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  Recent  Papers

 Papers published during 2007 

 

     Low-temperature synthesis/densification and properties of Si2N2O prepared with Li2O additive

              Q. F. Tong, J. Y. Wang, Z. P.  Li, Y. C.  Zhou J. Europ. Ceram. Soc., 27 (2007) 4767–4772

Abstract

Dense Si2N2O was successfully synthesized using 2 mol% Li2O as an additive by a hot-pressing method at 1500 oC. Compared to other metal oxide additives, Li2O can significantly decrease the sintering temperature of Si2N2O, which is ascribed to the lower melting point of Li2O–SiO2 and the formation of less viscous liquid phase. Increasing Li2O content has no apparent influence on the mechanical and dielectric properties of dense Si2N2O, which is due to the easy evaporation of Li2O at sintering temperature. The mechanical properties of Si2N2O with Li2O additive are comparable to those of Si2N2O synthesized with other additives. The as-prepared bulk Si2N2O with 2 mol% Li2O additive exhibits both low dielectric constant (6.17 at 1 MHz) and loss tangent (0.0008 at 1 MHz) and combines good mechanical performance, indicating it is a potential high-temperature structural/functional material.

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    Trend in crystal structure of layered ternary T-Al-C carbidesc(T = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, W, and Ta)

              Jingyang Wang, Yanchun Zhou, Ting Liao and Zhijun Lin J. Mater. Res., Vol. 22, No. 10, Oct 2007:2685-2690

Abstract

Layered ternary T-Al-C ceramics containing early transition metal Sc, Zr, and Hf, crystallize with the TnAl3Cn+2 formula, while others containing neighbor elements Ti, V, Cr, Nb, Mo, W, and Ta yield the Tn+1AlCn formula. Ternary TnAl3Cn+2 ceramics are structurally characterized by NaCl-type TC slabs being separated by Al4C3-type AlC layers. In the present study, we suggest that the ability of forming the TnAl3Cn+2
carbide could be traced back to the structure mismatches between the TC, Al4C3 and TnAl3Cn+2 compounds. Ternary carbides following the TnAl3Cn+2 formula experience small lattice mismatches and strain energies. Moreover, the discrepancy between crystal structures of TnAl3Cn+2 and Tn+1AlCn is interpreted by lattice mismatch and the produced strain energy for the ternary T-Al-C ceramics. We also present close relationships between the atomic radii of transition metal and lattice mismatch, as well as the strain energy. The proposed method is not only helpful to explain the trend in crystal structure of T-Al-C based ceramics, but may be also general to predict the crystal structure of layered compounds constructed by alternatively stacked structural units.

 

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    Synthesis, Microstructure, and Property of Cr2AlC  Review

              Zhijun LIN, Yanchun ZHOU and Meishuan LI  J. Mater. Sci. Technol., Vol.23 No.6, 2007:751-746

Abstract

Cr2AlC is an unusual layered ternary ceramic that combines the merits of both metals and ceramics. The
salient properties of Cr2AlC are strongly related to its bonding characteristics and microstructures. Synthesis,
microstructure, and property of Cr2AlC are reviewed in this paper. First, theoretical calculations and physical
properties are introduced. Then, the processing of Cr2AlC ceramic in both bulk form and thin films and their basic mechanical properties are summarized. Atomic-scale characterizations of Cr2AlC, as well as the microstructural relationships among Cr2AlC, Al8Cr5, and AlCr2 were achieved using a series of transmission
electron microscopy (TEM) techniques. Moreover, high-temperature oxidation and hot corrosion behaviors
of Cr2AlC were investigated by means of thermogravimetric analysis, X-ray diffraction, Raman spectroscopy,
scanning electron microscopy, and TEM. Mechanism of the excellent high-temperature corrosion resistance of Cr2AlC is discussed based on systematic microstructural analyses. Finally, concise conclusions are drawn.

