Embrittlement of Copper Wire Due to Oxygen Krhkost bakra zaradi kisika L. Kosec, V. Gontarev, B. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana N. Mlakar, Kolektor, Idrija An example of the reversible oxygen embrittlement of copper is deseribed in the paper. This phenomenon is combined vvith the drastic reduetion of ductility and workability. It appeared at the lovv temperature annealing (500 C) of copper in the nitrogen atmosphere vvith a lovv oxygen concentration (5 ... 6ppm), when diffusion of oxygen in copper took plače preferentially on grain boundaries. During the eooling to the surroundings temperature oxygen precipitated from the saturated solution in the form of copper oxide (Cu20) on the grain boundaries, thus the conditions for the intergranular dimple fracture have been created. Key vvords: oxygen, grain boundary diffusion, supersaturation, precipitation, intergranular dimple rupture, reversible embrittlement due to oxygen. V prispevku je opisan primer reverzibilne krhkosti bakra zaradi kisika. Pojav je povezan z drastičnim zmanjšanjem duktilnosti in preoblikovalne sposobnosti bakra. Nastal je pri nizkotemperaturnem (500°C) žarenju bakra v dušiku z majhno koncentracijo kisika (5 ... 6 ppm), med katerimi je prišlo do prednostne difuzije kisika po kristalnih mejah bakra. Med ohlajanjem na temperaturo okolice je kisik iz nasičene raztopine precipitiral v obliki bakrovega oksida (Cu20) na kristalnih mejah in ustvaril pogoje za interkristalni prelom z jamicami. Ključne besede: kisik, difuzija po kristalnih mejah, prenasičenje, izločanje, intergranularni jamičasti lom, krhkost zaradi kisika. 1. Introduction Copper and some its alloys represent of high duetile and well cold workable materials. These properties can be usually obtained by annealing in the proteetive atmosphere. But there exist frequent exceptions. They are numerous since many products are made by advanced technology of the bulk shaping instead of machining. Limited ductility in the bulk shaping allowed only a certain amount of plastic deformation. Further plastic deformation initiated cracking till final fracture of material (Fig.l). These problems are often caused by oxygen vvhich concentration could be detected by the metallographic analyse of oxide inclusions or chemical"'21. The copper oxide inclusions vvell fol-low the deformation of metal if extreme degrees are not exceed-ed. Oxygen in the solid solution vvhich simultaneously hardens copper and reduces its ductility is harmful. In some cases the chemically measured differences in the oxygen concentration betvveen duetile and brittle copper are very small, even vvithin the measuring error. In sueh cases oxygen is expected to be concen-trated on certain sites in the microstructure, for instance on the grain boundaries, but it could be detected only by an analitical in situ method. Before rolling, the copper wire of 12.8 mm in diameter have been annealed for 1 hour at 500"C in the nitrogen atmosphere vvith 5 ... 6 ppm of oxygen. During the annealing proces the average concentration of oxygen in the copper inereased from ap-proximately 0.001% to approximately 0.002%. Already after first or second pass through the grooved roll (round - square) the surface cracked. It vvas an obviously sign that further rolling vvas not possible any more. The cracks vvere approximately in the radial direetion vvith characteristic changes of direetions on the short sections. In single areas the surface damages vvere so intensive that even some small metalic parti-cles split off. The vvire vvith the limited ductility had the same strength and the yield stress as that vvhich vvas be shaped into the demanding sections. The reduced ductility vvas explained by fracture surfaces and by the careful analysis of the microstructure. The contraction of the copper vvire before annealing vvas ap-proximately 90%, and it vvas reduced to less than 30% after the annealing process. Essential difference betvveen the tvvo vvires vvas in the form of fracture. Not annealed copper vvire exhibited duetile transgranular dimpled fracture vvith characteristic deep unidireeted dimples (Fig. 6). On the fracture surface of the test bar broken in the air, the adsorbed carbon and oxygen have been measured (Fig. 9). After annealing the ductility vvas rapidly reduced vvhile the fracture vvas completely intercrystalline. Intergranular fracture surface consisted of many fine and shal-low dimples vvith inclusions of copper oxyde (Fig. 2-5). High oxygen concentration on that fracture surface vvas proved by the AES analysis. Oxygen vvas distributed obviously deeper under the fracture surface when compared vvith the no-tannealed copper (Fig. 10). The oxygen concentration on the surface corresponded to the composition Cu;0 and vvas rapidly reduced away from the grain boundaries. The initiation of cracks in the annealed copper is Figure 1: Cracks on copper wire surface after cold rolling (first step of reduetion); 10()x Slika 1: Površina bakrene žice /. razpokami po prvi redukciji pri hladnem valjanju; l()()\ Figure 3: Intergranular dimple rupture in copper wire resulting from microvoids coalescence at grain boundaries (Z = 2?'!); 200\ Slika 3: Intergranulama jamničasta površina preloma bakrene žice (Z = 25r/r); 200x Figure 2: Intergranular dimple rupture in copper wire resulting from microvoids coalescence at grain boundaries; 100x Slika 2: Intergranulama jamničasta površina preloma valjane bakrene žice: 10()x Figure 4: Detail of intergranular rupture surface with copper oxyde inclusions in dimples (fig.3); 6000x Slika 4: Detalj intergranularne prelomne površine z vključki Cu O " v jamicah (siJ); 6000\ /"**"' t--- ) / J^ f Figure 5: Small cracks on grain boundaries and copper oxyde precipitates; 20()x Slika 5: Kratke razpoke na kristalnih mejah s precipitati bakrovega oksida; 2()0x Figure 6: Copper oxyde precipitates on grain boundary: 6000x Slika 6: Precipitati bakrovega oksida na kristalni meji: 6000x connected vvith the inclusions of copper oxide on the grain boundaries (Fig. 3,5). As reference, also the surface of copper vvire vvhich has been covered vvith thin layer of corrosive prod-ucts during storing has been analysed. The composition vvas not the same on the vvhole surface. On one section of the surface the chemical composition of the corrosive products corresponded to the CuO copper oxide (Fig. 13). The layer is thin and it adheres to the unchanged metal at a high oxygen concentration gradient. In the other surface area. there vvas found u layer vvith high carbon concentration and it vvas thicker than that rich in oxygen (Fig. 14). The oxygen embrittlement of copper exhibited re-versibility. Annealing in the vacuum (5.10"6mbar. 85()"C, 10 hours) essentialy increased (Z = 75%) the copper ductility. The fracture surface of that annealed copper vvas predominantly transgranular dimpled ductile fraetured vvith a very small amount of residual intergranular dimpled fracture (Fig. 8). The chosen annealing conditions in the vacuum vvere obviously not so Figure 7: Ductile fracture of copper vvire (Z = 90%); l()()()x Slika 7: Duktilni prelom bakrene žice (Z = 90%): l()()()x Figure 8: Fracture surface of copper vvire after vacuum annealing (Z = 90%); 200x Slika S: Prelomna površina žice po žarenju v vakuumu (Z = 75%): 200x favourable enough to remove ali the oxygen accumulated in the copper during the annealing in the nitrogen atmosphere. The not uniform removal of oxygen vvas proved also by the AES analy- 100 80 j A A P f li t 4 6 8 10 12 Sputter time Iminl K Figure 9: Copper, oxygen and carbon concentration distribution on fracture surfaces of high ductility copper (Z = 90%) Slika 9: Profil koncentracij bakra, kisika in ogljika na prelomu bakra z veliko duktilnostjo (Z = 90%) 100 80 60, o