Jingmei Tao

Microstructure and properties of super martensitic stainless steel microalloyed with tungsten and copper

Abstract The microstructure and properties of super martensitic stainless steel (SMSS) microalloyed with tungsten and copper were studied by means of optical microscopy, dilatometer, X-ray diffraction and tensile tests. The results showed that the microstructure of SMSS, after quenching and tempering, was a typical biphase structure with tempered martensite and reversed austenite dispersedly distributed in the martensite matrix. W and Cu were added into the SMSS to reduce the transformation temperature (Ms) and improve the strength and hardness of the matrix by grain refining and solid solution strengthening. Thermo-Calc calculations confirmed that M23C6 compound and Laves phase were precipitated during tempering in the investigated steel. Compared with traditional SMSS, the steel microalloyed with W and Cu exhibited better mechanical properties.

Influence of plastic deformation on evolution of defect structures, microhardness and electrical conductivity of copper

Abstract Samples of pure Cu cylinder were plastically deformed by the combination of cold forging (CF) and cold drawing (CD) at liquid nitrogen temperature. X-ray diffraction measurements indicate that an increase in deformation strain leads to a decrease in crystallite size and an increase in twin densities for the CF and CD processed ultrafine grained samples. Dynamic recovery is suggested to start during the deformation process and leads to a decrease in dislocation density at large deformation strains. The increase in twin density could compensate the loss of microhardness because of the decrease in dislocation density. The electrical conductivities of CF+CD samples were tested through standard four-probe method, all of which are higher than 92% the International Annealed Copper Standard. The results suggest that the strength of pure Cu could be improved and still keep its relatively high electrical conductivity by introducing deformation twins into its microstructure.

Mechanical Properties of Nanocrystalline Cu and Cu-Zn Using Tensile and Shear Punch Tests

Shear punch test (SPT) has been used to study the mechanical properties of Cu, Cu–10 wt.% Zn, Cu–20 wt.% Zn and Cu–30 wt.% Zn after ball milling with an average grain size in the range of 33-12nm. The strain rate sensitivity (SRS) and physical activation volume have been determined. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. This suggests that the thermally activated process in nanocrystalline (nc) metal/alloys is different from the conventional forest dislocation cutting mechanism. The stacking fault energy (SFE) of Cu-Zn alloys decreased with the adding of Zn, and deformation twins are anticipated to introduce into the nc Cu-Zn alloys during process of ball milling. Dislocations could accumulate along the TBs and carry the plastic strain, so the ductility of nc Cu-Zn alloys could be improved.

The thermal stability of nanocrystalline Cu prepared by high energy ball milling

Full density nanocrystalline (NC) Cu with average grain size of 33nm was prepared through high energy ball milling. Effects of annealing on microhardness and activation volumes (V*) were studied. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. The much higher micro-hardness of as-prepared Cu sample of 1.7GPa was not detected to decrease after annealing at 773K for 1h with corresponding small value of activation volumes of 22.6. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volumes. The considerably higher microstrain and impurities stemming from high energy ball milling should be responsible for the relatively higher thermal stability of NC Cu. During annealing process, the strain release process occurred prior to the grain growth process and the impurities hindered the grain coarsening process, therefore, the NC Cu has a relatively higher thermal stability. The present investigation demonstrates that the thermal properties of NC materials are determined by not only the grain size but also the microstructure of grain boundaries.