Microstructure, Deformation, and Property of Wrought Magnesium Alloys

Abstract

Pure magnesium (Mg) develops a strong basal texture after conventional processing of hot rolling or extrusion. Consequently, it exhibits anisotropic mechanical properties and is difficult to form at room temperature. Adding appropriate alloying elements can weaken the basal texture or even change it, but the improvement in formability and mechanical properties is still far from expectations. Over the past 20 years, considerable efforts have been made and significant progress has been made on wrought Mg alloys at the fundamental and technological levels. At the fundamental level, textures formed in sheets and extrusions of different alloy compositions and produced under different strain paths or thermomechanical processing conditions are relatively well established, with the assistance of the advanced characterization technique of electron backscatter diffraction. At the technological level, room temperature formability of sheet has been significantly improved, and tension-compression yield asymmetry of extrusion is also remarkably reduced or eliminated. This paper starts with an overview of dislocations, stacking faults and twins, and deformation of single crystals of pure Mg along different orientations and under different loading conditions, followed by a review of microstructure (texture and grain size) and deformation of polycrystalline pure Mg with different textures, grain sizes, and loading conditions. With this information as a base, texture, grain size, and deformation of polycrystalline Mg alloy sheets and extrusions produced under different processing conditions are systematically examined and compared. Remaining and emerging scientific and technology issues are then highlighted and discussed in the context of texture and grain size. The need for better-resolution diffraction and spectroscopy techniques is also discussed in the relationship between texture change and grain boundary solute segregation.

Lean Wrought Magnesium Alloys

“…The differences in CRSS between the deformation mechanisms [ 4 ] also amplifies the texture development during directional forming processes (and subsequent heat treatments), leading to pronounced basal textures in various investigated Mg sheets and profiles [ 13 , 27 ]. Such a strong structural anisotropy is reflected in the mechanical properties, e.g., the ratio of tensile yield strength (YS) and compressive yield strength (YSc) [ 28 ], as well as in the forming behavior of these products.…”

“…Grain refinement is well known for its positive influence on the strength and deformation behavior of practically all metallic materials, particularly for Mg. This property is important for an improved performance during forming operations [ 36 ] and is often regarded as the main strengthening mechanism in dilute Mg alloys [ 4 ]. The effect of grain refinement is typically described via the Hall–Petch relationship, which predicts a significant rise in YS for fine-grained structures.…”

“…value). It is well known that the intense texture occurring in Mg alloys is counterproductive for this type of forming; therefore, extensive efforts were made to reduce the texture intensity in Mg sheet materials [ 4 ].…”

“…While it has been demonstrated repeatedly that Mg wrought alloys can show a wide spectrum of mechanical, chemical and physical properties [ 3 , 4 , 5 ], multiple reasons for their limited use can be found. Particularly noteworthy is the small number of industrially available alloying compositions, lack of stock material, the use of high-priced alloying elements, and last but not least, a lack of practical knowledge regarding material processing and utilization [ 6 , 7 , 8 ].…”

“…Nevertheless, steady progress has been made; nowadays, Mg products can be produced at high speeds [ 9 ] and can exhibit remarkable mechanical properties [ 10 , 11 ]. In recent years, alloying concepts using only low amounts of alloying elements have shown impressive results in a broad range of topics [ 4 , 12 , 13 ]. This design approach attempts to make full use of the alloying elements and tries to combine the processing and material response in the best possible way.…”