佐藤 英一

Processing of NiAl-Cr based eutectic in-situ composite via internal stress superplasticity has been explored. The alloy is heat treated to produce spherodized Cr particles in NiAl matrix. Results of thermal cycling creep experiments indicate that internal stress of considerable magnitude are generated between the NiAl matrix and Cr particles, which aids the external applied stress in deforming the alloy in viscous manner. The origin of internal stress is attributed to the difference in thermal expansion behavior of the constituent phases. At low stresses, the thermal cycling creep rates are higher than the isothermal creep rates. However, the observed thermal cycling creep rates are an order of magnitude lower than values predicted by an internal stress superplasticity model based on micromechanics. The possible reasons for this discrepancy are discussed.

Two types of micromechanics models for CTE-mismatch superplasticity have been proposed independently. They are developed about metal matrix composites and monolithic polycrystals. The present study compares and unifies these two models. Using a newly proposed geometrical factor, similar constitutive equations at low and high applied stresses are derived quantitatively. The unified model can be applicable to explain the CTE-mismatch superplastic behavior in any metal matrix composites and monolithic polycrystals.

Superplastic deformation behavior was examined in undoped and doped high purity 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with fine and coarse grains. In undoped 3Y-TZP, fine grain materials below 0.3 mum showed two deformation regions with a stress exponent of two, a grain size exponent of one and an apparent activation energy of 370 kJ/mol at high stress region and three, one and 630 kJ/mol at low stress region, while coarse grain materials above 1 mum showed two deformation regions with a stress exponent of one, a grain size exponent of three and an apparent activation energy of 450 kJ/mol at high stress region and three, one and 550 kJ/mol at low stress region. On the other hand in alumina doped high purity 3Y-TZP, both the fine and coarse grain materials showed two deformation regions with a stress exponent of one, a grain size exponent and an apparent activation energy of 520 kJ/mol at high stress region and two, one and 590 kJ/mol at low stress region. The region of diffusional creep region with a stress exponent of one was first observed in 3Y-TZP. Deformation mechanism maps have been drawn for undoped and doped TZP, and the obtained deformation mechanism maps can predict these deformation regions.

Internal stress superplasticity in NiAl based intermetallics was investigated. The alloys were NiAl-32vol%Cr and NiAl-9vol%Mo in situ composites, which were annealed at high temperatures to spheroidize the refractory metal phase. Under thermal cycling creep conditions, the alloys exhibited characteristics of internal stress superplasticity, i.e. at low stresses, the thermal cycling creep rates were much higher than the isothermal creep rates and the corresponding stress exponent was close to one. These results were compared quantitatively with the predictions of a theoretical model of internal stress superplasticity. Superplastic elongation of about 200% was attained during a thermal cycling tensile creep test.

The creep behavior in a discontinuous fiber-reinforced metal-matrix composite without other interfacial relaxation mechanisms than matrix plastic flow has been examined. The existence of steady-state creep is revealed from the following four points First, it is pointed out that the variation principles do not prove the existence of the energy minimum state and the creep termination. Second, it is shown that for viscous matrix material, creep continues with the same strain as elastic, which is important and does not change the stress distribution. Third, finite element method (FEM) analysis reveals steady-state creep. Without the constant dilation option, an incorrect result of the creep termination is obtained. At last, the creep behavior of AL-AI,Ni in situ composite is measured experimentally. At high stresses, a stress exponent of 5 and an activation energy of 130 kJ mol(-1) are obtained, showing plastic accomodation, while at low stresses, 1 and 85 kJ mol(-1) are obtained showing diffusional accomodation. (C) 2000 Elsevier Science S.A. All rights reserved.

Plastically accommodated creep in an inclusion bearing material (particle- or discontinuous fiber reinforced metal matrix composite) without any interfacial relaxation mechanisms has been examined. For a material with an elastic-viscoplastic matrix, the non-uniform strain rate in steady state creep is derived using Eshelby's solution for elastic strain outside an inclusion. The obtained creep strain increment is impotent and does not generate any additional internal stress. During this creep deformation, a dislocation comes in from one direction and goes out in another direction, so that no dislocation nor internal stress but a heterogeneous plastic strain remains in the material. The concrete trajectory of the dislocations climbing over a cylindrical inclusion is calculated and illustrated.

