Diffuse scattering and microstructure of Fe-1.83C(%) martensite aged at room temperature was studied by TEM. The intensity distribution of diffuse scattering in reciprocal space was determined by double-tilt technique. The results show that the shape of martensite diffraction spot becomes a diffuse spike after aging, and its 3-dimensional profile appears to be two head-on 4-sided regular pyramids. Fourier analysis of the diffuse scattering suggests that martensite undergoes a spinodal decomposition during aging,the tweed-like modulated structure is composed of a dominant wave with wavelength of about 1 nm and non-dominant waves with longer wavelengths. For the first time it is found that the modulated structure of Fe-C martensite does not grow even after aged for several months.The reason is considered to be due to the strong elastic interactions among carbon-enriched and depleted regions.Correspondent: REN Hiaobing, Presently with Department of Physics, Nanjing, University, Nanjing 21008)

Electron diffraction technique is applied to determine the crystal structure of a conventional ordered phase found in an aged high carbon(1.83%) martensite. Experimental data of good quality are obtained to ensure the reliability of the structure determination. The ordered phase is found to be an off-stoichiometric compound isomorphic with γ'-Fe_4N, it is thus called γ'-Fe_xC (I)(x= 4－11). Its cell dimension is (a_M and c_M are the length of a and c axis of martensite unit cell, respectively). The ordered phase is coherent with martensite, their orientation relationship is (001)Ⅰ∥(001)M,[100]Ⅰ∥[110]M. Diffraction patterns calculated based on γ'-Fe_xC (I) are in agreement with experimental observations.Correspondent: REN Hiaobing, Presently with Department of physics, Nanjing University, Nanjing 210008

Thin foil samples of TiNi shape memory alloy,furnace cooled, were studied by selected area electron diffraction (SAD). Metastable x-phase, characterized by (1/2) <001> and (1/8) <210> superlattice diffraction spots,was newly discovered. It is confirmed by the combination of SAD and reduced cell program analyses that x-phase belongs to monoclinic crystal system. Its lattice parameters were determined to be:a=0.669 nm, b= 0.603 nm, c= 0.951 nm and β=95°.Correspondent:ZHAO Hingzhong, lecturer, Faculty of Metallic Materials, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074

The room temperature ductility of Fe_3Al was improved by a suitable amount of Cr addition to the alloy, so as to form an oxide protective film.XPS study shows that the oxide film has a moderate amount of Al_2O_3, which is dense and protective enough to restrain and retard the reaction of environmental water vapour on Al. Thus the own intrinsic toughness of the alloy will fully be developed.Correspondent: QIAN Hiangrong, professor, Shanghai Iron and Steel Research institute, Shanghai 200940

Effects of P, Zr and B on the solidification behaviour of directionally solidified IN738 superalloy have been studied by the methods of directional solidification plus rapid quenching and at various solidification rates. With the increase of P content in the alloy, the breadth of solid-liquid biphase region and temperature interval may be obviously enlarged;the residual liquid phase in interdendritic region increased; the dendritic structure developed;the secondary dendritic arms become more numerous and thinner, and the interfacial area between solid and liquid extended. The solute segregation of the alloy is different between addition of only P and of P together with Zr to the alloy. With alloy containing P, Zr and B, the formation of (γ+γ') entectic and precipitation of (γ+boride) eutectic in its front may be promoted.Correspondent: HU Zhuangqi, professor, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015

The formation of γ' rafts and their rotation in the necking zone during high temperature stress-rupture tests of NASA IR100 single crystal superalloy were investigated.It was found that γ' precipitates linked each other in the direction of perpendicular to the stress axis just after the beginning of the stress-rupture test. Perfect and regularγ' rafts formed when the stress-rupture test lasted for about 50 h. After observing the longitudinal sections of ruptured bars by SEM the γ' rafts in the necking zones were found to be not perpendicular to the stress axis, but rotated around the stress axis. The mechanisms of γ' raft formation and rotation were put forward in this paper.Correspondent: GUO Xiping, lecturer,State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072

An existence of the spinodal decomposition of carbon was first observed at the initial stage of lower bainite transformation in Si-containing steel by using selected area diffraction and dark field image under TEM. The decomposition is similar to that of martensite tempering at initial stage. It is believed that the bainite transformation may be formed by the shearing mechanism.Correspondent: ZHANG Xiyan, Faculty No.401, Northwestern Polytechnical University, Xi'an 710072

The {110} cleavage fracture of Ni_3Al after charging hydrogen has been investigated by in situ TEM observation technique. The dislocations which piled up at the {111} crack tip generate high stress concentration to nucleate microcrack in {110}. In addition,hydrogen decrease the cohension of {110}.Correspondent: LIN Dongliang, professor, Department of Materials Science, Shanghai Jiaotong University.Shanghai 200030