Y.H. He

Improvement in mechanical and oxidation properties of TiAl alloys with Sb additions

Abstract The objective of this study is to improve the mechanical properties and oxidation resistance by control of both alloy addition and microstructure in two-phase TiAl alloys based on Ti–48Al (at%). It is found that with the addition of up to 0.15Sb, the room temperature bending deflection and fracture strength ( σ b ) can be enhanced significantly. For example, σ b reaches as high as 1345xa0MPa in the alloy of Ti–48Al–0.15Sb. These improved properties are attributed mainly to the refinements of the colony grain size and the interlamellar spacing with the addition of Sb. Additionally, the high-temperature oxidation resistance can also be substantially increased in the presence of 0.15Sb, being higher than those for Ti–48Al and Ti–48Al–2Cr–2Nb. The cause for this increase is suggested to be the formation of a protective layer of Al 2 O 3 stabilized by Sb.

Effect of heating rate on pore structure of porous FeAl material

Abstract The sintering behaviour of elemental powders of Fe and Al was investigated at various heating rates (0˙5–10°C min–1). It was found that the formation of porous FeAl material was accompanied by the volume expansion during the formation of the intermediate phase Fe2Al5. The final pore structure of porous FeAl material depends strongly on the heating rate; the higher the heating rate, the higher the volume expansion and the larger the porosity and the maximum pore size. It has been found that the pore structures produced with different heating rates experienced different formation routes.

Catalytic growth of metallic tungsten whiskers based on the vapor-solid-solid mechanism.

Metallic W whiskers with tip diameters of 50-250xa0nm and lengths of 2-4xa0µm have been successfully synthesized in large quantities using Co-Ni alloyed catalysts. The relatively low growth temperature of 850u2009°C and the large catalyst size (over 100xa0nm) suggest that the growth of the W whiskers must be governed by the vapor-solid-solid mechanism. Our results show that the vapor-solid-solid model is suitable not only for the growth of nano-scaled whiskers with diameters below 100xa0nm, but also for submicro-scaled whiskers with diameters well above 100xa0nm. This technique has great potential to synthesize well controlled metallic whiskers.

Structural characteristics and high-temperature oxidation behavior of porous Fe-40 at.%Al alloy

Porous FeAl alloy was prepared by Fe and Al elemental reaction synthesis. The cyclic oxidation evolutions of porous Fe–40xa0at.%Al alloy at the elevated temperatures of 600 and 800xa0°C in air were studied and compared to porous materials of Ti, Ni, and 316L stainless steel. It has been shown that the oxidation of porous Fe–40xa0at.%Al alloy fitted a power law and no spallation was found after cyclic oxidation for 202xa0h in air. The porous FeAl alloy exhibits much better oxidation resistance than other porous materials, such as Ti, Ni, and 316L stainless steel, indicating that the porous Fe–40xa0at.%Al alloy has the highest structural stability at the elevated temperature in the oxidation atmosphere. Thus, the porous FeAl alloy can be used as filtering materials at elevated temperature in the oxidation atmosphere.

Preparation and Pore Structure Stability at High Temperature of Porous Fe-Al Intermetallics

Porous Fe-Al intermetallics with different nominal compositions (from Fe-8xa0wt.% Al to Fe-50xa0wt.% Al) were fabricated by Fe and Al elemental powders through reaction synthesis. The effects of the Al content on the pore structure properties, and the comparison of pore structure stabilities at high-temperatures among the porous Fe-Al intermetallics and porous Ti, Ni, 316L stainless steel samples, were systematically studied. Results showed that the open porosity, maximum pore size, and permeability vary with the Al content. Porous Fe-(25-30xa0wt.%) Al intermetallics show good shape controllability and excellent pore structure stability at 1073xa0K in air, which suggests that these porous Fe-Al intermetallics could be used for filtration at high temperatures.

Effects of Al content on porous Fe–Al alloys

Abstract Porous Fe–Al alloys with the nominal composition ranging from Fe–20 wt-%Al to Fe–60 wt-%Al have been fabricated by Fe and Al elemental powder reactive synthesis. The effects of the Al content on the pore properties of resultant porous Fe–Al alloys were systematically studied. It has been found that the volume expansion, the open porosity and the permeability can be manipulated by varying the Al content and that their maximum values are reached at Fe–45 wt-%Al. Their mechanical properties suggest that they are strong enough for the filtration applications.

Criterion to control self-propagation high temperature synthesis for porous Ti–Al intermetallics

Abstract The criterion to control self-propagation high temperature synthesis for the fabrication of porous Ti–Al intermetallics was established from the prereaction model and the thermal balance condition. The criterion equation reflecting the relation between the solid reaction layer thickness correlation parameter and the ignition temperature was deduced to be p3u200a=u200aC1/(Tad−T0)+C2, according to which the near net shaping synthesis of porous Ti–Al intermetallics was realised, which was significantly important for the industrial productions and applications of this high performance novel porous material. The slight difference between theoretical predictions and experimental data was analysed according to the model analysis.

Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10−5 s−1 to 2 × 10−3 s−1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10−5 s−1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.

Mechanical properties of porous Fe–Al intermetallics

Abstract The effects of Al content and porosity on the mechanical properties of porous Fe–Al intermetallics, particularly at high temperatures, were determined using scanning electron microscopy, X-ray diffraction and mechanical properties testing. Results show that the optimal mechanical properties are achieved at 20 to 30 wt-%Al content. The tensile and bend strengths decrease with increasing Al content at 35 to 60 wt-%Al content. The quantitative relationship between porosity and strength for porous Fe–Al intermetallics can be described as σbu200a=u200a181(1−θ)3.3. The strength of porous Fe–Al intermetallics increases at medium temperature with the increase in temperature from room temperature. The maximum tensile strength (50.2 MPa) is achieved at 600°C, which is almost 36.8% higher than that at room temperature.

Investigation to impurity content and micromorphology of high purity titanium powder prepared by molten salt electrolysis

Abstract In this work, the high purity Ti powder was prepared by molten salt electrolysis. The impurity content and micromorphology of Ti powder were systematically studied. The results showed that the main impurities in electrolytic Ti were Mn, Al, Cr, Fe, Ti, N, C and O, and the impurity originations and distributions were disclosed. Increase in the initial soluble Ti ion concentration can reduce the impurity content effectively. The electrolysis devices should be made by Ti alloy, which enables the impurity contents in Ti powder to be much lower than electrolysis devices made by other materials. After the electrolysis, the electrolytic Ti at the upside of cathode was platy bulk and dense, while the downside was loose powder. High current density and low soluble Ti concentration could produce dendritic and fine electrolytic Ti powder, and conversely, the electrolytic Ti powder was coarse polygon.