Superplastic deformation behavior was examined in high-purity 3mol% Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with fine and coarse grains, and their deformation mechanism map was drawn. The fine grain material below 0.3m showed two deformation regions with stress exponent 2 at high stresses and 3 at low stresses, while the coarse grain material above 1m showed two deformation regions with stress exponent 1 at high stresses and 3 at low stresses. These deformation regions can be predicted by the obtained deformation mechanism map.

Metal Matrix Composite containing Fe₂Tb_0.3Dy_0.7 particles in Al matrix is strengthened by the effect of magnetostriction. The composite is fabricated by powder metallurgy, and the average diameter and the volume fraction of the particle are 2μm and 5%, respectively. The magnetostriction induced by the magnetic field is relaxed at high temperature. Turning off the magnetic field at room temperature produces the residual stress. It is found that the compressive stress induced in the matrix develops the large tensile strength.

A composite consisting of Al matrix and giant magnetostriction particles was fabricated by powder metallurgy. The composite was heat treated under a magnetic field, where the particles induced giant magnetostriction, and after turning off the magnetic field at room temperature, the matrix possessed compressive residual stress. Tensile tests revealed that the above magnetization heat treatment improved the yield strength of the composite. The result proposed the new magnetostriction strengthening mechanism.

We performed two experiments to clarify the grain-refining mechanisms of Mg-Al system alloys by superheat treatment of molten alloys. Numerous Al4C3 nucleants were formed in the molten commercial-purity alloy of AZ91 by superheat treatment. The high-purity alloy was made of 99.99% Mg and 99.999% Al. Grain refinement occurred without superheat treatment. A small amount of impurity carbon included in the pure Mg and Al reacted with Al to form Al4C3 compounds. Carbon is an important element for grain refining of commercial-purity AZ91 alloy. Pure carbon powder with an argon gas carrier was therefore blown into the molten AZ91 alloy instead of harmful C2Cl6. Numerous Al4C3 nucleants were formed in the molten alloy.

Polycrystalline materials having crystallographic anisotropy show internal stress superplasticity under thermal cycling conditions. The deformation mechanism is analyzed using a theoretical model based on continuum micromechanics including the effects of crystallographic texture. The model is experimentally verified through the thermal cycling creep tests using polycrystalline zinc having two kinds of fiber-texture, and agrees well with the experimental results.

Although high temperature deformations of dispersion strengthening alloys and short fiber reinforced metal matrix composites have been studied independently, we should understand them as an unified form of that of inclusion bearing materials, noticing accommodation of misfit strains between matrix and inclusions. The present study reports the experimental examination that spherical-Al2O3-particle-dispersed Al-matrix-composite fabricated through pressurized casting shows different deformation modes depending on the strain rate with the respective accommodation processes.

Steady-state creep in a dispersion hardening alloy and discontinuous fiber reinforced metal matrix composite without other interfacial relaxation mechanisms than matrix plastic flow has been examined. For a material with an elastic-viscoplastic matrix, the non-uniform strain rate in the steady state is derived using Eshelby's equivalent inclusion method. The obtained creep strain increment is impotent and does not generate any additional internal stress. During this creep deformation, a dislocation comes in from one direction and goes out towards another direction, so that no dislocation nor internal stress but a heterogeneous plastic strain remains in the material. The concrete trajectory of dislocations climbing over a cylindrical inclusion is calculated and illustrated.

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.

Stress sensitivity and activation energy for superplastic deformation were investigated for silicon nitride consisting of rod-shaped grains. It was found that both the stress sensitivity and the activation energy increased during the deformation. In addition, an abnormal stress increment, observed in the superplastic deformation, was analyzed by a viscous flow model taking into account grain rotation. It was demonstrated that the viscous flow model would be effective to explain the stress increment during superplastic deformation. The principle deformation mechanism is considered to be the non-Newtonian viscous flow of grain boundary glassy phase. At the last stage (more than 200% elongation), however, superplastic behavior is considered to be mainly controlled by the solution-precipitation with 2-dimensional nucleation. (C) 1999 Elsevier Science S.A. All rights reserved.