 

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    High-temperature oxidation and hot corrosion of Cr2AlC

              Z.J. Lin, M.S. Li, J.Y. Wang, Y.C. Zhou  Acta Materialia  55 (2007) 6182–6191

Abstract

High-temperature oxidation and hot corrosion behaviors of Cr2AlC were investigated at 800–1300 oC in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr2AlC is about 800 C, which is 400 oC higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr2AlC displayed excellent hightemperature oxidation resistance with parabolic rate constants of 1.08×10 -12 and 2.96×10 -9 kg2 m -4 s -1 at 800 and 1300 oC, respectively.  Moreover, Cr2AlC exhibited exceptionally good hot corrosion resistance against molten Na2SO4 salt. The mechanism of the excellent high-temperature corrosion resistance for Cr2AlC can be attributed to the formation of a protective Al2O3-rich scale during both the high-temperature oxidation and hot corrosion processes.

 

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    Improved Cr2O3 adhesion by Ce ion implantation in the presence of interfacial sulfur segregation

              M.S. Li, P.Y. Hou   Acta Materialia  55 (2007) 443–453

Abstract

As-polished and preoxidized Ni–20Cr alloys were Ce-implanted with a dosage of 1×10 17  ions/cm2, then subsequently oxidized at 1050 oC in air. The oxide adhesion and the extent of sulfur segregation at the oxide–alloy interface were determined, respectively, using tensile pull testing and scanning Auger microscopy with an in situ scratch device. The critical load for oxide failure was the lowest on the unimplanted Ni–20Cr, and was slightly higher on those with implantation made into a preformed oxide. Oxides that formed directly on Ce-implanted Ni–20Cr never failed under the pull test, which showed the strongest scale adhesion; however, similar amounts of interfacial sulfur, which segregated from the alloy during oxidation, were found at all interfaces. Ce additions were also found to reduce the oxidation rate and affect the extent of voids at the scale–alloy interface. It is suggested that the change in the oxide growth mechanism reduces the number of interfacial voids and, unlike Al2O3, these factors are more important for Cr2O3 scale adhesion than sulfur segregation to the scale–alloy interface.

 

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    Layered stacking characteristics of ternary zirconium aluminum carbides

              Z.J. Lin, L.F. He, M.S. Li, J.Y. Wang, Y.C. Zhou  J. Mater. Res., Vol. 22, No. 11, Nov 2007

Abstract

Layered stacking characteristics of ternary Zr–Al–C carbides were investigated using scanning transmission electron microscopy (STEM). Three previously unknown compounds, i.e., Zr4Al3C6, Zr5Al6C9, and Zr7Al6C11 were identified. The present study extends the structural information of ternary Zr–Al–C ceramics. The influence of the thickness of the NaCl-type Zr-C slab on the elastic properties of ternary Zr–Al–C ceramics is discussed based on first-principles calculations. In addition, direct atomic-resolution observations illustrate the process for forming the unique layered crystal structures of ternary Zr–Al–C ceramics. These results also provide insights into the formation mechanism of layered ternary Zr–Al–C carbides.

 

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    Synthesis and Characterization of Bulk Zr2Al3C4 Ceramic

              Lingfeng He, Zhijun Lin, Jingyang Wang,Yiwang Bao, Meishuan Li,Yanchun Zhou

              J. Am. Ceram. Soc., 90 [11] 3687–3689 (2007)

Abstract

Polycrystalline Zr2Al3C4 was fabricated by an in situ reactive hot-pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum
carbides. The mechanical properties of Zr2Al3C4 at room temperature were measured (Vickers hardness of 10.1 GPa, Young’s modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa .m1/2). The electrical resistivity and thermal expansion coefficient were determined as 1.10 μΩ.m and 8.1×10 -6  K -1, respectively.

 

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    In Situ Synthesis and Properties of Ti3AlC2/TiB2 Composites

              Chao Li, Meishuan Li, Yanchun Zhou, Jie Zhang, Lingfeng He, J. Am. Ceram. Soc., 90 [11] 3615–3620 (2007)

Abstract

In order to improve the mechanical properties of Ti3AlC2, nearfully dense Ti3AlC2/TiB2 composites were synthesized using Ti, Al, graphite, and B4C powders as the initial materials. Compared with monolithic Ti3AlC2, the composites exhibit a much higher strength (for the compressive strength, from initial 723 MPa to maximal 2205 MPa; for flexural strength, from initial 340 MPa to maximal 861 MPa), and the strengthening effect can be held at least up to 1100 oC. Moreover, besides the enhancement of the elastic modulus and hardness of Ti3AlC2, the introduction of a TiB2 phase makes a positive contribution to its electrical conductivity.

