Kazuo Kitagawa

Structure and properties of ultra-fine grain Cu–Cr–Zr alloy produced by equal-channel angular pressing

Abstract The structure, thermal stability and properties are investigated of a Cu–Cr–Zr alloy with ultra fine grains (UFG) of 160 nm diameter produced by severe plastic deformation through equal-channel angular pressing (ECAP). Special attention is paid to optimization of multi-functional thermal, electrical and mechanical properties of this alloy by aging after ECAP. Fatigue life and cyclic response under strain-controlled experiments are investigated aiming at clarification of mechanisms of plastic deformation and fracture in the precipitation hardened ECAP materials. It is shown that the precipitation strengthened UFG structure remains stable both under elevated temperatures as high as 500°C and under cyclic loading at room temperature. Substantial improvement of fatigue life is evidenced in comparison with conventional coarse-grain materials. The appearance of cyclic softening is noticed and its nature is discussed in terms of dislocation–particle interaction and possible dissolution of precipitates during fatigue.

Serrated plastic flow during nanoindentation of a bulk metallic glass

The results of nanoindentation tests of bulk glassy Pd40Cu30Ni10P20 using a specially designed instrument with high time and spatial resolution are presented. Pronounced serrations of the indenter penetration depth are observed. The parameters of serrated flow (the number of serrations, their amplitude and duration) are dependent on the duration of the loading force pulse.

Fatigue properties of 5056 Al-Mg alloy produced by equal-channel angular pressing

Abstract The fatigue behaviour of the fine-grain 5056 Al-alloy processed by equal-channel angular pressing (ECAP) is explored. This material exhibits a slightly enhanced fatigue life at low stress amplitudes. However, no improvement in the fatigue limit is observed. Fatigue performance is discussed in terms of fatigue life, crack nucleation and propagation. Structural changes during fatigue are investigated by transmission electron microscopy. It is shown that the fine structure achieved during processing is unstable and tends to relax with cycling, resulting in local recovery of the pre-deformed material. Structure relaxation during fatigue is supposed to provoke notable cyclic softening which is particularly pronounced at higher applied stresses. It is found that the crack growth rate is greater in the fine-grain ECAP material than in its coarse-grain counterpart. The latter is attributed to the roughness-induced crack closure and crack deflections which is more significant in conventional alloy. The improvement of fatigue properties at low-cyclic regime is believed to be due to a higher resistance to crack nucleation in the fine-grained material having a larger yield stress value.

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

The fatigue performance of the fine-grain 5056 Al-Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is assessed in both stress- and plastic strain-controlled experiments. Compared to its conventional counterpart, the ECAP material exhibits a high tensile and low-cyclic fatigue strength under constant stress amplitude. However, its fatigue life under strain-controlled conditions is notably shorter than that of the O-temper specimens. Despite severe pre-straining of the specimen during ECA-pressing, cyclic softening was found to be rather small. It is shown that the mechanical characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperatures (150°C, 15 min) after fabrication. Such heat treatment is supposed to recover partially the grain boundary region, which has been most heavily distorted during processing. Mechanisms of fatigue in ECA-processed materials are discussed within a framework of a simple one-parametric model of dislocation kinetics.

Cyclic response of ultrafine-grained copper at constant plastic strain amplitude

The investigation in the fatigue behaviour of ultrafine-grained copper produced by severe plastic deformation was performed in fully reversed tension/compression under constant plastic strain amplitudes. It is shown that UFG copper possesses unusual fatigue properties which are determined by both the small grain size and the specific non-equilibrium state of GBs. The experimintal results can be summarised as follows: - UFG materials show a clearly pronounced saturation stage; - no cyclic softening is observed during cyclic deformation; - in comparison with conventional polycrystals, UFG matarials reveal a high saturation stress and significant Bauschinger effect; - fatigue properties depend on internal structure of UFG materials which is strongly influenced by a thermal treatment. The saturation stress decreases and the Bauschinger energy parameter increases after short (3 min) annealing at 473 K as a result of recovery of non-equilibrium GBs.

Atomic force microscopic study on surface morphology of ultra-fine grained materials after tensile testing

The surface morphology of tensile tested ultra-fine grained copper and nickel produced by severe plastic deformation is investigated on different scale with a help of atomic force microscopy. A key role of grain boundaries in surface patterning and plastic flow is emphasized. The presence of two mechanisms contributing to a total strain was found (1) microscopic dislocation slip limited to the grain interior and (2) grain boundary shearing (sliding) of the mesoscopic scale in the plane of maximum shear stresses. This grain boundary sliding which appears at room temperature, results in a macroscopically observed characteristic shear band relief that is often observed on the surface of deformed ultra-fine grain metals.

