By the analysis of the dynamic processes of the movement of dislocations through a crystal lattice, a unified theory of collection dislocation motion including both thermal activation and viscous drag effects is proposed. The theory is able to describe the average behaviour of dislocations which move at low and high velocities with a simple equation.The experimental dislocation velocity─stress relation data of KCl crystals are fitted with the theory. Good consistency is obtained. The theory can be used as the start point in establishing the constitutive laws of macroscopic plastic deformation of crystalline materials.Correspondent. CHENG Jingyi, associate professor, Department of Mechanics and Mechanical Engineering,University of Science and Technology of China, Hefei 230026

Self─consistent charge discrete variational Xα cluster method (SCC─DVM ─Xα) is used to calculate the conduction electron densities of metals. Comparing with experimental data of positron annihilation rate,a relation between electron density and positron annihilation rate,and an expression of enhancement factor were obtained. Based on this relation,the fractions of core electron annihilation are discussed.Correspondent: WANG Jingyang, Institute of Metal Research ,Chinese Academy of Sciences, International Centre for Materials Physics, Shenyang 110015.

The hydrogen embrittlement of a spheroidized high carbon steel sheet has been investigated over a range of stress states from uniaxial to equibiaxial tension. For a given stress state, data based on locally determined fracture strains show a decrease in ductility for cathodically charged specimens compared with the corresponding uncharged condition. The loss of ductility caused by hydrogen increases with an increasing degree of biaxiality of the stress state. Metallographic and fractographic examinations show that the fracture of both charged and uncharged sheets is a consequence of void nucleation (due to the decohesion between carbide particles and ferrite matrix),void growth,and void link─up. These fracture processes are accelerated by hydrogen,especially in equibiaxial tension. The above results are discussed in terms of previously proposed modles and postulated mechanisms in steels.Correspondent: FAN Yunchang, senior engineer, Institute of Materials Science and Engineering, Shijiazhuang Railway institute, Shijiazhuang 050043

Hydrogen accumulation on inclusions, grain boundary and near a loaded notch tip in 28CrMoTiB steel was measured using ion microprobe mass analyzer. The results showed that accumulation of hydrogen on inclusions of TiN, Al2O3, TiO, MnS and compound oxide was 10─20, 8─14, 10, 10─17 and 13─14, respectively. The accumulation of hydrogen on surface of the specimen and the grain boundary was 1.2 and 7.3, respectively.There were two peaks of hydrogen accumulation ahead of a loaded notch tip and correspounding accumulation were 2.5 to 4.5. On grain boundary,there was no segregation for B,P and C,however,positive segregation for Ti,Cr,Mo and O,and negative segregation for Mn and S.Correspondent: YU Guanghua,Department of Materials Physics,University of Science and Technology Beijing, Beijing 100083

Hydrogen accumulation near notch tip of austenitic steel 21Cr7Ni9MnN under Ⅰ＋Ⅱ mixed mode loading has been studied by KYKY ion microprode mass analyser (IMMA) and ADINA non─linear finite element quantitive compution. It is found that there exist two accumulation peaks of hydrogen concentration,one at notch tip and the other far apart from it,either in the cracking direction or in the maximum plastic zone. With the increasing of mode II component,the peak of hydrogen accumulation far from the notch tends to drop and move towards the notch tip. The change of the initial hydrogen concentration in the steel has no dramatical influence on the two peaks of hydrogen accumulation near the notch tip. The redistribution of hydrogen concentration is caused mainly by the gradient distribution of plastic strain and hydrostatic stress near the notch tip.Correspondent：CHEN Lian, Professor,Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015

A transparent model alloy,succinonitrile-1.0 wt─% acetone,is directionally solidified to investigate the dynamic response of primary dendritic spacing to the variation of the solidification parameters (i.e., growth velocity and temperature gradient).The lower and upper limits of the allowed range of primary spacing,as a function of growth velocity, are then experimentally obtained. It shows that the lower and upper limits of the allowed range are very sharp, but average primary spacing is remarkably history─dependent not only on growth velocity but also on temperature gradient. The experimental result of the lower limit is in good agreement with that predicted by the modified Hunt─Lu model, while the result of the upper limit is apparrently larger than that by the Hunt─Lu model.Correspondent: DING Guolu, Faculty No.403, Northwestern Polytechnical University, Xi'an 710072

The influence of temperature, heating rate and strain rate on the tensile and yield strength of 30CrMnSi at the temperature range from 100℃ to 900℃ has been investigated. At low temperature region the performance parameters of material with a rise of temperature is dropped slowly and the heat treated condition has a great effect on the behavior of material, but the effect of elevating temperature rate can not be neglected. At high temperature region the material strength reduced greatly is not changed notably with the increase of temperature, heating rate and the heat treated condition. At the middle temperature region,the performance of material is fast dropped with the increase of temperature and the effects of the elevating temperature rate and the heat treated condition are not obvious.Correspondent: WANG Chunkui, associate professor,Institute of Mechanics, Chinese Academy of Sciences,Beijing ,100080

Study has been carried out of the growth behaviour of short crack under spectrum loading. Meanwhile,fatigue damage and life prediction under spectrum loading have also been made.The results show that both single─edge short cracks and short cracks from notchs under spectrum loading can propagate at higher rate below the long crack growth threshold,and indicate the same growth tendency as short cracks under constant amplitude test. However, short crack growth rate under spectrum loading is slightly lower than that of constant amplitude loading. Furthermore, growth rate for long crack under spectrum loading is even lower than that of constant amplitude loading. According to the statistical approach and the constant amplitude fatigue tests, the prediction for fatigue damage under ice spectrum loading has been made, and a reasonable method for fatigue damage under ice spectrum loading has been put forward.Correspondent: ZHANG Guangping,Institute of Metal Research,Chinese Academy of Sciences, Shenyang 110015