JOURNAL OF MATERIALS SCIENCE LETTERS 21, 2 0 0 2, 617 – 619 Brittle intergranular fracture of Cu polycrystals containing liquid B 2 O 3 particles H. SATO, N. TAKEOKA, T. FUJII, S. ONAKA ∗ , M. KATO Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 226-8502, Japan E-mail: onaka@iem.titech.ac.jp Copper single crystals containing small SiO 2 , GeO 2 or B 2 O 3 particles have often been used to investigate ef- fects of second-phase particles on various properties of materials [1–6]. These oxide particles are commonly amorphous and spherical in shape, and the alloys con- taining such particles are considered to be simplified model materials of dispersion-hardened alloys. Among these dispersed particles, only the B 2 O 3 particles have a unique character in that these become liquid-like above certain intermediate temperatures for the solid Cu ma- trix [1, 6–8]. This is because the viscosity of B 2 O 3 decreases rapidly with increase in temperature [9, 10]. Several characteristics of the “liquid” B 2 O 3 particles have been observed clearly for the Cu-B 2 O 3 alloys [6– 8, 11, 12]. Using the Cu single crystals containing the B 2 O 3 par- ticles, effects of liquid particles on high-temperature yield strength have been discussed [6, 8]. It has been reported that the liquid B 2 O 3 particles are no less effec- tive 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 B 2 O 3 particles on high- temperature deformation and fracture behavior of poly- crystals have not been fully understood. In the present study, we will examine these effects by tensile tests of the Cu-B 2 O 3 alloy polycrystals. Inherent effects of the liquid particles will be discussed by comparing the re- sults of the Cu-B 2 O 3 alloy polycrystal with those of the Cu polycrystal without dispersoids and the Cu-GeO 2 al- loy polycrystal containing plastically non-deformable GeO 2 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 GeO 2 and B 2 O 3 particles were produced by internal oxidation of a Cu-Cu 2 O-Al 2 O 3 mixed powder [12, 13] at 1123 K and 1273 K for 24 h in vacuum, respectively. The volume content of oxide par- ticles calculated from the alloy concentration is about 0.9% for both the Cu-B 2 O 3 and Cu-GeO 2 alloys. Ten- sile specimens with gage length of 10 mm were spark- cut 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 ∗ Author to whom all correspondence should be addressed. sheet rolled and annealed at 1123 K for 24 h, ten- sile specimens without dispersoids were also prepared. The Cu specimen also had equiaxed grains and gage length of 10 mm. For the Cu-B 2 O 3 , Cu-GeO 2 and Cu specimens, the sizes of grains and oxide particles on grain boundaries were measured with a scanning elec- tron microscope (SEM) and a light microscope (LM) as summarized in Table I. The area fraction of the oxide particles on grain boundaries was about 1% for both Cu-B 2 O 3 and Cu-GeO 2 . Tensile tests were performed in an argon-gas atmo- sphere with an Instron-type testing machine at 800 K using an initial nominal strain rate of 10 -4 s -1 . The testing temperature of 800 K was selected since sig- nificant intermediate temperature embrittlement is ex- pected for Cu and Cu alloys [13]. After the tensile tests, microstructural observations were carried out on the fracture surfaces and side surfaces of the specimens by SEM and LM. Fig. 1 shows stress-strain curves for the Cu, Cu- B 2 O 3 and Cu-GeO 2 specimens. Intergranular fracture occurred in all test samples. The tensile strength of Cu- GeO 2 is higher than in Cu and Cu-B 2 O 3 . The fracture strain of Cu-GeO 2 is larger than in Cu and Cu-B 2 O 3 . Apparently, Cu-B 2 O 3 is most brittle. Under the testing conditions of 800 K and 10 -4 s -1 , grain-boundary sliding (GBS) is actively operative for the Cu specimen and intergranular fracture occurs as a result of GBS [13]. However, when plastically non- deformable particles exist on grain boundaries, these suppress GBS and the amount of GBS becomes much less than that for the alloy without the particles [14]. Although possible diffusion around the particles can reduce the suppression effect, the kinetic evaluation has shown that the diffusional relaxation around the GeO 2 particles hardly occurs under the present testing T A B L E I The specimens used in present study Volume content Grain Particle of oxide particles diameter diameter Specimen (vol.%) (μm) on GB (μm) Pure Cu – 320 – Cu-0.28 wt%Ge 0.9 320 3.0 Cu-0.065 wt%B 0.9 200 3.6 0261–8028 C  2002 Kluwer Academic Publishers 617