The effect of Cr additive on the threshold stress in region I superplastic behavior of AI-Cu eutectic alloy is investigated using high-purity alloys with various Cr contents ranging from 0.002 to 0.26 mass%. The threshold stress increased largely with increasing Cr content up to 0.05 mass% and exhibited a tendency toward a possible saturation. It decreased with increasing temperature and grain size. The temperature dependence was similar to that of grain boundary segregation of impurity atoms. Transmission electron microscopy did not reveal the presence of fine precipitates at the grain and phase boundaries. Therefore, it was proposed that segregating Cr atoms are pinning grain boundaries, inhibiting grain boundary migration that accompanies grain boundary sliding and bringing about the threshold stress.

Three main theoretical models of internal stress superplasticity proposed by Greenwood and Johnson [Proc. R. Soc. A, 1965, 283, 403], Sherby ct al. [Mater. Sci. Technol., 1985, 1, 925], and Sate and Kuribayashi [Acta metall. mater., 1993, 41, 1759] have been expanded to include the temperature dependence of the average strain rate. It is revealed that the constitutive equations of these models are equivalent regarding the thermally activated kinetics, and that the apparent activation energy of internal stress superplasticity is equal to 1/n (where n is the stress exponent of isothermal power-law creep) times that of isothermal power-law creep. This theoretical prediction has been experimentally verified by the thermal cycling creep tests using the same temperature profiles with several equivalent temperatures in a Be particle-dispersed Al matrix composite. The obtained apparent activation energy was 22 kJ/mol, which is approximately 1/7 limes that of isothermal creep. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.

Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) is common superplasticity ceramics and its deformation behavior at high temperatures is influenced strongly by trace impurities such as silica and alumina. Low purity materials more than 0.1% show only one region of stress exponent 2, while high purity materials show two deformation behavior region with stress exponent 2 at high stresses and 3 at low stresses. In the present study, another deformation region with stress exponent 1 at lower stresses is observed in high purity 3Y-TZP, by means of constant cross-head speed compression tests and constant load compression creep tests.

Superplastic deformation is one of the most effective method to obtain anisotropic silicon nitrides. The superplastically plane strain deformed silicon nitride exhibited a highly anisotropic microstructure, where rod-shaped grains tended to be aligned pseudo two-dimensionally in the plane normal to the compression direction. Bending strength and fracture toughness were increased substantially by the deformation process when a stress was applied in the extruding direction. It was considered that these improvements were mainly due to effective operation of grain bridging and pull-out by the grain-alignment.

Three main theoretical models for internal stress superplasticity (Greenwood and Johnson, Sherby et al., and Sate and Kuribayashi) were reexamined with respect to the temperature profile of the thermal cycling. Assuming a mismatch strain rate obeying a power-law, we concluded that three constitutive equations were essentially identical except for constant terms. It is predicted that the apparent activation energy of internal stress superplasticity is equal to 1/n (n: stress exponent of isothermal power-law creep) times that of isothermal power-law creep. This theoretical prediction has been experimentally verified by the thermal cycling creep tests using the same temperature profiles with several equivalent temperatures in Al-Be eutectic composite.

3mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with various levers of Al2O3 doping and free from SiO2 contamination were prepared and the superplastic behavior and the microstructures were examined. High-purity TZP with less than 0.07wt%Al2O3 had two deformation regions: the low stress region had a stress exponent of three and an apparent activation energy of 630 kJ/mol, and the high stress region had correspondingly two and 460 kJ/mol. On the other hand, TZP containing more than 0.12wt%Al2O3 had only one region which had a stress exponent of two and an activation energy of 480 kJ/mol. High resolution transmission electron microscopy revealed that no amorphous grain boundary phase was produced even with 0.18wt%Al2O3 doping. EDS analyses near grain boundaries revealed that Y segregated at the grain boundaries with denuded zones of 30 nm width that may be created during cooling from the sintering temperature.

