| 其他摘要 | The development of new materials requires penetrating insight into their deformation mechanisms. Material computation contributes to such understandings, especially for the processes that are hardly reachable experiementally; meanwhile it exhibits a combination of high efficiency and convenience.
Plasticity is one of the most important mechanical behaviors of metals. Plastic defor-mations are mainly carried by dislocations and deformation twins. Employing molecular dynamics, the present thesis discusses two important issues closely related to plasticity, dislocation dipole transformations in fcc metals, and the deformation twinning mechanism in TiAl.
Work-hardening is one of the bases that enable metals to become structural materials. However, the mechanism of hardening is still “the most difficult remaining problem”, since various hardening theories fail to provide comprehensive explanations to such a phe-nomenon. Recent TEM observations and dislocation dynamics simulations show that, dur-ing single slip at the early stage of hardening, there is a wealth of dislocation reactions, in which edge dislocation dipoles may play an important role. But the observations need to be conducted under adequate resolution. Unfortunately, TEM observations are limited by resolution and due to the lack of long-range strain field of the dipole, several crucial fea-tures were left unattended. MD simulations, on the other side, are capable of revealing the atomic details in the processes of dislocation reactions, whereas previous methods of simu-lations are suspected due to the initial configurations.
In this study, dipoles are constructed in a new procedure, thus revealing that narrow dipoles are rather complex objects. Their size associated with the lack of long-range strain field makes them undetectable under TEM, but contrary to common belief, narrow edge dipoles neither collapse nor vanish. For Al, Cu and Ni with face-centered cubic structure and TiAl with L10 structure, dislocation dipoles would transform into stable and highly rearranged configurations. Point defect of various types, such as vacancies, self-interstitial rows and dipole loops are formed in large concentrations by direct reaction of dislocations under single slip during shear deformation. The by-products of such transformations and reactions are believed to provide nucleation sites for dislocation entanglement and self-patterning under single slip. They show in addition that, further to radiation damage, dislo-cation interactions can also constitute an important source of various point defects, even in an initially defect-free crystal.
TiAl-based alloys are becoming important materials for jet engine applications. How-ever, the problem of low ductility at room temperature is not yet solved, and one of the so-lutions might be to activate more twinning mechanisms. Simulations in systems with TiAl/Ti3Al interfaces show that the interfaces assist deformation and result in various be-haviors under different conditions. Regarding the complex stress states at the grain boundaries and within the grains in real cases, the present study applied hydrostatic pres-sure during the shear of the TiAl crystal to find the essential influence of external pressure on the twinning behavior. When sheared along the pseudo-twinning direction under hydro-static compression, the lattice forms an L11-structured pseudo-twin, while true twin grows when sheared identically but under hydrostatic tension. The extended γ-surface developed in the present study explains such behaviors satisfactorily.
A new mechanism for deformation twinning of TiAl by shearing along the two pseudo-twinning <211] directions of a {111} plane was revealed by atomistic simulations. Under zero pressure or hydrostatic tension, in order to thicken by one layer a true twin makes use of the correlated movement of five distinct 1/6<112> partials on two adjacent {111} planes. With a total twinning Burgers vector of 2/3<211], the present twinning mechanism yields a shear strain of 2√2, four times as large as either under conventional 1/6<112] twinning or in the 1/6<211] L10 to L11 transformation. The operation of the multi-stepped twinning mechanism in TiAl is a consequence of strong atomic ordering; it should not depend on the type of interatomic potential provided this describes the ordering energy reasonably well. Furthermore, since the essence of this mechanism is of crystallo-graphic nature, it is in principle restricted neither to TiAl nor to the L10 structure. |
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