ΘΕΜΑ: Ομιλία από την Ass. Prof. Seyda Polat
Την Τρίτη 08 Σεπτεμβρίου και ώρα 12:00 στην Αίθουσα Συνεδριάσεων B' όροφος
του πολυώροφου κτιρίου του Τμήματος Μηχανολόγων & Αεροναυπηγών
Μηχανικών, θα πραγματοποιηθεί ομιλία από την Ass. Prof. Seyda Polat του
Πανεπιστημίου Kocaeli University of Turkey με τίτλο: " A summary on the
investigation of the fatigue and fracture behavior of Cu-2.55Ni-0.55Si alloy".
ABSTRACT
Among the present copper based alloys, Cu-Be alloys are the most commonly
used ones due to their good mechanical and electrical properties. However, the
beryllium metallurgy is highly toxic and anti-stress relaxation properties of Cu–Be
alloys are poor when used at elevated temperature. Therefore, replacing the Cu–
Be alloys by novel copper alloys is a trend subject. Precipitation hardenable CuNiSi
(Corson) alloys are among the candidate materials due to their high strength and
hardness, good electrical and thermal conductivities, wear and corrosion
resistances. These properties can be further enhanced by the addition of Cr, Zr,
Co, Fe, Al, Mg.
In this study, fatigue behaviour of Cu-2.55Ni-055Si alloy was investigated. Alloy
contains also 0.25 Zr and 0.25 Cr (wt-%) in order to enhance the mechanical
properties compared to typical Corson alloys. Alloy was produced as billet
material, hot forged at 880 °C and then cooled in air. Microstructure of the alloy
was examined using light microscope (LM) and it consists of rough facetted Zr2Si
crystals, globular shaped elemental Cr phase and very fine Ni2Si precipitates
within twinned α-Cu matrix. In the first stage of the mechanical tests, hardness
measurements and tensile tests were carried out and the results showed that; (i)
matrix had a macro hardness value of 153 HV1, (ii) its yield and tensile strength
values were 520 MPa and 745 MPa, respectively. In the second stage, the fatigue
behaviour of the alloy was investigated. Fatigue specimens were prepared
according to ISO 1099 standard, while constant amplitude loading was applied at
a stress ratio of 0.1. The criterion set for the termination of the fatigue tests was
either the fracture of the specimen or the limit of 5E6 fatigue cycles. Different
stress levels were selected, aiming at deriving the S-N curve of the material. The
S-N diagram involves the data and fitted curve according to Weibull statistics.
According to the derived fitting parameters of the Weibull regression analysis, the
fatigue limit was calculated as 170 MPa. All tested surfaces were examined by
scanning electron microscope (SEM) to determine the fracture behaviour. In
general, the secondary phase particles (especially facetted crystals) contributed to
crack propagation and fracture surfaces exhibited typical tracks indicating dimple
rupture, intergranular/transgranular rupture and fatigue rupture as a function of
applied σmax. At both 400 and 300 MPa, fracture surfaces of the alloy had very
narrow zone having fatigue tracks (striations) and exhibited mixed fracture of
dimple and transgranular ruptures in its final fracture region. However, at σmax
values lower than 250 MPa, it was observed that; (i) the striations spread to a
larger area on the fracture surface, (ii) the fracture mode changed to
intergranular rupture not only at the grain boundaries but also at twinned
surfaces.