Kotobu Nagai

Progress in cold roll bonding of metals

Abstract Layered composite materials have become an increasingly interesting topic in industrial development. Cold roll bonding (CRB), as a solid phase method of bonding same or different metals by rolling at room temperature, has been widely used in manufacturing large layered composite sheets and foils. In this paper, we provide a brief overview of a technology using layered composite materials produced by CRB and discuss the suitability of this technology in the fabrication of layered composite materials. The effects of process parameters on bonding, mainly including process and surface preparation conditions, have been analyzed. Bonding between two sheets can be realized when deformation reduction reaches a threshold value. However, it is essential to remove surface contamination layers to produce a satisfactory bond in CRB. It has been suggested that the degreasing and then scratch brushing of surfaces create a strong bonding between the layers. Bonding mechanisms, in which the film theory is expressed as the major mechanism in CRB, as well as bonding theoretical models, have also been reviewed. It has also been showed that it is easy for bcc structure metals to bond compared with fcc and hcp structure metals. In addition, hardness on bonding same metals plays an important part in CRB. Applications of composites produced by CRB in industrial fields are briefly reviewed and possible developments of CRB in the future are also described.

Application of the Kocks–Mecking model to tensile deformation of an austenitic 25Cr–19Ni steel

Abstract Stress–strain relationships obtained by tensile test below room temperature for an austenitic 25Cr–19Ni steel were analyzed by using the Kocks–Mecking model to make clear the effects of temperature and strain rate on flow stress. A temperature range used here is between 77 and 296 K, a strain rate range between 10−9 and 10−2 s−1 and true strain below 0.2, where structure evolution depends on strain but scarcely on temperature and strain rate. This means that work-hardening rate is almost independent of test temperature and strain rate in the above ranges. Crosshead-arresting tests were performed to obtain flow stresses at 10−9 s−1 and the results suggested that the athermal stress could hardly be determined from the measurement of stress relaxation behavior at low temperatures. Flow curves obtained by the above deforming conditions are successfully described by using the Kocks–Mecking model with minor modifications. That is, we have claimed that the work-hardening consists of the thermal stress and the athermal stress. It should be noted that the flow curves for as hot-rolled specimens and for annealed specimens can be well simulated by changing the athermal stress.

Phosphorus-induced dislocation structure variation in the warm-rolled ultrafine-grained low-carbon steels

Abstract Dislocation structures in warm-rolled low-carbon steels with ultrafine ferrite grains (∼1 μm) are analyzed by both transmission electron microscopy (TEM) observation and the modified Warren–Averbach procedures for X-ray diffraction profiles. In good accordance with the TEM observation results, the dislocation density is derived by the X-ray diffraction profile analysis to be 1.06×10 14 and 1.98×10 14 m −2 , in the as-rolled 0.15C and 0.15C–0.1P steel, respectively. In contrast to the straight dislocation segments in the 0.15C steel, tangled and curved dislocations are observed in the phosphorus steel at both the as-rolled and annealed conditions. The larger q value and smaller dislocation arrangement parameter, M=R e ρ , are observed in the phosphorus steel. Phosphorus in the low-carbon steels tends to prompt the ratio of screw dislocation component and the dislocation arrangement with a stronger interaction during plastic deformation. Not only the dislocation density, but also the dislocation interaction behaviors control the changes of yield strength during annealing the as-rolled steels at 723 K. The modified Warren–Averbach X-ray diffraction profile analysis may be useful in characterizing the dislocation interaction behavior quantitatively.

Role of deformation twin on texture evolution in cold-rolled commercial-purity Ti

