Hideo Yoshinaga

The high temperature deformation mechanism in titanium carbide single crystals

In order to investigate the high temperature deformation mechanism in titanium carbide, single crystals were grown by the r.f. floating zone technique and were deformed by compression at temperatures from 1280 to 2273 K and at strain rates from 3.9 × 10−5 to 6.3 × 10−3s−1. The crystals exhibited a marked work-softening phenomenon which is characteristic of covalently bonded crystals such as germanium and silicon, and the phenomenon became less clear with increasing temperature and with decreasing strain rate. The mechanical equation of state at the yield stress can be expressed by λ> = AτcGmexp−QRT where m = 10 andQ = 240 kJmol− in the low temperature range at and below 1510 K and where m = 5.3 andQ = 470 kJmol−1 in the high temperature range at and above 1840 K. It is considered that the deformation in the low temperature range is controlled by the Peierls mechanism and the deformation in the high temperature range by the diffusion of carbon atoms.

Determination of high-temperature deformation mechanism in crystalline materials by the strain-rate change test

Abstract A new method is proposed to determine whether the effective stress for the viscous or quasi-viscous motion of dislocations exists or not in the deformation of crystalline materials. The crosshead speed is changed on the way of deformation and the work-hardening rate is measured immediately after the change. It is theoretically predicted that the work-hardening rate depends on whether the effective stress is negligible or not. Applying the method to high-temperature deformation of pure aluminum and Al-5.4 at.% Mg alloy, it is found that the effective stress is negligible in pure aluminum at all temperatures tested and in the alloy at temperatures where the dynamic strain aging occurs, while the stress is appreciable in the alloy at temperatures where the solute atmosphere dragging occurs.

Grain boundary structures in silicon carbide: verification of the extended boundary concept

A grain boundary layer of ca. 0.5 nm in thickness is present in B+C added SiC and SiC without any sintering aids. Since these materials do not show a significant strength-decrease at high temperatures, Ikuhara et al. presumed that the layer is not a second phase of sintering aids or impurities but a reconstructed structure formed to reduce the high energy of the grain boundary, and they called such a boundary an extended boundary. The concept of the extended boundary, however, has not yet been generally accepted for lack of convincing evidence. In the present work, the elements analysis of the boundary layer was made and some additional collateral verifications were conducted in order to inspect the extended boundary concept.

Deterioration behavior of a multi-phase vanadium-based solid solution alloy electrode

Abstract Deterioration behavior of an electrode made of V 4 TiNi 0.65 Co 0.05 Nb 0.047 Cr 0.058 was studied by means of impedance spectroscopy, scanning electron microscopy (SEM) and impedance spectroscopy. The reaction resistance related to the lowest frequency semi-circle increased considerably and dischargeability became worse with cycling. The double layer capacitance for the same semi-circle became smaller after 50 cycles. The SEM observation of the cross-section of the cycled electrodes indicated that voids were formed around alloy particles embedded in a matrix of Cu powder, and crack formation and dissolution of the TiNi second phase proceeded with cycling. These phenomena indicate that dissolution of the second phase caused loss of reaction sites and TiNi networks as a current collector.

High-temperature deformation mechanism in substoichiometric titanium carbide - correlation with carbon vacancy ordering

In order to clarify the reason for the marked nonstoichiometry-effect on the mechanical properties of TiCx, the order structure has been investigated in a wide range of CTi atom ratios, x, from 0.59 to 0.95 by the transmission electron microscopy. It is found that the as-grown crystals are weakly or strongly short-range ordered (SRO) depending on x and have a long-range ordered structure (LRO) at x = 0.59 at room temperature. As temperature rises, the degree of order tends to decrease, and around 1300 K, where a marked x-effect on mechanical properties was observed, the diffuse electron scattering due to SRO becomes very weak for x > 0.85, but still strong for x = 0.75, and there exist micro domains of LRO for x = 0.59. From the order structure and the additional experiment of aging effect on the initial deformation behaviour, it is concluded that the x-dependence of deformation behaviour can be understood as an ordering effect of carbon vacancies.

