Issue 35

W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 434-440; DOI: 10.3221/IGF-ESIS.35.49 436 MEF4A with a magnifying power from 100x up to 1000x. For determination of the microstructure and phase identification of precipitations in the tested alloy the observations were performed under a scanning/transmission electron microscope S/TEM Titan80-300(FEI) at 245÷300kV in classic TEM operation system with image resolution below 0.1µm. The preparations for observations were made in the form of foils using mechanical thinning and ion polishing. Fractographic tests of specimen fractures after hot tensile test were performed using a scanning electron microscope with resolution of 2µm at accelerating voltage of 20kV. For micro-analysis of precipitations revealed on fractures of the tested specimens an X-ray micro-analyser EDX was used. The tests performed on a scanning microscope allowed recording the topographic images of fractures with a magnifying power from 200x up to 15000x. Figure 1 : Strength (R m ) vs. CuNi2Si alloy tension temperature. Figure 2 : Elongation (A) and reduction of area (Z) vs. CuNi2Si alloy tension temperature. R ESULTS AND DISCUSSION he results of the static hot tensile test in the examined range of temperature and at the strain rate v r = 2mm/min have enabled the effect of temperature on the mechanical properties of the CuNi2Si alloy to be determined. The obtained results of elongation and reduction of area allowed the determination of the range of temperature in which reduced plasticity occurs. The results of investigations of mechanical properties after hot tensile tests of the tested alloy are presented in diagrams (Fig. 1, 2). The rise of temperature from 100°C up to 500°C has been found to result in insignificant reduction in R m from 282 MPa to 239 MPa, while the rise of deformation temperature from 550°C to 800°C results in sudden decrease in strength from 189 MPa to 36 MPa (Fig.1). The effect of deformation temperature on the value of elongation and reduction of area of the tested alloy is presented in the diagram (Fig. 2). The curves of elongation (A) and reduction of area (Z) show the range of temperature in which reduced plasticity of the test alloy occurs. The minimum value of elongation and reduction of area is shown the CNCS alloy during deformation at 550°C. The course of the elongation vs. tension temperature curve is variable. With increase in the tension temperature from 350 to 500°C, the elongation reduces suddenly to reach its minimum at 550°C. A further rise of the deformation temperature affects the increase in plasticity of the tested alloy (Fig. 2). The dependences of the reduction of area and elongation on temperature have been found to be similar (Fig.2). The increase in deformation temperature from 350°C to 450°C results in significant decrease in the reduction of area from approx. 27% to approx. 9%. In the temperature range between 550 and 600°C, the minimum reduction of area of the tested alloy equal to approx. 1% is observed. The rise of alloy deformation temperature in the range of (600 ÷800°C) results in increase in the reduction of area. After tension at 800°C, the CuNi2Si alloy reaches its maximum elongation of 92.5%. The results of metallographic investigations have enabled the impact of structure of the tested alloy in an as hot-forged state and after supersaturation from 940°C as well as after hot deformation on the mechanism of initiation and propagation of its cracking to be assessed. The results of observations are shown in microphotographs (Fig. 3÷8). T