First principles calculations of alloying element diffusion coefficients in Ni using the five-frequency model

The diffusion coefficients of several alloying elements（Al,Mo,Co,Ta,Ru,W,Cr,Re） in Ni are directly calculated using the five-frequency model and the first principles density functional theory.The correlation factors provided by the five-frequency model are explicitly calculated.The calculated diffusion coefficients show their excellent agreement with the available experimental data.Both the diffusion pre-factor（D 0） and the activation energy（Q） of impurity diffusion are obtained.The diffusion coefficients above 700 K are sorted in the following order：DAl〉DCr〉DCo〉DTa〉DMo〉DRu〉DW〉D Re.It is found that there is a positive correlation between the atomic radius of the solute and the jump energy of Ni that results in the rotation of the solute-vacancy pair（E 1）.The value of E 2-E 1（E 2 is the solute diffusion energy） and the correlation factor each also show a positive correlation.The larger atoms in the same series have lower diffusion activation energies and faster diffusion coefficients.

Microstructure and of Mechanics Microwave Boriding

Microwave boriding layer microstructure of carbon steels and its diffusion mechanics were studied. The results show that the existence of microwave field in the boriding can＇t change the growth mechanics of boriding layer. Compared with conventional boriding, if the treatment temperature and time remain constantly, the descend rate of the boriding layer thickness with the increase of carbon content of steel is smaller. The diffusion activation energy ofT8 steel is 2.6× 10^5 J/mol between the temperature of 750 ℃ and 900 ℃ in microwave field, which is in the same order of conventional boriding.

EFFECT OF NITROGEN ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALLOY HK50

The volume fraction of the lamellar carbide cell in HK50 alloy may be increased with the in- crease of nitrogen content over 0.065%.The habit plane of l(?)mellar carbide is {111}_γ.The distribution of nitrogen changes no more before or after the precipitation of lamellar structure. The diffusion activation energy of carbon reduces remarkably with the increase of nitrogen content.It is believed that the lamellar carbide cell is harmful to the high temperature creep and impact properties of the alloy.

Effect of Rare Earth Elements on Kinetics of Salt Bath Vanadizing

Adding rare earth into permeating agent has an obvious catalytic effect on vanadizing on steel surface, and the vanadizing rate can increase about 30%～40%. The case depth ( x ) of the samples which have undergone different periods of vanadizing time at 950 ℃ was measured. These depth values ( x ) and its corresponded time ( t ) were substituded into the experimental formula, i.e., x n=Kt (ln x=(1/n) ln K+(1/n) ln t ), and were processed by mono linear regression. It is found that x and t have the relationship of x 2=Kt . Addition of rare earth can promote reaction of the permeating agent, and increase vanadium potential of the agent. Rare earth, as a strong reductant, makes the oxide on the steel surface reduced, and thus activates the steel surface. Permeating of rare earth into steel and the VC layer intensifies the crystal fault density, and, together with its excellent chemical activation, makes carbon atoms diffuse easily. These functions of rare earth can decrease the diffusion activation energy of the carbon atoms, and therefore has catalytic effect on permeation.

EFFECT OF RARE EARTH ON VANADIZATION PROCESS IN BORAX BATH

This paper investigates the effect of rare earth elements on the kinetic process, microstructure and mechanical properties of vanadization in borax bath. The results show that addition of rare earth elements to vanadizing agent has obvious catalytic effect on the rate of vanadization, which has been enchanced by 30%. The wear and corrosion resistance of vanadium carbide layer were prompted by the addition of rare earth to the agent. Through increasing vanadium potential of the agent, activating the surface of workpieces and decreasing the activation energy of diffusion of carbon, rare earth elements accelerate the rate of vanadization process.

THEORETICAL PREDICTION OF PROEUTECTOID FERRITICTRANSFORMATION IN HYPO-PROEUTECTOID STRUCTRAL STEELS

Proeutectoid ferrite with carbon content xo precipitating from austenite in a multicomponent steel at temperature T is supposed to be equivalent to proeutectoid ferrite with the same carbon content precipitating from austenite in Fe-C binary system at temperature T'.is described as the temperature difference of proeutectiod ferrite formation, and can be calculated from the Fe-X diagrams and the equilibrium temperature A3. By introducing Tf and basing on the thermodynamic model for Fe-C binary alloy, the driving force for phase transformation from austenite to proeutectoid ferrite in multicomponent steels has been successfully calculated. Through the Johnson-Mehl equation and using the data hem known TTT diagrams, the relationship between the chemical composition and the intedecial edenly packeter as well as activation energy for proeutectoid ferrite formation can be calculated. The starting curves of proeutectoid ferritic transformation calculated in this way in some hypo-proeutectoid structural steels agree well with the erperimental data.

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony induced embrittlement kinetics in the nick- el-chromiunl steel. Based on these, the activation energy and frequency factor of diffusion of antimony vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.