Grain boundary segregation in 〈110〉 symmetrical tilt bicrystals of an Fe-3%Si alloy

Grain oriented silicon steel is characterized by the presence of a sharp {l_brace}110{r_brace} texture, which is produced by secondary recrystallization. It has been pointed out that, in an Fe-3%Si alloy in which there were precipitates, the evolution of {l_brace}110{r_brace} texture is associated with the special grain boundary migration characteristics of {Sigma}9 type coincidence boundaries. It has also been reported that, in Fe-3%Si alloys in which there were precipitates, evolutions of {l_brace}110{r_brace} , {l_brace}110{r_brace} , {l_brace}100{r_brace} textures are associated with the special grain boundary migration characteristics of {Sigma}9, {Sigma}5 and {Sigma}7 type coincidence boundaries, respectively. On the other hand, auger electron spectroscopy has been used to investigate grain boundary segregation in symmetrical tilt bicrystals of Fe-Si alloys containing phosphorus and carbon. The results showed that phosphorus and carbon segregated at grain boundaries. Segregation of silicon was not observed, probably because the segregation of phosphorus and carbon dominated at grain boundaries. In order to make the basic background of the selective grain boundary migration clear, the present authors have investigated the grain boundary segregation in Fe-3%Si alloys with symmetric tilt-boundaries, using Auger electron spectroscopy. In this study, {Sigma}9 coincidence boundaries and general boundaries were prepared for the investigation.

Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

Abstract The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the Mo M 4,5 NN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the Mo N 2,3 VV Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molydenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.

Dislocation Motion in the Field of a Random Distribution of Point Obstacles: Solution Hardening

Deformation rates of crystals, according to many experimental and theoretical studies, are controlled by thermal activation processes for surmounting various barriers. So far, the problems have been discussed using the following implicit solutions: (1) the processes are quasi-static and do not involve any dynamical aspects; (2) the frictional forces produced in materials are neither extremely large nor small. If the frictional forces due to the material are very small, the hypothesis (1) does not hold any more. This is the case of deformation of superconductors as described in the next chapter. Weertman [3.1] dissussed creep of ice on the basis of visco-elastic deformation theory developed by Eshelby [3.2]. According to ultrasonic attenuation experiments [3.3], B is 107 times as large as that for metals. In this case, although the frictional force is still proportional to the velocity of dislocations, the time of motion in the area between barriers becomes longer than that needed to surmount each of them. In other words, the barriers are of no importance for the discussion of the rate of deformation. Accordingly, such a linear visco-elastic deformation may be put aside in the present discussion. As for studies of plastic deformation of crystalline materials, the past treatments of the case, where the frictional forces are very small, should be carefully reconsidered. In the present chapter, we discuss problems within the above framework, (1) and (2), and those outside of this will be discussed in the next chapter.

Dislocations in bcc Metals and Their Motion

Copper (and its alloys) and iron or steel form two groups of metals that have been utilized since the ancient times. More recently, aluminium and its alloys have found many applications, thus forming three important metallic groups. When we compare the low-temperature flow stresses of these three kinds of metals, we find that that of Fe is much higher than the other two: at liquid nitrogen temperature usual steels can no longer be plastically deformed without fracture. From the fact that Fe has body-centered cubic (bcc) structure while Cu and Al have face-centered cubic (fcc) structure, such a difference in low-temperature plasticity is considered to be due to the difference in crystal structure. In fact, while all pure fcc metals exhibit weak temperature dependence of the yield stress, metals with bcc structure show, without exception, a strong temperature dependence. In this chapter, we describe characteristic plastic behavior of bcc metals and its interpretation.

Dislocation Motion Controlled by the Peierls Mechanism

Reflecting the periodicity of the crystal lattice, any dislocation in any crystal has its own intrinsic resistance to glide, because the self-energy of the dislocation changes with the periodicity of the lattice. This periodic potential energy with respect to the dislocation position is called the Peierls potential, after the researcher who first estimated this potential theoretically for a simple model [5.1]. The stress necessary for the dislocation to surmount this potential, without the aid of thermal energy, is called the Peierls stress. This chapter is concerned with theoretical treatments of the dislocation glide controlled by the Peierls potential.