The effect of Al2O3 doping of around 0.1 wt % on superplastic behavior was studied in 3 mol % yttria-stabilized tetragonal zirconia polycrystal (TZP) which was free from SiO2 contamination and had a grain size of 0.4 mu m. Compression creep tests revealed that high-purity TZP with less than 0.07 wt % Al2O3 had two deformation regions: the low stress region had a stress exponent of three and an apparent activation energy of 640 kJ/mol, and the high stress region had two and 460 kJ/mol. On the other hand, TZP containing more than 0.12 wt % Al2O3 had only one region which had a stress exponent of two and an activation energy of 480 kJ/mol. The region of diffusion control with a stress exponent of one was not observed in any samples. High resolution transmission electron microscopy revealed that no amorphous grain boundary phase was produced even with 0.18 wt % Al2O3 doping. Energy dispersive X-ray spectroscopy near grain boundaries revealed that yttrium was segregating at the grain boundaries with denuded zones of 30 nm width, which were created during slow cooling from the sintering temperature. (C) 1999 Kluwer Academic Publishers.

Thermal cycling creep behavior in fiber-reinforced composites was investigated using a directionally solidified Al-Al3Ni eutectic composite. A superplastic elongation of 120% was obtained during a thermal cycling tensile creep test. Compression creep tests were performed under an external stress applied either parallel or perpendicular to the growth direction. The average strain rates for the two directions exhibited the characteristics of internal stress superplasticity: those at low stresses were much higher than the corresponding isothermal creep rates and were proportional to the applied stress. In the case of transverse loading, the thermal cycling creep rate was explained quantitatively using the previously reported internal stress superplasticity model for particle-dispersed composite. In the case of longitudinal loading, it was much lower than that predicted using the model because of the difference in the stress state and the relaxation process. However, thermal cycling creep had very low activation energy, which is a unique characteristic of internal stress superplasticity. © 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

The three-dimensional microstructural development of silicon nitride ceramics that exhibit superplastic elongation (up to epsilon = 1.34) was analyzed using a stereological analysis method. According to the microstructural change from randomly oriented grains to aligned grains along the tensile direction, the average orientation angle between the tensile axis and the major axis of a grain decreased monotonously as the strain increased. The average grain aspect ratio remained almost constant up to epsilon = 0.88 and then started to increase, Based on the microstructural development, three different modes of the change in the grain configuration-i.e., grain rotation, grain elongation, and grain translation-were considered. It is suggested that the contributions of the three modes vary according to the microstructural development during the deformation.

Materials containing second phases show superplastic behavior during thermal cycling (internal stress superplasticity). In the present study, this behavior has been investigated by thermal cycling creep tests with a triangular wave having fixed heating and cooling rates, using a Be-particle-dispersed Al matrix alloy. The thermal cycling creep rates were much higher than the isothermal creep rates and proportional to the applied stress at intermediate stresses. It was interpreted by the previous theoretical model of internal stress superplasticity, and the values of the thermal cycling creep rate were about 60% of the theoretical one. The discrepancy was explained by the transition after each temperature reversal until achieving the quasi-steady slate stress distribution during thermal cycling. (C) 1997 Acta Metallurgica Inc.

The stress relaxation in an inclusion bearing material at high temperatures under an external load has been examined. Independently from the analysis by Mori ct al. (Acta mater., 1997, 45, 429), it has been ascertained that the material reveals steady-stare creep even with only matrix plastic flow (multiaxial power-law creep) operating, through both variational principles analysis and finite element method (FEM). The characteristic of nonuniform stress distribution, which brings about steady-state creep, has been discussed. The dependence of the steady-state creep rate on inclusion aspect ratio and volume fraction has been also obtained through FEM anlaysis. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Vickers indentation in silicon nitride, which was produced by superplastic tensile deformation of 150%, was conducted. The specimen had a highly anisotropic microstructure, where rod-like grains aligned along the tensile direction, and showed unusual crack systems. In the case of the indentation whose diagonals point to 0 degrees/90 degrees (0 degrees direction is the tensile direction), two V-shaped surface cracks were propagated from each 90 degrees edge and one crack from each 0 degrees edge. In the case of the 45 degrees/45 degrees indentation, surface cracks emanated from the edges of the indent and deflected towards the tensile direction. Subsurface crack systems also showed unique shapes, differing from a cross-shaped crack system in isotropic brittle materials. Three-dimensional crack systems were estimated on the basis of the observation, and the crack evolution process was considered. Furthermore, the mechanical properties of the anisotropic silicon nitride were measured: and showed superior bending strength and fracture toughness in specific directions, compared to the undeformed material. (C) 1998 Elsevier Science S.A. All rights reserved.

