Microstructure investigations on two different aluminum wrought alloys after very high cycle fatigue D. Schwerdt a,⇑ , B. Pyttel a , C. Berger a , M. Oechsner a , U. Kunz b a State Materials Testing Institute and Institute for Materials Technology, Technische Universität Darmstadt, Grafenstr. 2, 64283 Darmstadt, Germany b Physical Metallurgy, Institute of Materials Science, Technische Universität Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany article info Article history: Received 16 April 2012 Received in revised form 18 December 2012 Accepted 10 January 2013 Available online 21 January 2013 Keywords: Aluminum alloys Very high cycle fatigue Fatigue strength Crack initiation Microstructures abstract Fatigue studies were conducted under load control with a servo-hydraulic testing machine with smooth specimens of the aluminum wrought alloys EN AW-6056-T6 (R m = 399 MPa) and EN AW-6082-T5 (R m = 356 MPa) up to a maximum number of N =2  10 8 and 10 9 cycles, respectively. The results show for both aluminum alloys that the fatigue strength decreases with increasing number of cycles after the kneepoint of the S–N-curve and that approximately at this point a transition of the crack initiation site from the surface to the subsurface occurs. All fractured specimens were investigated with a scanning electron microscope (SEM). Large defects like primary intermetallic particles could not be found at the crack initiation sites. The difference between the subsurface non-defect crack initiation sites of both alloys is to be found in the fractographic structure. The internal crack initiation site of EN AW-6056-T6 shows a multiplicity of cleavage-like planes contrary to the flat area found at the alloy EN AW-6082- T5. These results were already presented in [1]. The scope of this present paper is new microstructural investigations and the formulation of a failure model which respects the results of different scales of microscopy. Electron back scatter diffraction analysis (EBSD analysis) and transmission electron micros- copy (TEM) were done for both alloys to characterize the microstructure and to compare unstressed and tested materials. These differences are discussed with regard to the differences in failure mechanisms. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction It is well known that the fatigue strength of materials with face- centered cubic lattice, as for instance aluminum, decreases with increasing number of cycles. Only a few sources are found in the public literature describing the cyclic strength of technical alumi- num alloys [e.g. 2–4] in the VHCF-region and only few of them in- clude investigations of the failure mechanisms [5–7]. The crack initiation site for aluminum wrought alloys was detected at the surface of the specimens [3,4,7] and close below the surface [2,5,6] in the VHCF-region. However, there are internal crack initi- ations which are not related to defects. Here the fracture mecha- nisms are more influenced by microstructure, interface and micro plasticity [3]. Within the scope of this paper, two different wrought alumi- num alloys (EN AW-6056-T6, EN AW-6082-T5) were investigated to determine fatigue strength and failure mechanism at number of cycles up to N =2  10 8 and 10 9 , respectively. 2. Methods The investigations at EN AW-6056-T6 were done at smooth, mechanically polished specimens [1,8] at a stress ratio of R = 0.1. The smooth specimens of EN AW-6082-T5 were only precision- turned. They were investigated at R = 0 in previous studies [9]. At that time it was of interest to investigate the fatigue behavior with a technical surface condition. Fatigue tests were conducted under load control with a servo-hydraulic testing machine at a frequency of 400 Hz. The temperature of the specimens of EN AW-6056-T6 were checked on the first tested specimens in the very high cycles region (f = 400 Hz) about 10 times and also at two specimens fati- gued at higher load (f = 50 Hz) for about 5 times with a thermom- eter with surface device. A linear regression was carried out to achieve a double r–N line with N ¼ N k ðr na =r Nk Þ k ð1Þ and N ¼ N k ðr na =r Nk Þ k ð2Þ Eq. (1) describes the r–N-line for cycle numbers smaller than the kneepoint N k and (2) the r–N curve in in the VHCF-region. It 0142-1123/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijfatigue.2013.01.004 ⇑ Corresponding author. Tel.: +49 6151 164572; fax: +49 6151 166051. E-mail address: schwerdt@mpa-ifw.tu-darmstadt.de (D. Schwerdt). International Journal of Fatigue 60 (2014) 28–33 Contents lists available at SciVerse ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue