佐藤 英一

The creep behavior at an ambient temperature of typical h.c.p., b.c.c. and f.c.c. metals and alloys of annealed states were surveyed. Cubic metals and alloys demonstrated negligible creep strain under all stress ranges. In contrast, h.c.p. metals and at toys demonstrated significant creep behavior. In particular, Ti-6Al-4V alloy and CP-Ti metal showed accumulated large creep strains. The difference among metals and alloys were negligible, for Mg and Zr. After being cold rolled, Ti-6Al-4V alloy and CP-Ti metal showed less significant creep behavior.

Marked creep behavior is shown by commercially pure titanium (CP-Ti) at an ambient temperature below the yield stress. Creep behavior of CP-Ti was investigated at stress levels below yield stresses with special emphasis on low temperatures down to ambient temperature. The apparent activation energy was determined as about 10 kJ/mol; the apparent stress exponent is about 5.0. Based on these creep parameters, the ambient-temperature creep region was drawn in the Ashby-type deformation mechanism map of annealed CP-Ti. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A cryogenic tank made of carbon fiber reinforced plastic (CFRP) shell with aluminum thin liner has been designed as a liquid hydrogen (LH2) tank for an ISAS reusable launch vehicle, and the function of it has been proven by repeated flights onboard the test vehicle called reusable vehicle testing (RVT) in October 2003. The liquid hydrogen tank has to be a pressure vessel, because the fuel of the engine of the test vehicle is supplied by fuel pressure. The pressure vessel of a combination of the outer shell of CFRP for strength element at a cryogenic temperature and the inner liner of aluminum for gas barrier has shown excellent weight merit for this purpose. Interfaces such as tank outline shape, bulk capacity, maximum expected operating pressure (MEOP), thermal insulation, pipe arrangement, and measurement of data are also designed to be ready onboard. This research has many aims, not only development of reusable cryogenic composite tank but also the demonstration of repeated operation including thermal cycle and stress cycle, familiarization with test techniques of operation of cryogenic composite tanks, and the accumulation of data for future design of tanks, vehicle structures, safety evaluation, and total operation systems. (c) 2005 Elsevier Ltd. All rights reserved.

A new application of superplasticity was proposed in the manufacturing process of metal foams. Preform sheets were manufactured using superplastic 5083 aluminum alloy sheets through accumulative roll-bonding (ARB) process. Microcellular aluminum foam plates with 50% porosity were produced through solid-state foaming under the superplastic condition. The cell shape was oblate spheroid, which is effective to reduce the thermal conductivity. The present aluminum foam plates have a potential as an excellent heat insulator.

Fracture behavior under multiaxial stress state of polycrystalline alumina was studied from the view point of an artificial crack propagation and fracture from a natural flaw. The former was studied by mixed-mode fracture toughness tests; asymmetric four-point bending and diametral compression techniques were carried out using precracked and notched specimens. The latter was studied by biaxial fracture tests in compression and torsion loading; multiaxial fracture statistics was applied to the measured fracture envelope. The ratio K-IIC/K-IC obtained from the biaxial tests was higher than that obtained by the mixed-mode fracture toughness tests. It revealed that the fracture from an artificial flaw does not simulate the fracture from a naturall flaw in polycrystalline ceramics.

The primary creep behavior at ambient temperature of typical h.c.p., b.c.c. and f.c.c. metals and alloys of annealed state was surveyed and the deformation mechanism of CP-Ti was discussed through transmission electron microscopy. Only h.c.p. metals and alloys demonstrated significant creep at ambient temperature. Arrays of straight screw dislocations of b=1/2 < 2110 > and < 2110 > direction were observed in the crept CP-Ti.

Using commercial AZ31 magnesium alloy sheets, we produced a foamable preform sheet containing titanium hydride (TiH2) powder through diffusion-bonding and hot-rolling of four cycles. Heating the preform sheets in Ar atmosphere, we obtained closed-cell magnesium alloy foams with various porosities. The foamed specimen at 883 K showed the maximum porosities of 77%.

