佐藤 英一

This work has successfully proposed a solution to produce robust Nb-interlayer-inserted Ti-6Al-4V/Si3N4 joints optimized for a maximum operating temperature of 873 K; transient liquid phase bonding (TLPB) of Ti-6Al-4V/Nb side was carried out with Cu and Ni fillers to suppress brittle intermetallic compounds (IMCs), whereas brazing of Nb/Si3N4 side was performed using a highly ductile Ti-added Ag-rich filler for effective residual-stress relaxation. A sound yet simple one-step bonding process incorporating simul-taneous TLPB and brazing was achieved with a relatively short holding time of 10 min at 1213 K. TLPB of Ti-6Al-4V/Nb side with Cu and Ni foils of 2-mu m-thick each as a laminated filler suppressed brittle Ti -based IMCs and developed a homogenized microstructure consisting mainly of (alpha + )-Ti beta via isothermal solidification. Meanwhile, brazing of Nb/Si3N4 side with 100-mu m-thick SILVER-ABA filler (92.75Ag-5Cu-1Al-1.25Ti mass%) foil enhanced interfacial bonding with sufficient total Ti content and accommodated residual stress better than conventional eutectic Ag-Cu-based fillers, and it was verified by finite element analysis with consideration of materials' temperature-dependent elasto-plastic properties. All joints with a bonding area of 10 mm x 10 mm were tested via symmetrical four-point bending from room temper-ature (RT) to 873 K fractured from Nb/Si3N4 side. When re-heating the joints from RT to 673 K, frac-ture initiation gradually shifted from Si3N4 towards interfacial-compounds/Si3N4 interface and bending strengths maintained-220 MPa as weakening of SILVER-ABA filler was compensated by residual-stress relaxation in Si3N4. When tested at 873 K, joints fractured mainly across the Ag-rich solid solution in a ductile manner and bending strength degraded by-20% to 171 MPa as weakening of SILVER-ABA filler dominated.(c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

The pressure reduction of transient liquid phase bonding (TLPB) technique with sound Ti-6Al-4V similar joint is reported. Eliminating brittle intermetallic compounds (microstructural homogenization) and voids at the interface (void closure) is critical for sound joints with bending strength above the yield strength of Ti-6Al-4V. These two goals are achieved by adjusting the holding time (0.5-12 h) and bonding pressure (0.001-0.3 MPa) at a fixed bonding temperature of 950 C below the phase transus of the substrates. The maximum bending strength of 1570 MPa is obtained with joint having a homogenized microstructure without any voids under joining condition held for 12 h with 0.03 MPa corresponding to 3% of the conventional TLPB technique. The holding time for microstructural homogenization is reduced with 0.3 MPa bonding pressure by liquid push-off from the interface. Low-pressure TLPB is recommended for cost reduction and ensuring shape accuracy of complex geometry components.

A novel two-step bonding of Ti-6Al-4V/Si3N4 joint was developed with Nb interlayer as residual-stress reliever via low-pressure transient-liquid-phase bonding (TLPB) of Ti-6Al-4V/Nb side prior to active-metal brazing of Nb/Si3N4 side. While 1.75 mass% of Ti in a 50-µm-thick CUSIL-ABA® filler was sufficient for sound bonding at Nb/Si3N4 side when brazed at 1103 K for 10 min, one-step-brazed joints with bonding area of 10 × 10 mm2 were prone to failure at the Ti-6Al-4V/Nb side due to brittle Cu-Ti intermetallic compounds (IMCs). Replacing brazing of Ti-6Al-4V/Nb side with TLPB using pure Cu and Ni foils as filler at 1213 K for 180 min eliminated the formation of brittle IMCs via homogenization of (α + β)-Ti; bending strength increased to 193 MPa with residual-stress-induced failure from Si3N4 ceramics. Finally, effectiveness of stress-accommodation via Nb interlayer and filler's plastic flow was quantitatively verified with reasonable fidelity by finite-element analysis incorporating temperature-dependent elasto-plastic properties.

It is desirable to develop high temperature shape memory alloys (HTSMAs) with a martensitic transformation start temperature (Ms) above 100??C and a recoverable strain of about 4??6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We previously demonstrated facilitation of detwinning for Ti??Zr??Ni??Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti??(15??20)Zr??49.7Pd (at%) and surrounding Ni-containing compositions are candidates of HTSMAs having low variant reorientation stress. On the other hand, the aging condition for precipitation strengthening was not optimized, since the candidate alloys show a complex precipitation behavior of two types of precipitates, Ti2Pd-type and H-phase. Therefore, in this study, the effects of aging on precipitation behavior, martensitic transformation temperatures, and shape memory properties were investigated for one of the candidate alloys, Ti??20Zr??49.7Pd, and an excellent shape memory effect with Ms above 130??C, a low reorientation stress around 200 MPa, a high plastic deformation stress around 1800 MPa, and a large recovery strain of 4.5% was achieved after an optimum aging treatment. On the other hand, short-range ordering of solute atoms occurs just above the reverse transformation temperature and decreases Ms, which would limit the number of shape recovery operations when the alloy is used as a device. [doi:10.2320/matertrans.MT-M2022034]

