Electron Microscopy Study On Twin-Twin Interactions In Deformed Mg-Gd-Zn Alloy

In this thesis,the deformation microstructure in the solution-treated Mg-5Gd-1Zn alloy(at.%)subjected to pin-impact deformation and that in the Mg-5Gd alloy(at.%)cold rolled at room temperature have been examined by means of transmission electron microscopy(TEM),during which special twin-twin interaction configurations of {10(?)1}-{10(?)2}and{10(?)-12}-{10(?)2} types have been found and subsequently investigated in detailed with the aim to understand their interaction processes and mechanisms.The main research contents and the results obtained are summarized as follows:The {10(?)1}-{10(?)2} intersection occurring in the Mg-5Gd-1Zn(at%)alloy when subjected to pin-impact deformation at room temperature has been characterized in detail by means of TEM technique.The results show that the intersection process is actually accomplished by inducing a secondary {10(?)1} twinning in the obstacle {10(?)2} twin under the action of the incident {10(?)1} twinning shear,and as a result,a kind of pseudo-twinning relationship between the induced secondary {10(?)1} twin and the incident {10(?)1} twin,which is same to a real {10(?)2} twinning relationship,can be established.Being as pseudo-twinning plane,the corresponding {10(?)2} lattice planes in both {10(?)1} twins are well coincided with each other.Based on the twinning dislocation theory,an adequate explanation of the twin-twin interaction phenomenon has been given,through which one can understand that,the good thickness correspondence between incident twin and induced twin is due to the effective response for incident twinning shear transferring into the obstacle twin once they encounter.As one of the most common deformation products in hexagonal system,twinned structure has a definite crystallographic relationship with respect to its matrix.Crystallographic calculations can therefore be applied to analyze various twinning relations with geat ease.Based on matrix transformation method,various crystallographic relationships including in the characteristic {10(?)1}-{10(?)2} twin intersection configuration have been clarified.Interaction of coaxial {10(?) 2} twins leading to occurrence of de-twinning and the related structural variations in the Mg-5Gd alloy(at.%)cold rolled at room temperature have been characterized by transmission electron microscopy(TEM).Based on TEM observation results,possible reason for the de-twinning involved in twin-twin interaction is analyzed.It is shown that the de-twinning can be described as a reverse secondary twinning process occurred in a primary twin,proceeding into the twin from its boundary.