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A novel procedure of ODS alloy fabrication was examined using a model alloy in the Fe Al O ternary system. Nano-sized Al oxides were precipitated within the α-Fe matrix through a reaction between Fe Al and Fe2O3 powders. Two types of oxides, FeAl2O4 and γ-Al2O3, were found to precipitate during sintering at 1000 °C and the subsequent annealing at 800 °C. The oxides showed sluggish coarsening at 800 °C, which then provided high resistance to coarsening for the α-Fe grains. Microstructure observation and Vickers hardness measurement indicated that the homogeneous dispersion of oxides was achieved through the present procedure without ball milling.

The relation of creep rupture life to minimum creep rate, namely Monkman-Grant (MG) relation is examined by using a creep database of modified 9Cr-1Mo steel. The database covers wide ranges of creep rupture life up to 1.2 × 105 h and of minimum creep rate down to 7.6 × 10−8 h−1. The MG plot exhibits a low overall value of MG exponent (p = 0.85) together with wide scatter of data points at long rupture life. Causes of the low p value and the wide scatter are discussed in the present study paying attention to change in creep curve shape with creep rupture life. There are four regions H, M, L1 and L2 showing different creep behavior in the steel. Regional MG relations are examined also in each region. Values of p are less than unity and change among the four regions. The p value is especially low (0.62) in long-term region L2. The regional p values less than unity result in the low overall p value of 0.85. The especially low regional value of p in the long-term region causes the wide scatter of data points in the overall MG plot. A modified Monkman-Grant equation (Eq. (29)) is proposed taking account of creep deceleration rate A (=−1/(dln e ˙ /de), where e ˙ and e are the creep strain rate and the creep strain) in primary creep and creep acceleration rate B (=1/(dln e ˙ /de)) in tertiary creep. The rates characterize shape of a creep curve. The modified equation can explain the difference of p values among the four regions.

Grade 91 steel is widely used as steam pipes in ultrasupercritical (USC) steam boilers. In residual creep life assessment of the pipes by calculation, one needs creep rupture life of the steel as a function of stress and temperature in a time range longer than 105 h. Four regions with different creep rupture characteristics appear in a stress versus creep rupture life diagram of the steel. Main steam pipes made of the steel are used in a long-term region with low values of stress exponent and activation energy for creep rupture life (referred to as region G in this paper). Creep rupture lives of the steel in this region vary from heat to heat depending on their prior austenite grain size. This paper proposes a grain size-dependent equation representing creep rupture life of the steel in region G. The equation is verified with creep rupture data up to 232,833 h at 600 °C. Region G is absent in some heats with a large grain size. The equation can rationalize the absence in the heats. In a stress versus creep rupture life diagram of grade 92 steel, there is the same long-term region G. In the region, a creep rupture life of each heat is dependent on its grain size as is the case in grade 91 steel. The proposed equation accords well with the creep rupture lives of the grade 92 steel in region G.

Larson-Miller (LM) and Orr-Sherby-Dorn (OSD) parameters are widely used when formulating and predicting creep rupture life tr. The LM constant C and the activation energy Q in the OSD parameter characterize temperature dependence of tr. Q = QLSD (QLSD: activation energy for lattice self-diffusion) and C = 20 are often assumed in the formulation. Creep rupture datasets of 11 heats of nickel alloy A617, 304H and 304J1 stainless steels, 6 heats of A3004N aluminum alloy and four Mg-Al alloys are formulated with the parameters for determining C and Q of each dataset. Based on the correlation between C and Q among similar materials, it is discussed what is the C value equivalent to Q = QLSD. It is also examined how the C value changes with melting temperature of the material. The value of C equivalent to Q = QLSD varies from 14 to 9.5 depending on average values of log(rupture life) and of reciprocal temperature of the dataset. These values of C point out that C = 20 is not equivalent to Q = QLSD. The equivalent C values are similar in all the materials independently of their melting temperatures.

The liquidus projection and the invariant reactions associated with the liquid phase in the Zr-Si-Al ternary system were experimentally determined. Seventeen primary phase regions were identified from metallographic observation and analysis of the as-cast microstructures. There are at least sixteen invariant reactions and three local maxima associated with congruent melting in the ternary system. The ZrSi primary solidification region covers a wide range of compositions in the Zr-Si-Al ternary system. A variety of the solidification paths were described.