The residual-stress induced failure of Ti-6Al-4V/Si3N4 joints brazed with two different Si(3)N(4 )ceramics possessing different intrinsic properties was elucidated through experimental and finite-element (FE) thermo-mechanical simulations incorporating the local elasto-plastic properties of the as-received Ag-Cu-Ti filler and the brazing zone characterized by nano-indentation. All tested joints fractured mainly from the ceramics due to residual stress, and their bending strengths increased when using Si3N4 ceramics of higher intrinsic strength. FE-analysis based on the as-received filler's properties overestimated the nominal bending strengths by approximately 100 MPa in both joints; nano-indentation revealed that the depletion of ductile Ag-Cu phase and growth of hard Cu-Ti intermetallic compounds reduced the plastically deformable thickness after being brazed, whereby they resulted in higher residual stress in the ceramics. Finally, the validity of FE-estimated bending strengths was enhanced when considering the effective plastically deformable thickness of filler (30 mu m) instead of the as-received state (50 mu m).

The present study aims to understand the high-temperature bending fatigue behavior at the stress-concentrated Y-shape junction of a component made of Tyranno SA3 fiber/BN interphase/SiC matrix composite. The SiC/SiC CMC component was processed via a hybrid technique by combining Chemical Vapor Infiltration (CVI) and Polymer Impregnation and Pyrolysis (PIP) processes. Two different types of tests, namely, point-load-bending-static-test and point-load-bending-fatigue-test, were performed at 1100 °C. The point-load-bending-static test revealed quasi-ductile or shear failure owing to the lower elastic modulus of the PIP-SiC matrix. The first matrix-cracking occurred when the high-stress region reached approximately 180 MPa, which was facilitated by the pores and cracks that remained after the PIP process around the intricate Y-shape junction of the sample. The point-load-bending-fatigue tests were performed under the maximum of the fatigue stresses ranging from 180 to 290 MPa. When the maximum of the fatigue stress was lower than 60% of the static strength, multiple matrix-cracking were pronounced such that the component did not fail and withstand 1000 fatigue cycles. At higher stresses of 250 and 290 MPa, delamination due to shear stress generation at the fiber tows interface and degradation of the BN interphase ultimately led to the fracture of the component at low fatigue cycles of less than 100. Moreover, the fracture morphology of the 290-MPa-fatigue sample showed brittle failure, which was due to matrix-crack-assisted oxygen embrittlement. In addition, a brief comparison between the point-load-bending-fatigue test and the conventional tension-compression fatigue test was discussed.

© 2020 Elsevier Ltd Hypervelocity impact experiments were performed on polycarbonates to investigate the progress of the impact-induced damage process inside the polymer material. Damage evolution and stress wave propagation associated with hypervelocity impact were observed by two imaging methods using ultra-high-speed cameras. The first was two-directional (side-view and rear-view) scattered light imaging. Time-resolved images obtained provided information about the three-dimensional time evolution of not only the damage development, but also the damage texture. The second was cross-polarized shadowgraphs. In this measurement, propagation of the stress field was clearly visualized. The results obtained in this study clearly indicate that the combination of various imaging methods, where each one can provide different information, is a useful way to obtain insights into the damage process and mechanism associated with hypervelocity impact.

The interfacial microstructure evolution and mechanical behavior of Ti-6Al-4V/Si3N4 joints with insertion of a 2 mm-thick Nb interlayer were studied by brazing with Au96.5Ni3Ti0.5 (mass%) alloy as a heat- and corrosion-resistant filler. When the joints were brazed at 1323 K with holding times of 0, 5, and 10 min, multilayered Au4Ti, Au2Ti, AuTi, AuTi3, and Au2Nb3 intermetallic compounds were formed at the Ti-6Al-4V/Nb interface, whereas two distinct regions consisting of Au2Nb3 layer and Au-Ni solid solution were predominantly developed at the Nb/Si3N4 interface. Joints brazed with a 0 min holding time fractured, with initiation at the reactionlayer/Si3N4 interface and propagation into the Si3N4 with a concave morphology because of large residual stress; the joints exhibited a maximum average room-temperature bending strength of 53 MPa.

A novel deployable rocket nozzle utilizing superelasticity was proposed in this study. Ti-4.5Al-3V-2Fe-2Mo alloy (SP-700) sheets were heat-treated to have appropriate alpha/beta ratio so that the sheet shows superelasticity at room temperature. A miniature nozzle model was fabricated through thinning, cutting, and welding processes of the sheets. Folding-deployment tests of the model were conducted in addition to finite element analyses of its folding behavior. The feasibility of the new concept of superelastically-deployable sheet structure was successfully verified.

