E. Dogan

THE ROLE OF DEFORMATION MODES ON DUCTILITY AND DYNAMIC RECRYSTALLIZATION BEHAVIOR OF AZ31 Mg ALLOY AT LOW TEMPERATURES

Tensile ductility and dynamic recrystallization of AZ31 (Mg-3Al-1Zn) alloy samples were investigated at the temperature range of 25°C–200°C. Samples were cut from a strongly textured, hot rolled AZ31 plate from three different orientations in order to activate alternate deformation mechanisms during uniaxial tensile loading. Dynamic recrystallization (DRX) behavior was found to be highly dependent on the active deformation modes at temperatures between 100°C and 200°C, where non-basal slip mechanisms (prismatic and pyramidal ) retards DRX whereas basal slip promotes DRX. The final microstructure and elongation to failure vary with active deformation modes at moderately elevated temperatures (>50°C), while at room temperature elongation to failure seems to be insensitive to the deformation modes. There is a significant indication that twinning initiated shear localization and induced an earlier fracture at elevated temperatures. More homogenous deformation until necking was observed when prismatic slip was the main active deformation mode.

Ductility Enhancement in Mg Alloys by Anisotropy Engineering

A mean-field theory suggests that certain forms of plastic anisotropy hinder ductile damage accumulation. Here, a proof-of-concept is presented in the case of Mg–Al–Zn alloys. Textures produced by severe plastic deformation are compared with the as-received rolling texture in terms of their anisotropy-ductility correlations at ambient temperature. The 3D plastic anisotropy is characterized in each material using compression specimens. The ductility is characterized using tensile bars. A micromechanical model is introduced to rationalize the trends in terms of the anisotropy effect on ductility (AED) index. Here, this index is tuned via texture manipulations at fixed chemical composition and grain size. The main finding suggests that plastic anisotropy can be engineered to aid ductility.

Microstructural Design of Mg Alloys for Lightweight Structural Applications

Under a carefully selected starting texture, Mg-3Al-1Zn (AZ31) has been successfully processed via equal channel angular processing (ECAP) at 150°C under multiple passes to obtain an ultra-fine grained bulk material. The multiple ECAP passes at 150°C decreased the grain size of bulk as-received AZ31 from 25 µm to ~0.8 µm. Upon grain refinement, the tensioncompression yield asymmetry decreased, and the yield strength increased to the level of 6000 series Al alloys. Detailed electron backscatter diffraction (EBSD) analyses clearly indicated that the formation of compression twins initiated deformation localization via softest basal slip. Subsequently, dynamic recrystallization initiated within the compression twins for temperatures at 150°C and below. Thus, the preferential sites for softest basal slip cause local softening and DRX, accumulating throughout the material in the form of large shear bands which lead to failure during ECAP.

Effect of Tensile Twinning on Low Temperature Shear Formability of Mg-3Al-1Zn Alloy

Mg-3Al-1Zn (AZ31) alloy samples with different starting textures have been processed using Equal Channel Angular Processing (ECAP) at 150°C and 200°C. The common temperature limit for ECAP of wrought AZ31 alloys has been reduced from 200°C to 150°C by suppressing tensile twinning activity. Twinning activity was reduced by either changing the starting texture or increasing the temperature. A crystal plasticity model was utilized to gain a better insight into the operating deformation mechanisms during ECAP. Suppression of tensile twinning and pronounced prismatic slip activity resulted in a crack-free, homogeneous deformation at 150°C. In addition, twinning, when exists, was shown to have a significant effect on the promotion of DRX and shear band formation.