First-principles description of planarfaults in metals and alloys

Detta är en avhandling från Stockholm : KTH Royal Institute of Technology

Sammanfattning: Phase interface and stacking fault are two common planar defects in metallic materials. In the present thesis, the interfacial energy and the generalized stacking fault energy of random alloys are investigated using density functional theory formulated within the exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA).The interfacial energy is one of the key physical parameters controlling the formation of the Cr-richα’ phases during the phase decomposition in Fe-Cr ferrite stainless steels. This decomposition is believed to cause the so-called “475°Cembrittlement”. Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475oC embrittlement. The effect of Al on the interfacial energy and the formation energy of Fe-Cr solid solutions are studied in this thesis. The interface between the decomposed Fe-rich α and Cr-richα0 phases carries a positive excess energy, which represents a barrier for the process of phase separation. Our results show that for the α-Fe70Cr20Al10/α0-Fe100−x−yCryAlx (0≤x≤10, 55≤y≤80) interface, the Al con-tent(x) barely changes the interfacial energy. However, when Al is partitioned only in the alpha phase, i.e. for the α-Fe100−x−yCryAlx/α0-Fe10Cr90 (0≤x≤10, 0≤y≤25) interface, the interfacial energy increases with Al concentration due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying.The intrinsic energy barriers (IEBs) on theγ−surface (also called generalized stacking fault energy, GSFE) provide fundamental physics for understanding the plastic deformation mechanisms in face-centred cubic (fcc) metals and alloys. In this thesis, the GSFEs of the disordered Cu-X (X=Al, Zn, Ga, Ni) and Pd-X (X=Ag, Au) alloys are calculated. Studying the effect of segregation of the solutes to the stacking fault planes shows that only the local chemical composition affects the GSFEs. Based on the calculated GSFE values, the previously revealed “universal scaling law” between these IEBs is demonstrated to be well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, the addition of Al, Zn and Ga increases the twinnability, while adding Ni decreases it. Aluminum and gallium yield similar effects on the twinnability. Our theoretical predictions are in line with the available experimental data. These achievements open new possibilities in understanding and describing the plasticity of complex alloys.We investigate theγ-surface of paramagneticγ-Fe as a function of temperature. At ambient conditions, the fcc lattice is thermodynamically unstable with respect to the hexagonal close-packed (hcp) lattice, resulting in negative intrinsic stacking fault energy (ISF). However, the unstable stacking fault energy (USF), representing the energy barrier along theγ-surface connecting the ideal fcc and the intrinsic stacking fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recently developed plasticity theory, the overall effect of temperature is to move the plastic deformation mode of the paramagnetic fcc Fe from the stacking fault formation regime (T <<300K) towards maximum twinning (T≈300K) and finally to a dominating full-slip regime (T >>300K). Our predictions are discussed in connection with the available experimental observations.The same methodology is used to establish theγ-surface of Fe-Cr-Ni alloys as a function of chemical composition and temperature. We fix the concentration of Cr at 20 at.%. Nickel is found to increase the intrinsic stacking fault (SFE), unstable stacking fault (USF) and unstable twin fault (UTF) energies. The theoretical SFE versus chemistry and temperature trends agree well with experiments. Both USF and UTF decrease with increasing temperature. The calculated IEBs are used to establish the temperature and composition dependence of the deformation modes in Fe-Cr-Ni alloys. Stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective SFE below∼18mJm−2, which is in good agreement with the observed upper limit of the SFE for the TRIP (transformation-induced plasticity) mechanism. Alloys with SFE above this critical value show both twinning and full slip at room temperature and surprisingly, even the SFE is very high, twinning remains a possible deformation mode even at elevated temperatures, which is in line with observations

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