DISTRIBUTION OF Cu AND ITS EFFECT ON MICROSTRUCTURE OF Cu-BEARING STEEL

In order to investigate the distribution of Cu and its effect on the microstructure of Cu-bearing steel, a series of mild steels containing different contents of Cu are developed by vacuum electric arc furnace. These steels are annealed at 1 260℃, 1 100 ℃ and 1 000℃ respectively for one hour and followed by furnace cooling to room temperature to simulate the heat treatment before the rolling process. The results show that Cu did not obviously segregate in annealed steels. The scanning electron microscope （SEM） observation show that the main microstruetures in Cu-bearing steel are ferrite and pearlite; The volume fraction of pearlite in steel increase with increasing Cu content. The grain size reduces with the decrease of annealing temperature. The results of energy dispersive X-ray analysis （EDXA） suggest that the Cu content in pearlite is higher than that in ferrite, which means that the microstructure-segregatian of Cu exists. However, the cast specimens show that Cu content in MnS and S-rich phase is very high, and Cu changed the distribution of/vinS in steel. In addition, the optimal Cu content in steel between 0.2%-0.4% and the optimal annealing temperature between 1 100-1 200℃are determined by the economical and practical principles.

Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttriastabilized zirconia

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries（GBs） and the solidvapor interfaces（free surfaces）, each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies（differently）as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.

Theoretical study of the effects of alloying elements on Cu nanotwins

Nanotwins form in many metallic materials to improve their strength and toughness.In this study,we thoroughly studied the alloying effects of 10 common metal and nonmetal elements on Cu nanotwins by density functional theory(DFT).We calculated the segregation energies to determine if Cu nanotwins attract both the metal and nonmetal alloying elements;these segregation energies were then decomposed to mechanical and chemical components.The Cu-Sn bonds are different from other metal alloying elements,and the strong bond between Cu and the nonmetal element results in the negative values of the chemical contribution.Furthermore,the temperature and concentration have different effects on the nanotwin formation energy of the metal and nonmetal alloying elements.As determined by the Generalized Stacking Fault Energy,Al and nonmetals can inhibit the migration of Cu nanotwin boundary,and the effects of Li,Mg,and Sn are opposite.Our theoretical study serves as the foundation for the engineering nanotwin structures through alloying elements,the elements that may lead to new alloy compositions and thermomechanical processes,and are important complements to the experimental research.