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Attempts were made to improve the mechanical properties of an orthorhombic Ti 2 AlNb-based Ti-22Al-27Nb intermetallic alloy through microstructural and compositional modifications, and the dispersion of fine TiB particulates. A Ti-22Al-27Nb alloy with a prior B2 grain size ranging from 8 μm to 49 μm was successfully obtained using spherical α 2 particles as obstacles to grain growth. The finest grained material showed an excellent combination of room temperature tensile strength (around 1,000 MPa) and tensile ductility (more than 15 %). Transition metal elements such as Mo, V and W for a portion of the Nb in the Ti-22Al-27Nb were substituted. The guideline for this compositional modification required that the beta phase stability in the modified alloy be equal to that of the Ti-22Al-27Nb. It was found that the substitution of 2 % W for 7 % Nb was quite effective in increasing tensile strength at temperatures above 923 K and reducing the steady state creep rate and primary creep strain. The Ti-22Al-27Nb alloy-based particulate composites reinforced with 6.5 % TiB were successfully produced with the gas atomization P/M method. The dispersion of extremely small TiB was very uniform. Most of the mechanical properties of the composites, including the yield stress, tensile strength, Young's modulus, resistance to creep, and high cycle fatigue strength were greatly superior to those of the matrix alloy.

The Ti-6Al-4V alloy has been widely used for structural materials due to high specific strength property, however it is difficult to cast because of high reactivity at high temperature. In this study casting characteristics and post-casting heat treatment were studied in the cast Ti alloy. The cast alloy was prepared using high-frequency induction heating and horizontal vacuum centrifugal casting to reduce reaction with crucible and mold by minimizing overheating of the molten metal and casting fast. In the thin casting with a thickness of 2 mm, α’ martensite was observed, and in the casting with a thickness of 4~8 mm, α+β Widmanstätten was observed. As the thickness of the casting increased, the grain size, α-lath width, and α-case thickness increased. Accordingly, as the thickness of the casting increased, the hardness of the matrix decreased and the hardness of the surface increased. In addition, when the casting was heat treated below the β-phase transformation temperature, the α-lath width increased, and when quenched after heat treatment in the β-phase transformation temperature region, an α’ martensite was formed. On the other hand, when the casting was heat treated in the atmosphere, it was confirmed that the elongation was very low because hydrogen and oxygen in the atmosphere were induced. In addition it was confirmed that casting defects can be removed through HIP.

Microstructure evolution and the creep fracture characteristics of dissimilar welds between Ni-based superalloy Inconel 740H and High Cr ferritic steel Gr.92 were investigated. The dissimilar welds were fabricated by shielded metal arc (SMA) welding process with Ni-based austenitic Inconel 182 superalloy. Postweld heat treatment at 760 o C for 4 hours was conducted to temper the fresh martensite formed in the heat-affected zone (HAZ) of Gr.92, and to form γ΄ strengthener in Inconel 740H. The weld interface between the Gr.92 and weld metal was characterized by a discrete line with minimal inter-diffusion, whilst the weld interface on the Inconel 740H side showed an unmixed zone having epitaxial columnar growth from the base metal. It was observed that the fresh untempered martensite formed at the Gr.92-sided coarse-grained HAZ next to the weld interface during PWHT. This may be due to higher Ni contents diffused from the Ni-based weld metal. A sharp increase in hardness was detected at the coarse-grained HAZ of Gr.92 while the hardness minimum occurred at the fine-grained and intercritical HAZ (FG/IC HAZ). Fracture after creep at 650 o C/80 MPa and 700 o C/35 MPa occurred at the FG/IC HAZ of Gr.92, commonly known as a type IV fracture. This may be attributed to the strain localization at the FG/IC HAZ due to the smaller grain size as well as the lowest hardness. The elemental migration across the weld interface was not significant. It was also found that a needle-like Nb-rich δ-phase formed at the interdendritic region of the weld metal after creep at 700 o C/35 MPa.