Ryuta Onodera

Dislocation emission from a crack tip in MgO thin crystals

Abstract Dislocation emission from the tip of a crack in MgO thin crystals has been studied based on the geometry of crack tip dislocations observed using high voltage electron microscopy (HVEM). The effect of crack tip shielding due to the dislocations was also calculated using three-dimensional (3D) Bueckner–Rice weight function theory for the crack–dislocation interaction. Dislocation image analyses showed that dislocations ahead of the crack tip observed in the present study were almost right-handed (R-H) screw dislocations lying on the (0 1 1) plane, while in the crack wake many of the dislocations were left-handed (L-H) on the (0 1 1) plane. Formation of such dislocation configuration can be understood by the emission of dislocation loops from sources at a crack tip, where crack jogs, crack kinks and intersections of crack planes with free surfaces may act as significant sources for dislocation emission. 3D stress analysis exhibits that the largest component of crack tip stress intensities induced is mode I shielding type, and that mode II component is not negligible in the present case, suggesting the induction of the crack tip shielding for the mixed stress modes of I and II through the dislocation emission from the sources indicated above.

Effect of heat treatments on the portevin-le chatelier effects in aluminium-silver alloys

Abstract The effect of solute content and heat treatment on occurrence of the P-L effect were investigated in AlAg alloys. The activation energies estimated from values of the critical strains were nearly equal to that for solute migration in AlMg alloys, which means that the P-L effects in AlAg alloys were caused by the same mechanism as in AlMg alloys. The P-L effects in Al-0.5 and 1.0 at.%Ag alloys occurred in the furnace cooled specimens only, on the other hand in 2.5 and 4.0% alloys it occurred in the quenched specimens only. In order to explain these results it seems better to adopt the cluster theory rather than the Cottrell theory.

The relation between the Portevin-Le chatelier effect and the solid solubility in some binary alloys☆

Abstract Effects of composition and temperature on the occurrence of the Portevin-Le Chatelier effect were investigated in the literature and the experiments in some alloys. These investigations showed that the alloy systems were divided into two groups according to the minimum solute concentration below which the P-L effect did not occur. The first group consisted of alloy systems in which the P-L effects occurred at the solute content less than 1 at.%, and their common feature was that they had small solubilities of solute. In the second group the P-L effects did not occur at the solute content less than 5 at.% and the solute solubilities were large. Based on these results a new theory for the P-L effect that the precipitates or the short range ordered regions interacted with the dislocations, was proposed and compared with the Cottrell theory.

HVEM observation of crack tip dislocations in silicon crystals

Abstract Dislocation configurations near the tip of a crack in Si crystals has been investigated by using a high voltage electron microscope (HVEM). A ( 1 10 ) crack was introduced into a (001) silicon wafer by using indentation method at room temperature, and the specimen was annealed at 823 K to emit dislocations from the tip of the crack under the presence of residual stress due to the indentation. An array of dislocations was seen not only in front of the crack tip but also in the crack wake. The dislocations were emitted on the ( 1 1 1 ) plane which is oblique to the ( 1 10 ) crack. The effect of crack tip shielding due to the dislocations is analyzed to be mainly mode I. Dense dislocation region is also found near the tip, suggesting the occurrence of dislocation multiplication around the crack tip.

HVEM observations of dislocation structures near a crack tip in MgO crystals

Abstract Dislocation structures near the tip of a crack in MgO thin plate crystals have been investigated using a high voltage electron microscope (HVEM). In (001) thin plate crystals, a (010) crack is most commonly observed and an array of dislocations is seen along the [100] direction in front of the crack tip. The dislocations in the array are those of (011[ 0 1 ¯ 1 ] or ( 0 1 ¯ 1 )[011] slip systems and their Burgers vectors have large screw components. Dislocation image analyses have been applied to determine the signs of their Burgers vectors, and the screw dislocations on the (011) plane are found to be left handed and those on the ( 0 1 ¯ 1 ) plane are right handed. The effect of crack tip dislocations on fracture behavior is qualitatively examined and clarified to be a mode I shielding type so as to accommodate the tensile stress concentration near the crack tip.