Boron segregation behavior during tempering at 600 and 790 °C was examined in a boron-doped 9Cr-steel by using high-resolution SIMS imaging (nano-SIMS). Boron segregation that developed at prior austenite grain boundaries (PAGBs) during austenitizing was found to decrease reduced during tempering because the equilibrium concentration of B segregation for austenite is larger than that for ferrite so that the segregated boron atoms diffused into block and packet boundaries. The size and the volume fraction of the carbide precipitates at PAGBs were smaller than those at packet boundaries, suggesting that boron segregation at PAGBs suppresses the carbide precipitation upon tempering.

YB4/YB6 two-phase alloys were prepared by arc-melting of high purity elemental materials. The Young's moduli of YB4 and YB6 were evaluated by nanoindentation technique. During loading of nanoindentation, pop-in events were clearly observed for YB4 and YB6, suggesting that both have some capability of dislocation emission at room temperature. TEM observation depicts that dislocations have formed in YB4 and YB6 due to the thermal stress that developed during cooling following arc-melting. The Burgers vectors of these dislocations were identified to be [001] for YB4, and 〈100〉 for YB6. Potential slip systems that are indicated to operate were (100)[001] for YB4 and {100}〈001〉 for YB6.

Heat-to-heat variation of creep strength is significant in grade 91 (Gr.91) steel, but the causes of the variation have not been well understood yet. In the present paper, creep rupture data of 14 heats of Gr.91 steel were analyzed paying attention to their chemical compositions and microstructures. The longest creep rupture lives analyzed are 2 × 105 h at 500 and 550 °C and 105 h at 600 °C. The causes of the heat-to-heat variation are different, depending on creep test conditions. At low temperature and high stress (creep rupture life of 104 h at 500 and 550 °C), creep rupture strength increases with increase of hardness after tempering. This suggests strengthening by a fine subgrain microstructure developed during normalizing and subsequent tempering. At higher temperature and intermediate time range (104 h at 600 °C), creep rupture strength depends on Cr concentration of the heats in addition to the hardness. This finding suggests an important contribution of recovery process of the subgrain microstructures to creep strength of the steel. In long-term creep (2×105 h at 550 °C and 105 h at 600 °C) creep rupture strength primarily increases with increasing grain size of the heats. This suggests that grain boundary sliding is an important deformation mode at low strain rate because of fine grain size usual with Gr.91 steel. Specifications on Ni concentration and N%/Al% ratio are newly introduced in the type II version of Gr.91 steel. They are not effective to eliminate a heat with low creep strength.

Al3Nb is known as a high oxidation resistant material, while it is quite brittle. As the fracture toughness of Al3Nb single crystal and its dependence on the composition are not obtained, the micro-sized fracture testing proposed by Suzuki et al. was performed. Al3Nb single crystal micron-order size cantilevers with a chevron-notch were fabricated in a grain of two-phase polycrystalline alloys by using FIB (Focused Ion Beam). From the load-displacement curves during the bending by a nanoindenter, the average value of fracture toughness of Nb-rich Al3Nb is evaluated to be 2.90 MPam1/2, while the fracture toughness of Al-rich Al3Nb is also evaluated to be 2.82 MPam1/2. From this result, the fracture toughness of Al3Nb is less dependent on its Al/Nb ratio. Furthermore the fracture toughness of Al3 (Nb, V) was evaluated to be 2.82 MPam1/2.The fracture toughness of Al3Nb is seemingly insensitive to V addition.

