Hisashi Sato

Effects of grain-boundary GeO2 particles on intergranular fracture of Cu-GeO2 polycrystals

The Cu-GeO2 alloy polycrystals containing plastically hard GeO2 particles are tensile tested to study intergranular fracture behavior of the alloys at intermediate temperatures. The effects of large GeO2 grain-boundary particles on the intergranular fracture are discussed using the specimens containing the particles of the fixed size (3 μm in diameter) and different area-fractions. The ductility of the Cu-GeO2 alloy polycrystals is larger than that of Cu polycrystals. The grain-boundary GeO2 particles improve the ductility by suppressing grain-boundary sliding. The grain-boundary voids to cause theintergranular fracture preferentially nucleate between the grain-boundary GeO2 particles. The ductility of the Cu-GeO2 alloys increases with increasing the area fraction of the grain-boundary GeO2 particles. The area-fraction dependence of the ductility is explained by considering the amount of GBS as a criterion of the intergranular fracture.

Brittle intergranular fracture of Cu polycrystals containing liquid B2O3 particles

Copper single crystals containing small SiO2, GeO2 or B2O3 particles have often been used to investigate effects of second-phase particles on various properties of materials [1–6]. These oxide particles are commonly amorphous and spherical in shape, and the alloys containing such particles are considered to be simplified model materials of dispersion-hardened alloys. Among these dispersed particles, only the B2O3 particles have a unique character in that these become liquid-like above certain intermediate temperatures for the solid Cu matrix [1, 6–8]. This is because the viscosity of B2O3 decreases rapidly with increase in temperature [9, 10]. Several characteristics of the “liquid” B2O3 particles have been observed clearly for the Cu-B2O3 alloys [6– 8, 11, 12]. Using the Cu single crystals containing the B2O3 particles, effects of liquid particles on high-temperature yield strength have been discussed [6, 8]. It has been reported that the liquid B2O3 particles are no less effective hardening centers than plastically non-deformable particles [6]. The origin of the strength is understood to be the presence of coherent matrix-particle interfaces in materials and the attractive interactions between glide dislocations and the particles [6, 8]. However, effects of the grain-boundary liquid B2O3 particles on hightemperature deformation and fracture behavior of polycrystals have not been fully understood. In the present study, we will examine these effects by tensile tests of the Cu-B2O3 alloy polycrystals. Inherent effects of the liquid particles will be discussed by comparing the results of the Cu-B2O3 alloy polycrystal with those of the Cu polycrystal without dispersoids and the Cu-GeO2 alloy polycrystal containing plastically non-deformable GeO2 particles on grain boundaries. The as-rolled Cu-0.28 wt%Ge and Cu-0.065wt%B alloys were annealed at 1123 K for 24 h to obtain equiaxed grains. The GeO2 and B2O3 particles were produced by internal oxidation of a Cu-Cu2O-Al2O3 mixed powder [12, 13] at 1123 K and 1273 K for 24 h in vacuum, respectively. The volume content of oxide particles calculated from the alloy concentration is about 0.9% for both the Cu-B2O3 and Cu-GeO2 alloys. Tensile specimens with gage length of 10 mm were sparkcut from the alloys and annealed at 1123 K in vacuum for 24 h to remove oxygen dissolved in the Cu matrix during the internal oxidation. Using the 99.99wt%Cu