Isothermal oxidation behavior of Mo/Mo5SiB2in-situ composites containing small amounts of Al was investigated under an Ar-20%O2 atmosphere in the temperature range of 1073–1673 K. The Mo/Mo5SiB2in-situ composites, (Mo-8.7mol%Si-17.4mol%B)100-xAlx (x=0, 1, 3, and 5mol%), were prepared by Ar arc-melting, and then homogenized at 2073 K for 24 h in an Ar-flow atmosphere. Without addition of Al, Mo/Mo5SiB2in-situ composite exhibits a rapid mass loss at the initial oxidation stage, followed by passive oxidation after the substrate is sealed with borosilicate glass in the temperature range of 1173–1473 K, whereas it exhibits a rapid mass gain around 1073 K. On the other hand, small Al additions, especially of 1 mol%, significantly improve the oxidation resistance of Mo/Mo5SiB2in-situ composites at temperatures from 1073–1573 K. The excellent oxidation resistance is considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides. The protective oxide scales contain crystalline oxides, and the amounts of the crystalline oxides obviously increase with Al concentration.

The effect of heat treatments (aging or annealing) on microstructure was investigated for rapidly solidified ribbons of near-stoichiometric TiCo. In as-spun ribbons, it was observed by TEM that an equiaxed grain structure was developed and its crystal structure had been already B2-ordered, while a small amount of a second phase, Ti2Co, finely disperses in grains and along grain boundaries. Some grains were dislocation-free but others contained curved or helical dislocations and prismatic loops having a Burgers vector parallel to <100> directions. By annealing the as-spun ribbons at 700°C for 24h, the dislocation density was obviously increased compared with that of the as-spun ribbons, while grain growth appears to occur slightly. The increase of the dislocation density in the annealed ribbons is believed to result from the condensation and/or absorption of supersaturated vacancies. Therefore, the TEM observation results indicate that a large amount of supersaturated thermal vacancies were retained in the TiCo ribbons by the rapid solidification.

Relaxation behavior of supersaturated vacancies in water-quenched and rapidly solidified B2 type CoAl and NiAl with the stoichiometric composition was studied by differential scanning calorimetry (DSC). In both CoAl and NiAl quenched from 1773 K, a single exothermic peak due mainly to the recovery of supersaturated vacancies appeared near 950 K. On the other hand, the exothermic peak was also observed in as-spun rapidly solidified CoAl and NiAl ribbons, and the temperature range of the peak was much broader than that of quenched-in bulks, suggesting that the peak in the rapidly solidified ribbons results from not only a single recovery process of vacancies but also those of some other lattice defects. TEM observation showed that a large amount of dislocations were introduced in the NiAl ribbon and that comparatively many voids as well as an amount of dislocations were introduced in the CoAl ribbon.

In B2-type FeAl, supersaturated vacancies retained upon rapidly quenching are absorbed near surface during an aging treatment, being agglomerated into nano- to meso-clusters through the absorption process. Eventually, surface morphology is self-patterned in nano-order by the vacancy clustering. If FeAl was not quenched from high temperature or plastic strain remained near surface, the surface self-patterning never occurs, indicating that the change in surface morphology is caused by the clustering of supersaturated vacancies. The clusters have specific shape with cluster surfaces faceted toward {100} planes. Thus, the shape of the clusters formed near surface is controllable by changing surface orientation. Vacancy cluster size and its distribution density can be also controlled by varying the concentration of supersaturated vacancies and/or the clustering condition. These indicate that the vacancy clustering is a unique process to efficiently pattern the surfaces of metals, alloys and intermetallics in nano-scale.

Rapidly solidified ribbons of B2-ordered Fe-40, 45 and 50mol%Al were produced by a conventional single-roll melt-spinning method. The lattice parameters of as-spun ribbons are fully restored by annealing at 723 K for 24 h. This suggests that large numbers of supersaturated thermal vacancies are removed by the heat treatment. After the heat treatment, it is found that clustering of the supersaturated vacancies leads to a large number of pores that have a few hundreds nm or less in diameter near the surfaces, thus creating nanoporous surfaces. DSC measurements show irreversible exothermic peaks due to vacancy clustering. Vacancy complexes such as dislocations and pores are also observed inside the ribbons by TEM. The volume fraction of the overall vacancy complexes shows Al concentration dependence, demonstrating that defect structure formed by clustering of the excess vacancies is controllable by changing Al concentration in rapidly solidified FeAl ribbons.

Pulverization behavior and microstructure evolution with hydrogenation in hydrogen absorbing Ta-Ni intermetallic-based alloys, such as Ta2Ni with Ta solid solution (Tass), TiMn2 with TiMn and Nb3Al with Nb solid solution (Nbss), are investigated to elucidate the mechanism of the hydrogen pulverization. Ta-10at.%Ni consisting of Ta solid solution (Tass) and Ta2Ni Laves phase is pulverized to coarse powder over 100 μm in hydrogenation. Crack propagation occurs preferentially in the brittle Ta2Ni phase rather than in the ductile Tass phase. When the volume fraction of brittle Ta2Ni increases with increasing Ni content, hydrogen pulverization is enhanced. The lattice parameter of Tass increases by hydrogenation, while it does not change in Ta2Ni. In addition, nano-sized regions with Moiré patterns are produced in Tass and Debye rings corresponding to tantalum hydride β-TaH appear in the diffraction pattern. These features are very similar to those of TiMn2 based alloy and Nb3 Al based alloys in the literature. Based on the present results along with those in the literature it is concluded that the hydrogen pulverization is attributable to (1) the absorption of a large amount of hydrogen in constituent phase(s), (2) the large strain introduced by lattice expansion and the hydride formation, and (3) the ease of crack nucleation and propagation in brittle constituent phase(s).