The applicability of radio frequency glow discharge (rf-GD) sputtering to the surface finishing of nanoindentation specimens was examined for pure Al, Cu, and Ni. Comparisons were made between specimens that had been subjected to the following surface finishing: (1) rf-GD sputtering, (2) electropolishing, and (3) mechanical polishing with 0.25 µm of diamond suspension. It was suggested from the nanoindentation behavior and the quality of electron backscattered diffraction patterns that residual damage on the surface of the rf-GD sputtered samples was as small as that of electropolished samples, while some damages remained on the surface of the mechanically polished samples. The areal roughness within a 1-μm square of the rf-GD sputtered surface was comparable to that of the mechanically polished surface and slightly larger than that of the electropolished surface. Nonetheless, the difference of the roughness within the range of this study has no practical impact upon nanohardness and elastic modulus evaluation. Pop-in events occurred in the electropolished and rf-GD sputtered specimens, but not in the mechanically polished samples. The critical load for pop-in to occur was slightly smaller in rf-GD sputtered sample than in electropolished samples.

The present paper studies stress, temperature and heat dependence of creep rupture life of 9Cr-1.8W-0.5Mo-VNb (Gr. 92) steel with creep rupture data of 4 heats reported in National Institute for Materials Science Creep Data Sheet 48B. In creep of Gr. 92 steel there are four regions, H, M, L and G, showing different stress and temperature dependence of creep rupture life. Creep rupture life of Gr. 92 steel increases with increasing hardness after tempering in Regions H and M (high and medium stress), and with increasing prior austenite grain size in long-term Region G. Its creep rupture life is insensitive to the heats in low-stress Region L. Creep rupture lives of the steel are formulated in each region as a function of stress, temperature and a metallurgical variable determining its creep rupture life. Two types of breakdown in creep strength (substantial decrease in stress exponent for creep rupture life) take place in Gr. 92 steel; one from Region M to Region L, and the other from Region M or L to Region G. Since Gr. 92 steel is used in Region G in steam power boilers, the second breakdown to Region G is of engineering importance. Causes of the second breakdown are discussed with creep rupture data of four grades of creep strength enhanced ferritic steels.

The three-phase, (Mo,Nb) ss /(Mo,Nb) 5 Si 3 /(Mo,Nb) 5 SiB 2 , Bcc/T 1 /T 2 microstructures that develop in Mo–Nb–Si–B alloys have been examined in arc cast and directionally solidified samples to identify the phase sequencing during solidification. A Mo-32.6Nb-19.5Si-4.7B (at.%) alloy was directionally solidified using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/h. The microstructure of the directionally solidified alloy is characterized by an elongated T 2 phase surrounded by bcc and T 1 phases with an interwoven morphology From the evaluation of the microstructures in arc cast ingots of several alloys at a constant 32.6 at%Nb composition, the path of the liquidus valleys with decreasing temperature has been determined to intersect at a common point that establishes the solidification reaction as a eutectic. The incorporation of the experimental results into a computational thermodynamic analysis provides insight on the partitioning of components within each phase during solidification.

The oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy (38Mo–30Ti–17Si–10C–5B, at.%) was systematically studied in the temperature range of 700–1000 °C. Temperature dependence of oxidation kinetics and oxide scale formation was revealed for this alloy. The oxidation kinetics at 700 °C featured an initial transient stage of weight gain and then switched to weight loss. At 800 °C, the oxidation kinetics featured an initial transient stage with fast weight loss, a steady stage with small weight loss, and an acceleration stage leading to catastrophic failure. At 900 and 1000 °C, the alloy exhibited a rapid transient weight loss as followed by a decreased weight loss fitting to parabolic kinetics. The scales that formed at 700–1000 °C mainly consisted of rutile TiO2 and amorphous SiO2. The high viscosity of SiO2 at 700 and 800 °C caused the formation of porous and microcracked oxide scales while the occurrence of viscous flow during oxidation at 900–1000 °C promoted the formation of dense and protective scales.

In this study, the crystallographic orientation relationship between bcc precipitates and a Ti5Si3 matrix in Mo–50Ti–14Si–6C–6B (at.%) alloy was established. The alloy microstructure consisted of the bcc solid solution, Ti5Si3, Mo5SiB2, and TiC phases. The metal-rich Ti5Si3 phase was formed during a solidification stage, followed by the precipitation of the bcc phase within the Ti5Si3 matrix in the subsequent annealing process. The orientation relationship between the two phases was described by the expression: 112 bcc / / 01 1 ‾ 0 Ti 5 Si 3 and 3 ‾ 11 bcc / / 0001 Ti 5 Si 3 . The formation of a low-energy interface between the two phases was explained by the edge-to-edge matching of their corresponding lattice planes at the faceted interface.

