Tetsuji Noda

The role of microstructural instability on creep behavior of a martensitic 9Cr-2W steel

The microstructural instability during creep and its effect on creep behavior were investigated for a martensitic 9Cr-2W steel. The steel was developed as a low radioactive steel suitable for fusion reactor structure. Creep testing was carried out at 873 K for up to 15,100 ks (4200 hours). The creep curve consisted of transition creep, where creep rate decreased with time, and acceleration creep, where creep rate increased with time. During creep, microstructural instability, such as the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains, was observed to occur, which resulted in softening but no hardening. The transition creep was a consequence of the movement and annihilation of excess dislocations, resulting in the decrease in dislocation density and the increase in martensite lath size with time. The acceleration creep was a consequence of a gradual loss of creep strength due to the microstructural instability which occurred from the initial stage of creep.

Relation between microstructure, properties and spark plasma sintering (SPS) parameters of pure ultrafine WC powder

Abstract A combined experimental/numerical methodology is developed to fully consolidate pure ultrafine WC powder under a current-control mode. Three applied currents, 1900, 2100 and 2700 A, and a constant pressure of 20 MPa were employed as process conditions. The developed spark plasma sintering (SPS) finite-element model includes a moving-mesh technique to account for the contact resistance change due to sintering shrinkage and punch sliding. The effects of the heating rate on the microstructure and hardness were investigated in detail along the sample radius from both experimental and modeling points of view. The maximum hardness (2700 HV10) was achieved for a current of 1900 A at the core sample, while the maximum densification was achieved for 2100 and 2700 A. A direct relationship between the compact microstructure and both the sintering temperature and the heating rate was established.

Frequency effect on pulse electric current sintering process of pure aluminum powder

Pure aluminum (Al) powder was sintered by means of a pulse electric current sintering (PECS) process at various pulse frequencies. The effects of pulse frequency on the density, electrical resistivity and tensile properties of the compacts were investigated. The results showed that the effects were not significant. From the activation energy calculation in the sintering process and scanning electron microscopy observation of compacts and transmission electron microscopy observation of interfaces between powder particles, the pulse frequency effects on PECS process and microstructure were not observed.

The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels

The effect of W on dislocation recovery and precipitation behavior was investigated for martensitic 9Cr-(0,l,2,4)W-0.1C (wt pct) steels after quenching, tempering, and subsequent prolonged aging. The steels were low induced-radioactivation martensitic steels for fusion reactor structures, intended as a possible replacement for conventional (7 to 12)Cr-Mo steels. During tempering after quenching, homogeneous precipitation of fine W2C occurred in martensite, causing secondary hardening between 673 and 823 K. The softening above the secondary hardening temperature shifted to higher temperatures with increasing W concentration, which was correlated with the decrease in self-diffusion rates with increasing W concentration. Carbides M23C6 and M7C3 were precipitated in the 9Cr steel without W after high-temperature tempering at 1023 K. With increasing W concentration, M7C3 was replaced by M23C6, and M6C formed in addition to M23C6. During subsequent aging at temperatures between 823 and 973 K after tempering, the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains occurred. Intermetallic Fe2W Laves also precipitated in the δ-ferrite grains of the 9Cr-4W steel. The effect of W on dislocation recovery and precipitation behavior is discussed in detail.

Alloy composition selection for improving strength and toughness of reduced activation 9Cr-W steels

Abstract Alloy composition selection for improving creep rupture strength and toughness of reduced activation 9Cr-W steels was investigated. Creep rupture strength increased but toughness decreased with increasing W concentration. Minor additions of V, Ta and B stabilized the fine structure of lath martensite and markedly increased strength without any significant degradation of toughness.