The phenomena of two-liquid phase separations are significantly influenced by the gravity on the ground because of the difference in the densities of the constituent components, particularly, in the case of liquid alloys with critical mixing. In this paper, experimental techniques and results are reported for the measurements of the electrical resistivity for typical liquid alloys with critical mixing, such as Bi-Ga, under microgravity by the use of a rocket S520-19 belonging to ISAS (Institute of Space and Astronautical Science, Japan), It was found that the temperature coefficient of the electrical resistivity, on cooling of the homogeneous liquid phase, increases with the approach to the critical temperature. This trend under microgravity by the rocket experiment is more pronounced compared to the trend of the reference experiment on the ground. In addition, the supercooling of homogeneous liquids under microgravity is larger than that on the ground. These differences are explained by the difference in;he degree of the growth of concentration fluctuations; the concentration fluctuations are far greater under microgravity than on the ground. Therefore, it is found to be very important to study the process and the critical phenomena of two-liquid phase separations under microgravity. Measurement of electrical resistivity is an effective method to obtain informations about the process, the critical phenomena, and the supercooling of two-liquid phase separations in liquid alloys with critical mixing.

Materials containing second phases show superplastic behavior during thermal cycling (internal stress superplasticity). As one of its characteristics is independence of matrix grain size, there is a possibility of superplastic forming of coarse grain materials, which is impossible to be formed by fine structure superplasticity. In the present study, thermal cycling creep behavior of sufficiently annealed Al-Be eutectic alloy and directionally solidified Al-Al3Ni eutectic alloy is investigated. These materials have shown the internal stress superplastic behavior (stress exponent one) during thermal cycling at low stresses. The strain rate under this condition has relations to heating and cooling rates and to the shape of second phase. Experimental results imply the possibility of internal stress superplastic forming of a composite with a single crystal matrix through thermal cycling.

The effect of Cr additive on the threshold stress in region I superplastic behaviour of Al-33.3%Cu alloy is investigated at various temperatures. Threshold stress and apparent activation energy for superplastic now are found to increase with increase in Cr content. Transmission electron microscopy does not reveal the presence of precipitates on the grain boundaries and it is proposed that segregating Cr atoms are pinning the gain boundary, inhibiting the gain boundary migration and enhancing the threshold stress.

The effect of Cr additive on the threshold stress in region I superplastic behavior in Al-33% Cu eutectic alloy is investigated using high-purity alloys with various Cr contents ranging from 0.002 to 0.26wt%. The threshold stress increases largely with increasing Cr content up to 0.05% and exhibits a tendency for a possible saturation. It decreases with increasing temperature and grain size. Transmission electron microscopy does not reveal the presence of fine precipitates at grain and phase boundaries. Therefore, it is proposed that segregating Cr atoms are pinning grain boundaries, inhibiting grain boundary migration accompanied by grain boundary sliding and bringing about the threshold stress.

Materials containing second phases show superplastic behavior during thermal cycling (internal stress superplasticity). A self-consistent and quantitative model based on continuum micromechanics was proposed: For a body consisting of a creeping matrix and a non-creeping inclusion, which was loaded during temperature changing, a stationary internal stress distribution and the overall creep behavior were calculated. This theoretical model was verified by thermal cycling, compression creep tests using an Al-Be eutectic alloy consisting of an Al matrix and Be inclusions. The temperature profile was a triangular-wave obtained by induction heating and gas cooling. Under intermediate applied stresses, the thermal cycling creep rates were much higher than the isothermal creep rates and depended proportionally on the applied stresses as predicted by the model. In a Ni base superalloy, the internal stress superplasticity was first demonstrated. The material was a directionally solidified superalloy, Mar-M247, consisting of gamma phase matrix and gamma' phase precipitates.

