the specimen surface in an early stage by using an optical microscope, which is connected to the 
testing machine by a moveable frame. In addition to the optical microscopy study, the specimens 
were removed from the testing machine to examine the cracked surface in the SEM applying electron 
channelling contrast (ECC) and EBSD. 
Figure 1 shows a schematic representation of the procedure carried out. The fatigued specimen is 
mounted in a specifically designed specimen holder and first studied in electron channelling contrast 
by which the individual grains become visible (see Fig. 2). For the EBSD measurements of the 
crystallographic orientations, the specimen has to be tilted for 70°. Part of the elastically scattered 
primary electrons hits the lattice planes under an angle according to Bragg’s law: 
n-A=2-d-sin® 1) 
where A denotes the electron wave length, © the Bragg angle and d the lattice spacing. The elec- 
trons which are diffracted under these conditions yield a Kikuchi pattern on a phosphorous screen 
which is mounted in front of the specimen. The geometry of the Kikuchi pattern corresponds to the 
local crystallographic orientation and can be evaluated by software manually and automatically (Ori- 
entation Imaging Microscopy). In this study the program COS (Crystal Orientation Software) from 
TSL was used, which yields absolute orientation data, misorientation angles and the rotation axis and 
pole figures. 
nina specimen surface | orientation n matrix M_ 
€ 
phosphors clectron beam ! or X (Rd) Y(Ta) Z (Sn) {he pro 
SIT camera screen 100 Mn M2 Mis the devel 
: 010 | Man M2 M2 
001 ' Ma Mx Mn 
with: 
- absolute orientation See 5 Kossel cone Ra = loading direction N 
- misorientation angles BSE detector . . 
Ta = transversal direction 
Sp = surface normal 
Fig. 1: Schematic illustration of EBSD technique . 
The orientation matrix M which is depicted in Fig. 1 defines the three-dimensional orientation of the 
crystal axes [100], [010] and [001] with respect to the specimen axis X,Y,Z. The matrix elements 
Mj; can be calculated from the cosines of the angles between specimen axis and the bce crystal axis. 
The orientation relationship between adjacent grains is characterised by misorientation angle 6 
describing the rotation between M; and M; at two grains i and j. In the present study high-angle 
(6 215°) and low-angle (8 <15°) grain boundaries are distinguished. More detailed descriptions of 
the EBSD technique are given in ref. (5) and (6). 
Figur 
Exemplary results and discussion 
During fatigue testing (Ac/2 = 600 MPa, f = 1 Hz, R = -1) small cracks initiate transgranularly as 
well as intergranularly. Typical SEM micrographs (ECC), which were taken after 1000 cycles (20% lien 
of the number of cycles to failure Nr), are represented in Fig. 2. Thi 
160