Wanming Lin

Microstructural Evolution of Surface Layer of TWIP Steel Deformed by Mechanical Attrition Treatment

A nanocrystallihe layer was synthesized on the surface of TWIP steel samples by surface mechanical attrition treatment (SMAT) under varying durations. Microhardness variation was examined along the depth of the deformation layer. Microstructural characteristics of the surface at the TWIP steel SMATed for 90 min were observed and analyzed by optical microscope, X-ray diffraction, transmission and high-resolution electron microscope. The results show that the orientation of austenite grains weakens, and armartensite transformation occurs during SMAT. During the process of SM AT, the deformation twins generate and divide the austenite grains firstly; then α-martensite transformation occurs beside and between the twin bundles; after that the martensite and austenite grains rotate to accommodate deformation, and the orientations of martensite and between martensite and residual austenite increase; lastly the randomly oriented and uniform-sized nanocrystalline layers are formed under continuous deformation.

Structural characteristics of nanocrystalline copper after carbon ion implantation.

A gradient structure was produced in a pure copper plate by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer was reduced to nanoscale and the grain size increased gradually along the depth of the treated sample. In situ transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observation was performed on the nanocrystalline copper after implantation of carbon. Carbon atoms first precipitated along the edges of the copper substrate or at the surface, then formed amorphous carbon layers. Subsequently, onion-like fullerenes were formed under electron-beam irradiation. The effects of ion implantation, electron beam irradiation, nanostructure of the substrate and interaction of C and Cu atoms on the formation of the onion-like fullerenes are discussed.

EFFECT OF VELOCITY OF BALLS ON THE STRAIN AND STRESS OF LOW CARBON STEEL SURFACE LAYER DURING SMAT

By means of finite element method, the effect of velocity of balls on the strain and stress of low carbon steel surface layer during the course of surface mechanical attrition treatment (SMAT) are investigated. The effect of different impact velocity on strain rate and grain refinement mechanism is also analyzed. Calculation results confirm that there exists severe plastic deformation in the surface layer: strain, strain rate and stress gradually decrease along the depth of the treated sample during SMAT, which is in agreement with the microstructures observed in corresponding locations. Strain and strain rate play an important role in the grain refinement process and the resultant grain sizes upon plastic deformation.

Solid-Phase Decarburization of High-Carbon Ferromanganese Powders by Microwave Heating

Solid-phase decarburization of high-carbon ferromanganese powders (HCFPs) was conducted using calcium carbonate powders (CCPs) as a decarburizer by microwave heating. Solid-phase decarburization kinetics was investigated by isothermal method. The results show that the HCFPs show excellent microwave absorption at a higher average heating rate of 80 °C/min, while CCPs exhibit poor microwave absorption at a lower heating rate of 5–20 °C/min; the heating characteristics are in-between when HCFPs and CCPs are mixed. The average heating rates of the mixture are 32.14, 31.25, 31.43, and 30.77 °C/min when the mixture is heated up to 900, 1000, 1100, and 1200 °C, respectively. The good microwave absorption property of the mixed material lays the foundation for the solid-phase decarburization of HCFPs containing CCPs. Solid-phase decarburization of HCFPs containing CCPs is a first-order reaction by microwave heating. Apparent activation energy of solid-phase decarburization is 55.07 kJ/mol, which is far less than that of ordinary carbon gasification reaction and that of solid-phase decarburization under the same decarburization condition by conventional heating. It indicates that microwave heating not only produces thermal effect, but also has non-thermal effect.