Journal of the Japan Society of Powder and Powder Metallurgy

The effect of W addition on microstructure and creep properties of sintered 25Cr-20Ni-1.3Nb austenitic heat resistant steel is investigated. The 25Cr-20Ni-1.3Nb sintered compact had a structure in which fine spherical NbC was uniformly dispersed in the γ-Fe matrix grains and Cr₂₃C₆ was discontinuously precipitated in a thin film at the γ grain boundaries. In addition, the creep life of those compacts was lower than that of the cast material of the same composition. The addition of W resulted in higher creep properties than the cast material. The improvement in creep properties is due to the reduction in void ratio and grain growth. Fine W powders fill the pore between coarse powders of 25Cr-20Ni-1.3Nb and this leads to an increase in packing density. The increase in W addition caused a change in NbC precipitation from fine particles at intragranular to a film-like shape at grain boundaries. Therefore, the disappearance of fine NbC in the matrix grains, which had suppressed grain growth due to the pinning effect, leads to grain growth and improves the creep property.

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A single-phase RuB₂ (orthorhombic, space group Pmmn) polycrystalline material was successfully synthesized by the arc melting method. The condition for obtaining a single-phase RuB₂ material is dependent on the composition and form of the raw materials, namely using the composition of atomic ratio Ru:B = 1:2.1 and using granules Ru and B as the raw materials. The lattice constants of a single-phase RuB₂ obtained by the arc melt method are a = 4.645(1), b = 2.865(1), c = 4.046(1) Å, V = 53.8(1) Å³. A thermogravimetric-differential thermal analysis (TG-DTA) for RuB₂ was carried out from room temperature to 1473 K. The oxidation reaction of RuB₂ begins at about 570 K, and the weight gain rate of final oxidation is 29%. Interestingly, the RuB₂ material was found to have a significantly lower oxidation resistance than Ru metal. The product after heating up to 1473 K in air atmosphere is a mixture of RuB_1.1 (RuB) and Ru phases, and B₂O₃ which is probably produced in an amorphous state. The values for electrical resistivity of RuB₂ are in the ranges from 23.3 × 10⁻³ to 102.2 × 10⁻³ Ω ‧ cm.

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