Crack tip dislocations in silicon characterized by high-voltage electron microscopy

Abstract The nature of crack tip dislocations and their multiplication processes in silicon crystals have been examined by using high-voltage electron microscopy. Cracks were introduced by the Vickers indentation method at room temperature, and the specimen indented was annealed at high temperatures to induce dislocation generation around the crack tip under the presence of residual stress due to the indentation. In the specimens annealed, fine slip bands with the step heights of around 1 nm were formed along the {111} slip planes near the crack tip. The crack tip dislocations observed were characterized by matching their images to those simulated, and it was found that two different slip systems were activated even in the early stage of dislocation emission. With the increase in the number of crack tip dislocations, more complicated dislocation configurations such as dislocation tangles were formed around the crack tip, showing the beginning of multiplication of crack tip dislocations which causes effective crack tip shielding.

The portevin-le chatelier effects in Cu-Ti, Cu-P and Cu-Si alloys

Abstract The presence or the absence of the Portevin-Le Chatelier effects in Cu-P, Cu-Ti and Cu-Si alloys were investigated as functions of solute content, temperature and heat treatment. The serrated flows were observed in all the systems but in the Si bearing alloys it appeared at higher solute content and with less height of serration than in the other two. The effect of heat treatment was observed only in Cu-Ti alloys; the P-L effects in 0.5 and 0.75% Ti alloys occurred at lower temperature for annealed specimen than for quenched specimen and in alloys with higher solute content the reverse was true. In Cu-Ti and Cu-Si alloys the minimum temperature above which the P-L effect occurred depended strongly upon solute content. The P-L effect in Cu-Ti alloys occurred at considerably higher temperature than in other copper base alloys. It was concluded that these results were explained more rationally by the cluster theory than the Cottrell theory.

A phenomenological theory for the Portevin-Le chatelier effect

Abstract Computer calculations were done to explain phenomenologically the Portevin-Le Chatelier effect. A model used for a simulation of tensile test was that a deformed zone connected in series with a spring which have a composed stiffness of the machine and the specimen, was pulled at a constant speed. The constitutive equation of the deformed zone was formulated based on the consideration that (1) the flow stress was a sum of effective stress and two kinds of internal stress, in which the one was dependent on strain and the other on strain rate, and (2) the stress mentioned last had a negative rate sensitivity and responded to a strain rate change not simultaneously but with a delay time. The calculated results agreed quantitatively with experimental results in regard to the effects of machine stiffness, work hardening rate and crosshead speed upon stress amplitude, Luders strain and strain rate.

Crack tip dislocations and their shielding effect in MgO thin crystals

Crack tip shielding by dislocations has been studied using the method of 3-dimensional stress analyses in connection with dislocation structures observed by HVEM in MgO thin crystals. A screw dislocation array corresponding to the plastic zone of 45°-shear-type is commonly observed along the [100] direction ahead of the (010) crack in (001) thin plate crystals. The 3-dimensional stress analyses show that the main component of local stress intensity factor induced by the crack tip dislocations corresponds to mode I while mode II and III components are negligible in spite that they are screw dislocations. The mode I component induced is negative, which implies that the dislocations observed are of shielding type so as to accommodate the tensile stress concentration near the crack tip. The value of fracture toughness of MgO crystals is also discussed based on the stress analyses.

HVEM Study of Crack Tip Dislocations in Silicon Crystals

The present paper describes the nature of crack tip plasticity in silicon crystals examined by high voltage electron microscopy (HVEM) and atomic force microscopy (AFM). Firstly, AFM images around a crack tip are presented, where the formation of fine slip bands with the step heights of one or two nanometers is demonstrated. Secondly, crack-tip dislocations observed by HVEM are exhibited, where it is emphasized that dislocation characterization is essential to consider the relief mechanism of crack-tip stress concentration.