| Magnesium is the lightest of all commonly used structural metals, with a density approximately two thirds that of aluminum and one quarter that of steel. Recently, magnesium alloys are attracting increasing attention in the automotive and aerospace industry for weight reduction. However, the use of magnesium alloys is still very limited due to their poor ductility and inadequate strength. Fundamentally, the ductility and strength of metallic materials are highly related to the slip of dislocations, the evolution of twin boundaries and the propagation of cracks.In the case of ductility improvement, alloying elements can be added to modify the generalized stacking fault energies(GSFE) and the cleavage energies(γcarck), with the goal of changing the behaviour of dislocations and cracks. In this study, the effects of structural relaxation on the GSFE of hexagonal-close-packed system from first-principle calculations were discussed in detail. Then, the alloying elements effects on the GSFE of }0001{ 0110><, }0011{ 0211>< and }2211{ 3211>< slip systems, the stacking fault energies(SFE) of I1 and the γcarck of }0001{ plane were simulated. Based on the simulated results, the effects of alloying elements on the behaviour of basal dislocations, the activation of non-basal dislocations, and the intrinsic ductility under “Mode I” were discussed.The most soft deformation modes in magnesium alloys are basal slip and }2110{ twin. Traditionally, the strengthening of magnesium alloys is fulfilled by impeding the motion of basal dislocation. However, studies on the mechanism of }2110{ twin boundary impeding are limited. In this study, the }2110{ twin boundary segregation energies(ESeg) and solute-diffusion activation enthalpies(EAct) of various alloying elements were simulated, using first-principle method. Moreover, the interaction between }2110{ twin and basal stacking faults, and the evolution of }2110{ twin boundary were simulated. Then, the strengthening effects of basal stacking faults on the }2110{ twin were discussed.The major conclusions can be summarized as follows.① To simulate the GSFE relatively precisely, a relaxation along the z direction, perpendicular to the slip plane, should be adopted, especially for the high-index slip plane. When the atoms on the two sides of the slip direction are unsymmetrical(e.g., }2211{ 3211>< slip system), a relaxation along the y direction, parallel to the slip plane and perpendicular to the slip direction, is essential. These findings are also applicable to the calculation of GSFE in other hexagonal close packed(HCP) systems such as Co, Ti, Zn and Zr.② Addition of Ag、Al、Dy、Er、Gd、Ho、Lu、Mn、Nd、Sc、Sm、Y、Zn and Zr increase the the γcarck of }0001{ plane, which means the propagation of cracks might be suppressed by the addition of these elements. Addition of Al, Bi, Ca, Dy, Er, Ga, Gd, Ho, In, Lu, Nd, Pb, Sm, Sn, Y and Yb decrease the SFE of stacking I1, which means the density of stacking I1 might be increased by the addition of these elements, leading to higher ductility by the activation of }2211{ 3211>< dislocations, and leading to higher strength by the impending of }2110{ twin boundaries. For the }0001{ >< 0110 slip system, addition of Al, Bi, Dy, Er, Ga, Gd, Ho, In, Lu, Nd, Pb, Sc, Sm, Sn, Y, Yb and Zr facilitate the sequential faulting mechanism, which means the density of stacking I2 might be increased by the addition of these elements, leading to higher strength by the impending of }2110{ twin boundaries. For the }0011{ 0211>< slip system, addition of Ca, Dy, Er, Gd, Ho, Lu, Nd, Sm, Y and Yb decrease the unstable stacking fault energies(γus) dramatically, which means the critical resolved shear stress(CRSS) of this slip system might be reduced by the addition of these elements. For the }2211{ 3211>< silp system, it is found that addition of Ag, Al, Ca, Dy, Er, Ga, Gd, Ho, Li, Lu, Nd, Sm, Y, Yb and Zn soften the }2211{ 3211>< silp and increase the intrinsic ductility of magnesium, based on the discussions on the nucleation and dissociation of }2211{ 3211>< dislocation, and the competition between }2211{ 3211>< nucleation and basal crack propagation.③ A design map, Fig 5.6, based on the twin boundary strengthening mechanism was proposed, in which ESeg and EAct are taken into account. The elements lying at the lower left quarter of the map, including Nd, Sm, Gd, Dy and Yb, show the most effective }2110{ twin boundary strengthening effects, by means of twin boundaries segregation.④ The formation mechanism of BP interface is discussed. The Burgers vector(b) of twin dislocation on the }2110{ twin boundary is so small, ~0.498?. which means it can not escape from the lattice when encounter a free surface. As a result, a BP(basal-prism) segment is generated to accommodate the strain. The BP segment can be widened by absorbing twin dislocations.⑤ The feature of the dislocations on the }2110{ twin boundary and the dislocations on the BP interface are characterized under a relative large system, including ~2 million atoms. The results reveal that both of them exhibit a curvy feature, which means they can bypass a obstacle like the lattice dislocations.⑥ Molecular dynamics simulations reveal that both the stacking I1 and I2 show great strengthening potentials on the }2110{ twin. The strengthening effects of stacking I1 is stronger than that of I2... |
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