Predictions as to 105 h creep rupture strength of grade 91 steel have been made recently. The predicted values are examined with long-term creep rupture data of the steel. Three creep rupture databases were used in the predictions: data of tube products of grade 91 steel reported in National Institute for Materials Science (NIMS) Creep Data Sheet (NIMS T91 database), data of T91 steel collected in Japan, and data of grade 91 steel collected by an American Society of Mechanical Engineers (ASME) code committee. Short-term creep rupture data points were discarded by the following criteria for minimizing overestimation of the strength: selecting long-term data points with low activation energy (multiregion analysis), selecting data points crept at stresses lower than a half of proof stress (σ0.2/2 criterion), and selecting data points longer than 1000 h (cutoff time of 1000 h). In the case of NIMS T91 database, a time–temperature parameter (TTP) analysis of a dataset selected by multiregion analysis can properly describe the long-term data points and gives the creep rupture strength of 68 MPa at 600 °C. However, TTP analyses of datasets selected by σ0.2/2 criterion and cutoff time of 1000 h from the same database overestimate the data points and predict the strength over 80 MPa. Datasets selected by the same criterion from the three databases provide similar values of the strength. The different criteria for data selection have more substantial effects on predicted values of the strength of the steel than difference of the databases.

We performed in situ compression of interstitial-free steel nanoblades using transmission electron microscopy (TEM) in order to determine the relation between the evolution of the dislocation structures and the flow stress during deformation. In the early stage of deformation, the sample deforms elastically with a few dislocation motions. The dislocation multiplication processes have been discussed. Remarkable plastic softening with increasing dislocation density is observed after the maximum stress is reached, which can be understood as a situation in which the dislocation density is the dominant factor affecting the softening based on the Johnston–Gilman model.

Phase equilibrium between the α-Nb5Si3 and β-Nb5Si3 phases has been studied in the Nb–Mo–Si ternary system. The high temperature β-Nb5Si3 phase is stabilized by Mo additions to yield a relatively narrow α/β two-phase field in the Nb–Mo–Si ternary system. The enthalpy of the α–β phase transformation has been evaluated from the experimentally determined two-phase field boundaries lines in Nb-xMo-37.5Si (x = 0–10) alloys as 21 (±3) kJ/mol-atom. The β-Nb5Si3 phase exhibits a temperature dependent solubility, which results in the formation of Nbss precipitates both in the α- and β-Nb5Si3 matrices. The orientation relationship between Nbss and Nb5Si3 has been identified by transmission electron microscopy. Lattice parameter variation of the β-Nb5Si3 phase suggests that the departure of the β-Nb5Si3 phase from stoichiometry toward (Nb + Mo) compositions is derived from the anti-site substitution of Nb and Mo atoms for Si sites.

Microstructure and creep behavior of a commercial ZrO 2 dispersion-strengthened Pt alloy were investigated. Emphasis was placed on examining the correlation between the minimum creep rates and the applied stress in the temperature range of 1273–1773 K. The ODS Pt alloy exhibited a highly elongated grain structure that had developed by a thermo-mechanical processing. The interfaces between ZrO 2 particles and the Pt matrix were found to be incoherent. An attractive interaction appeared to be present between dislocations and ZrO 2 particles. The detachment controlled dislocation creep model was used to explain the steady state creep behavior. The presence of ZrO 2 inclusions was also beneficial for maintaining the grain structure and the subgrains, which then provides increased resistance to high temperature creep deformation.

Possibilities of heat resistant alloys based on a C15 Laves phase and an FCC phase have been examined in the Ir-Pt-Y ternary system. Although the Ir solid solution phase (A1) and the Ir2Y phase (C15) are not in equilibrium in the Ir-Y binary system, this equilibrium is attained by small Pt additions to the binary system. High temperature compressive strength of an A1/C15 monovariant eutectic alloy was found to be much lower than that of Ir-15Nb, an Ir based γ/γ' alloy. Low strength of the present alloys is attributed to the absence of effective strengthening mechanisms that operate in the A1 phase; for Y is hardly dissolved within the A1 phase, by which solution hardening and precipitation hardening are not available.

