Grain Refinement Mechanism of the As-Cast and As-Extruded Mg–14Li Alloys with Al or Sn Addition

Abstract

Abstract:The microstructures of the as-cast and as-extruded Mg-14 wt. % Li-1 wt. % Al (LA141) and Mg-14 wt. % Li-2 wt. % Sn (LT142) were observed by optical and scanning electron microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The effects of Al and Sn on the grain refinement on the Mg-14Li alloy were investigated. In addition, the mechanism of grain refinement on the as-cast and as-extruded alloys was discussed from the view of the solute effect and heterogeneous nucleation effect via edge-to-edge matching model. The results showed that the average grain sizes of the as-cast LA141 and LT142 alloys were similar due to the close solute effect of 1.1 wt. % Al and 1.8 wt. % Sn, while, in the as-extruded alloys, the average grain size of LT142 was over two times finer than that of LA141. This was attributed to the reason that Li 2 MgSn particles can serve as heterogeneous nucleation sites for the β-Li matrix during the process of dynamic recrystallization (DRX), but LiMgAl 2 cannot serve the same way. Therefore, Sn can act as a more effective grain refiner for the Mg-14Li alloy compared to Al.

Study on Microstructure and Properties of Bimodal Structured Ultrafine-Grained Ferrite Steel

“…The dense and protective rust layer could form easier in the bimodal ferrite steel than in dual-phase steel. This was mostly because a large number of defects existed in martensite, which facilitated the occurrence and development of corrosion [5,6]. Meanwhile, the Figure 7b shows that the Volta potential of martensite was higher than the Volta potential of the surrounding ferrite and that the Volta potential difference between martensite and ferrite was large, which improved the electrochemical activity of the dual-phase microstructure and accelerated the progress of corrosion [19,20].…”

“…Particularly, when the cold-rolled samples were annealed at 650 • C for 40 min, the yield strength (517 MPa) of the bimodal microstructure significantly improved, while the total elongation remained at a high level of 26%, which were attributed to the coordinated action of fine-grained strengthening, back-stress strengthening, and precipitation strengthening [16]. On the one hand, the refinement of bimodal ferrite grains and the increase of dislocation density with tensile strain caused an increase in stress, i.e., fine-grained strengthening, which was caused by the increase in grain boundaries [6,25,26]. On the other hand, the soft lamellae of coarse grains start plastic deformation first during the tensile process.…”

“…Microstructural refinement is one of the most important methods of metallic materials strengthening to yield lightweight components with improved performance [6][7][8]. In particular, extremely high yield strength was achieved in ultrafine-grained or nanograined steels (UFG/NG), and the yield strength resembled the Hall-Petch relationship [9,10].…”

Developing Improved Mechanical Property and Corrosion Resistance of Mg-9Li Alloy via Solid-Solution Treatment

“…As seen in Figure 2a, the cast alloy presents the typical duplex-phase structure, including the bright lath phase and dark matrix phase. As has been widely reported [30,31], the lath phase in the Mg-Li duplex alloy is the Mg-enriched α phase (Mg-Li solid-solution phase) with a hcp structure, which is usually named as α-Mg phase briefly. The matrix phase of the Mg-Li duplex alloy is the Li-enriched β phase (Li-Mg solid-solution phase) with a bcc structure and is usually named as β-Li phase briefly.…”

Development of a High Strength Mg-9Li Alloy via Multi-Pass ECAP and Post-Rolling

“…The increase in dislocation density leads to the sub-grain boundary becoming the nucleation point of recrystallization, eventually growing up to from new grains [18,41]. Therefore, dynamic recrystallization takes place preferentially in β-Li phase, which may lead to a limited strengthening and the potential softening of the alloy [42].…”