Liming Peng

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg17Al12, and Mg2Sn phases. The amount of Mg17Al12 and Mg2Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.

High Cycle Fatigue of Cast Mg-3Nd-0.2Zn Magnesium Alloys

This paper investigates the high cycle fatigue properties of a recently developed high-strength cast magnesium alloy [Mg-3Nd-0.2Zn (all compositions in wtxa0pct except when otherwise stated)] with varied Zr contents for grain refinement (NZ30K) and the influence of heat treatment conditions. The NZ30K alloy containing 0.45Zr and heat treated to the peak-aged T6 condition [14xa0hours at 473xa0K (200xa0°C)] shows the highest fatigue strength, about 100xa0MPa, which is about 25xa0pct higher than that of commercial AZ91D-T6 alloy. In the absence of casting flaws, the high cycle fatigue properties of the NZ30K alloy strongly depend on its grain size and heat treatment conditions. The dependency of fatigue strength on grain sizes follows the Hall–Petch relationship. The NZ30K alloy also shows a significant response to heat treatments. The fatigue strength increases in a near linear fashion with increasing yield strength of the material through heat treatment.

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

Abstract The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.

Ductility Improvement of Mg-Nd-Zr Cast Alloy by Trace Addition of Zn

This paper reports a significant increase in tensile ductility in solution-treated Mg-Nd-Zr cast alloy by trace addition of 0.2wt.% Zn. The improvement is accompanied by the occurrence of wavy slip lines on the surface of fractured tensile specimens, which are caused by the activation of non-basal slips including and or non-basal dislocations, while these non-basal slips are hardly observed in Mg-Nd-Zr alloy without Zn addition.

Effects of Alloying Elements on Creep Properties of Mg-Gd-Zr Alloys

The minimum creep rate and microstructures of aged samples of Mg-Gd-Zr alloys, with and without alloying additions of Zn and/or Y, have been investigated in the present work. The creep tests were performed at 523xa0K (250xa0°C) and under 80 to 120xa0MPa, and the microstructures before and after creep tests were characterized using scanning electron microscopy, transmission electron microscopy, and the high-angle annular dark-field imaging technique. It is found that dislocation creep predominates in the steady-state creep stage for all alloys. The Mg-2.5Gd-0.1Zr (at.xa0pct) alloy, strengthened by the β′ precipitates, has minimum creep rates in the range 1.0xa0×xa010−8 to 3.8xa0×xa010−8xa0s−1 under 80 to 120xa0MPa. The addition of 1.0xa0at.xa0pct Zn to the Mg-2.5Gd-0.1Zr alloy reduces the 0.2xa0pct proof strength and increases the minimum creep rate, resulting from the formation of γ′ basal plates at the expense of β′ precipitates. The replacement of 1.0xa0at.xa0pct Gd by Y in the Mg-2.5Gd-1.0Zn-0.1Zr alloy leads to a substantial reduction in the minimum creep rate, even though it does not cause much change to the 0.2xa0pct proof strength. The reduced minimum creep rate is attributed to a much lower diffusivity of Y atoms than Gd in the solid magnesium matrix. An increase in the Gd content from Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr to Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr leads to a denser distribution of precipitates, a higher 0.2xa0pct proof strength, and a further reduction in the minimum creep rate.