Fatigue life of fine-grain Al /Mg /Sc alloys produced by equal-channel angular pressing

The effect of severe plastic deformation through equal-channel angular pressing (ECAP) on the fatigue life of Al � /Mg � /Sc alloys with different concentration of magnesium is discussed. Both high-cyclic and low-cyclic fatigue properties have been assessed in terms of the fatigue strength, fatigue ductility, cyclic stress � /strain curves, etc. It is shown that the addition of Sc significantly increases the thermal stability of the fine-grain structure formed during ECAP. However, only modest improvement of tensile and cyclic mechanical properties have been achieved via ECAP of Al � /Mg � /Sc alloys in comparison to their ordinarily fabricated counterparts. The results of present investigation show that the grain refinement of precipitation hardenable Al-alloys through severe plastic deformation can not be effective for enhancement of their static or cyclic strength. It is argued that early shear banding play an important role in cyclic degradation of ECAP Al � /Mg � /Sc alloys. # 2002 Elsevier Science B.V. All rights reserved.

NANOMETER GRAIN BOUNDARY SLIDING IN CU : 011 SYMMETRIC TILT BOUNDARIES, MISORIENTATION DEPENDENCE AND ANISOTROPY

Abstract Sliding on [011] symmetric tilt boundaries in Cu has been studied between 323 and 584 K, using an electron microscope technique and bicrystals of an aged Cu-1.05%Fe-0.45%Co alloy. The b.c.c. FeCo particles formed by aging on boundaries suppress the sliding so that it amounts to less than 1 nm. The sliding has been detected by observing a change in the direction of Moire fringes formed by the matrix {111} and the particle {110}. Finely dispersed f.c.c. FeCo particles in grains abutting boundaries completely prevent crystal dislocations playing any role in boundary sliding. The sliding increases with increasing temperature and saturates to a value determined by the size and distribution of the boundary particles and the applied shear stress. A boundary with a higher energy can slide at a lower temperature, as low as 350 K. A curve showing the ease of sliding against the misorientation angle is similar, with cusps, to the energy vs misorientation angle curve. The sliding parallel to the tilting axis, [011], occurs at lower temperatures than that in the direction normal to the tilting axis.

Acoustic emission during cyclic deformation of ultrafine-grain copper processed by severe plastic deformation

Abstract Acoustic emission (AE) is investigated during cycling at a constant plastic strain amplitude of ultrafine-grain (UFG) copper produced by severe plastic deformation, aiming at detailed characterization of AE in the time and frequency domain and clarification of fatigue damage mechanisms. The results of AE analysis in UFG copper are compared with those obtained in conventional polycrystals and single crystals. It is shown that fatigue damage and corresponding AE in UFG copper is controlled by three main mechanisms: large-scale shear banding, mode I tensile crack opening and mode II shear crack propagation. No AE that could be associated with ordinary dislocation mechanisms typical for conventional coarse-grain metals was detected in the UFG state. A close connection between strain localization, crack nucleation and grain boundaries in UFG materials is emphasized. It is shown that annealing at a moderate temperature of UFG materials fabricated by severe plastic deformation can reduce the degree of strain localization, modify the fatigue mechanisms and improve the fatigue life significantly.

Cyclic response of fine grain 5056 Al–Mg alloy processed by equal-channel angular pressing

Abstract The cyclic response of the fine-grain 5056 Al–Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is investigated in plastic strain controlled experiments with a plastic strain amplitudes e pl ranged from 5×10 −4 to 10 −2 . This material exhibits a fairly high yield stress of 400 MPa, however, its strain controlled properties appear worse than those of the O-temper specimens as is evidenced by the Coffin–Manson plot. The cyclic softening was found to be small with a visible tendency to increase with e pl . It is emphasized that the mechanical properties of the ECAP materials are strongly influenced by their susceptibility to strain localisation. It is shown that the engineering characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperature (150°C, 15 min) after fabrication. This annealing is supposed to recover the most heavily distorted grain boundary region, facilitating the dislocation mobility to carry out the imposed plastic strains.