1 Introduction2 Description of topologically close-packed structures2.1 Coordination polyhedra2.2 Atom layers2.3 Structure units2.4 Classification of phases3 Experimental high-resolution electron microscopy3.1 Preparation of specimens3.2 Imaging conditions of HREM images3.3 Micro and convergent beam electron diffraction4 New topologically close-packed metallic phases4.1 σ-related new phases: H, F, K, J phases4.2 Laves-related new phases: C, C_1 phases4.3 Summary5 Microdomain structures5.1 Domain structures in o-related phases: translation domain boundaries- {100}σ, {110}σ, {1(?)0}_H. (001)_H, boundary joints; rotation domains——28°-σ, 28°-H5.2 Domain structures in Laves-related phases: translation domain boundaries-(001)_μ, (001)_L, ((?)1)_L, (111)_μ, ((?)2)_μ, boundary joints; rotation domains; domains in C phase6 Antiprisms with long-range orientational order7 Conclusions

The TEM analysis of the microstructure of two GH36 superalloys undergone different regimes of heat treatment was conducted. The precipitates were identified as VC, M_(23)C_6 and NbC. The orientation relationship between the precipitates and γ matrix is shown as follows:[011]_(v C)//[011]_M//[011]_(M_(23)C_6)(111)_(v C)//(111)_M//(111)_(M_(23)C_6)(200)_(v C)//(200)_M//(200)_(M_(23)C_6)After normal heat treatment, the VC precipitates and distributes as fine dispersed particles of size from several to ten odd nm, and minimum ones are down to 2.8 nm alone examined by Moire pattern. The VC may coagulate and grow in a few nm larger and more M_(23)C_6 may precipitate along grain boundary after superimposed aging at elevated temperature. This microstructure and grain boundary precipitation may reduce the yield and stress rupture strengthes of the alloy, but improve the notch sensitivity of stress rupture test.

Experimental confirmation by analysis of single crystal electron diffraction spots is presented, showing that the Cr_7C_3 in superalloys is of the orthorhombic crystalline structure. In the Cr_7C_3, the {013} growth twin was discovered by the electron diffraction patterns and the secondary twin might be possibly found. The {013} twin is observed in Cr_7C_3 film. The interrelation of higher twin in Cr_7C_3 has also been discussed.

An observation was carried out on the feature of microstructure, fracture and worn surface of the Al-base composite prepared by rheocasting with graphite, SiC and SiO_2. The a solid solution, Si, CuAl_2 and allied eutectics are found to nucleate and to grow up along the surface of particulates added during solidification of the compslurry. The short-range movement of the particulates may distribute more uniformly in matrix owing to rapidly raising temperature of the composlurry before casting. Over the particulate, a transition layer connects with the matrix, and the active element Mg accumulates more or less. Thus, the firm bond is joined between the particulate and matrix, and it makes no more pore and crack on the microstructure of fractured or worn surface.

A study was made of the lump-like composite structure, which is observed to be composed of lower bainite, dislocation martensite and austenite, appearend on the bainite transformation under different austenizing temperatures and continuous cooling conditions in the 55SiMnMo steel. It may be controlled because its volume percentage increases with the increase of cooling rate and decreases with the decrease of austenizing temperature. The effect of it on the mechanical properties of the steel has also been examined. It may retard the propagation of fatigue crack, especially, more effectively with certain lower ΔK. It may also contribute the strength and toughness for the steel owing to its dislocation martensite and lower bainite.

TAN Yanchang; HE Bin; ZHAO Yuhua (Northeast Institute of Technology, Shenyang)

The fatigue crack propagation mechanism (FCPM) in the nodular graphite cast iron of different microstructures with various proportions of pearlite and ferrite at distinct ΔK levels has been studied by SEM so as to construct their own FCPM charts. The FCPM of the ferritic nodular cast iron is shown to be: transgranular and intergranular fracture→striation and intergranular fracture→striation→striation and dimple rupture; that of pearlitic one to be: transgranular fracture and striation→striation and cleavage fracture; and that of one containing bull's eye ferrite to be: transgranular and intergranular fracture→striation and intergranular fracture→striation and dimple rupture. The correlation between the transition of FCPM and structure or properties as well as the reason why the highest percentage of intergranular fracture observed in the nodular cast iron with 60% ferrite have also been discussed.