Aluminum foams having extremely low densities offer a large potential for lightweight structural materials. New manufacturing process without expensive aluminum alloy powder has been developed using conventional bulk aluminum alloy sheets. Preform plate containing blowing agent particles is first manufactured through accumulative roll-bonding (ARB) process. By heating the preform plate, closed-cell aluminum foams having various porosity and cell morphology are produced. It was revealed that ARB processing condition is significantly important to produce suitable aluminum foam with high porosity and uniform pore distribution. Present manufacturing process also possesses a potential to apply to many other metal and alloy foams.

The primary creep behavior at room temperature of typical hcp, bee and fcc metals and alloys in the annealed state are surveyed. Cubic alloys show negligible creep under all stress ranges, while cubic metals demonstrate very low creep. In contrast, hcp metals and alloys demonstrate significant primary creep. In particular, Ti-6Al-4V alloy and CP-Ti metal show considerable creep and accumulated large creep strain of more than 1% in 90 s and 1600 s, respectively, under 0.9 of 0.2% proof strength. As the creep strain rate decreases with time, this creep is regarded as primary creep. (C) 2004 Elsevier B.V. All rights reserved.

Dispersing reinforcements in a ductile matrix is one method of strengthening metallic materials at high temperatures. Although this method generally strengthens the material, it is predicted that concomitant diffusion weakens it when heterogeneous stresses around the reinforcements are accommodated by rapid diffusion along the matrix/reinforcement interface. Here we show weakening with increasing volume fraction of reinforcements under low strain rates and strengthening under high strain rates using Ti/TiB(w) composites at 1473 K. The criterion of weakening or strengthening is established by the diffusional-accommodation rate considering the accommodation process of the misfit strain between the matrix and reinforcements by diffusion. (C) 2004 Elsevier B.V. All rights reserved.

A giant-magnetostriction particle reinforced tin-matrix composite, Sn/Terfenol-D, was fabricated, and the relaxation behavior of internal stress caused by magnetostriction of the particles was directly observed as an average strain change of the composite with and without an external stress. The diffusional relaxation under zero external stress was not observed. During steady-state creep, the internal stress by magnetostriction suppressed dislocation movement and removal of the magnetostriction activated the dislocation movement. This implies that the plastic relaxation of internal stress was observed directly from the average strain change. (C) 2004 Published by Elsevier B.V.

Closed-cell Al-Si alloy foam was produced from two bulk alloy strips. Preform sheet containing titanium hydride (TiH2) particles was first manufactured through accumulative roll-bonding (ARB) processing. Large ARB cycle number was effective to achieve uniform distribution of the TiH2 particles in the Al-Si matrix. Following high temperature foaming tests revealed that high porosity and small pore size can be obtained from the preform prepared through large cycle number. These results indicate that a suitable ARB processing condition enables to optimize the cell microstructure of the metal foams.

We investigated compressive creep behavior of TiB reinforced Ti matrix composites of 10 and 20 vol.% TiB at 1473 K. Ti/20TiB displays higher or lower creep strength than TOMB depending, respectively, on whether the strain rate is higher or lower than the diffusional-accommodation rate. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A new manufacturing process for closed-cell aluminum foams was developed using bulk aluminum sheets as a starting material. Aluminum preform containing titanium hydride powder was successfully produced through accumulative roll-bonding (ARB). Heating the preform sheets under the controlled temperature conditions, we obtained foamed aluminum plates of about 40% porosity. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

M-V is a three-staged solid-propellant satellite launcher developed by the Institute of Space and Astronautical Science (ISAS) of Japan to support various Japanese space science missions. It launches a two-ton class payload into low earth orbit. Although the initial two launches were successful, the third launch of M-V failed to put an X-ray observatory satellite into the orbit because the attitude control was lost for 20 sec in the first-stage flight. From the various data, it turned out that the heat-resistive throat-insert of the nozzle of the first stage fractured and dropped-off, and the hot gas damaged the nozzle throat. And the equipments for attitude control neighboring the nozzle were damaged by the hot gas blew out from the side of the damaged nozzle. After careful review, ISAS and the Space Activity Commission concluded that the fracture of the insert was most likely caused by an embedded or surface crack in the throat insert made of brittle graphite. Because further study came to the conclusion that the performance of up-to-date non-destructive inspection is not enough to surely detect all the detrimental cracks embedded in the graphite billet of the size of this insert, ISAS determined to use inserts made of 3D carbon-carbon composite material. After the redesign and verification by static firing tests, M-V satellite launcher successfully returned to the flight in May 2003, injecting HAYABUSA spacecraft for asteroid sample return mission into a correct orbit.