ANODIC PASSIVATING FILMS ON IRON IN PHOSPHATE AND BORATE SOLUTIONS

Ellipsometric measurements and chemical analyses have been carried out of anodic passivating films on iron in phosphate and borate solutions at pH ranging from 1.85 to 11.50. In neutral solutions the passive film consists of an inner oxide barrier layer and an outer hydroxide deposit layer: the optical constant was 3.0—0.5i for the barrier layer and 1.8—O.li for the deposit layer. The barrier layer is potential-dependent and its thickness increases linearly with the potential developing at the steady state a high electric field 5.6 6 V/

Mechanical properties of several advanced Tyranno-SA fiber-reinforced CVI-SiC matrix composites

A recently developed SiC fiber, Tyranno-SA (2D plain-woven), was used as the reinforcement in several SiC/SiC composites. The composites were fabricated by chemical vapor infiltration (CVI) process. The mechanical properties and fracture behaviors were investigated using three-point bending test. The Tyranno-SA fiber possesses rough fiber surface with pure SiC surface chemistry, which may result in strong fiber/matrix bonding and fiber sliding resistance. Various pyrolytic carbon (PyC) and SiC/PyC interlayer coatings were applied in the composites to modify the mechanical properties of the interface. The interlayers were deposited by isothermal CVI process. The test results revealed a close PyC layer dependence of the strength of the composites. The ultimate flexural strength (UFS) increased with the increasing of the PyC layer thickness up to 100 nm, and then, kept at similar level till 200 nm. The Tyranno-SA/SiC composites exhibited relatively high proportional limit stresses due mainly to the large Youngs modulus of the fiber. Fiber pullouts were observed at the fracture surfaces of all the interlayered composites. Attractive promising was exhibited on further improvement of the mechanical properties of the composites through further improvement of the interfacial properties and the matrix densification process.

Atomic scale analysis of self assembled GaAs/AlGaAs quantum dots grown by droplet epitaxy

In this letter we have performed a structural analysis at the atomic scale of GaAs/AlGaAs quantum dots grown by droplet epitaxy. The shape, composition, and strain of the quantum dots and the AlGaAs matrix are investigated. We show that the GaAs quantum dots have a Gaussian shape and that minor intermixing of Al with the GaAs quantum dot takes place. A wetting layer with a thickness of less than one bilayer was observed.

Materials selection for reduced activation of fusion reactors

Abstract The induced activity of thirty elements which are assumed to compose the fusion reactor first wall and shield has been calculated as a function of the neutron fluence. The evaluation of the radioactivity was made to criteria for surface dose rate and waste disposal. The potential materials for a low activation first wall are strictly limited to light elements found in ceramics and composites, based on rapid access for maintenance after long-term operation. Al is a promising low activation material for neutron fluences of −2 . Mo is not favorable for the first wall material because of the production of long lived radioactive nuclides. However, the activity decay of Mo is very fast if Mo is used as the shield material and the fluence does not exceed 1 MW·y·m −2 .

Optimum alloy compositions in reduced-activation martensitic 9Cr steels for fusion reactor

Abstract In order to obtain potential reduced-activation ferritic steels suitable for fusion reactor structures, the effect of alloying elements W and V on the microstructural evolution, toughness, high-temperature creep and irradiation hardening behavior was investigated for simple 9Cr-W and 9Cr-V steels. The creep strength of the 9Cr-W steels increased but their toughness decreased with increasing W concentration. The 9Cr-V steels exhibited poor creep rupture strength, far below that of a conventional 9Cr-1MoVNb steel and poor toughness after aging at 873 K. It was also found that the Δ-ferrite should be avoided, because it degraded both the roughness and high-temperature creep strength. Based on the results on the simple steels, optimized martensitic 9Cr steels were alloy-designed from a standpoint of enough thoughness and high-temperature creep strength. Two kinds of optimized 9Cr steels with low and high levels of W were obtained; 9Cr-1WVTa and 9Cr-3WVTa. These steels indeed exhibited excellent toughness and creep strength, respectively. The 9Cr-1WVTa steel exhibiting an excellent roughness was shown to be the most promising for relatively low-temperature application below 500°C, where irradiation embrittlement is significant. The 9Cr-3WVTa steel was the most promising for high temperature application above 500°C from the standpoint of enough high-temperature strength.