Seiichi Karashima

Sliding behaviour and dislocation structures in aluminium grain boundaries

Abstract Dislocation structures of grain boundaries whose sliding behaviour had already been studied, have been observed by transmission electron microscopy of the slid grain boundaries. The ordered (coincidence) and random boundaries exhibited quite different sliding behaviour and dislocation structures. In the case of ordered boundaries, sliding was difficult and remarkable slide-hardening was shown. Transmission electron microscopy evidenced the presence of a number of EGBDs in these grain boundaries. In contrast, sliding of random boundaries easily took place showing slight slide-hardening. EGBDs were hardly observed in random boundaries. A systematic change in grain boundary misorientation during sliding was observed. An off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations. These results can be qualitatively explained by a dislocation model of sliding in which the absorption of lattice dislocations into the grain boundary is considered t...

Deformation behaviour and dislocation structures upon stress reversal in polycrystalline aluminium

Abstract In order to obtain information about the dislocation mechanism for the Bauschinger effect, the compressive flow behaviour of polycrystalline aluminium pre-strained by tension was investigated at temperatures between room temperature and 450°C. The change in dislocation structures during compression was also examined by transmission electron microscopy. The work-hardening rate was smaller at an early stage of compression than just before the stress reversal. This tendency became more remarkable with increasing temperature; at temperatures above 150°C the plastic deformation proceeded at a constant flow stress at an early stage of compression. The structural observations and some complementary experiments revealed that cell walls and sub-boundaries, which had been developed by pre-straining at low and high temperature respectively, were unstable against the stress reversal. This result implies that the dissolution and re-formation of cell walls and sub-boundaries occur during the reversed straining. Furthermore, it was found that, at both low and high temperatures, the total dislocation density decreased by about 16% at an early stage of the reversed straining. These structural changes are considered to be the origin of the work-hardening behaviour mentioned above. It is also concluded that, in addition to the reversed motion of free isolated dislocations within cells or sub-grains, the dissolution of cell walls or sub-boundaries upon the stress reversal is closely related to the Bauschinger effect at least in metals in which cells or subgrains are formed during pre-straining.

Grain boundary hardening and segregation in alpha Iron-Tin alloy

Abstract Microhardness measurements and Auger electron spectroscopy (AES) have been made to study the relation between grain boundary hardening and segregation in alpha iron-tin alloy. It has been found that the grain boundary hardening depends on the grain boundary misorientation and the amount of tin segregation to grain boundaries also strongly depends on the misorientation, increasing with the tilt angle. The excess grain boundary hardening by addition of tin to alpha iron is concluded to be caused by the grain boundary segregation of tin. Grain boundary segregation diagram has been proposed to quantitatively predict the structure-dependent intergranular segregation.

Misorientation dependence of grain boundary sliding in 〈1010〉 tilt zinc bicrystals

Abstract The misorientation dependence of grain boundary sliding and effects of grain boundary structure and crystal deformation on sliding have been studied by experiments using 〈1010〉 tilt zinc bicrystals with different misorientations. The results have shown that grain boundary sliding strongly depends on the misorientation and is difficult in exact- or slightly off-coincidence boundaries. A cusp found near a tilt angle of 55° is explained by the existence of a 56·6°〈1010〉/〉/Σ9 near-coincidence boundary. The observations are interpreted on a dislocation mechanism for sliding which is based on the movement of crystal lattice dislocations along the grain boundary by a combination of climb and glide.

Creep Mechanism of AI-Mg Alloys at High Temperatures

AbstractDislocation density, ρ, during steady-state creep of A1–5.54 at.-%Mg alloy was measured as a function of applied stress (oc= 14.7–68.6N/mm2) and of test temperature (T=573–673 K). Effect of the concentration of magnesium (C=0.52–5.54 at.-%) on ρ, was also studied at 573 K. From the present results together with the data on mechanical behaviour reported previously, it is deduced that the steady-state creep rate in these alloys under the condition where m = 1 and n ≃ 3 (m being the effective-stressexponent of dislocation velocity, and n the applied-stress exponent of steady-state creep rate) can be calculated by the solute-atmosphere dragging model using values of p and of the effectivestress which were obtained experimentally as a function of σc, T, and C.