Dislocation character and operative slip systems in α-Nb 5 Si 3 were examined by transmission electron microscopy. Two-phase alloys comprised of (Nb) and α-Nb 5 Si 3 were used in this study. Although few dislocations were present in the α-Nb 5 Si 3 phase of a pre-deformed alloy, many developed after 15% of compressive deformation at 1673 K. Two types of the Burgers vectors were identified for α-Nb 5 Si 3 : 5 Si 3 at 1673 K were determined as {011)

A Laves phase Ir2Y is found to exhibit a temperature dependent solubility, which yields Ir3Y precipitation within the Ir2Y matrix of an arc-melted Ir-30at.%Y alloy. Microstructure observations have confirmed that the precipitation of Ir3Y has already started in the as-cast state. Upon annealing of an as-cast Ir-30at.%Y, the volume fraction of Ir3Y precipitates increases and finally reaches about 50%. The orientation relationship between Ir3Y and Ir2Y has been identified as: (0001)Ir3Y//(111)Ir2Y and [21 _ 1 _ 0]Ir3Y//[1 _ 10]Ir2Y. The orientation relationship is rationalized by the atom matching at the habit plane between the two phases.

A C15 Laves phase Ir 2 Y that forms in the Ir-Y binary system develops off-stoichiometry toward the Ir-rich composition at high temperatures, and its range of homogeneity becomes narrow with decreasing temperatures. One consequence of this solubility behavior is the formation of Ir 3 Y precipitates within the Ir 2 Y matrix of an Ir-30mol%Y alloy. The orientation relationship between Ir 3 Y and Ir 2 Y has been identified as: (0001) Ir3Y // (111) Ir2Y and [2110] Ir3Y // [110] Ir2Y . The similarity in the crystal structures of the two phases yields a plate-like Ir 3 Y precipitate showing a typical Widmanstatten structure. The growth of the precipitates has been suggested to follow the ledge mechanism. The interfacial character between the two phases has been identified in this study.

Nanoindentation measurements of the grain interiors of an ultra-fine grained (UFG) pure Al produced by equal channel angular pressing were taken to evaluate the contribution of the matrix strength. Specimens were subjected to 0, 1, 2, 4, and 8 passes at ambient temperature. The nanohardness of the deformed samples was always higher than that of the undeformed sample 0P in the range of the indentation depth that was investigated, suggesting a strengthening of the matrix in the UFG Al. The increase in hardness that was contributed by the matrix to the macroscopic scale hardness was significantly large in about 40% of the deformed samples. The microstructural characterization and the deformation response analysis with the pop-in event during indentation suggested that the strengthening of the matrix originated from dislocation strengthening and some other presumable factors in the grain interiors.

The phase equilibria and oxidation behavior of Ir–Y binary alloys have been investigated. A phase diagram at the Ir-rich end in the Ir–Y binary system has been proposed on the basis of experimental results. Iridium is found to dissolve little Y in the FCC solid solution. The pure-Ir and Ir–Y binary alloys exhibit severe weight loss upon air-exposure at 1373 K with elapsed time. Protective oxide layers are not formed on the surface of the alloys with Y up to, at least, 20 at.%. The kinetics of the oxidation is controlled by the formation and sublimation of Ir oxides at the surface.

Dislocations and planar faults are found to develop in the Mo5SiB2 phase during elevated temperature annealing. The dislocations formed in Mo5SiB2 of annealed alloys are mostly edge dislocations with the Burgers vectors of 〈100], 1/2〈111], and 〈110]. For the Mo-rich Mo5SiB2 phase that formed in a Mo–10Si–20B alloy, a significant amount of constitutional vacancies is introduced during solidification, and the excess vacancies aggregate and collapse into dislocations. At the same time, these dislocations can act as the heterogeneous nucleation sites for the subsequent Mo precipitation. For the Mo-lean Mo5SiB2 phase that formed in a Mo–13Si–28B alloy, the constitutional defects are not significant for the development of dislocations.