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    Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C Composite

              Zhijun Lin, Meishuan Li, Jingyang Wang, Yanchun Zhou  J. Am. Ceram. Soc., 90 [12] 3930–3937 (2007)

Abstract

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr2AlC, AlCr2, Al8Cr5, and Cr7C3. The orientation relationships and atomic-scale interfacial microstructures among Cr2AlC, AlCr2, and Al8Cr5 are presented. ?This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na2SO4 salt. The parabolic rate constants for the oxidation in air at 1000oC, 1100oC, and 1200oC are 3.0×10 -12, 6.2×10 -11, and 6.2×10 -10 kg2 (m4 . s) -1, respectively, while the linear weight gain rates for the hot corrosion of Na2SO4-coated samples at 900oC and 1000oC are, respectively, 1.210 -3 and 4.410 -3 mg (cm2 . h) -1. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.

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    Transient oxidation behavior of nanocrystalline CoCrAlY coating at 1050 ℃

              LIU Guang-ming, LI Mei-shuan, MA Jing-hui,DU Nan, ZHOU Yan-chun

             Trans. Nonferrous Met. Soc. China., 17(2007):595-599

Abstract

Nanocrystalline CoCrAlY overlay coating was prepared on M38G superalloy by magnetron sputtering deposition. To investigate the oxidation behavior and phase transformation of alumina during oxidation, the oxidation experiments were conducted at 1 050 ℃ for various time in the range of 5−180 min. The phase compositions of the oxide scales were investigated by using glancing angle X-ray diffraction(XRD). The microstructure analysis of oxide scales was carried out by means of scanning electron microscopy(SEM). The growth process of metastable alumina at the grain boundaries and transformation to stable alumina were discussed. The results show that at the initial oxidation stage the mixture of δ-Al2O3, γ-Al2O3 and α-Al2O3 is formed on the sample surface rapidly. Especially, δ-Al2O3 and γ-Al2O3 prefer growing at the grain boundaries of CoCrAlY coating. With increasing oxidation time, δ-Al2O3 and γ-Al2O3 transform to θ-Al2O3, afterwards θ-Al2O3 transforms to α-Al2O3 gradually. After 180 min oxidation, θ-Al2O3 transforms into α-Al2O3 completely.

 

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    Effects of CeO2 applied to preformed oxide scales on subsequent oxidation of Fe–20Cr at 1000oC

              M. Zhu, M.S. Li, Y.C. Zhou Corro. Engin, Sci & Technol., 42(1)2007:73-79

Abstract

In order to explore the relationships between rare earth elements existing in oxides and growth rate and adhesion of oxide scales, CeO2 thin films were prepared on Fe–20Cr alloys after preoxidation at 1000 oC; subsequent isothermal and cyclic oxidation was carried out, and the oxide adhesion was determined by using the tensile pull test. The results demonstrated that similar to the effects of CeO2 on the as polished Fe–20Cr alloy, the application of CeO2 to the preformed oxides could decrease the subsequent oxidation rate, improve the cyclic oxidation resistance of the alloy and raise the adhesion strength of the oxide scales. These beneficial effects of the applied CeO2 decreased with increasing pre-oxidation time. The fact that virtually none of CeO2 applied on the preoxidised preformed layer reached the metal/oxide interface suggests that the ‘sulphur trapping effect’, through which CeO2 may act, is minimal in the present system.

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    Thermal stability of Ti3AlC2/Al2O3 composites in high vacuum

              J.X. Chen, Y.C. Zhou, H.B. Zhang, D.T. Wan, M.Y. Liu Mater. Chem. & Phys., 104 (2007) 109–112

Abstract

The thermal stability of Ti3AlC2/Al2O3 composites in high vacuum was investigated. On the sample surface, Ti3AlC2 can decompose with the formation of TiC0.67 and gaseous Al at first, then Ti evaporation results in the irregular morphology of non-stoichiometric TiCx. At the same time, the amount of Al2O3 particles becomes less with increasing the soaking time. Finally, Al2O3 particles disappear and a layer of non-stoichiometric TiCx forms on the sample surface. But the bulk Ti3AlC2/Al2O3 composites always keep stable during the vacuum treatment.

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