The ultra-fine grained (UFG) aluminum with the grain size of 260 nm was fabricated by annealing for the severely plastic deformed A1100 alloy. This UFG aluminum showed the 0.2% proof stress (σ 0.2) of four times the stress that the conventional Hall-Petch relation showed. In this study, for the UFG aluminum, the fine-grained (FG) aluminum with the grain size of 960 nm and the coarse-grained (CG) aluminum with the grain size of 4.47 µm, dislocation density change during the tensile deformation was investigated by the In-situ XRD measurement using SPring-8. The dislocation density changed in four stages with increase in strain. The first stage was the elastic deformation region and the dislocation density hardly changed. Only in the CG aluminum, this stage was hardly observed and the stress in which the dislocation began to multiple (σ I) was almost 0 MPa. In the second stage, the dislocation density rapidly increased to ρ II in which plastic deformation became possible at constant strain rate. In the third stage, the change became moderately. In the fourth stage, the dislocation density rapidly decreased by the fracture of test pieces. Additionally, the σ 0.2-σ I were followed the conventional Hall-Petch relation regardless of grain size.

A {001} 100 oriented aluminum single crystal was deformed by nine cycles of accumulative roll bonding which corresponded to a total equivalent strain of ε=7.2. The formation of {4 4 11} 11 11 8 component in deformation texture was examined by using electron back-scatter diffraction method. The specimen processed by five cycles ARB (ε=4) developed four variants of {123} 634 orientation in the deformation texture. Between the five and nine cycles, the four variants of {123} 634 rotated toward {4 4 11} 11 11 8 and {112} 111 orientations. The specimen after the ninth cycle had a weak texture which was mainly composed of {4 4 11} 11 11 8 , {112} 111 and {123} 634 components. The results obtained in the present study indicate that {4 4 11} 11 11 8 and {112} 111 become the primary and secondary stable orientations in the specimens deformed by accumulative roll bonding to the extremely high strain. The occurrence of the cyclic ratcheting which was reported by Heason et al. was also investigated. The cyclic ratcheting, which causes the formation of {4 4 11} 11 11 8 in the center layer, did not have much effect on the {001} 100 oriented single crystal.

The enhancement of strength of a Cu 1.4 massNi 0.25 massP 0.1 massZr alloy, in which the Ni and P contents are about twice larger than those in commercial Cu Ni P alloys, has been tried by means of combining accumulative roll bonding (ARB) process by 7 cycles and aging treatment at 350 to 450°C. For the sake of comparison, the mechanical properties of the alloy conventionally cold rolled to a reduction of 50 and 90 and aged at 350 to 450°C have also been examined. The grain sizes of the Cu Ni P Zr specimens deformed by 7 cycle ARB process and 90 cold rolling were refined down to about 0.4 mm and 4 mm, respectively, and the fractions of high angle grain boundaries in the specimens were nearly the same, about 45. Initial aging, subsequent ARB process or 90 cold rolling and re aging at 400°C produced the Cu Ni P Zr alloy highly strengthened. The alloy, initially aged, then ARB processed and re aged, had a tensile strength of 780 MPa, an elongation of 6 up to failure and an electrical conductivity of 56 IACS. The differences in yield strength among the re aged specimens after 50 and 90 cold rolling and ARB process are explained by the differences among the dislocation density, grain size and inter precipitate spacing. (Received December 20, 2010; Accepted June 3, 2011) Keywords: copper nickel phosphorus alloy, accumulative roll bonding process, tensile property, grain refinement strengthening, precipitation strengthening, dislocation strengthening

Internal friction (Q -1 ) of nickel sheets highly deformed up to an equivalent strain (e) of 4.8 by the Accumulative Roll Bonding (ARB) process was investigated in order to clarify the damping mechanism of the ultrafine grained materials produced by severe plastic deformation. Although the strength increased with increasing number of the ARB cycle (N ), the maximum value of Q -1 was obtained at N＝4 (e＝3.2). At relatively small e about 1, where dislocation cell structure was formed, relatively small Q -1 below 5×10 -3 was obtained. The dislocations within the cell walls appeared to be tightly pinned at high density of nodes or other dislocations. In the middle range of e, where the ultrafine grains having diameter lager than 0.2 mm were formed, a high valued of Q -1 greater than 5×10 -3 was obtained. In the ultrafine grains, dislocations without the pinning by nodes or other dislocations were observed. At the largest e of 4.8, where the grain size was as small as 0.15 mm, the Q -1 was smaller than 4×10 -3 . The distance of the dislocation motion under vibration stress seemed to be small due to the fine grain size. The change in the Q -1 with number of the ARB cycles was attributed to the changes in the dislocation density and the distance of dislocation motion under vibrating stress which is controlled by the pinning points and the grain boundaries.

Using Vickers hardness measurements and TEM observation, this study investigated the aging behavior of ultrafine-grained UFG Al-0.5%Si-0.5%Ge alloy fabricated using six-cycle ARB processing. The hardness of the starting specimen of ultrafine-grained Al-0.5%Si-0.5%Ge alloy is about 2.3 times higher than that of the coarsegrained specimen. The starting UFG specimen showed ultrafine grains with mean size of 156 nm. The hardness of a UFG specimen aged at 473 K decreased monotonously with increasing aging time. From TEM observations of the microstructure of UFG specimen aged after long-term aging at 473 K, results show that the mean size of ultrafine grains increased significantly with increasing aging time. Furthermore, few precipitates exist inside gains, although many coarse precipitates exist at grain boundaries. The hardness of UFG specimen aged at 373 K showed the maximum value. From TEM observations, results showed many elongated ultrafine grains, with many precipitates formed on the grain boundaries and inside grains of the specimens aged at 373 K. These results suggest that precipitation hardening strongly affected the hardness in this alloy because it has many elongated ultrafine grains. Many fine Si-Ge precipitates formed inside grains of the UFG specimen aged at 373 K in long-term aging.