Papers by Author: Zong Chang Liu

Abstract: It is significant theoretically to study the nucleation laws of the phase transformation products of the supercooled austenite. The different iron and steel materials were used to study the nucleation of pearlite, bainite and martensite by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. The results show that, from the high-temperature zone to the low-temperature zone, the nucleation of the transformation products of the supercooled austenite is a gradually evolutionary process from two-phase nucleation to single-phase nucleation. Pearlite nucleates in the austenitic grain boundary, bainite nucleates preferentially in the grain boundary and sometimes in the grain interior and martensite nucleates preferentially in the interface and generally in the grain interior. The preferential nucleation sites are the interface, and with the decrease of the transformation temperature, they gradually shift to the defects in the grain interiors, which accord with the general rules of the phase transformation nucleation.

Abstract: The precipitation of copper during aging at 600oC in high-purity Fe-Cu alloy was examined by means of transmission electron microscopy (TEM).Nano-scale copper-rich clusters with a B2-like structure were observed during heat treatment. These micro structural features play an important role in precipitation strengthening. In addition, the precipitation process has been analyzed in terms of the evolution of microstructure by a Monte Carlo method. A description of the coherent precipitation of copper in iron, based on a vacancy diffusion mechanism, thermally activated jump frequencies and cohesive energy is discussed in order to deal with simultaneous precipitation of metastable and stable phases in Cu-containing steel during aging. This analysis gives an estimation of the precipitation dynamics, as well as the evolution of Cu precipitates across a wide range of temperatures.

Abstract: Microstructure evolution of Fe-1.18%Cu high purity steels during solution and aging was investigated under high-resolution electron microscope (HREM). In addition, the aging strengthening mechanisms were discussed based on the microstructure observation. The results show that there were lots of Cu atom clusters in ferrite matrix during solid solution and aging initial stages, subsequently, Cu-rich metastable Fe-Cu particles precipitate at the aging strength peak. It is found that the intense strengthening is controlled by the coherency relationship of Fe-Cu metastable phase with matrix that forms the obstacle of the dislocation motion, while the decrease of strength after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging strengthening in Cu bearing high purity steels Thus our TEM observation results are in reasonably agreement with some previous assume.

Abstract: 0MnVTiNb, 12Cr1MoV, 20Cr2Ni4, 35CrMo, 40Cr, 42CrMo, 60Si2CrV and T8 steels and Fe-1.2C alloy were used to study the morphology and formation mechanism of martensite by metallographic microscope, QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope after they were austenized at different temperature and then quenched respectively. The results show that the martensite of low-carbon steel is lath martensite, the martensite of high-carbon steel is plate martensite, and the martensite of medium-carbon steel is the integrated microstructure of lath martensite and plate martensite. With the increase of carbon content, the morphology of martensite in steel evolves from lath shape to plate shape, the distribution of martensite slices changes from in parallel to with crossing angle, and the substructure evolves from high density dislocations and stacking faults to twin crystals. The martensite in steel can nucleate in the austenite crystal grain interior as well as along the austenite crystal grain boundary. It is proposed that the volumetric strain energy in martensite transformation is the essential reason of the different morphologies of martensite.

Abstract: The precipitate behavior of copper in the high purity structural steel was investigated by means of transmission electron microscope (TEM), and aging hardening mechanism was investigated based on the corresponding phase transformation mechanism. The results show that lots of Cu rich clusters exist in supersaturated ferrite matrix in solid solution, which evolve to B2-like structure during aging. It is found that the hardening in the initial stage is controlled by the coherency relationship of the B2-like structure with matrix that forms the obstacle of the dislocation motion, while the decrease in hardness after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging hardening in Cu bearing high purity steels.

Abstract: The precipitation of copper during aging at 650oC within ferrite in high-purity Fe-1.03wt%Cu steel was examined by transmission electron microscopy, and the influence of precipitation particles on property of experimental steel was investigated. The microstructure and the corresponding diffraction patterns of different zone axis were analyzed. Nano-scale copper-rich clusters with B2-like structure and high density dislocation around precipitate was observed during either solution treatment or aging. Nano-scale metastable precipitates and high density around them were found to play the most important role for increasing steel strength.