Various investigations have been attempted to improve the low temperature ductility of Nb-silicides by microstructural control while they show superior high temperature strength. Present authors have focused on the microstructure evolution through the eutectic and eutectoid reactions in Nb-rich portion of Nb-Si binary system, and with small amounts of additives (Zr or Mg) alloys large Nb grains with fine silicide (α-Nb 5 Si 3 ) particles have been obtained, which is attractive for high temperature use. For further understanding of this phenomenon, the present study has two objectives; one is to apply the advanced solidification technique for further microstructure control, and the other is to investigate the effect of co-existence of Zr and trace amount of Mg on the microstructure evolution during the eutectoid reaction in terms of the interfacial energy between phases. EBSD analysis revealed that uni-directionally solidified alloy show the same crystallographic orientation relationship (O.R.) between Nb and α-Nb 5 Si 3 with that in arc-melted alloy having the same composition. On the other hand, Mg-doped alloy containing Zr shows an O.R. which was not observed in previous works. This implies that Mg doping is effective to control the interfacial energy between Nb and α-Nb 5 Si 3 even in Nb-Si-Zr alloys. Two-step heat-treatment is found to be effective to obtain finer microstructure, and a further investigation on the controlling factors of eutectoid decomposition will provide a proper route to well-controlled microstructures.

Unidirectional solidification experiments were performed on the (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems. The effect of solidification rates on microstructure development was studied using an optical floating zone melting furnace within the solidification rates from 10 to 200 mm/h. For the binary invariant eutectic alloy, a planar eutectic microstructure forms at 10 mm/h, while cellular eutectic microstructures develop at the higher solidification rates. The orientation relationship between Nb and Nb3Si formed upon planar eutectic solidification is determined as: {1 1 0}Nb//{1 1 0)Nb3Si, and 〈1 1 2〉Nb//〈0 0 1]Nb3Si. For the ternary univariant eutectic alloys, the formation of cellular eutectic microstructures is prevailing for all over the rates of solidification. The growth of Nb3Si is faster than that of (Nb). The size distribution of (Nb) particles spreads widely in an asymmetric manner. The cell size variation against solidification rates is in a reasonable agreement with the theoretical prediction.

The (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems were unidirectionally solidified in an optical floating zone melting furnace. The effect of solidification rates on microstructures of the alloys is investigated. The coupled growth of (Nb) and Nb3Si is observed in the binary alloy solidified at 10 mm/h. The ternary alloys exhibit the development of discontinuous (Nb) particles within the Nb3Si matrix. The unidirectionally solidified alloys show improved fracture toughness and enhanced high temperature compressive strength as compared with the arc-melted counterparts. The role of an aligned eutectic microstructure in toughening is discussed from the existing models based on the crack bridging and the crack deflection.

The critical cooling rate for glass formation was measured for an Fe-based easy glass forming alloy using a wedge casting technique combined with direct temperature measurements and finite element heat transfer analysis. For an alloy with a nominal composition Fe 48 Cr 15 Mo 14 Y 2 C 15 B 6 , the critical cooling rate was measured to be in the range 140–190 K/s. The low critical cooling rate indicates that this alloy may be suitable for use in thermal spray processes. The main crystalline phase competing with glass formation during continuous cooling appears to be a M 23 C 6 type phase.

Dual phase (α+β) copper–tin alloys were synthesized from elemental nanoparticle precursors following consolidation, annealing and quenching steps. The precursors were derived from the sequential reduction of tin and copper salts in ethanolic solution. The dual phase microstructure exhibits a duplex character with a mixture of Cu-rich and Sn-rich (α+β) regions.

Design of two-phase alloys consisting of niobium solid solution (A2) and α-Nb 5 Si 3 (D8 l ) phases in the Nb-Ti-Si ternary system is pursued for ultra-high temperature structural applications. Compositional and annealing conditions are determined for the formation of A2/D8 l lamellar microstructure via eutectoid decomposition of Nb 3 Si (tP32) phase in the Nb-Si binary and Nb-Ti-Si ternary system. Addition of titanium is found to result in increased room temperature toughness, but decreased high temperature strength. Enhancement of mechanical properties is achieved by applying the directional solidification technique. Mechanical properties of the alloys are discussed from microstructural point of view.