The evaluation of the threshold stress of hot salt stress corrosion cracking (HSSCC) of Ti alloy TC11 in temperature range from 350 to 500℃ under laboratory condition and of its susceptibility influenced by five different surface conditions was made. In comparison among them, it was believed that the susceptibility of Ti alloy TC11 to HSSCC may be effectively improved by anodizing its surface owing to retarding the aggressive materials diffused through the oxide layer into substrate. The surface treatment of shot peening may also improve the resistance to HSSCC, especially, at temperature below 430℃. However, no more improvement over its surface resistance by the compressive stress is noticed in increasing temperature.

The crystallization kinetics of Pd(Fe,Co,Ni)_(0.05-0.15)Si_(0.17) and Pd_(0.84) Si_(0.16) metallic glasses has been monitored by determining the time dependence of electrical resistance during isothermal annealing below their own crystallization temperatures. It is found that the total drop of electrical resistance in the crystallization process is 10—20% generally, but the metallic glasses Pd-Fe-Si seemed to be something special and their electrical resistance was measured to be directly related to their Fe content. The electrical resistance of the metallic glasses Pd_(0.78)Fe_(0.05)Si_(0.17) increases during isothermal crystallization, yet the reason is not clear. From the viewpoint of kinetics, an addition of Fe, Co or Ni into Pd-Si system may increase their crystallization activation energy and decrease their crystallization mode coefficient, thus, makes them more stable owing to retarding the nucleation. It is believed that the stability of metallic glasses would be considered by both thermodynamic and kinetic basises other than the crystallization temperature.

The homogeneous viscous flow behaviour of six Pd-Si base metallic glasses under heat at same rate has been invetigated by tensile method. The dynamic viscosity and the activation energy of dynamic viscous flow in their rapid flow region have been evaluated. The results showed that the maximum flow rate and the lower viscosity at 30-35℃ above T_g are presented in the Pd-Si base metallic glasses with an addition of third metal Cu, Ag or Ni. The dynamic viscosity of eutectic Pd_(82)Si_(18) at a close level of temperature,10—20℃ above T_g, is obviously lower that of another two binary metallic glasses. On the basis of the free volume model and the entropy model for the viscous flow, the proposal would be made of that the viscous flow behaviour of the metallic glasses is related to their thermal stability and degree of short-range order.

The controlled roiling experiments into the dual-phase γ+a region of the Nb-V microalloy steel have been carried out using a pilot roiling mill. The effect of finishing roiling temperature on the microstructure and mechanical properties of the steel was examined. The yield strength components containing the solid solution strengthening, σ_s, precipitation strengthening, σ_p, dislocation strengthening, σ_d, subgrain strengthening, ck_s l~(-1/2) and grain strengthening, k_yd~(-1/2), were estimated so that a new expression is found as:σ_y=σ_i+σ_s+[σ_p~2+σ_d~2+(ck_s l~(-1/2))~2]~(1/2)+k_yd~(-1/2)The σ_y calculated is in quite agreement with the experimental results.

The flow stress of eight alloy structural steels at high strain rate and high temperature has been studied using a cam plastometry compression machine capable of imparting certain given amount of deformation at a constant strain rate. The tests of the deformation temperature, strain rate and amount of deformation ranged between 850—1150℃, 5—80s~(-1) and 5—69% respectively. The effects of them on the flow stress of the steels were examined. The analysis by non-linear regression and comparison between the mathematical models of flow stress with two types of temperature term were conducted. Thus, a mathematical model of flow stress with great confidence of the eight alloy structural steels is proposed, and its regression coefficient is also recommended.

On the basis of the previous works by the authors, a continuous approach to the calcination-hydration activated sintering of the natural MgO-CaO series materials of high purity was studied with putting much emphasis upon the effect of the ratio of MgO/CaO on the sintering behaviour and upon the resistance of grogs to disintegration as well. It is found that the densification of the two-phase materials is practically completed when the specimens fired at 1400℃ and developed the highly direct-bonded basic grogs of fine microstructure at 1600℃, while that of one-phase end members grows a lot of exaggerated grains with moderate intragranular pore formation resulting in lowering of the density. The activated sintering of the calcination-hydration specimens seems to be related to the adsorption-desorption of OH~- by CaO and MgO. The resistance to disintegration decreases with the increase of their CaO content.