To enhance the space mission capabilities, it is required to make the components of spacecraft more reliable with higher efficiency. At the same time, most of current RCS (Reaction Control Subsystem) thrusters are metallic, and their specific impulse (Isp) is restricted to lower value by the temperature limit of the chamber material. Moreover, conventional metallic chamber requires to be coated, which may invade the reliability of the thrusters, in some cases. To overcome this situation, authors have studied the possibility of utilizing the thrusters made of ceramics and conducted the firing test successfully with the chamber made Of Si3N4. Still, it is not adequate to make all of components including injector and nozzle with Si3N4, especially in cases where the thruster is large in size. It would be disadvantageous in terms of cost and weight. In this paper, we discuss the way to adhere Si3N4 to other materials such as CMC (Ceramic Matrix Composite) and metal. The basic idea to adhere these materials is confirmed through a number of hardware tests including strength tests and firing tests.

Composite creep deformation was analyzed, based on a continuum plasticity representation of the matrix, in an ideal composite at high temperatures in the case of negligible interfacial diffusion and sliding. A general formula of the steady-state creep strain rate was derived for a composite consisting of an ellipsoidal rigid reinforcement and a creeping matrix with a stress exponent of one. A closed-form solution was then derived for a composite with a cylindrical reinforcement under pure shear deformation in a two-dimensional analysis. The resultant creep deformation satisfies the requirements of impotency, volume conservation and interfacial continuity. Traces of two types of edge dislocations were analytically drawn; they show that dislocations climb over the reinforcement, retaining no dislocations either in the matrix or at the interface. Also, two types of dislocations simultaneously climb up and down at any portion in the matrix through dislocation core shuffling without long-distance diffusion. Finally, it was concluded that plastically-accommodated creep is characterized by two types of dislocations that simultaneously climb over a reinforcement, generating a heterogeneous creep strain increment without long-distance diffusion.

A new inspection system using ultrasonic phased array technique has been developed for a graphite block for a nozzle throat of a solid rocket motor, through solving difficulties arising from high attenuation and scattering properties of graphite and anticipated cracks specified neither in position nor in direction. Inspecting procedure using this system has been developed to detect cracks at any positions for any directions within a reasonable inspection time.

Elastic-plastic properties of closed-cell metal foams were derived using continuum micromechanical model (the equivalent inclusion method and the mean-field approximation). Assuming a hydrostatic internal stress in the cell, the effect of the gas pressure was incorporated into the calculation. The model was developed to the foams with isotropic and anisotropic geometries. The analytical results were compared with the available experimental data and the unit-cell model. Except for foams with high porosity, the micromechanical model agreed well with the experimental elastic-plastic behavior of both isotropic and anisotropic foams. On the other hand, the unit-cell model failed to estimate the mechanical properties of anisotropic foams such as lotus-structured porous metals. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A steady-state creep model of a metal matrix composite was proposed considering the accommodation processes of the misfit strain between the matrix and reinforcements in order to comprehensively explain dispersion strengthening, composite strengthening and composite weakening. The proposed model was verified experimentally using the model composites, Ti/TiB in situ composites of 5, 15 and 20vol.%TiB, which have a good interfacial bonding, suitable size and aspect ratio of TiB for a moderate diffusional accommodation rate, and no fine oxides. A sigmoidal curve in a double-logarithmic scale of stress and strain rate was observed in the creep of Ti/15TiB at 1123 K and was classified into a complete-diffusional-accommodation region corresponding to composite weakening, a diffusional-accommodation-controlled region and a plastic-accommodation-controlled region corresponding to composite strengthening with increasing stress. The activation energies in the three regions were close to those of the volume, interface and volume diffusion of Ti, respectively. In the plastic-accommodation-controlled region, the strain rate decreased with increasing volume fraction, while near the complete-diffusional-accommodation region, the strain rate scarcely changed. The strain rate of the transverse creep was ten times that of the longitude creep in the plastic-accommodation-controlled region, while the difference was scarcely observed near the complete-diffusional-accommodation region. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Effects of internal stress superplasticity on solid-state foaming process were examined using Al-8.69Si alloy and pure zinc compacts produced by the powder metallurgical (P/M) route. Isothermal and thermal cycling compression creep behaviors revealed that composite CTE (coefficient of thermal expansion)-mismatch superplasticity was induced in P/M Al-Si alloy, however, no difference was shown in the solid-state foaming. On the other hand, the foaming rate of P/M zinc was enhanced by anisotropic CTE-mismatch superplasticity. The cell morphology of the foamed zinc has anisotropy due to the original powder compact produced by hot-extrusion.

