Rheological Solidification Behavior and Mechanical Properties of AZ91-Sn Alloys

Abstract

The solidification and tensile deformation behaviors of rheo-cast AZ91-Sn alloys were revealed to study the effects of Sn alloying on improvement of AZ91 alloy’s mechanical properties. Two kinds of Mg17Al12 phases precipitated from the supersaturated magnesium matrix during rheo-solidification were observed: coarse discontinuous precipitates (DP) at grain boundaries and small-sized continuous precipitates (CP) inside grains. With increasing Sn content, the amount of Mg17Al12 phases was increased whilst the amount of Al atoms in the matrix was decreased. Due to the higher melting point of Mg2Sn than Mg17Al12, Mg2Sn precipitated earlier from the melt, and therefore provided heterogeneous nuclei for Mg17Al12 during the eutectic reaction. Due to grain refinement and solid solution strengthening, AZ91-2.4Sn (mass%) gained 52% increase in tensile strength and 93% increase in elongation compared with pure AZ91 alloy. The higher-density twins and microcracks induced by Sn alloying relaxed stress concentration during plastic deformation, so the fracture mode was transformed from cleavage fracture of pure AZ91 alloy to ductile fracture of AZ91-Sn alloys.

The Evolution of Microstructure, Mechanical Properties and Fracture Behavior with Increasing Lanthanum Content in AZ91 Alloy

“…Stanford et al studied the effect of CPs on the tensile behavior of AZ91 alloy and found that that the age hardening was conducted by both twinning deformation and the precipitation of CPs [24]. We also revealed the strengthening mechanism of the CPs and the DPs in AZ91-Sn alloys in our previous studies [12]. However, there is still a lack of understanding regarding the precipitate distribution and its influence on the mechanical properties of AZ91 alloy with lanthanum alloying.…”

“…AZ91 alloy with 0.5 mass%, 1.0 mass% and 1.5 mass% lanthanum addition (nominal chemical composition) were investigated as the experimental materials, which were designated AZ91-0.5La, AZ91-1.0La and AZ91-1.5La, respectively. The AZ91 alloy (AZ91D, Zhongtai, China) and Mg-La master alloy (10 mass%, Boyue, China) were weighed, mixed and melted under protective atmosphere (Ar + 1.5 vol.% SF 6 ) at 690 • C and processed into semi-solid slurry using a rheo-treatment machine [12]. The melt was then held at 685 • C for 30 min in a steel crucible, and the solidification process was performed by immersing the crucible in cooling water.…”

“…Sn, Sb, Bi, etc. are usually introduced in the alloying of AZ alloys to improve their mechanical performance [11][12][13]. Previous results indicated that rare earth elements were relatively more effective in optimizing AZ91's mechanical performance than other elements [8,14].…”

“…Mg 17 Al 12 continuous precipitates (CPs) with plate-shaped morphology and Mg 17 Al 12 discontinuous precipitates (DPs) with lamellar morphology were formed as partially divorced precipitates during the aging treatment [21,22]. Although the CPs and the DPs both have a bcc structure, they play distinct roles in the improvement of mechanical properties [12,22]. Jain et al reported that both CPs and DPs could effectively conduct precipitation strengthening at different temperatures [23].…”

Effect of Necklace-Type Distribution of SiC Particles on Dry Sliding Wear Behavior of As-Cast AZ91D/SiCp Composites

“…The development and application of high-strength and light-weight metallic materials have received much attention in recent years owing to the increasing concerns of world environmental issues, such as air pollution, energy overconsumption, resource shortage, etc. Among various light-weight metallic materials, magnesium and its alloys exhibit great potential because of their low density, high specific strength and stiffness, good machinability, and so on, which were therefore considered as the next generation of green engineering structural materials [1][2][3][4][5]. Nevertheless, the absolute strength and elasticity modulus of magnesium alloys are relatively low, which could not meet the requirements of practical applications [6][7][8][9].…”