Texture evolution of a commercial-purity titanium (CP-Ti) during cold rolling was studied by means of x-ray diffraction (XRD) and electron back-scattered diffraction (EBSD). Twinning was identified to significantly contribute to deformation up to reductions of about 50%. Based on initial texture of the material investigated and twinning modes available in hexagonal close-packed (HCP) structures, the measured texture evolution can be interpreted in terms of (i) compressive twinning ({11 ¯ 2 2}〈11 ¯ 2 ¯ 3 〉) within the two dominant initial texture components B ({0001}〈10 ¯ 1 0〉±40°TD) and E ({0001}〈11 ¯ 2 0〉±40°TD) and (ii) followed by tensile twinning ({10 ¯ 1 2}〈10 ¯ 1 ¯ 1 〉) in the then-favorably reoriented twinned part. Reduction of grain size at high deformation inhibits further twinning and results in a stable texture evolution driven exclusively by dislocation slip. During cold rolling, the crystals of the initial texture component B first rotate to orientation M ({01 ¯ 1 0}〈2 ¯ 1 ¯ 1 2〉) by compressive twinning (primary), and then orientation M rotates to orientation D ({0001}〈11 ¯ 2 0〉) by tensile twinning (secondary). Meanwhile, the crystals of the initial component E first rotate to the orientation M ′ ({14 ¯ 5 3}〈6 ¯ 5 ¯ 1 3〉) by compressive twinning (primary), and then orientation M ′ rotates to the orientation A ({0001}〈10 ¯ 1 0〉) by tensile twinning (secondary). At higher deformation level, twinning was significantly depressed by strongly refined grain size, which resulted in the elimination of the transient texture components caused by slip. These results are useful for the prediction and control of the texture in titanium.

Subsurface crack initiation in high cycle fatigue of Ti5Al2.5Sn extra-low interstitial alloy at liquid helium temperature

Abstract Subsurface fatigue crack initiation in a Ti5Al2.5Sn extra-low interstitial alloy at liquid helium temperature was characterized. The subsurface crack initiation occurred at low stress level and neither defects nor foreign material were detected at the subsurface initiation site. The subsurface crack initiation site was composed of “small grains” of diameter about 5–10 μm. The concentration of aluminium at the initiation site was much lower than in the bulk. From the viewpoint of the microstructure this alloy had partly a two-phase region (small α grain region). It was composed of small α grains with low aluminium content and an iron-enriched β phase at their grain boundaries. The size, morphology and composition of the subsurface crack initiation site fitted those of the small α grain region. Hence it is concluded that the subsurface crack was initiated in the small α grain region.

Low absorbed energy ductile dimple fracture in lower shelf region in an ultrafine grained ferrite/cementite steel

The curve of absorbed energyvs test temperature for an ultrafine grained ferrite/cementite steel showed a transition from an upper shelf energy to a lower shelf energy,i.e., a transition from an energy-absorbent ductile mode to an energy-absorbent brittle mode. Some dense and small-sized dimples were observed in the lower shelf region. A significant and consistent difference existed between ductile-to-brittle transition temperature and impact energy transition temperature, that is, low absorbed energy but undergoing a ductile dimple fracture at a certain low temperature.

Progress of Laminated Materials and Clad Steels Production

Laminated materials and clad metals have received much attention in the industrial production due to the superior mechanical properties different from those in any of the constituent materials. Clad metal is a composite metal plate generally made by bonding a metal such as stainless steel plate to another metal such as carbon steel or low alloy steel plate. Clad metal not only has sufficient strength required of structural materials but provides other functions including resistance to heat and corrosion. As a result, the application of clad metals can significantly save precious alloying elements and reduce the cost. Therefore, clad metals have become an increasingly interesting topic in a variety of industrial fields. In this paper, fabrication technique and evaluation on mechanical properties of laminated metals have been briefly overviewed. In addition, the applications of laminated materials including clad metals are reviewed and the prospect of clad metals in the future is also described.

Cyclic deformation, dislocation structure, and internal fatigue crack generation in a Ti-Fe-O alloy at liquid nitrogen temperature

AbstractTo clarify the internal fatigue crack generation in a Ti-Fe-O (near α-type) alloy, microstructures, internal fatigue crack initiation sites, and dislocation structures in samples fractured during high-cycle fatigue tests at liquid nitrogen temperature were studied. The alloy contained two kinds of elongated α-phase microstructures, i.e., recovered α grains and recrystallized α grains. Untested samples contained mobile dislocations in recovered α grains, but in recrystallized α grains, any dislocations were observed. Internal crack initiation sites were formed transgranularly and were related to the recrystallized α grain region, judging from their morphology, size, and chemistry. Dislocations in recovered α grains were rearranged after cyclic loading in either

Transmission electron microscopy study of high cycle fatigue deformation in Ti5Al2.5Sn extra-low interstitial alloy at cryogenic temperatures

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