Evolution of Nanostructure Through Thickness of Deoxidized Low-Phosphorous Copper by Three-Layer Stack Accumulative Roll-Bonding.

A nanostructured deoxidized low-phosphorous copper (DLPC) was fabricated by three-layer stack accumulative roll-bonding process. Three sheets of 1 mm in thickness, 30 mm in width and 300 mm in length were stacked up and roll-bonded to thickness of 1 mm by two-pass cold rolling. The bonded sheet was cut in three pieces of same length, then stacked up and roll-bonded to the thickness of 1 mm again. The evolution of nanostructure through thickness with three-layer stack ARB were investigated in detail. It was found that the microstructure has been evolved from a dislocation cell structure to a nano grained structure with the proceeding of ARB cycles. The average grain thickness of 45 µm in initial decreased to 170 nm after 7 cycles of the ARB. The heterogeneity in microstructure through thickness was also largely decreased by the ARB. These results suggest that three-layer stack ARB is an effective process for a formation of nanostructure of DLPC alloy.

Microstructural Evolution of a Nanostructured Complex Copper Alloy Processed by Accumulative Roll-Bonding of Oxygen Free Copper and DLP.

The accumulative roll-bonding (ARB) process using different copper alloys of oxygen free copper (OFC) and dioxide low-phosphorous copper (DLPC) was performed up to six cycles at ambient temperature without lubrication. A complex copper alloy sheet'in which OFC and DLPC alloys are stacked alternately each other was successfully fabricated by the ARB process. The microstructural evolution and texture development of the complex copper alloy with proceeding of the ARB were investigated by electron back scatter diffraction (EBSD) measurement. The specimen after 1 cycle showed significantly inhomogeneous microstructure in thickness direction, however, the inhomogeneity decreased gradually with increasing the number of ARB cycles. In addition, the grains became finer with the proceeding of the ARB. Resultantly, after 6 cycles, the specimen exhibited an ultrafine grained structure in which the grains above 65% were surrounded by the high angle grain boundaries above 15 degrees. On the other hand, there was no difference in texture development between OFC and DLPC in almost all specimens. In addition, the texture development did not depend on positions in thickness direction; the rolling texture such as {112}<111> and {011}<211> components developed strongly at all regions.

Microstructure and mechanical properties of ultrafine grained complex copper alloy fabricated by accumulative roll-bonding process.

Accumulative roll-bonding (ARB) process using dissimilar copper alloys was performed up to six cycles (-an equivalent strain of 4.8) at ambient temperature without lubricant for fabrication of a new complex copper alloy. The dissimilar copper alloy sheets of oxygen free copper (OFC) and dioxide low-phosphorous copper (DLPC) with thickness of 1 mm were degreased and wire-brushed for the ARB process. The sheets were then stacked together and rolled by 50% reduction so that the thickness became 1 mm again. The sheet was then cut to the two pieces of same length and the same procedure was repeated up to six cycles. A new sound complex copper alloy sheet in which OFC and DLPC are combined each other was successfully fabricated by the ARB process. The tensile strength of the copper alloy increased with increasing the number of ARB cycles, reached 492 MPa after six cycles, which is about three times of the initial material. The average grain size was 12.6 µm after the 1st cycle, but it became 1.5 µm after six cycles. Microstructures and mechanical properties of the complex copper alloy fabricated by the ARB were investigated in detail.

Fabrication of nanostructured deoxidized low-phosphorous copper processed by three-layer stack accumulative roll-bonding.