Dissociation of lattice dislocations in coincidence boundaries

The possible dislocation reactions in coincidence boundaries with Σ ≦19 in fee and b cc crystals were systematically analysed in consideration of energetics according to the coincidence site lattice theory. The elastic energy reduction during dissociation of lattice dislocations suggests to be more easily absorbed into coincidence boundaries with large Σ-values. Transmission electron microscopy showed a large difference in the absorption rate between coincidence and random boundaries in crept specimens. The absorption of the lattice dislocation depends strongly on its Burgers vector even in the same grain boundary. Experimental results showed that most of EGBDs observed in the coincidence boundaries were of the Burgers vectors which give rise to smaller energy reduction due to the dissociation reaction.

Dependence of phosphorus segregation on grain boundary crystallography in an FeNiCr alloy

Abstract Relation between the amount of P-segregation and grain boundary crystallography in an FeNiCr alloy was studied. A grain boundary etching method was utilized for analyzing P-segregation, and the crystallography of grain boundaries was characterized by making channelling pattern and trace analysis. Macroscopic geometric characteristics of grain boundaries which were found to be effective in suppressing the amount of P-segregation are (1) low-angle disorientation (2) σ3 orientation relation, (3) \ gs3 (quasi-twin) orientation relation (4) boundary orientation close to the orientation of the coherent twin boundary,(5) boundary plane having low indices, and (6) boundary straightness. It was suggested that an essential geometric factor on the atomic scale which controls the amount of P-segregation in high-angle boundaries is their coherency or free volume.

High-temperature creep rate and dislocation structure in a dilute copper-aluminium alloy

Abstract In order to make clear the dislocation mechanism for high-temperature creep deformation, creep rate, g3, activation area, A ∗ , internal stress, σ i and density, ρ and arrangements of dislocations were examined during high-temperature creep (creep condition: 480°C, 3.0 kg/mm 2 ) of copper—0.56 at.% aluminium alloy specimens subjected to various degrees (0 ~ 6%) of prestrain, e p , at room temperature. With an increase in e p from 0% to 6%, e decreased from 7.5 × 10 −7 / sec to 1.4 × 10 −8 / sec , σ and ρ increased from ~2.0 kg / mm 2 to ~2.9 kg / mm 2 and from 5.8 × 10 8 / cm 2 to 1.3 × 10 8 / cm 2 , respectively. However, A ∗ had a tendency to decrease only slightly from ~700 b 2 to ~570 b 2 ( b = the Burgers vector). From these results e p dependence of g3 could be interpreted self-consistently using a general strain-rate equation for high-temperature creep deformation. Experimental results on e p -dependence of σ i and of dislocation structure showed that σ i can be related to x by the equation σ i ≈ 0.2 Gb / x where x is the average dislocation spacing in cell boundaries and G the shear modulus. It was concluded that the internal stress is determined by the resistance which glide dislocations suffer at the time of cutting through dislocation tangles within cell boundaries and that creep deformation is rate-controlled by the glide motion of jogged screw dislocations within the cell boundaries.

Work-hardening rates during the high temperature creep of aluminium determined from the instantaneous strain on sudden stress changes

Abstract The work-hardening rates during the creep of polycrystalline aluminium were determined from instantaneous plastic strains in stress change tests in the temperature range 455–650 K under stresses for which the steady state creep rates lie within the range 5 × 10 −7 − 2 × 10 −5 s −1 . The true work-hardening rates h were estimated by extrapolating the apparent values which were determined from the instantaneous plastic strains occurring during about 0.02 s after the stress increases ranging from 0.1 to 1 MPa. The values of h are of the order of Youngs modulus and depend markedly on temperature and stress. The need not only to improve the existing theories of recovery creep but also to extend the present fine structure observations is suggested.

Effect of testing modes on deformation behavior at stages prior to the steady states at high temperatures in class I alloys

Abstract Types of primary creep curves were compared with types of the initial portion of stress-strain curves in conventional tests of α-iron alloys containing 1.5 and 1.8 at.% molybdenum at 1 074 and 1 124 K. Four types appeared in the creep- and also in the conventional tests. When the strain rates or stresses in the steady state were the same in both kinds of tests, types of the primary stage of creep curves had a certain correspondence with types of stress-strain curves. Conventional tests can be used instead of creep tests, or vice versa , for revealing deformation characteristics in the initial stage of deformation, so long as the steady state strain rates or stresses are the same in these two kinds of tests.