Coarse-grained Copper Research Articles

Control of intergranular fatigue cracking by slip homogeneity in copper I: Effect of grain size

Recent studies of long-life fatigue in polycrystalline copper have shown fractures to initiate primarily at grain boundaries. This is considered surprising in the light of early work where crack nucleation in persistent slip bands seemed to be the rule. To investigate the causes of this behavior, studies habe been carried out on polycrystalline copper with emphasis on the following factors: grain size (Part I), method of starting the test, frequency and mode of test control (Part II). The results show that the cyclic plasticity is the controlling factor in grain boundary initiation and propagation of fatigue cracks. It was found that transgranular cracking can be produced by factors which promote strain localization, and reducing the grain size of the specimens in constant-amplitude tests is one of them. In small-grain-sized metals, most of the applied plastic strain is concentrated in a few persistent slip bands, some of which develop into cracks with continued cycling. Use of coarse-grained structures causes strain homogenization and therefore intergranular cracking. For coarse-grained copper, a step is gradually built up at a sensitive grain boundary as a result of the “homogeneous” cyclic slip, at which point the cracking occurs in the same manner as proposed by Kim and Laird for high strain fatigue.

Development of Cube Texture in Coarse Grained Copper

Cube oriented deformed material has been detected in the rolling texture measured on edge sections of heavily rolled coarse grained copper. The level of intensity is low, and seems to be independent of rolling reduction over the range 93–98%. Recrystallization textures show an increasing strength of Cube with rolling reduction. The evidence is consistent with the idea that Cube nuclei are created by a mechanism similar to that proposed by Dillamore and Katoh, and the strength of the recrystallized Cube texture depends on oriented growth. Prior grain size effects are briefly examined and it is shown that deformation textures are less sharp in large grain size compared with small grain sized copper at similar strains. It is likely that the effect of grain size on cube texture formation arises from grain size dependent texture changes in the vicinity of the deformed Cube oriented material.

Embrittlement of copper due to segregation of oxygen to grain boundaries

The tensile and creep properties of oxygen free OF- and oxygen saturated OS-polycrystalline copper have been investigated in the temperature range 25 to 500 °C. Oxygen increases the yield strength by a factor of 2 or 3, by solid solution hardening, and for coarse-grained copper, causes a severe embrittlement, particularly under creep conditions. Both direct and indirect evidence indicate that the embrittlement is caused by the segregation of oxygen to the grain boundaries in copper, thus promoting grain boundary decohesion and intergranular fracture. Auger electron spectroscopy is used to indicate the presence of oxygen at the grain boundaries in OS-Cu. The embrittling effects of oxygen are reversible in the sense that both tensile and creep ductility are restored when oxygen is removed.

Relation between the crushability and the mineralographic and petrographic composition in ores from the copper pyrites deposits of the urals

1. The mechanical and strength properties of ores and rocks are governed by their mineralogical and petrographic compositions, the quantity of sulfides (pyrite, chalcopyrite, sphalerite, and bornite), and their degree of recrystallization and grain sizes. 2. Increase in the ore mineral contents of medium and coarse-grained copper pyrite ores promotes weak cohesion of the minerals and the development of cracks dueto dynamic shocks. 3. Increase in sulfide content improves the crushability of the ore. 4. In calculating the para meters for blasting operations we must take account of the mineralogical composition of the ores, their metal content, and the grain-size composition of the minerals. For pyrite ores the most reliable criterion in the calculations in the total quantity of sulfides (sum of sulfides), which can be fairly accurately and rapidly determined from the chemical analysis data.

Grain-Boundary Relaxations in Copper-Gold Alloys

Strong damping and modulus effects have been observed using a torsional pendulum in very pure, coarse-grained copper, gold, and seven completely miscible copper-gold alloys. In Cu and Au a high-temperature relaxation peak increases with increasing grain size and stabilizes at 470 and 230 degC, respectively, above the orthodox grain-boundary damping peaks. Related damping peaks were also measured in the alloys with activation energies distributed between Cu and Au, decreasing with increasing Au content. All alloys showed stronger relaxation effects than pure Cu and Au, with a maximum at cu3Au. It is suggested that this high-temperature peak results from grain-boundary sliding and that the relaxation time and strength in Cu and Au are controlled by irregularities in the grain boundaries. Adsorption of solute atoms on the alloy boundaries will influence these irregularities and facilitate greater sliding.

Measurement of Equilibrium Concentrations of Vacancies in Copper

The net added fraction of thermally-generated atomic sites, $\frac{\ensuremath{\Delta}N}{N}$, has been measured, during equilibrium heating to near the melting point, in a coarse-grained copper bar of 99.999% purity. The linear thermal expansion, $\frac{\ensuremath{\Delta}L}{L}$, and the x-ray lattice expansion, $\frac{\ensuremath{\Delta}a}{a}$, as measured simultaneously, agree within the measurement error of \ifmmode\pm\else\textpm\fi{}1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ throughout the temperature interval between 56 and 850\ifmmode^\circ\else\textdegree\fi{}C. At 1000 and at 1075\ifmmode^\circ\else\textdegree\fi{}C, $\frac{\ensuremath{\Delta}N}{N}=3(\frac{\ensuremath{\Delta}L}{L}\ensuremath{-}\frac{\ensuremath{\Delta}a}{a})$ is (0.9\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ and (1.9\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$, respectively. Using expected limits for the binding energies of vacancy clusters, it is concluded that $\frac{\ensuremath{\Delta}N}{N}$ consists almost entirely of monovacancies. An assumed value of $(1.5\ifmmode\pm\else\textpm\fi{}0.5)k$ for the formation entropy, combined with these concentrations, yields a monovacancy formation energy of (1.17\ifmmode\pm\else\textpm\fi{}0.11) eV. The ratio of this formation energy to the activation energy for self-diffusion is therefore about 0.57, near the ratios found for the other noble metals. The results imply a monovacancy migration energy of (0.88\ifmmode\pm\else\textpm\fi{}0.13) eV. The present results are compared with the results of a wide variety of other investigations of defects in copper which include: (1) theoretical calculations, (2) quenching and annealing, (3) thermal diffusion, (4) annealing after irradiation, and (5) annealing after cold work. In certain cases apparent disagreement is found. However, it is concluded that in no case is there a sufficiently firm body of experimental data available either to confirm or to contradict the present monovacancy data. The need for further definitive experiments in these areas is emphasized.