Silicon nitride (Si3N4) ceramics with rod-shaped beta-Si3N4 grains show superplasticity with microstructural change from randomly oriented grains to aligned grains along the tensile axis. At the same time, grains seem to elongate considerably on the two dimensional. Stereological analysis has been developed to obtain the three dimensional (3-D) distribution of radius, aspect ratio and orientation angle from the measured two dimensional distribution on a sectioning plane. The calculated 3-D distributions show that the aspect ratio remains almost constant up to a strain of 0.88, and then starts increasing. The orientation angle decreased monotonously. Based on this microstructural change, a dominant deformation mechanism is determined as grain boundary sliding at the early stage and anisotropic grain growth induced by deformation at late stage. Grain rotation (alignment) also acts throughout the deformation.

The first measurement under microgravity of electrical resistivity rho (or resistance R) was performed for a liquid Bi-Ga system during a parabolic rocket flight. Specially designed Ti cells were used, The real time data of rho (or R) were obtained on earth during a cooling process from the homogeneous liquid phase (HLP) to the two-liquid phase (TLP) by means of telemetry. The rho decreases moderately, and its temperature coefficient, (d rho/dT)/rho, increases abruptly with the approach to binodal temperatures in a cooling process from HLP to TLP, particularly for liquid alloys near the critical concentration. The (d rho/dT)/rho shows a maximum at the critical composition. This decreasing tendency of rho continues even in supercooled states of HLP up to the onset of TLP separation. The supercooling under microgravity is distinctly larger than that obtained under gravity on earth.

The sintering behavior of alpha-alumina powders doped with magnesia (500 or 1500 ppm) and yttria (0, 500, or 1500 ppm) was investigated using constant-heating-rate dilato-metric experiments, The apparent activation energies for the intermediate stage of sintering were 740, 800, and 870 kJ/mol for 0, 500, and 1500 ppm yttria doping levels, respectively; these were independent of magnesia doping, Yttria-doped powder compacts exhibited systematic anomalous second peaks in the densification rate curves at certain grain sizes which were determined only by yttria doping levels, Before the anomalous peak, with lower yttrium contents at grain boundaries, yttrium in an atomic state delays densification and raises the apparent activation energy, Beyond the peak, with higher yttrium contents at grain boundaries, yttria-rich precipitation delays the densification, Within the peak, yttrium segregation near the saturation level enhances densification.

A general method is presented for solving stereological problems for particle systems consisting of spheroids having variable size, shape (aspect ratio) and orientation provided that the orientation has axial symmetry. The kernel function, which connects the probability density function (distribution) of the spheroids and that of the sectioning ellipses on the observation planes along the symmetry axis, is calculated theoretically. The spatial distribution of the spheroids can be calculated from the experimentally measured planar data of the ellipses through the inverse operation of the kernel function. Using this method, the spatial size, shape and orientation distributions of grains were calculated in as-sintered and unidirectionally elongated silicon nitride samples consisting of rod-shaped beta-Si3N4 grains. The calculated spatial distribution shows that the aspect ratio remains almost constant during the deformation although it appears elongated upon planar observation owing to a superficial effect of grain alignment along the tensile axis.

Materials containing second phases show superplastic behavior during thermal cycling (internal stress superplasticity). The authors recently proposed a quantitative model using continuum micromechanics theory for the superplastic behavior. In the present study, this theoretical model has been verified by compression creep tests under thermal cycling conditions using a Be particle dispersed Al matrix alloy as an ideal material. The thermal cycling creep rates were much higher than the isothermal creep rates and proportional to the applied stress at low stresses. However they approached the isothermal creep rates at high stresses. This behavior agrees well with a theoretical prediction, and the value of the thermal cycling creep rate at low stresses coincides with the theoretical one. Therefore it was concluded that the internal stress superplasticity model was applicable to particle dispersed metal matrix composites.