Sandwich panels for high temperature applications were fabricated using superplastic diffusion bonding of aluminum foam core. Superplastic 5083 aluminum alloy sheets and closed-cell aluminum foam ALPORAS were used as skins and a core. Compressive stress of 0.35 MPa at 823 K for 10 min in vacuum resulted in the penetration of the cell wall into the superplastically-deformed skin, and thus solid state diffusion bonding was achieved. EDS measurement confirmed the diffusion of magnesium from the 5083 alloy to the ALPORAS foam regions. The bonding strength measured by vertical tensile tests at room temperature was 32% of the original ALPORAS foam.

The steady-state creep behavior of 10 vol% spherical Al2O3 particle-reinforced Al-5 at% Mg matrix composites fabricated by ingot metallurgy was investigated in the compression mode, The composites had a moderate diffusional accommodation rate and were free of fine dispersoids, e.g. oxides or precipitates. Threshold stress behavior in the low-strain-rate region below the diffusional accommodation rate was observed despite the fact that there were no fine dispersoids. In the high-strain-rate region, power-law creep controlled by plastic accommodation was observed. while linear creep controlled by diffusional accommodation was not observed around the diffusional accommodation rate. It was explained that because strengthening by the spherical particles through load transfer is quite small, diffusional-accommodation-controlled creep can be scarcely observed. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The appearance and vanishing of the load transfer effect in the steady-state condition of composites' creep were analyzed via consideration of two accommodation processes, diffusion and plastic, which are inevitable for composites to continue creep deformation. Creep experiments were performed using model materials, Ti-5, 15 and 20 vol%TiB in situ composites, which have moderate diffusional-accommodation-controlled creeps, good interfacial bonding and no fine dispersions. With higher strain rates than the diffusional-accommodation-controlled creep, the stress exponent of all composites was 4.5, similar to that of alpha-Ti, 4.3, and the strain rates decreased with increasing volume fraction of the reinforcements. Variations of the strain rate agreed with the prediction by the self-consistent potential method, and thus the load transfer effect was confirmed in this region. With strain rates near the diffusional-accommodation-controlled creep, the stress exponents of Ti-15 and 20vol%TiB decreased to 2. With a strain rate lower than the diffusional-accommodation-controlled creep, the strain rates of the composites did not decrease with increasing volume fraction, and vanishing of the load transfer effect was observed.

The steady-state creep behavior of metal matrix composites was analyzed via consideration of two accommodation processes, diffusion and plastic, which are inevitable for the materials to continue creep deformation. The creep experiments were performed using a model material, in situ TiB fiber-reinforced pure alpha-Ti matrix composite, which has a good interfacial bonding, a moderate diffusional-accommodation rate and no fine oxide dispersions. A sigmoidal curve of strain rate and stress relation in a double logarithmic plot was observed, indicating the presence of three deformation regions: plastic-accommodation-control region, diffusional-accommodation-control region and complete diffusional-accommodation region, at high, middle and low stresses, respectively. The activation energies in the three regions were close to those of volume, interface, and volume diffusion of alpha-Ti, respectively.

