Electron Microscopy Study On The Deformation Microstructures And Involved Interaction In Mg-Y-Nd-Zn Alloy

In this thesis, the deformation microstructures in solution-treated and LPSO strengthened Mg-Y-Nd-Zn alloys subjected to compression at room temperature have been examined by means of a combination of optical microscopy, electron backscattered diffraction(EBSD) and transmission electron microscopy(TEM), through which some important issues have been investigated, including deformation twinning or kinking structures, twin-twin interactions and the effect of long-period stacking ordered( LPSO) structures on twinning and kinking features. The main research results obtained are summarized as follows:In the solution-treated Mg-Y1.1-Nd0.4-Zn0.8 alloy(at.%) subjected to compression to a strain of 10% at room temperature, (?) twins were found to be formed abundantly as dominant deformation product. Occurrence of three typical interactions characterized by blocking, penetrating and crossing morphologies have been revealed for two co-zone variants of (?) twins(denoted by T1 and T2, respectively) when they encountering each other. The growing fronts for T1 and T2 can contact alternately to form a zig-zag shaped interface consisted of facets parallel to the twinning planes of T1 and T2, respectively. The crossed structure characterized by the crossing of T2 by T1 has been clearly revealed for the first time, according to which a penetration mechanism associated with basal slipping of dislocation and kinking formation has been proposed.A kind of uncommon deformation twin band of (?) high-index type has been found to form after the alloy being compressed to a strain of 10%. TEM observations shows that the twin interface of this twin band is not along the (?) plane which is theoretically required for the (?) twinning, but is actually nearly parallel with (?) plane of the matrix. Inside the (?) twin band, secondary twinning substructure and clear evidence associated with the occurrence of a small angle of lattice reorientation can also be observed. It follows that the formation of the (?)twin band can be attributed to a double twinning process, during which necessary accommodation of small angle of lattice rotation should involve. Based on this formation mechanism along with analysis of crystallographic twinning geometry, other possible twin bands of (?)?(?)?(?) and (?) types can be predicted for Mg alloys. These twin bands of high-index types can be regarded as a type of apparent twinning band, whose morphological characteristics and their boundary orientation should be similar to those of their corresponding primary twinning bands.Twin formation in the deformed Mg-1.1Y-0.4Nd-0.8Zn(at.%) alloy has been studied by a combination of EBSD and TEM examinations. The results show that the existing 14 H type LPSO structure has less influence on the twinning mode when the alloy subjected to compressive deformation, but can result in a decrease of twin size and occurrence of intersecting steps in the twinning surfaces as well. Concerning the effect of pre-existing LPSO structure on the (?)twinning process, two kinds of growth features have been observed. When the (?) twin growth proceeds in a region distributed with thin and widely-spaced LPSO slabs, the twinning can cross the LPSO layers by bending them to an angle of 3.7°. When encountering thick and closely-spaced LPSO slabs, the twinning can not accomplish a crossing process directly, but instead can initiate a twinning process again involved in nucleation and growth in the Mg matrix beyond these thick LPSO slabs. This process can occur in sequence such that deformation twinning appears to propagate forward.For the Mg-1.1Y-0.4Nd-0.8Zn alloy with 14 H LPSO structure, it has been revealed that kinking is main deformation mode when subjected to compressive deformation at room temperature. The formation of kinking bands results from slipping of basal dislocations and arraying of these dislocations to walls. The misorientation angle of a kink boundary is proportional to the dislocation density involved in kinking. Largeangle kinking can be accomplished via formation of more than one set of basal dislocation walls. It has observed that, subsequent to the formation of a large-angle kinking band, (?) twins turn to nucleate at the kink boundary and to grow into the kink band so as to release the accumulated stress concentration associated with the kinking.