Room-Temperature Deformation Behavior and Dislocation Fine-Structures of β-Mg₁₇Al₁₂ Micropillar Single Crystal—The Strengthening Phase in AZ-Series Low-Density High-Strength Mg Alloys

Magnesium alloys are attracting increasing interest because of their low density, high strength, and good corrosion resistance. Among them, the Aluminum-zinc alloyed Magnesium alloys, known as the AZ series Mg alloys (such as AZ91 alloy containing Mg-9 wt.% Al-1 wt.%Zn-0.3 wt.%Mn), are the most popular as-cast Mg alloys and have been widely used in automotive and aerospace fields owing to their excellent mechanical properties. It has been generally accepted that the β-Mg₁₇Al₁₂intermetallic phase precipitated in the α-Mg matrix with continuous or discontinuous morphology caused by the high Al addition is highly influential on the performance of the AZ-series alloys. Because of that, a great deal of effort, both experimental and computational, has been devoted to investigating this β-Mg₁₇Al₁₂ phase of its thermal stability, its formation during solidifications, and its effect on room- and elevated-temperature strength, corrosion resistance performance, castability performance, and so on. However, surprisingly, very little study has been focused on the plastic deformation properties of the β-Mg₁₇Al₁₂ itself, and none of them has ever revealed the operative slip system, the most fundamental information of the plastic deformation property, at room- or high-temperature. The β-Mg₁₇Al₁₂ phase has an α-Mn-like cryptographic structure, which is a complex crystal structure of the A12 type in the Strukturbericht symbol in the space group of I<u>4</u>3m. Although the crystal structure is based on the body-centered cubic lattice and shares the BCC type symmetry, for example, the shortest Burgers vector is the same as the BCC structure as <b><i>b</i></b>= 1/2[111], the unit cell (containing 58 atoms) is much larger than a normal BCC cell and the operative slip systems are totally deferent to those in normal BCC alloys, as revealed in our previous study on the deformation behavior of α-Manganese. Hence, it is not only of great importance in industry but also of great interest in material science to know whether the β-Mg₁₇Al₁₂ exhibits plastic deformability at ambient temperature, which slip system will be activated during the deformation, and what the dislocation structure looks like in these slip systems.<br/>In the present study, we investigate the deformation behavior of single crystal of the β-Mg₁₇Al₁₂by micropillar compression at room temperature as a function of crystal orientation and specimen size. In spite of the repeated reported brightness in bulk deformation, plastic deformation is observed in micropillar single crystal specimens at room temperature for all loading axis orientations. Three slip systems, {110}&lt;-110&gt;, {110}&lt;-111&gt;, and {110}&lt;001&gt; are identified to be operative at room temperature depending on the loading axis. The CRSS values for all the identified slip systems are very high (~1 GPa) and decrease slightly with increasing pillar size, following the well-known ‘smaller is stronger’ size effect in the micropillar compression test. We also make an in-depth investigation of the dislocation structure with the ‘weak-beam dark field’ transmission electron microscopy (TEM) photography technique accompanied by the atomic-resolution scanning transmission electron microscopy (STEM) technique focusing on the dislocation core structure. Dislocations for all three slip systems are confirmed to dissociate into partial dislocations. The experimentally determined dissociation scheme agrees well with our simulation based on the generalized stacking fault energy calculation. These new findings on the room-temperature deformation behavior and the dislocation structure of the β-Mg₁₇Al₁₂will contribute to the understanding of the mechanical properties and can be applied to get useful strategy to further improve the performance of the AZ-series Mg alloys strengthened by this intermetallic phase.