Y. Matsukawa

Mechanisms of stacking fault tetrahedra destruction by gliding dislocations in quenched gold

The destruction processes of stacking fault tetrahedra (SFTs) induced by gliding dislocations were examined by transmission electron microscopy (TEM) in situ straining experiments for SFTs with edge lengths ranging from 10 to 50 nm. At least four distinct SFT destruction processes were identified: (1) consistent with a Kimura–Maddin model for both screw and 60° dislocations, (2) stress-induced SFT collapse into a triangular Frank loop, (3) partial annihilation leaving an apex portion and (4) complete annihilation. Process (4) was observed at room temperature only for small SFTs (∼10 nm); however, this process was also frequently observed for larger SFTs (∼30 nm) at higher temperature (∼853 K). When this process was induced, the dislocation always cross-slipped, indicating only screw dislocations can induce this process.

Stress-induced amorphization at moving crack tips in NiTi

In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glas...

On the features of dislocation–obstacle interaction in thin films: large-scale atomistic simulation

Large-scale atomistic modelling has demonstrated that the dynamic interactions of dislocations in thin films have a number of remarkable features. A particular example is the interaction between a screw dislocation and a stacking fault tetrahedron (SFT) in Cu, which can be directly compared with in situ observations of quenched or irradiated fcc metals. If the specimen is thin, the dislocation velocity is slow, and the temperature is high enough, a segment of the original SFT can be transported towards the surface via a double cross-slip mechanism and fast glide of an edge dislocation segment formed during the interaction. The mechanisms observed in the simulations provide an explanation for the results of in situ straining experiments and the differences between bulk and thin film experiments.

Defect Structures Introduced in FCC Metals by High-speed Deformation

Variation in defect microstructures introduced by compression of three fcc metals, Al, Cu and Ni, was investigated over a wide range of strain rate, from 10 m 2 to 10 6 /s. Dislocations formed under high-speed deformation are randomly distributed, whereas dislocations formed under low-speed deformation develop into cell structures, the transition between the two being at a strain rate of 10 3 . Dislocations of opposite signs are equally mixed in high-speed deformation, whereas grouped dislocations in low-speed deformation are composed of unbalanced numbers of dislocations of opposite signs. In high-speed deformation vacancy clusters are formed at high density all over the matrix, whereas in low-speed deformation only a few numbers of vacancy clusters are formed in the area of localized distribution of dislocations, the boundary between these two characteristics being at the transition of the nature of dislocation distribution. In the high-speed deformation vacancy clusters are formed by the aggregation of deformation-induced vacancies, whereas in low-speed deformation they are produced directly by dislocation reaction during deformation. Stress during high-speed compression has been estimated to increase to more than 10 GPa. A model of plastic deformation that produces vacancies at high concentration is proposed, in which high-speed plastic deformation proceeds without involving dislocations.

Electron irradiation effect on phase transformation in TiNi shape memory alloy

Abstract The sensitivity of phase transformation in TiNi shape memory alloy to electron irradiation has been investigated by means of HVEM at room temperature. In the present study, the phase transformation of this alloy finally went to the amorphous phase. The amorphization process can be categorized according to the following three types. (1) The B2 phase transformed to the amorphous phase successively with the increase of dose. (2) The martensitic phase transformed to the B2 phase at the beginning of irradiation. The transformation to the amorphous phase progressed successively. (3) The martensitic phase transformed to the amorphous phase directly. The amorphization progressed irregularly. The critical dose for the amorphization in all three processes has been investigated. The critical doses for the amorphization were quite sensitive to the micro-chemical composition; the dose of the stoichiometric composition was relatively high.

Effects of alloying elements (Sn, Nb, Cr, and Mo) on the microstructure and mechanical properties of zirconium alloys

The alloying effects of Sn, Nb, Cr, and Mo on zirconium alloys were elucidated and compared. Electron backscatter diffraction, transmission electron microscopy, tensile test, and fractographic observation were jointly utilized to carry out detailed microstructural characterization and mechanical property evaluation. Results show that Mo is the most effective among these elements from the viewpoints of strengthening and reducing grain size. The strengthening mechanism for each element is also discussed. The order of solid-solution strengthening of these alloying elements is Cr > Nb > Sn, and the sequence is Cr ≈ Mo > Nb when precipitation strengthening is considered. Further, as far as the ability to impede dislocation motion is concerned, the sequence is Mo > Cr > Nb > Sn. The experimental results demonstrate that minor amount of Mo addition in zirconium alloys is greatly effective in strengthening the alloy and reducing the grain size.

