As-cast microstructure and its changes during heat treatment for Ti-44.9Al(at.-%) and Ti-44.3Al-3Cr(at.-%) have been investigated.The addition of Cr increased the amount of TiAl(γ) phase in the binary Ti-Al alloy and changed the α_2 lamellae into block shaped particles.Cr accelerated the discontinuous coarsening process of the lamellar α_2+γ,by which the finer new-growing lamellar grains were formed.Therefore,addition of Cr and long-time heat treatment at 1150℃ are an effective way to refine the large lamellar grains of α_2+γ in as-cast alloy.Cr promoted the formation of an ordered bcc β_2 phase.Correspondent:ZHENG Yunrong,professor,Laboraiory No.4,Institute of Aeronautical Materials,Beijing 100095

Based on the fact that the scattering lengths of Fe and Cr atoms to neutron are quite different,the site occupation of Cr in D0_3 type Fe-28Al-5Cr alloy has been determined by means of neutron difractometry.The experimental results indicate that Cr atoms are only substituted for Fe atoms and occupy the sublattice of the next nearest neighbour of Al atoms.Decrease in NNNAPB energy leads widening of NNNAPB in the superlattice dislocation and,therefore,cross slip becomes easier and the deformation ability is improved.This is confirmed by the investigation of the surface slip line and dislocation configuration under optical and transmission electron microscope respectively.Correspondent:YANG Wangyue,Department of Materials,University of Science and Technology Beijing,Beijing 100083

Interface ledge and retained austenite fine structure in lower bainite of 40CrMnSiMoVA and 65Si2MnWA steels have been studied by TEM.A large number of faults have been found in retained austenite and these faults correspond to the interface ledges.The results show that it is very possible that ferrite plate of lower bainite thickens by broad face of the interface ledge shearing on the austenite fault face but not side migrating of the ledge.Correspondent:ZHANG Xiyan,Faculty No.401.Northwestern Polytechnical University,Xi'an 710072

Based on a mechanism proposed for diffusion of a solute atom-vacancy complex the diffusion coefficients and activation energies of some solute atoms,which diffuse anomalously fast in HCP crystals,are calculated.The mechanism could explain this phenomenon very well.Correspondent:CHEN Ning,Department of Physical Chemistry,University of Science and Technology Beijing,Beijing,100083

Transformation mechanism and discontinuous coarsening of lamellar structure were studied by cooling Ti-47.8at.-%Al alloy fromα region to room temperature and then aging in the two phase region.The lamellar transformation process relates to aging temperature and cooling rate,at which α_2 single phase forms. A model of lamellar transformation kinetics was developed.The stability of lamellae decreases with decreasing interlamellar spacing and discontinuous coarsening occurres during aging.Correspondent:TIAN Huahang,assistant,Laboratory No.8,Department No.1,Beijing University of Aeronautics and Astronautics,Beijing 100083

Based on the micro-structural feature of single crystal Ni-base superalloy,two micro-models have been proposed to calculate and analyze its thermal misfit,especially,the critical resolved shear stress of the two-phase superalloys.The results show that the macro-critical resolved shear stress can be obtained by the finite element method which takes thermal misfit into consideration,and the method can be used to analyze further the superalloys with high γ'volume.The effect of thermal misfit on change of γ'on tempering condition,and the primary quantitative result shows that the thermal misfit stress is one of the main driving forces on γ'coarsening.Correspondent:YUE Zhufeng,Post Box 350,Department of Mechanical Engineering,NorthwesternPolytechnical University,Xi'an 710072

The microstructure and phase transformation during annealing of rapidly solidified Al-Fe-Ti-C alloy have been studied by TEM,EDS and EELS.The initial microstructure consists mostly of α-Al,Al_6Fe particles and a smaller volume fraction of an amorphous phase.During annealing,the amorphous phase transforms into α_T-AlFeSi phase,the globular particles of Al_6Fe growth up and some needle-like Al_3Fe phase precipitates near the α-Al grain boundaries.At 673 K,96 h and 773 K,5 h annealing,the Ti and C dissolved in the α-Al during rapid solidification precipitates as a compound phase of TiC_079.And the formation process of TiC phase was discussed.Correspondent:TONG Xingcun,lecturer,Centre of Malerials Research,Zhengzhou Institute of Technology,Zhengzhou 450002

The effect of various factors which include grain size/grain shape,strain rate and preoxidation,on environmental embrittlement of Fe_3Al based alloys has been investigated.Fe_3Al specimens with refined grain or columnar grain structure exhibit improved ductility and strength in both oxygen and air,however,the alloys still demonstrate severe environmental degradation.The severity of embrittlement is identical in air and gaseous hydrogen.As the strain rate increases,the ductility of Fe_3Al in both air and hydrogen is improved.Preoxidation can partly eliminate the environmental effect in Fe_3(Al,Cr,B) alloy.Surfaee reaction is supposed to be the controlling step in environment induced embrittlement of Fe_3Al based alloys.Correspondent:WAN Xiaojing,professor,Shanghai University of Technology,Shanghai 200072