore, component design is often fatigue strength limited. The literature on crack initiation in Al alloys is contradictory. Crack initiation has been identified from slip band cracking,[2,3] grain boundary cracking,[2,4,5] and particle cracking.[4,5] However, short crack growth appears to normally occur along slip bands. The low fatigue resistance of high-strength, precipitation-hardened, Al alloys is believed to result from a high degree of microstructural instability to alternating stresses.[6] To and fro dislocation motion destroys the strengthening precipitates, leading to localized softening and therefore highly heterogeneous deformation.[7,8] The dominance of a particular initiation mechanism will depend, at least in part, on the surface deformation structure that develops, since it determines the surface roughening from which cracks may initiate, the degree of slip irreversibility, and therefore the damage accumulation rate.[9,10] Despite this, no studies have been performed on the deformation structures developed at the surface of a commercially important Al alloy. There remains considerable controversy over the nature and role of surface deformation structures developed by cyclic stresses. Much of the work has been undertaken on Cu and Cu-based solid solutions, where the cyclic strain is accommodated almost exclusively in persistent slip bands D.N. HANLON, Senior Research Fellow, formerly with the Department of Engineering Materials, The University of Sheffield, is with The Netherlands Institute for Metals Research, The Technical University of Delft, 26006A Delft, The Netherlands. W.M. RAINFORTH, Senior Lecturer, is with the Department of Engineering Materials, The University of Sheffield, Sheffield S1 3JD, United Kingdom. Manuscript submitted October 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
(PSBs).[8] However, as shown by a recent viewpoint set in Scripta Materialia,[11] there is no agreement as to whether the surface is mechanically harder or softer than the bulk. For example, Lukas et al.[10] examined a range of singlephase fcc metals and reported that for single crystals there are no specific near-surface structures and that PSBs or planar arrays of dislocations penetrate the whole crystal volume emerging at the free surface. However, in planar slip fcc metals, a 10 to 20 mm low dislocation density surface layer was found, while in wavy slip metals, a 1mm dislocation-free region lying between the matrix substructure and the surface was reported.[10] In contrast, Hong et al.[12] suggest that, for planar slip alloys, a complex three-layer structure exists, consisting of an outer layer with dislocation density higher or comparable to the bulk, a subsurface layer of low dislocation density, and finally a high disloc
Data Loading...