In order to estimate dependence of volume fraction of ceramic and metallic phase on composite's creep behavior, we fabricated Ti-TiB in situ composites whose volume fractions of TiB were 10, 24 and 40%. The steady-state creep rates of all composites were agreement with the estimation of continuum mechanic. Microstructure of Ti-40TiB, however is quite different from that of Ti-10TiB, because TiB whiskers are strongly connected each other in Ti-40TiB. This structure of Ti-40TiB seems to require the deformation off TiB to creep, and of creep behavior of composites with high volume fraction of reinforcements requires more extensive study.

Solid-state diffusion bonding (DB) was demonstrated for joining closed-cell aluminum foams (ALPORAS). A superplastic 5083 aluminum alloy sheet was inserted between the foams to assist the DB process. Microscopic observation revealed that the cell wall of the foams penetrated into the 5083 alloy sheet and their boundary partly disappeared, Energy dispersion X-ray spectrometer (EDS) confirmed the diffusion of magnesium element from the 5083 alloy to the aluminum foam regions. The bonding strength was evaluated by four-points bending tests. The obtained flexure stress was about 50% of the original foam at room temperature and was more than 60% at 423 K. The advantage of the DB process in the high temperature applications was discussed comparing with the adhesive bonding of aluminum foams. (C) 2002 Elsevier Science B.V. All rights reserved.

Elastic modulus, coefficient of thermal expansion and thermal conductivity of an anisotropic closed-cell metal foam were derived based on continuum micromechanics, Eshelby's equivalent inclusion method and Mori-Tanaka's mean-field approximation were used for the calculation. Assuming the eigenstrain in the cell phase, the effect of the gas pressure was incorporated into the calculation. Anisotropic cell structure was modeled by the cell of spheroidal shape. The analytical results were compared with previous Ashby's unit-cell model which has been widely applied to many cellular materials. It was revealed that the present micromechanics model has high advantage for high-density and anisotropic closed-cell metal foams. The analytical results agreed well with the experimental result of lotus-structured porous copper with anisotropic cell structure.

The rate-control processes depending on the temperature were investigated using TiB dispersed Ti matrix composite. The stress exponent was revealed to be 3.2 in the all temperature, while the activation energy changed from 515kJ/mol at higher temper to 81kJ/mol at lower temperature, suggesting the change in the rate-controlling phase from the ceramic phase (TiB)to the metallic phase (Ti).

Internal stress superplasticity (ISS) in a NiAl-Mo based eutectic alloy was investigated by conducting thermal cycling creep tests. The alloy was annealed at high temperatures to spheroidize the refractory metal phase. Under thermal cycling creep conditions, the alloy exhibited characteristics of ISS, that is, at low stresses, the thermal cycling creep rates were much higher than the isothermal creep rates and the corresponding stress exponent was close to 1. These results were compared quantitatively with the predictions of a theoretical model of ISS. The experimental thermal cycling creep rates agreed with the predicted values within an error less than one order of magnitude. Superplastic elongation of > 200% without fracture was attained during a thermal cycling tensile creep test. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Isothermal and thermal cycling creep behavior of a NiAl-Cr alloy was studied by compression tests. Al low stresses, the thermal cycling creep rates were faster than the isothermal creep rates. The corresponding stress exponent under the thermal cycling condition was close to one. At high stresses, the stress exponent and the creep rate under the thermal cycling condition approached the isothermal values. The result indicates the occurrence of internal stress superplasticity at low stresses. The origin of internal stress was attributed to the difference in thermal expansion behavior of the constituent phases. Further, the thermal cycling creep rate was found to increase with increase in temperature amplitude and heating/cooling rate. The above trends were compared with the predictions of an internal stress superplasticity model. Good formability of the alloy under the thermal cycling condition was demonstrated though carrying out the thermal cycling compressive creep test to a large strain level. (C) 2001 Elsevier Science Ltd. All rights reserved.

