Abstract: ： In this work, the effect of temperature on the microstructure evolution of vacuum brazed joint of TiAl alloy and 42CrMo was studied. The results have shown when the brazing temperature is under 870°C, no compound is formed at the boundary on the 42CrMo side. But when brazing temperature is over 870°C, a very narrow TiC compound film is found at the homologous site., while on the TiAl side, there always exists a Ti(Cu,Al)2 intermetallic compound layer at any brazing temperature. The elemental diffusion and reaction is the predominant factor of joint forming. The tensile fracture strength of brazed joint increases with the brazing temperature, the increase of the tensile strength of joint is due to the variation of the joint structure.
Abstract: Aiming for further improvement of mechanical properties of Co3AlC-based heat resistant alloys, microstructure control was conducted using optical floating zone (OFZ) melting. Unidirectional solidification was performed to align Co3AlC/a(Co) two-phase eutectic microstructure. Co3AlC single phase poly-crystal alloys were successfully fabricated for the first time by taking advantage of OFZ. Mechanical properties were evaluated for selected alloys by compression tests at ambient temperature, 1073 K and 1273 K. Excellent elevated temperature strength is achieved in Co3AlC single phase alloys and ductility is sufficiently improved in Co3AlC/a(Co) two-phase alloys.
Abstract: Investigation on the crystallographic orientation relationships among D022-Al3Ti,
A15-Mo3Al and high temperature bcc phase consisting of an Al-Mo-Ti ternary alloy with an equi-axed two-phase structure was conducted by FESEM/EBSD analysis. The grains of intermetallic phases have certain crystallographic orientations each other, while the crystallographic orientation distributions become random after a high temperature deformation. This strongly suggests the grain boundary sliding and grain rotating govern the high temperature deformation, which is consistent with the fact that the strain-rate sensitivity m is 0.3 or higher during a steady-state compressive deformation.
Abstract: The phase constitution, lattice parameter, martensitic and magnetic transformation
behavior and hardness of the Ni2MnGa-Cu2MnAl pseudobinary alloys designed as
(Ni2MnGa)x(Cu2MnAl)1-x were investigated in order to improve magnetic properties of Ni2MnGa. It was revealed that L21 Ni2MnGa and Cu2MnAl make a continuous solid solution of (Ni,Cu)2Mn(Ga,Al) when heat treated at 1073K, and that the lattice parameter of the L21 phase increases monotonously with increasing the compositional ratio x, that is, the amount of Cu2MnAl. Curie temperature TC also increases with increasing x. On the other hand, the martensitic
transformation temperature of Ni2MnGa seems to decrease rapidly by adding Cu2MnAl. Hardness of the alloys heat-treated at 1073K ranges from HV200 to HV370, and solution hardening was recognized by mixing. When heat treated at 773K, a phase decomposition from L21 phase to Cu9Al4 and b-Mn was confirmed in the Cu2MnAl-rich alloys. The phase decomposition causes a decrease in the lattice parameter of L21 phase and TC and a significant increase in hardness.
Abstract: Precipitation behavior of intermetallic phases in ferrite matrix is investigated by
transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co (at%) alloys with and without 0.3at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873K. It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free
alloy, the R-phase precipitates instead of I-phase at 873K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973K. The Laves phase formed at 973K transforms to the I-phase in the Si-added alloy but to the R-phase in the Si-free alloy during subsequent aging at 873K. The factors in controlling the phase stability of I-phase, R-phase and Laves phase precipitates
in Fe-based alloys are discussed by the atomic size ratio and electron concentration factor (e/a).
Abstract: Substructure development through aging and annealing treatments was studied for
rapidly solidified TiCo ribbons using TEM. In as-spun ribbons, equiaxed grain structure was developed and its crystal structure was B2-ordered immediately after melt-spinning, while a small amount of fine precipitates existed as second phase. Some grains were dislocation-free but others contained a certain amount of curved or helical dislocations and loops. The dislocation density in the ribbons annealed at 700 °C for 24 h was obviously higher than those in the as-spun ribbons and the ribbons aged at 200 °C for 100 h. The increase of the dislocation density in the annealed ribbons would result from the absorption of excess vacancies. Therefore, the obtained results indicated that a large amount of supersaturated thermal vacancies were retained in TiCo as-spun ribbons by the rapid solidification.
Abstract: The oxidation behavior of the equiatomic TiNi alloy was investigated at 600-900oC. The oxidation resistance of the TiNi alloy was better than the pure Ti alloy, but worse than the pure Ni alloy. The oxide scales consisted of TiO2, with and without embedded particles of TiNiO3 and Ni. The subscale matrix phase was (TiNi3+TiNi) at 600oC, TiNi3 at 700oC, (Ni+TiNi3) at 800oC, and Ni at 900oC. Oxidation was mainly governed by the inward diffusion of oxygen, and the outward diffusion of Ti and a bit of Ni. Detailed microstructures and the oxidation mechanism are proposed.
Abstract: Starting from elemental Co (99.9%), Ni (99.9%) and Sb (99.99%) powders, Co4-xNixSb12 powder mixture with different Ni concentration were subjected to mechanical alloying by using a planetary ball mill. Phase transformation during mechanical alloying process of Co-Ni-Sb system was studied in this paper. Ni substituted skutterudite could be synthesized by solid state reaction. With assistance of hot pressing for two hours, the as mechanically alloyed low Ni concentration powders could be completely transformed into skutterudite compound. The lattice parameter of Ni substituted skutterudite compound complies well with Vegard law when x is not more than 0.2. When Ni concentration is larger than the maximum solubility, NiSb2 compound forms and single phase skutterudite compound can not be obtained.
Abstract: Ten new title compounds were synthesized and their crystal structures refined from X-ray powder diffraction data. For the light rare earth (R = La, Ce, Pr, Nd, Sm) compounds, one of the copper sites is only partially occupied while the occupancies of this position are almost ignorable for the rare earth metals heavier than Gd including Y. The unit cell parameters show normal lanthanide contraction. The bonding energy and density of states of the La compounds are calculated. The results show that the La compound with partial Cu occupancy is more stable than
that without the extra Cu.
Abstract: Fe-Al-Ta alloys with the ternary Laves phase Ta(Fe0.5+x, Al0.5-x)2 are studied
experimentally with the objective of clarifying the effect of Laves phase precipitation and atomic ordering on the deformation characteristics of such Fe-Al-base alloys. The present study concentrates on the hardening effect of the Laves phase in ordered and disordered Fe-Al-Ta alloys with Al contents between 16 and 45 at.% showing the A2 disorder or the DO3 or B2 order. Ta has a low solubility in Fe-Al alloys which is beneficial for slowing down precipitate coarsening. Small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way. The alloys with 1% Ta exhibit a high oxidation resistance. The continuing work aims at adjusting the Al and Ta content for an optimum relation of high-temperature strength and low-temperature ductility with maintaining a sufficient oxidation resistance.