Hot compression experiments were carried out on rare earth (RE) added and RE-free Nb-containing steels by using a Gleeble simulator. Stress-strain curves obtained at various temperatures were analyzed to investigate the dynamic recovery and dynamic recrystallization softening behaviours. Morphology, size and number of precipitates in the both steels were examined by means of transmission electron microscopy (TEM). The results showed that, for the experimental Nb-containing steel, the grain size was fined by the RE addtion. In general, dynamic recrystallization cant occur in two steel under 40% deformation rates, and the deformation resistance of RE-containing steel is higher than that of RE-free steel in both the the austenite and ferrite temperatures range.While under the higher deformation rate, the dynamic recovery starting strains of the RE addition steel are higher than that of RE-free steel.It is also shown that the number of precipitate in the RE-containing steel more than that in the RE-free steel, which is due to the RE increasing nucleation rate and promoting Nb carbonitrides precipitation growth in the austenite region. Furthermore, the carbon activity may change by the RE addition, and thereby promote the precipitation strengthening of Nb-microalloyed steel.
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.
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.
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.
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.
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