Thermal cycling creep behavior of AI-Al3Ni eutectic alloy was studied by compression creep test. Three kinds of eutectic alloys were prepared; as-extruded alloy having polycrystal matrix and spherical Al3Ni phase, directionally solidified at 5 mm/h alloy having a single crystal matrix with short spheroidal Al3Ni phase, directionally solidified at 60 mm/h alloy having a single crystal matrix with long rod shaped Al3Ni phase. The strain rates of all specimens under thermal cycling conditions at low stresses were faster than the corresponding isothermal creep rates. However there is no significant difference at high stresses. The material having short spheroidal Al3Ni phase in the single crystal Al matrix had the lowest stress exponent (n=1) in thermal cycling creep at intermediate stresses, which is typical of superplastic behavior induced by internal stress, caused by a difference of thermal expansion between the matrix and inclusions.

The steady stale creep of a metal matrix composite is discussed considering the accommodation processes of strain mismatch around inclusions, i.e., accommodation by diffusion and that by non-uniform plastic deformation. The steady state creep was analyzed by Eshelby's equivalent inclusion method and by finite element method. Stress exponents, n, of unity and same as that of the matrix were predicted for the respective accommodation mechanisms. Creep behavior of an Al-Al3Ni directionally solidified in-situ composite was examined experimentally. The composite was made of a single crystal Al matrix and 10 vol.% Al3Ni fibers of 0.84 mu m diameter and about 100 mu m length. A region of n=1 at lower strain rates and an increase in n at higher strain rates were observed.

Two kinds of commercial, submicron-grained, nondoped alpha-alumina powders were cold-isostatically pressed and then sintered at a constant heating rate. Before the sintering, pre-treatments were executed at 820 or 920 degrees C for 50 h. The effects of the pretreatments on the densification behavior and the microstructural development are discussed. The powders contained some nano-sized alumina particles about 10 nm in diameter as well as submicron-sized primary particles about 0.1 mu m in diameter, which agglomerated in packs of 0.2-0.4 mu m diameter. The densification rate curves showed a low temperature shoulder during sintering attributed to these nano-sized particles. After pretreatments, the nano-particles disappeared, resulting in disappearance of the shoulder in the subsequent densification rate curves. These treatments, however, caused little change in the apparent activation energy in the intermediate stage sintering. For the weakly agglomerated powders, the bodies sintered with pre-treatments had coarser final microstuctures.

An approach to improve the accuracy of submicron particle size distribution measurement using a cuvette photocentrifuge (Horiba CAPA-700) has been applied to a commercial alumina powder and a standard quartz powder. This approach incorporates a correction for the light scattering from small particles, an improved raw data smoothing routine and a correction for the radial dilution effect. The results for the commercial alumina powder show very good correspondence with the supplier's X-ray sedimentometer data when corrected for light scattering and radial dilution; and allow a more complete data collection in the 0.3-0.03 mum range. For the standard quartz powder (BCR 66) a less satisfactory result is found, with the light-scattering corrected data giving a much finer particle size distribution than expected for the standard powder. Two possible reasons for this discrepancy, the nonsphericity of the quartz powder and an imaginary part of the refractive index of quartz due to impurities, are discussed along with the limitations of the approach for accurate submicron particle size measurement of polydispersed irregularly shaped particles.

Sintering behavior with and without external stress was investigated using undoped and doped (1500 ppm yttria) sub-micron alpha-alumina powders. The yttria doped alumina showed anomalous behavior with grain size around 0.9 mu m, a second peak in densification rate during free sintering and a stagnation followed by a steep increase in flow stress during sinter forging. These were thought to correspond to the creep rate plateau regime and to the saturation of grain boundary with yttrium. Yttrium segregation retarded the densification during free sintering, and made the flow stress higher during sinter forging. It also lowered the grain size at which the transition occurred in the rate controlling mechanism of deformation from grain boundary diffusion to interface reaction.