Strain Accommodation Effect Between Deformation Twins In Magnesium Alloys And Their Variant Selection Mechanism

This work aims to investigate strain accommodation effects and variant selection of deformation twins in magneiusm alloys subjected to various plastic processes. A series of tension, compression, and rolling experiments were carried out on several commercial AZ31 hot-rolled plates. In defomed samples, there were not only various twin patterns including multi-twins inside a grain, twin pair, twin chain, and long twin band, also secondary twins observed. The morphology and misorientation of the above twin patterns were examined using?in-situ? electron backscatter diffraction?EBSD? technique. The propensity and variant selection of twin?patterns? could be explained by a combined analysis of Schmid law and strain accommodation effect?m? criterion?.The trace analysis is an important supplenment to the orientation rotation method used to identify the active twin variant and is dependent on the observation plane. The trace angle of Para-positional variants with max Schmid factor?SF? was in a high angle range of 79-86° on the plane parallel to the c axis and compression direction, favoring for the identification. Such observed plane was found to be still suit for indetication of twin variant in rolled and extruded Mg alloys subjected to compression along different direction. By an analysis on stress along the tranverse direction during rolling, it was found that the two modified criteria reported earlier for twinning had the same stress state for rolling. A generalized Schmid factor obtained by incorporating the transverse stress could successfully explain not only the variant selection, also the propensity of twinning with changes of the initial orientation.Various {101₂} twin patterns were observed in an AZ31 plate subjected to compression along TD, such as mutil-twins inside a grain, twin pair and twin chains crossing neighboring grains. When compression direction was nearly parallel to the <101₀> of a grain, a pair of PP twin variants in the grain had very high SF among six possible variants. Therefore, both of them could be activated. Alternatively, When compression direction was parallel to the <112₀>, there were two pairs of ortho-position?OP? variants, i.e. four variants, having high and close SF. In this case, the two in OP of the four variants were frequently activated due to the best strain compatibility between them. The twin transfer at grain boundary resulted in the formation of twin pair. The twins of twin pair usually had high SF and high m? between them. However, a low SF twin could be also simulated by the other twin of high SF in the case of twin pair. The statistical results indicated that the strain accommodation effect between twin pairs was popular and could activate a certain amount of non-Schmid twins. Besides, high m? was not the consequence of Schmid law-dominated variant selection. The twin transfer was promoted with reduction of grain size and increase of basal texture intensity so that long twin bands were formed during rolling of a refined Mg alloy. These twins in long twin bands generally had high SF and high m??> 0.8? between the adjoining twins, indicating that the local strain accommodation effect not only influenced the variant selection of the adjoin twins, also promoted the formation of long twin bands. Moreover, the twin strain acrossing grain boundary was intuitively observed in image quality map by in-situ EBSD examination, which confirmed the twin transfer at grain boundary. In addition, various twin nucleation phenomena including single twin nucleating at the twin boundary and inside a grain were in-situ observed as well.A mass of {101₂}-{101₂} secondary twins were observed at the intersection of primary twins in a super-coarse grain in an AZ31 plate subjected to tension along ND. The statistical results indicated the relation between the crossing primary and secondary twin was experimentally found to be concentrated around 49.7°<9₀94>, which resulted from the variant selection of primary and secondary tiwn. It was found that the two primary twins intersecting with each other were generally from the OP variants, which have the best strain compatibility inside a grain. The secondary twin generally had a twin plane in OP with that of its parent and lower strain incompatibility with the crossing primary. The {10 1₁}-{10 1₂} secondary twins have four different misorientations with the initial matrix, corresponding four secondary twin variants. The activation rank of the four variants was dependent the specific defroamtion styles. However, the dependence could be not understood in terms of Schmid law or local strain accommodation effect alone. By compounding the Schmid law and the strain compatibility?m? criterion? between the primary and secondary twin variants, a composite Schmid factor mc was achived. It could better explain the changes of activation rank of the four variants with the deformation styles.