A nanostructured deoxidized low-phosphorous copper (DLPC) was fabricated by three-layer stack accumulative roll-bonding (ARB) process. The microstructural evolution and the variation of mechanical properties with three-layer stack ARB were investigated in detail. It was found that the microstructure has been evolved from a dislocation cell structure to ultrafine grained structure as the number of ARB cycles increases. In addition, the mean spacing of grain boundaries, which was 45 microm in initial material, reduced to 2.1 microm after 1 cycle, 360 nm after 3 cycles, 250 nm after 5 cycles, then 170 nm after 7 cycles, progressively. The fraction of high-angle grain boundaries after 1-cycle ARB was no more than 0.27, but it increased with the number of ARB cycles, and became surprisingly more than 0.7 after 7-cycle ARB. The tensile strength increased with the number of ARB cycles, and then after 7 cycles it reached about 600 MPa, which is about 2.5 times higher than that of the initial material. Therefore, the three-layer stack ARB is very effective for development of ultrafine grains and high strengthening of DLPC alloy.

Evolution of nano-grains in high purity copper by accumulative roll-bonding process.

The evolution of nano-grains in oxygen free copper with accumulative roll-bonding is investigated by TEM and EBSD analysis. The ultrafine grains developed in the sample after 8-cycle ARB. It was found that the mean spacing of grain boundaries, which was 63 microm in initial material, reduced to 5.5 microm after 1 cycle, then surprisingly 450 nm after 3 cycles. In addition, the fraction of high-angle boundaries in the sample after 1-cycle ARB was 0.32, but it after 3-cycle ARB was surprisingly more than 0.6. Texture development of the ARB processed samples is different depending on the number of ARB cycles.

Annealing characteristics of nanostructured Cu-Fe-P alloy processed by accumulative roll-bonding.

Annealing characteristics of a nanostructured copper alloy processed by accumulative roll-bonding (ARB) were studied. A nano-grained Cu-Fe-P alloy processed by 8 cycles of the ARB was annealed at various temperatures ranging from 100 to 400 degrees C for 0.6 ks. The sample still showed an ultrafine grained (UFG) structure up to 250 degrees C, however above 300 degrees C it began to replace by equiaxed and coarse grains due to an occurrence of the conventional static recrystallization. The hardness of the annealed copper decreased largely above 300 degrees C. These annealing characteristics of the UFG copper alloy were compared to those of a high purity copper.

Nanostructured bulk copper fabricated by accumulative roll bonding.

In this study, we tried to fabricate the nanostructured bulk copper alloys by a severe plastic deformation process. The sheets of copper alloys (OFC, PMC90, and DLP) were heavily deformed to an equivalent strain of 6.4 by the accumulative roll-bonding (ARB) process. The microstructure and the mechanical property of the fabricated specimens were systematically investigated. The microstructure was finely subdivided with increasing the equivalent strain by the ARB process. The severely deformed copper alloys exhibited the ultrafine lamellar boundary structure where the mean lamella spacing was about 200 nm. The strength significantly increased with decreasing the lamella spacing in the ARB processed copper alloys. Especially, the tensile strength of the DLP alloys ARB processed by 8 cycles (the equivalent strain of 6.4) reached to 520 MPa, which was about three times higher than that of same materials with conventional grain size of 10-100 microm. On the other hand, the total elongation greatly dropped only by 1 ARB cycle corresponding to an equivalent strain of 0.8, which was around 3%. However, the total elongation increased again with increasing the number of the ARB cycle, and it reached to 10% after 8 cycles. The recovery of the total elongation could be recognized in all studied copper alloys. The obtained stress-strain curves showed that the improvement of the total elongation was caused by the increase in the post-uniform elongation. It can be concluded that the nanostructured copper alloys sheets having high strength without a large loss of ductility could be fabricated by the ARB process.

Annealing characteristics of nano-grained oxygen free copper processed by accumulative roll-bonding process.

Annealing characteristics of nano-grained oxygen free copper processed by accumulative roll-bonding (ARB) were studied. A nano-grained oxygen free copper fabricated by 8 cycles of the ARB was annealed at various temperatures ranging from 100 to 300 degrees C for 0.6 ks. TEM observation revealed that the ultrafine grains still sustained up to 150 degrees C, however above 200 degrees C they were replaced by equiaxed and coarse grains due to an occurrence of the static recrystallization. The tensile strength of the copper decreased largely above 200 degrees C. These annealing characteristics of the copper were compared with those of a commercially pure aluminum.