The Microstructure Evolution Of Cu-rich Precipitate In RPV Model Steel

The samples for this study were taken from reactor pressure vessel(RPV) model steel having higher Cu content. The samples have 3 kinds of chemical composition. They were heat treatment of 0.5 h at 880 ℃ and water quenching, the samples were tempered at 660 ℃ for 10 h followed by air-cooling to room temperature. Samples were then isothermally aged at 370 ℃ and 400 ℃ for different times up to 6000 h. The Vickers microhardness was messured by microhardness tester( HV-10) with a load for 5 kg for 15 s. The precipitation of Cu-rich clusters and crystal structural evolution of Cu-rich precipitates in RPV model steels were investigated by means of extraction replica, TEM, HRTEM, STEM and EDS. The main conclusions are described as follows:(1) In the STEM mode, the diameters of Cu-rich precipitates in RPV model steels, which were separately thermal aged at 370 ℃ and 400 ℃ for 5000 h and 6000 h, are around 10 nm. The EDS results showed that, element Mn and the higher isothermal temperature will promote the diffusion and segregation of element Cu. Also, every Cu atomic plane will show along with two continuous iron atomic plane.(2) A Cu-rich precipitate in aged RPV model steel(4000 h at 400 ℃) was nearly spherical in shape with a diameter about 7 nm after cold rolled to 30% at room temperature. The corresponding fast Fourier transformation(FFT) of the region and the inverse FFT(IFFT) image showed a fivefold twinning structure. Direct evidence was found that the fivefold twinning occus via simulataneous emission of two Shockley partial dislocations from two particular α-Fe/Cu interfaces, and then the pileup tips of the twofold twin. It can be seen that pinned dislocations are predominantly located in the interface regions in which are marked as I1 and I2, respectively. Under applied stress action, it is possible that two perfect 60° dislocations located in I1 and I2 interfaces nucleate simultaneously. The dissociation of a 60° dislocation in a shear stress field gives rise to a 90 ° leading partial, immediately followed by the trailing 30° partial dislocation. Subsequently, TB1 and TB2 are formed almost simultaneous by Bα and Aβ 90° partial dislocation emission from the interfaces under the same conditions, respectively. When two glissile dislocations Bα on(1-1-1) plane and Aβ on(1-11) plane move away from the interface and collide, the 90° partial Bα is stopped by the TB2 and dissociated into the other dislocations. TB3 and TB4 are formed according to the dislocation reactions which are energetically favored on the pileup tips of the twofold twin. TB5 is also formed due to this mechanism. The orientation relationship between the α-Fe matrix and Cu-rich precipitate and cold rolling direction may be critical parameters for the fivefold twinning formation. It is also hoped that the results will be useful for the understanding of the twinning mechanism of Cu-rich precipitates in cold-rolled ferritic steels.