Dynamic observation of dislocation-free plastic deformation in gold thin foils

Ductile fracture of metals produces a thin foil portion, which is observable by transmission electron microscopy, at the fractured edge. The thin foil portion shows unusual deformation microstructure, which contains no dislocations, but contains vacancy-type point defect clusters at extraordinarily high density. Dynamic observation of the deformation process revealed that these defect clusters are produced in the portion of local heavy deformation; however, no dislocation motion was observed during the course of the heavy plastic deformation, constituting direct evidence that the unusual deformation microstructure is produced by plastic deformation without dislocations. Also, the deformation was found to involve 14% elastic deformation, indicating that the dislocation-free plastic deformation occurs under an extraordinarily high internal stress level of more than 10 GPa, which is comparable to the ideal strength of metals. Furthermore, during the dislocation-free plastic deformation, equal-thickness fringes were found to disappear temporarily, suggesting that instability of crystalline state under extraordinarily high internal stress level is a key factor for the mechanism of dislocation-free plastic deformation.

Defect structure of gold introduced by high-speed deformation

Abstract The effect of deformation speed on defect structures introduced into bulk gold specimens at 298 K has been investigated systematically over a wide range of strain rate from e ′=10 −2 to 10 6 s −1 . As strain rate increased, dislocation structure changed from heterogeneous distribution, so-called cell structure, to random distribution. Also, stacking fault tetrahedra (SFTs) were produced at anomalously high density by deformation at high strain rate. The anomalous production of SFTs observed at high strain rate is consistent with the characteristic microstructure induced by dislocation-free plastic deformation, which has been recently reported in deformation of gold thin foils. Thus, the results of the present study indicate that high-speed deformation induces an abnormal mechanism of plastic deformation, which falls beyond the scope of dislocation theory. Numerical analysis of dislocation structure and SFTs revealed that the transition point of variation of deformation mode is around the strain rate of 10 3 s −1 .

Dynamic observation of dislocation-free deformation process in Al, Cu, and Ni thin foils

Abstract Dislocation-free plastic deformation, which occurs under extraordinarily high internal stress comparable to ideal strength of metals, was discovered in thin foil portion produced by ductile fracture of fcc Au by dynamic observation of the deformation process [1] , [2] , [3] , [4] , [5] . In the present study, the deformation process of thin foil portion in other fcc metals (Al, Cu, Ni) was examined in the same manner. In all these fcc metals, production of vacancy-type point defect clusters was confirmed during deformation without dislocations. Also, the dislocation-free deformation was found to progress under extraordinarily high internal stress levels corresponding to 14% elastic deformation in Ni, 12% in Cu, and 4% in Al. Especially in Al, as temperature decreased, the number density of stacking fault tetrahedra produced during deformation increased, along with increasing of the detected elastic deformation. These results indicate that internal stress level is a key factor in generalizing the new theory regarding dislocation-free plastic deformation.

Development of advanced expansion due to compression (A-EDC) test method for safety evaluation of degraded nuclear fuel cladding materials

Expansion due to compression (EDC) test has been applied to evaluate the performance of nuclear fuel claddings where pellet-cladding mechanical interaction (PCMI) is introduced by swelling of fuel pellets and is triggered by the larger hoop deformation of the pellets, especially during accidental transients. The purpose of this study is to modify the EDC test to describe PCMI, specimen volume reduction and others. Ring-shaped specimens were cut from Zry-4 cladding tubes. Cylindrical metal pellets with 8 mm in diameter and 15 mm in maximum height were used as inner pellets. Expansion of the specimens due to the inner pellet compression was performed at room temperature. The experimental data were further analyzed by finite element method. Through the survey in the variation of the specimen and core, specimen size and inner pellet geometry were optimized. Excellent reproducibility with less error was confirmed. The uniaxial tension condition in the hoop direction up to the specimen failure was confirmed. Hoop stress–hoop strain curves were successfully derived.