A theoretical model of internal stress superplasticity is developed in a single-phase polycrystalline material with an anisotropic thermal expansion. Quasi-steady state creep equation during a thermal cycle is derived quantitatively based on continuum micromechanics. The model assumes that the generated mismatch strain is accommodated simultaneously by the plastic flow of the material. The linear creep deformation. which corresponds to internal stress superplasticity, is obtained at low applied stress region. and the creep rate depends on the crystallographic texture of the material. The validity of the model is experimentally verified using polycrystalline zinc which is a typical metal having large anisotropy in thermal expansion. The calculated strain rates using the texture information and the isothermal creep equation agree quantitatively well with the experimental results. The apparent activation energy of thermal cycling creep reveals 1/n (n: stress exponent of isothermal creep) of that of isothermal creep, which is one of the characteristics of internal stress superplasticity. Except for the factors attributable to the material geometry, the thermal cycling creep equation in the polycrystalline material is identical to that in a metal matrix composite. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The creep behavior of metal matrix composites was analyzed through considering two accommodation processes, diffusional and plastic, which are inevitable for the materials to continue creep deformation. The diffusional accommodation rate mainly depends on the size and aspect ratio of the reinforcements, while the plastic accommodation rate mainly depends on the volume fraction and aspect ratio of the reinforcements. It implies that the effect of the volume fraction appears only in the plastic accommodation control region, but not in the diffusional accommodation control region. In order to confirm this prediction, high temperature compression tests were performed using two model materials, in-situ TiB whisker reinforced pure alpha-Ti matrix composites, which had 5 and 20 vol.%TiB, respectively, with the same oxygen content the same moderate diffusional accommodation rate, good interfacial bonding and no fine oxide dispersions. The steady-state creep rate showed the power law creep behavior in the two composites except the low stress region. In this power law region, the creep rate of 20vol.%TiB composite was lower than that of 5vol.%TiB composite at the same condition. In the low stress region, the data of 20vol.%TiB composite showed lower stress exponent and the difference between the two composites was small.

Structural superplasticity, in magnesium alloys has been widely investigated by many scientists, but the internal stress superplastic deformation of magnesium have not been reported. This report describes internal stress superplasticity in polycrystalline AZ31 magnesium alloy through isothermal and thermal cycling compression creep tests. The alloy showed typical class-I creep behavior (n = 3) under the isothermal condition with the activation energy Qc (124 kJ/mol). On the other hand, the alloy showed linear creep behavior under the thermal cycling condition and the average strain rates were much higher than the corresponding isothermal creep rates. The apparent activation energy of thermal cycling creep was 49 kJ/mol, which is close to Q(c)/n. This is a typical characteristic of internal stress superplasticity. The experimental results were compared with the previous theoretical model quantitatively.

Lead based metal matrix composite containing giant magnetostriction particles, Terfenol-D, of 40vol% was fabricated through powder metallurgy process. The composite was heat treated under a magnetic field to produce residual stress due to the giant magnetostriction. The direction of the residual stress was changed by changing the direction of the magnetic field. Compression tests at room temperature revealed that the above thermo-magnetic treatment modified the yield strength of the composite; the treatment in parallel decreased the yield strength of 33% and that in perpendicular increased it of 33%. The strengthening mechanism based on micromechanics was discussed, and the prediction agreed well with the experimental results. The present new strengthening procedure allows a structurally isotropic material having anisotropic mechanical properties.

The creep behavior of inclusion bearing materials was analyzed through considering two accommodation processes, diffusion and plastic, which are inevitable for the materials to continue creep deformation. The creep experiments were performed using a model material, in-situ TiB fiber reinforced Ti matrix composite, which have a good interfacial bonding, the moderate diffusional accommodation rate and no fine oxide dispersions. A sigmoidal curve of (epsilon)over-dot and sigma relation in a double logarithmic plot was observed, which indicated the presence of three deformation region: plastic accommodation, diffusional accommodation control, and complete diffusional accommodation regions. The activation energies in the plastic accommodation and diffusional accommodation control regions were close to those of volume and interface diffusion, respectively.