© 2018 Elsevier B.V. The high-temperature deformation mechanism of the FeCrAl-ODS ferritic steel was investigated at 1000 °C for the creep loading perpendicular to the elongated and aligned grains. The strain rate was varied in the range from the order of 10−2to 10−7s−1. With decreasing strain rate from 10−2to 10−5s−1, creep mechanism shifts from conventional dislocation creep pinned by oxide particles to grain boundary sliding (GBS) assisted concomitantly by diffusional creep. With further decreasing strain rate to 10−7s−1, deformation mechanism is drastically changed; group of three grains can move cooperatively, and cooperative GBS (CGBS) was originally recognized. The threshold stress for onset of CGBS was designated as σthI(CGBS). Rate limiting process of CGBS is dominated by dislocation movement over the oxide particles so as to relieve the stress accumulation due to CGBS. The σthI(CGBS) for CGBS corresponds to one third of the conventional threshold stress for dislocation creep.

The microstructural mechanisms of dynamic anisotropic grain growth during superplasticity in a quasi-single phase Al–Mg–Mn alloy were characterized. The tensile superplasticity with 320% elongation was mediated by grain boundary sliding accompanied by rigid grain rotation with a limited crystallographic slip. The deformed sample exhibited a bimodal microstructure. Some grains maintained their original size and equiaxed morphology during superplasticity, whereas the others became elongated more than twice in aspect ratio and were composed of equiaxed subgrains that were aligned in the tensile axis. These microstructural features were possibly attributed to a rotation-coupled grain coalescence accompanied by grain boundary sliding.

<p>SLIM (Smart Lander for Investigating Moon) is the Lunar Landing Demonstrator which is under development at ISAS/JAXA. SLIM demonstrates not only so-called Pin-Point Landing Technique to the lunar surface, but also demonstrates the design to make the explorer small and lightweight. Realizing the compact explorer is one of the key points to achieve the frequent lunar and planetary explorations. This paper summarizes the preliminary system design of SLIM, especially the way to reduce the size.</p>

<p>Ceramic/metal brazing was investigated to produce light-weight and highly-efficient ceramic thrusters. Silicon nitride ceramic and metal bars were brazed using an Ag-based brazing material. Four-point bend tests were conducted at room and high temperatures to evaluate the strength of the brazed joints. Computational fluid dynamics (CFD) and finite element method (FEM) analyses were also performed to investigate the effect of the construction and shape of the joints on the stress distribution around them. It was demonstrated that brazing was a great candidate as the joining technique, and a 20 N ceramics/metal brazed thruster was successfully produced.</p>

Two-dimensional (2D) grain boundary sliding (GBS), which is useful for phenomenological understanding of superplastic and near-superplastic deformation, was achieved during a high-temperature shear test in oxide-dispersion-strengthened ferritic steel exhibiting anisotropic microstructure with largely elongated and aligned grains. In this study, 2D GBS, dislocation slip and subsequent micro structural evolutions were examined using surface markers drawn by focused ion beam and electron back-scattered diffraction analysis. In the near-superplastic state (region III), GBS was accommodated by transgranular dislocation activities initiating from grain protrusions or triple junctions into core areas, as described by the Ball Hutchison model. The accommodation mechanisms were determined by the microstructural correlation between GBS-triggered stress concentration and available slip orientation and were closely related to the angle theta between GBS and dislocation slippage. When theta was small, GBS tended to be accommodated by a group motion of dislocations belonging to &lt;111&gt; {110} or &lt;111&gt; {112} slip systems (slip-band type). When theta was large, GBS tended to be accommodated by intragranular dislocation accumulation, which led to the development of sub-boundaries along {110} planes via dynamic recovery (sub-boundary type); this would be the origin of continuous dynamic recrystallization. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

© 2017 The Authors. Published by Elsevier Ltd. Hypervelocity impact experiments have been performed on polycarbonate to observe the impact-induced damage process progressing inside polymer material. Impact experiments have been conducted by impacting a sphere made of several kinds of materials using a two-stage light-gas gun. The damage evolution and stress wave propagation associated with hypervelocity impact have been observed via shadowgraphy using an ultra-high-speed camera. Recoded images clearly demonstrate how damages form during the hypervelocity impact event. The obtained time evolution of penetration show that the behavior of stress wave propagation associated with impact vary according to the penetrating motion, and the impact velocity dependence of penetration length and shape change depending on the deformation and fracture behavior of impactor during penetrating process. From these results, it is indicated that not only the mechanical characteristics of impactor material but also the change of penetrating motion associated with the deformation and fragmentation of impactor are key factors for impact-damage formation.

