Issue 50

V. Kytopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 414-422; DOI: 10.3221/IGF-ESIS.50.35 417 both of these X-ray productions were taken into consideration, which means that the X-ray signal intensity used consists of the overlapping of characteristic and continuum X-ray of the material. In this way the applied manner of measurements exhibits a great advantage over the usually applied elemental X-ray microanalysis technique where the imposed measuring conditions and the precautions including atomic number, absorption and fluorescence factors (ZAF) are much more stringent [22]. In this study, the X-ray detection was made by using a Microspec 201-type wavelength dispersive spectro- meter with special analyzing crystals. This spectrometer was attached to a Cambridge-S4-10-type, SEM, installed in the Laboratory of Strength and Materials of the National Technical University of Athens. The edge-notched specimens, of dog-bone geometry, were loaded in tension using a specially designed servo-motor device and examined in-situ in the SEM-EPMA system. Fig.1 shows the geometry of the sampled region by SEM-electron beam ahead of the notch root of the specimen. The electronic scan frame and line set-up on the SEM were adequately selected to allow convenient measurements imposed by the conditions of X-ray signal collection. For instance, since electronic deflection of the beam can lead to defocusing of the X-ray spectrometer the desired line scans were performed by stepping the sample under a static electron beam. Figure 1 : Electron beam of scanning – stepping set up principle. The stepping procedure was made by means of a high-precision micro-positioning system of the loading stage. In this context, it is noted that the emitted X-ray beam intensity is strongly depended on the electron beam intensity and because of this the measurements were performed for fixed electron intensity. This was made by monitoring the electron beam by means of a special specimen current apparatus of high sensitivity ±10 -12 Amp attached to the SEM. Furthermore, because of the statistical nature of the X-ray emission phenomenon the counting error is independent of counting time, it is beneficed to record a pre-determined fixed number of counts rather than to operate for a fixed time period [10]. In this sense the measured X-rays emission intensity should be appropriately scaled with the corresponding time signal variation. For this reason the measurements were carried out by introducing a time-converted signal given as:   i em i o t I I constant (5a) i I  t  1  (5b) where I 0 is the predetermined fixed number of counts, I em i is the emission intensity from the “i” point on the surface of the specimen and t i is the corresponding measured specific accumulation time. Thus, the main measurement parameter in Eq. (5b), Ĩ , is the so-called specific accumulation rate. Taking into account, the discussion concerning Eq.(4), one can generally state that the specific parameter Ĩ , may reflect a measure of the damage-controlled microstructural integrity change in the material. The data presented are the result of the average of measurements taken from three specimens. Materials In this study, two MMCs were tested, namely the 8090-Al-Li and 2124-Al-Cu. Both were fabricated by liquid metallurgy. This method is the most economical to fabricate composite materials. The mixed molten metal at 710 o C was poured into