Issue 35

W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 434-440; DOI: 10.3221/IGF-ESIS.35.49 439 At the deformation temperature of 400°C, mixed fracture is observed. In addition to small areas showing typical plastic deformations, there are areas characteristic of brittle intergranular cracking. Trace effects of plastic deformation are also observed on the planes of cracking at the grain boundaries (Fig. 10). After tension of the alloy at 550ºC (A=3.2%, Z=0.7%), the brittle intergranular fracture with small plastically deformed areas is observed (Fig. 10). Micro-cracks occurring due to slide at the grain boundary are observed on the fractures as an effect of one of the probable intergranular brittleness mechanisms [2]. The characteristic “catchment areas” and faults formed during relocation of the grain boundaries and numerous precipitations are observed on the planes of brittle cleavage cracking (Fig. 11). Figure 10 : Mixed fracture after deformation of CNCS alloy at 400°C Figure 11 : Brittle intergranular fracture of CNCS alloy after deformation at 550ºC The following were found in the chemical composition: Cu, Ni, Cr, Si and Mn as well as S as contamination. The concentration of these elements is 50%Cu, 31.69%Cr, 1.79%Ni and 0.22%Si, respectively. The observed alloy brittleness is above all the effect of intergranular precipitation that occurs during heating and deformation of the alloy at 550°C, resulting in the occurrence of numerous non-coherent coagulated precipitations on the cracking planes. After tension at 800°C, there is a fracture of typical ductile nature in the tested alloy showing high plasticity (A=68.7%, Z=92.5%). The example of precipitation at the α solution grain boundary as well as the distribution of elements comprising this precipitation, revealed by means of a transmission electron microscope, is shown in the microphotograph (Fig. 12). a) b) c) Figure 12 : a) Precipitation at the boundary of α phase grains, b) dark field of the precipitation, c) linear distribution of elements in the precipitation.