© 2016The high-temperature deformation process of the recrystallized 16CrODS ferritic steel was investigated at 1000 °C for the stress loading perpendicular to the elongated grain structure. The strain rate was varied in the range from 1.0 × 10−2 to 1.0 × 10−5 s−1. At the strain rate over 1.0 × 10−4 s−1, deformation is dominated by the conventional dislocation creep. Decreasing strain rate from 1.0 × 10−4 s−1, grain boundary sliding becomes prominent. Accommodation process for the localized stress induced by grain boundary sliding could be dislocation creep at 1.0 × 10−4 s−1, and by diffusional creep at 1.0 × 10−5 s−1 or less. These were verified through the observation of void formation and localized strain accumulation by KAM map.

The mechanism governing grain boundary sliding (GBS) accommodated by dislocation and micro structural evolution in regions and III was studied to understand superplasticity. Two-dimensional GBS that occurred during high-temperature shear in oxide dispersion strengthened ferritic steel exhibiting an elongated and aligned grain structure was analyzed using surface markers drawn by focused ion beams. In addition, the accommodating dislocation structure was evaluated by electron back scattered diffraction and electron channeling contrast imaging. In the initial stage of deformation, GBS triggered dislocation slippage in "mantle" areas near grain boundaries. These mantles tended to appear around GBS-resistant areas such as curved boundaries and grain protrusions. Next, the mantle dislocations generated dislocation walls before forming low-angle boundaries (LABs) along {110} crystallographic planes via dynamic recovery at the core/mantle boundaries. Finally, secondary GBS or rigid rotation occurred at the newly formed LABs to compensate for the initial GBS and resulted in continuous dynamic recrystallization. These mantle dislocation activities and substructural evolution mechanisms were graphically modeled and validated by comparison with previous studies. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This paper describes a characteristic damage propagation mechanism in low-cycle creep-fatigue of Cu-0.7Cr-0.09Zr (mass%), as investigated by creep-fatigue tests including strain controlled fatigue and stress-holding type creep, and following microstructural observations by scanning electron microscopy (SEM). The total stress-holding time until rupture in the creep-fatigue test was shorter than one-tenth of the rupture life in the simple creep test, and the rupture life of the specimen in the creep-fatigue test was shorter than half of that in the simple fatigue test. The SEM images suggest that the connection between fatigue crack propagating along grain boundaries and intergranular creep voids rapidly accelerates crack propagation. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.

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High-temperature tensile deformation was performed using an oxide-dispersionstrengthened (ODS) ferritic steel, which has grain structure largely elongated and aligned in one direction, in the perpendicular direction. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

The ODS ferritic steels realize potentially higher operating temperature due to structural stability by the dispersed nano-size oxide particles. The deformation process and mechanism of 15CrODS ferritic steels were investigated at 1073 K and 1173 K for the cold-rolled and recrystallized conditions. Tensile and creep tests were conducted at the stress in parallel (LD) and perpendicular (TD) directions to the grain boundaries. Strain rate varied from 10(-1) to 10(-9) s(-1). For the LD specimens, deformation in the cold rolled and recrystallized conditions is reinforced by finely dispersed oxide particles. The dominant deformation process for the recrystallized TO specimen is controlled through the grain boundary sliding and stress accommodation via diffusional creep at temperature of 1173 K and lower strain rate less than 10(-4) s(-1). The grain boundary sliding couldn't be rate-controlling process at 1073 K for the as-cold rolled TO specimen, where a dynamic recovery of the dislocation produced by cold-rolling is related to the deformation process. (C) 2015 Elsevier B.V. All rights reserved.

Two-dimensional grain-boundary sliding (GBS) was achieved microscopically in an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure, which was deformed perpendicular to the long axis. At the border between superplastic regions II and III, microscopic deformation was observed using sub-micron grids drawn on the material surface using a focused ion beam. GBS was accommodated by intragranular deformations in narrow areas around grain boundaries, which has been predicted by earlier researchers as characteristics of the core-mantle model. These observations suggest that dislocations slip only in the mantle regions around wavy boundaries to relax the stress concentration caused by GBS during superplasticity.

Optical methods providing full-field deformation data have potentially enormous interest for mechanical engineers. In this study, an in-plane and out-of-plane displacement measurement method based on a dual-camera imaging system is proposed. The in-plane and out-of-plane displacements are determined simultaneously using two measured in-plane displacement data observed from two digital cameras at different view angles. The fundamental measurement principle and experimental results of accuracy confirmation are presented. In addition, we applied this method to the displacement measurement in a static loading and bending test of a solid rocket motor case (CFRP material; 2.2 m diameter and 2.3 m long) for an up-to-date Epsilon rocket developed by JAXA. The effectiveness and measurement accuracy is confirmed by comparing with conventional displacement sensor. This method could be useful to diagnose the reliability of large-scale space structures in the rocket development.