Kazuhisa Shibue

Influence of chlorine on the oxidation behavior of TiAlMn intermetallic compound

Abstract Excellent oxidation resistance of reactive-sintered titanium aluminides was reviewed and the mechanism of forming protective Al 2 O 3 film was studied. TiAlMn containing more than 200 mass ppm chlorine from raw titanium powder showed very little mass gain after oxidation from 1173 to 1398 K in air. The chlorine effect was found obviously in composition of α 2 + γ and γ regions in the TiAl phase diagram. The chlorine effect was found even in a plasma arcmelted TiAlMn alloy with chloride covering or oxidized in air containing a small amount of chlorine gas. Also, other halogen elements as fluorine and bromine had the same effect as chlorine. It was clear, in particular by surface analyses that chlorine existed in TiO 2 near the oxide/metal interface in the early stage of oxidation and decreased oxygen ion vacancies in TiO 2 . Then TiO 2 growth was interrupted and protective Al 2 O 3 scale was formed on the titanium aluminide.

Control of lamellar orientation in TiAl base alloy using a combination of heavy extrusion and reactive sintering

Polycrystalline TiAl alloy has been produced using heavy extrusion followed by reactive sintering. Many grains that have a lamellar orientation parallel to the extrusion direction were observed in as-reactive sintered materials. Texture measurements were conducted on the as-extruded and as-reactive sintered materials. A (1010) fibre texture was found for Ti in the as-extruded material and for the α 2 phase in the as-reactive sintered material. The deformation texture of Ti remained after reactive sintering. Thus, the control of crystal orientation of the elemental Ti powder is an effective method of controlling the lamellar direction of reactive sintered TiAl alloy.

Evolution of high aspect ratio grains in a TiAl-based alloy by directional grain growth

A concept of microstructural control named “directional grain growth” (DGG) has been proposed. In this method, local phase transition and grain growth are utilized for controlling grain shape and grain orientation in a restricted region for an inhomogeneous material in chemical composition which is given directionality by plastic deformation. In heat treatment for DGG, a seed grain grows preferentially in a shaped region without precipitates by being held at a relatively low temperature and then enlarges further by encroaching the surrounding region where precipitates vanish gradually with temperature elevating at a slow rate. Such a DGG heat treatment was applied to a Ti–45.6at %Al–1.7at.%Mn alloy fabricated by reactive sintering of an extruded mixture of elemental powders and contained inevitably directional inhomogeneity. As a result, a structure was obtained consisting of giant columnar grains of Al-bearing αTi phase of several mm in length, 6.0–7.0 in aspect ratio and a preferred orientation. The axis of the columnar grains was approximately perpendicular to [0001]α.

Nano-Scaled Microstructure and Tensile Behavior of Spray-Formed and Subsequently Worked Al-8Fe-2Mo-2V-1Zr Alloys

Room temperature tensile behavior of nano-structured Al-8Fe-2Mo-2V-1Zr alloy prepared by spray forming and subsequent hot extrusion (as-received) was investigated by tensile test in relation to microstructure and processing step. The microstructure of spray formed and extruded sample is composed of equiaxed grain structure of about 500 nm with finely dispersed intermetallic phases such as Al 3 (Fe, Mo, V, Zr) and Al 6 (Fe, Mo, V, Zr) at grain boundaries and grain interiors. The average particle size of dispersoids was measured to be 40 nm. Warm rolling of extruded sample accelerates an occurrence of dispersoids less than 20 nm. The elongation of warm rolled alloy is higher than that of as-received one, although yield strength decreases by warm rolling. It is suggested that warm rolling of as-received alloy causes the increase in elongation and decrease in yield strength due to decreasing content of solid solution strengtheners by activated diffusion process to form a precipitate in supersaturated Al matrix phase. Details will be discussed in correlation with XRD, SEM, TEM analyses.

Effect of oxidation treatment on wear resistance for TiAl-Mn intermetallic compound.

Wear resistance of reactive-sintered TiAl-Mn which was coated with oxide film was evaluated by pin-on-disk type wear test. Oxide film was coated by using oxidation reaction in air at 1173K. The two coated TiAl-Mn alloys show much less wear loss than uncoated ones. After wear test, oxide film remains partialy in the coated Ti-47.3Al-1.7Mn. On the contrary, oxide film of the coated Ti-43.2Al-1.6Mn is worn over. The low wear loss in the later is due to hard Ti-rich layer which is formed under oxide film produced during oxidation treatment. Thickness of Ti-rich layer increases with titanium content of TiAl-Mn or oxidizing temperature. It is supposed that Ti-rich layer is hardened by solutioning oxygen. Excellent wear resistance of the oxidized TiAl-Mn is caused by not only oxide film but also Ti-rich layer. This superior wear resistance surface can be obtained easily by oxidation in air of reactive-sintered TiAl-Mn containing chlorine.