Directionally solidified Nb-xMo-22Ti-18Si (x = 10, 20 and 30 mol%) and Nb-10Mo-yW-10Ti-18Si (y = 0, 5, 10 and 15 mol%) alloys were prepared. All of the alloys consisted of Nb solid solution and (Nb, Mo, (W,) Ti)5Si3 silicide. In compression tests performed on the Nb-xMo-22Ti-18Si alloys, a sample with x = 10 had the highest maximum stress at 1670 K, but it showed a sharp stress drop after reaching the maximum stress. Samples with x = 20 and 30 showed lower yield and lower maximum stress than that with x = 10, and they showed a small stress drop and an almost constant flow stress of 330 and 400 MPa, respectively. The sample with x = 20 showed a minimum creep rate (εm) of 9.6 × 10-7 s-1 at 1670 K at a stress of 200 MPa. The yield stress of the Nb-10Mo-yW-10Ti-18Si alloys increased with increasing W content, although the stress decreased gradually after reaching the maximum stress and showed no steady state deformation. The εm of the sample with y = 15 was 1.4 × 10-7 s-1 at 1670 K at a stress of 200 MPa, which was much slower than that of Nb-20Mo-22Ti-18Si.

An alloy composed of L12-type Zn3Ti was investigated in terms of phase stability and mechanical properties. Zn and Ti powders were mixed at a composition of Zn-25mol%Ti using a ball mill in Ar, and an ingot was made by melting the powders. Optical microscopy, X-ray diffraction analysis and thermogravimetry - differential thermal analysis were carried out. Mechanical properties were investigated by Vickers hardness tests at room temperature (RT) and compression tests from RT to 703K in Ar. It is found that (1) the alloy is mainly composed of L12 Zn3Ti, (2) the alloy has weak positive temperature dependence of strength, and (3) normalized strength by melting point is comparable to that of L12 Al3Ti-Cr alloys. L12 Zn3Ti has HV178 and is brittle at RT. Reaction temperatures of Zn-rich portion of the Zn-Ti phase diagram were also reinvestigated and a peritectic-reaction temperature between Zn3Ti and liquid + Zn2Ti is determined to be at 880K.

Phase stability of κ-phases (perovskite-type phases) was studied in the third-period transitionmetals (M: Mn to Cu) - aluminum (Al) - carbon (C) systems. The concentrations of transition metals (CM) were systematically changed from 60mol%Mn to 20mol%Ni+40mol%Cu along the periodic table under the constant nominal alloy compositions of CMl+CM2=60mol%, CA1=60mol% and Cc=20mol%. Alloys were prepared using mechanical alloying and hot pressing. Constituent phases were investigated by X-ray diffraction analysis, scanning electron microscopy and electron probe microanalysis. It is confirmed that Mn3Al(C,O)0,95, Fe3Al(C,O)0.87, Co3AI(C,O)0. 6 and Ni3Al(CO)0.26 exist in their ternary systems where oxygen is introduced during the process. Also, only one κ-phase is recognized in an alloy having an intermediate composition in every system. This suggests that continuous solid solutions between these κ-phases are formed in the pseudo-ternary systems. Composition dependence of lattice parameter is discussed.

Mechanical properties of E21 (Mn, Fe)3AlC-base alloys were investigated. The E21 crystal structure is closely related to L12, and thereby E21 compounds are expected to exhibit superior properties similar to those of the L12 Ni3AL. Nominal compositions are fixed to be 60mol%(Mn, Fe)-20mol%A1–20mol%C. Alloys were prepared by mechanical alloying and hot pressing. ICP chemical analysis, X-ray diffraction analysis, scanning electron microscopy and electron probe microanalysis were carried out for alloy characterization. Mechanical properties were evaluated by Vickers hardness tests at room temperature (RT) and compression tests from RT to 1273K. Strain rate dip tests were also carried out. It is found that most alloys are composed of two phases of E21 as the primary phase and graphite as the precipitates, and that the volume fraction of graphite increases with increasing Fe content. Hardness and 0.2% flow stress at RT are raised with increasing Fe content. At RT, 0.2% flow stress and fracture strength of Fe3A1C alloy used are 2.9GPa and 3.4GPa, respectively. An alloy containing 40%Mn-20%Fe shows weak positive temperature dependence of strength at 700–800K, similar to the observation in some Co3AIC alloys. Moreover, work-hardening coefficient of all alloys shows strong positive temperature dependence below 700K. These results suggest the occurrence of K-W related mechanism for plastic deformation in these alloys.

Ir should be used as an effective oxygen diffusion barrier (ODB) for ultrahigh temperature structural materials since If exhibits extremely low oxygen diffusivity. Oxidation resistance of Ir is, however, not good due to formation of gaseous oxide, IrO3, over 1390K. In this study, the improvement of oxidation resistance was aimed through alloy design of alloying with Al. IrAl is expected to form a self-healing multifunctional layered structure composed of Ir layer as ODB and A12O3 layer as a protective oxide (PO) on the Ir layer. Ar arc-melted IrAl alloy was crushed into powder, followed by hot pressing and heat treatment to remove Ir formed by eutectic reaction. Oxidation behavior was measured using thermogravimetry (TG) and differential thermal analysis (DTA) under the conditions of (1) dynamic heating of 0.167K/s and (2) isotherms at 1273K to 1673K in O2 environment. It was found that oxidation resistance is much improved by alloying with At and that the designed structure (PO/ODB) is formed on the IrAl substrate. Compressive mechanical properties were investigated from RT to 1873K: both the strength as a function of normalized temperature and specific strength are higher than those of pure Ir, NiAl, TiAl and Ni3A1. IrAl is promising as a advanced smart coating material equipping good oxidation resistance as well as high temperature strength.