| Twinning deformation is considered an important factor affecting the anisotropy and the asymmetry of the AZ31 magnesium alloy through the tests of microstructures and properties of the material with extrusion textures and compression textures. To understand the mechanism of twinning deformation of magnesium alloys, a new model was established to describe the atomic motion in {1012} twinning, {1011} twinning and {1013} twinning, and on the basis of which three new parameters have been proposed to measure the difficulty degree of twinning: The first parameter is the rotational angle of the QPAG units?; the second parameter is the relative displacement magnitude between the adjacent QPAG units?; the third parameter is the atomic deformation to overcome during twinning?. The later two parameters?and?can be used to measure the difficulty degree of twinning; the lager these parameters are, the harder the twinning occurs, which is consistent with the experiment results. It is not accidental that these two parameters can be used to measure the difficulty degree of twinning, because they are of true physics meaning for that the larger value of the relative displacement magnitude and the atomic deformation means the higher stress to overcome. In addition, the QPAG model also can be used to explain the interesting phenomenon about twinning, such as twin growth, detwinning and twin recrystallization. According to these results some conclusions have been drawn about the deformation behavior and the twinning mechanism of AZ31 magnesium alloy.?The AZ31 magnesium alloy samples homogenized at 723 K for 5 h were compressed to various strains at room temperature. The results show that the amount of twins is increased with the deformation, and the twin shape turn wider and bended from the original shape thin and straight. EBSD technique show that most of the twins are {1012} extension twins, and only a few of them are {1011} contraction twins. Static recrystallization occurred in the twins when the compressed samples were annealed with small recrystallized grains generated within the twins.?The QPAG unit model was established for the first time to reveal the law of atomic motion during twinning in AZ31 magnesium alloy. The study indicates that the atomic motion during twinning can be ascribed to the rotational motion of the QPAG units. Since there is no resistance within interior of the QPAG units, it can be considered that the twin resistance mainly comes from the relative motion between the adjacent QPAG units. The QPAG units rotate an angle of 15.9°during {1012} twinning, and the relative displacement magnitude between the adjacent QPAG units is 0.349a.?The QPAG unit model established in {1012} twinning is also suitable for describing the atomic motion in {1011} twinning, the atomic motion in {1011} twinning also can be ascribed to the rotational motion of the QPAG units. However, there exist two kind of QPAG units: one of them (type I) rotated only in the plane y-o-z;the other one (type II) not only rotate in the plane y-o-z, but also shift along the x axis for 0.5 a. These two kind of QPAG units are distributed alternatively. Both of these two kind of QPAG units rotate an angle of 14.9°during {1011} twinning, and the relative displacement magnitude between the adjacent units is 0.517a. Though the rotational angle in {1011} twinning is smaller than in {1012} twinning, the appearance of the type II QPAG units make the atomic motion in {1011} twinning become more complex and the relative displacement magnitude larger, which may be the essential reason that {1011} twinning is harder to occur than {1012} twinning.?The QPAG unit model is also suitable for describing the atomic motion in {1013} twinning. There exist totally two kind of alternately distributed QPAG units in {1013} like in {1011} twinning: one of them (type I) rotated only in the plane y-o-z;the other one (type II) not only rotate in the plane y-o-z, but also shift along the x axis for 0.5 a, the difference is that the QPAG units in {1013} twinning represent a“chiral”relation with that in {1011} twinning. Both of these two kind of QPAG units rotate an angle of only 6.3°during {1013} twinning. However, the relative displacement magnitude between the adjacent units is 0.539a, larger than the value in {1011} twinning, this may be the reason that {1013} twins are more infrequent to be observed than {1011} twins.?The rotational motion of the QPAG units during twinning has a property of transfer. The twinning atoms are required to overcome a certain deformation during twinning, and there exist a critical value about the deformation to overcome. When the deformation is before the critical value, the twinning atom will return to the original position once the stress is removed;when the deformation pass the critical value, the twinning will occur. The stacking fault atoms can not reach the normal position in the twin after twinning, and they arranged in a disordered state. Thermodynamically, these disordered atoms are in a high-energy state, and thus inclined to become the recrystallization sites. ?The twin boundaries can be regarded as composed of the QPAG units oriented between the matrix and twin gradually. The twin boundaries can move along the norm direction through the rotation of these QPAG units: twin growth occurs when the rotation of the QPAG units make the twin boundaries move to the matrix;detwinning takes place when the rotation of the QPAG units make the twin boundaries move to the twin. Basal slip in the magnesium always results in the grain rotation and the grain boundary movement. Because the twin boundary sliding is difficult to occur for the special atomic arrangement of the twin boundaries, dislocations will accumulate at twin boundary when the twin and matrix in a soft orientation, and thus the twin boundaries will become the nucleation zone of fracture.?The values of atomic deformation need to overcome in {1012} twinning, {1011} twinning and {1013} twinning are 0.024, 0.031 and 0.038, respectively. The order of these value is consistent with the observation in the experiment of the room temperature compression of the AZ31 magnesium alloy, that is, the {1012} twins are the easiest to form, followed by {1011} twins, and the {1013} twins are the hardest to form. In conclusion, the factor that determine the difficulty degree of twinning to occur is the atomic deformation need to overcome during twinning. |
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