Study On {11(?)1} Twin And Twin-twin Interactions In Deformed Mg-Y Alloy By Electron Microscopy

Deformation twinning often mediates plastic deformation in magnesium alloys.But unfortunately,due to the complexity involved in the twinning and twin-twin interaction processes,the relationship between twin-twin interactions and related mechanism have not yet been clarified.Therefore,it is necessary to reveal the growth feature of deformation twins and the related twinning interaction in order to give some important information on strengthen strategies and alloy designs for magnesium alloys.In this paper,the deformation microstructures involved with {11(?)1} twinning and twin-twin interaction in the Mg-10Y(wt.%)alloy rolled at room temperature have been investigated by means of transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM)and high angle annular dark field scanning transmission electron microscopy(HAADF-STEM).Based on experimental observations,the corresponding structural models for interpreting {11(?)1} twin growth and the related twin-twin interactions are constructed,through which diffraction analysis,structural adaptation factor analysis and Schmid factor theory have been applied to analyze the twin-twin interaction.The main results obtained are summarized as follows:1)TEM observations show that large amount of {11(?)1} twins can be formed in the form of long-straight strip in the Mg-Y alloy rolled at room temperature.The atomic resolution structure of the {11(?)1} twin boundary has been clearly revealed for the first time by means of HAADF-STEM imaging technique.It is clearly shown that the {11(?)1}twin boundary is a highly coherent structure and there is no any step structure,which indicate that it is difficult to form apparent twin boundary deviating from the standard{11(?)1} plane.On the basis of experimental observation,an atomic shear model was constructed which clearly reveals that the formation process ofa {11(?)1} twin involves not only atomic shear but also atom shuffle.2)The structure associated with(10(?)2)-(11(?)1)twin interaction structure has been observed and investigated by TEM in the deformed Mg-Y alloy.The shoulder-like configuration structure was found on both sides of the {10(?)2} twins near the interaction zone.Diffraction simulation,Schmid factor analysis and structural adaptation factor analysis have been applied to analyze the(10(?)2)-(11(?)1)twin interaction process.It is clearly revealed that a(10(?)2)twin can cross a(11(?)1)twin by inducing a(01(?)2)secondary twin inside the obstacle(11(?)1)twin and forming a definite twin-twin crossing configuration.Based on the atomic shear model of(11(?)1)twin which we have proposed,the continuous growth tendency for these twins involved in the(10(?)2)-(11(?)1)crossing interaction was analyzed.The result shows that,during the continuous twinning process for the interaction configuration,the non-uniform stress distribution is responsible for the epitaxial growth of(10(?)2)twins which can lead to formation of shoulder-like structure at the interaction position.3)The interaction behavior for co-zone {11(?)1} twins in the deformed Mg-Y alloy has been investigated by TEM.Schmidt factor analyzing and {11(?)1} twin modeling have been applied to explain the presence of step at the interaction position and the asymmetric twin growth in the interaction configuration as well.It has been clarified that the stress concentration resulted from the twin-twin interaction can have significantly different effects on the growth of both {11(?)1} twin variants in the interaction configuration depending on twinning direction.The twin interaction boundary has been identified to be a definite planar boundary whose ideal crystallographic habit plane should be identical to the(1120)plane of matrix.It is inferred that the interaction surface exists in the form of(11(?)2)t1//(11(?)2)T2.The mechanism of stress traversing has been proposed to explain why co-zone {11(?)1} twin interaction can form crossing configuration without forming secondary twin.Furthermore,it has been clearly illustrated that the local twinning conditions are associated with both Schmid factor and structural adaptation factor.The analysis by combination of Schmid factor analysis and structural adaptation factor is quite necessary for understanding the twinning conditions and twin growth processes.