Nobutaka Narita

The formation of deformation bands in f.c.c. crystals

Abstract The formation process of deformation bands, i.e. kink bands perpendicular to the primary slip direction and bands of secondary slip approximately parallel to the primary slip plane, and their stability in terms of dislocation configurations with low self-energy, has been studied on copper and Cu-1at.%Ge alloy crystals. These were deformed in tension and exhibited a long well-defined stage I. Particular emphasis is laid on the causal relation between the rapid activation of secondary slip and the transmutation of “local bend-gliding regions” and “coplanar slip zones” into kink bands and bands of secondary slip respectively.

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-impurity interactions and their role in the embrittlement of Fe alloy crystals charged with light elements

Abstract The effect of mobile impurity doping on fracture behaviour has been investigated using plasma charging of light elements for Fe-2.5 wt.%Si alloy crystals with particular attention to the role of crack-impurity elastic interactions. FeSi crystals are markedly embrittled by plasma charging of helium as well as hydrogen at around room temperature, this being accompanied by slow crack growth. Neon charging contributes little to the embrittlement, but argon charging does not contribute. The crystals are also embrittled by nitrogen charging in the tests at 450 K and exhibit slow crack growth during the tests. Elastic analyses indicate that crack-impurity interactions are induced not only by the applied KI field but also by the stress modification due to ambient impurities in the presence of a crack. The interactions serve effectively to concentrate mobile impurities ahead of a crack tip, leading to the increase in the local stress intensity kI. The effect of interstitial impurities on crack extension is discussed in connection with the modification of stress states due to impurities around a crack tip.

Structural and Resistivity Changes in YBa2Cu3Oy Ceramics by Heat-Treatment in Air

Effect of heat-tretment in YBa2Cu3Oy ceramics was investigated using the methods of electrical resistivity, TG, DTA and X-ray diffraction. In the heating process, a mass increase and a resistivity decrease are observed in the sample in the temperature range 630-780 K. Heat-treatment in air at temperatures above 780 K causes the marked increase of resistivity as well as the decrease of oxygen content. The YBa2Cu3Oy compound is decomposed gradually above 1200 K and completely at around 1290 K. The degradation and recovery of structural and transport properties by heat-treatment in air are also reported.

Effect of dislocation activities on crack extension in ionic crystals

Abstract The effect of dislocation activities on crack extension has been investigated using NaCl, KCl and NaClBr mixed crystals. In NaCl and KCl crystals {110} 1 0 > slip systems operate actively around a crack tip at room temperature, while the threshold stress intensity KIC for crack extension is very small, less than 0.1 MPa m 1 2 . When test temperature are lowered to 77 K, the dislocation activities are reduced and the KIC values are also increased at room temperature when the crystals are hardened by the partial substitution of Br for Cl in order to reduce dislocation activities. When temperatures are raised above 400 K, {001} 1 0 > slip operation in addition to the abundant operation of {110} 1 0 > slips place around a crack tip and the KIC values are markedly increased. Such peculiar effects of dislocation activities on KIC values are discussed in connection with the activities of shielding and antishielding dislocations estimated from computer simulations as well as experimental observations.

High Temperature Deformation and Textures in Oxide Superconductors

High temperature deformation and its role on texture formation were investigated using YBa2Cu3Oy(YBCO) and Bi0.7Pb0.3SrCaCu1.8Oy(BPSCCO) ceramics. The YBCO and BPSCCO ceramics are fairly ductile to deform over 50% in compressive strain at temperatures above 840°C and 720°C, respectively. In compression tests of YBCO and BPSCCO, their yield stresses are markedly decreased with increasing temperature and also with decreasing strain rate. After the tests, preferred orientations in the loading direction were examined by means of pole figures and Debye-Sherrer patterns. The results showed that the intensity of (00n) reflections was increased by high temperature deformation in both YBCO and BPSCCO specimens. This indicates that the YBCO and BPSCCO can deform by slip at high temperatures so as to develop deformation textures.

Heat Treatments After High Temperature Deformation in BPSCCO Sintered Specimens

Annealing behavior under various O2 partial pressures before and after high temperature deformation was investigated using Bi0.7Pb0.3SrCaCu1.8Oy (BPSCCO) superconductors with particular attention to the effect of oxygen absorption on transport properties. Mass increase due to oxygen absorption was observed during annealing and was much affected by the O2 partial pressure and the annealing temperature. The amount of absorbed oxygen increases as the Po2 is increased and also as the annealing temperature is lowered. The oxygen absorption promotes the generation of additional phases such as Ca2PbO4, leading to the degradation of transport properties. However, a little increase in oxygen content serves to improve transport properties, probably due to the increase of electron hole concentration through the reaction of 2Vo + O2 = 20 x 0 + 4h. Those opposite tendencies in the effect on transport properties indicate that an optimum condition of oxygen absorption is present for the improvement. From this point of view, some attempts are made to seek a suitable condition of annealing after high temperature deformation.

Transition from Stage I to II in the Work-hardening of FCC Crystals

The mechanism of transition from stage I to II in the work-hardening of fee crystals has been studied by using copper and its alloy crystals deformed in tension. Main points are: (1) Stage I terminates with the formation of kink bands, which were transmuted from local ‘bend gliding’ regions abundantly developed in a specimen, owing to the inhomogeneous operation of primary main slip, under the grip constraint of uniaxial deformation. (2) Once the walls of kink bands are built up, dislocations are blocked from further movement, being rapidly accumulated so that secondary slip systems are activated. Thus the swift transition from stage I to II occurs. (3) The work hardening of stage I has also been discussed in terms of excess dislocations constituting local bend gliding.

Heat Treatments After High Temperature Deformation in Oxide Superconductors

Annealing behavior in O2 flow after high temperature deformation was investigated using YBa2Cu3Oy(YBCO) and Bi0.7Pb0.3SrCaCu1.8Oy (BPSCCO) ceramics. High temperature deformation causes the degradation of transport properties. In deformed YBCO samples, when they are directly subjected to annealing at 900°C, a small amount of CuO takes place together with oxygen release. Generation of such additional phase can arise from structural instability due to the damage introduced by deformation. To stabilize the YBCO phase, deformed specimens were intermediately annealed at around 500°C for 3 hours on a way of heating up to 900°C. This intermediate annealing served not only the prevention of mass decrease due to high temperature annealing but also the suppression of structural degradation. Annealing behavior of BPSCCO samples is also reported.