Effects Of Alloying Elements On The Deformation Modes In Magnesium Alloys: First-principles Calculations And Experimental Measurements

The application of magnesium alloys is severely hindered by their relatively low strength and poor ductility. The addition of alloying elements is known to be a potential approach to improve the mechanical properties of magnesium alloys. Unfortunately, rational selection of alloying elements has long been a tough problem due to the insufficient understanding of the alloying effects on the deformation mechanisms. Dislocation slip and twinning are the most basically deformation modes in magnesium. Committing to the research on the effects of alloying elements on the deformation modes has an important guiding significance for the selection of alloying elements while designing Mg alloys, which further promotes the development of new Mg alloys with high performance.In this work, the solid solution formation energies for systems with alloying elements staying at a substitutional, a tetrahedral interstitial and an octahedral interstitial site were calculated. The most stable configuration of various alloying elements in Mg lattice has been determined. Based on the calculated results of solid solution formation energy, supercells associated with the elements of interest were constructed to compute the generalized stacking fault energy(GSFE) of 0110)0001(>< and 3211}2211{>< slip systems for various Mg-X alloys. The effect of alloying elements on the mobility of basal <a> dislocation, the nucleation tendency of pyramidal <c+a> dislocation and the stability of glissile <c+a> dislocation were discussed. The misfit volume of various alloying elements and the strengthening effect of Nd on the }2110{ twinning dislocation were computed. The factors affecting the strength of }2110{ twinning dislocation are discussed.Based on the calculated results, binary Mg-x Nd(x=0.03, and 0.18at%, and marked as 1# and 2#, respectively) alloys were developed. The mechanical behaviors of Mg-Nd alloys with various grain sizes and strong extrusion fiber texture were investigated through compression test at room temperature. Effects of grain sizes and the content of alloying element Nd on the critical resolved shear stress(CRSS) of <c+a> dislocation and }2110{ twinning were discussed. The effects of annealing heat treatment on the strength and strain fraction accommodated by slip and twinning of twin-structured Mg-Nd alloys were investigated and discussed.The major conclusions are summarized as follows:① Alloying elements Ag, Al, Bi, Dy, Er, Ga, Gd, Ho, In, Li, Lu, Mn, Nd, Pb, Sc, Si, Sm, Sn, Y, Yb, Zn and Zr prefer to stay at the substitutitional site. Alloying element H prefers to stay at the tetrahedral interstitial site. Alloying elements C, N and O show a preference to stay at the octahedral interstitial site.② Alloying elements Al, Bi, C, Dy, Gd, H, In, Lu, N, Nd, O, Pb, Si, Sm, Sn, Y and Yb show great potentials in decrease the intrinsic stacking fault energy of basal slip, thus improving the creep resistance of Mg alloys. At the same, these alloying elements can increase the density of I2 stacking faults, thus impeding the movement of dislocations during the latent deformation process.③ Alloying elements B, C, Ca, H, Li, N, Nd, O, Pb, Si, Sm, Sn and Yb can decrease the unstable stacking fault energy of 3211}2211{>< slip system, thus promoting the nucleation of <c+a> dislocation. Alloying elements Al, Bi, Dy, Er, Ga, Gd, Ho, In, Li, Lu, Nd, Pb, Sm, Sn, Y, Yb and Zn can decrease the intrinsic stacking fault energy of 3211}2211{>< slip system, thus increasing the stability of glissile <c+a> dislocations. Taking the nucleation tendency and mobility of pyramidal <c+a> dislocations into account, alloying elements Bi, C, N, O, Nd, Sm, Sn, Yb are promising in enhancing the ductility of magnesium alloys via modifying the stacking fault energy.④ Alloying elements Ag, Dy, Er, Gd, Ho, Lu, Mn, Nd, Sm, Tm, Y, Yb and Zn show great potentials in strengthening the }2110{ twinning dislocations among which Nd is the most unique one. The addition of 0.03at% and 0.18at% Nd can strengthen the }2110{ twinning dislocations for 3 MPa and 23 MPa at room temperature.⑤ For 1# alloy with average grain sizes in the range of 62~89 μm, the CRSS for twinning is in the range of 25~19 MPa. For 2# alloy with average grain sizes in the range of 33~63μm, the CRSS for twinning is in the range of 24~21 MPa. The CRSS for twinning decreases with the increase of grain size for both 1# and 2# alloys. For a similar grain size(~62μm), the CRSS for twinning of 2# alloy is 4 MPa lower than that of 1# alloy. The CRSS for twinning is not sensitive to the content of alloying element Nd.⑥ For 1# alloy with average grain sizes in the range of 62~70 μm, the CRSS for <c+a> dislocation is ~50 MPa, and it is not sensitive to the grain size. However, softening was observed for a grain size above ~89μm. For 2# alloy with average grain sizes in range of 33~63μm, the CRSS for <c+a> dislocation is in the range of 46~40MPa and it decreases with the increase of grain size. For a similar grain size(~62μm), the CRSS for <c+a> dislocation of 2# alloy is 8 MPa lower than that of 1# alloy, which indicating that alloying element Nd can effectively decrease the CRSS for <c+a> dislocation of Mg alloys.⑦ For twin-structured Mg-Nd alloys, the }2110{ twinning dislocation can be strengthened via annealing heat treatment, thus regulating the strain fraction accommodated by slip and twinning during deformation, and further enhancing the mechanical properties of Mg alloys.