Effects of samarium addition on as-cast microstructure, grain refinement and mechanical properties of Mg-6Zn-0.4Zr magnesium alloy

In this paper, the microstructural evolution, grain refinement and mechanical properties of as-cast Mg-6Zn-0.4Zr alloys with varying Sm contents（0, 2 wt.%, 4 wt.% and 6 wt.%） were investigated by using an optical microscope（OM）, a scanning electron microscope（SEM） equipped with energy dispersive spectroscope（EDS）, an X-ray diffractometer（XRD） and mechanical tests at room temperature, respectively. The experimental results indicated that the addition of Sm could obviously refine the as-cast grains, modify the eutectic morphology and affect the mechanical properties of the alloys. The main phases in Mg-6Zn-xSm-0.4Zr alloys included matrix α-Mg, Mg2Zn3, Mg（41）Sm5 and MgZ nS m. With Sm content increasing to 4%, the MgZ nS m phase was created, meanwhile, the morphology of some eutectic phases revealed apparently lamellar structure, which had a bad effect on the mechanical properties. In addition, the maximum values of ultimate tensile strength（UTS, 214 MPa） and elongation（EL, 7.42%） were simultaneously obtained from the alloy with 2% Sm. However, Sm addition had no obvious effects on the fracture behavior of the alloys, namely, the fracture pattern of Mg-6Zn-0.4Zr alloy belonged to inter-granular and brittle modes while the fracture regimes of all the Sm-containing alloys were dominated by the mixture of inter-granular and trans-granular modes.

Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

The objective of this study is to predict grain size and heat transfer coefficient at the metal-die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCASTTM, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting, Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interracial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interracial heat transfer coefficient for different locations.

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of 13-Mg17Al12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates （DP）, but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of AI dissolved in the matrix, which is required for the formation of continuous precipitates （CP）. This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time： DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.

Effect of Sr on Grain Refinement of Mg–3%Al Alloy Containing Trace Fe by Carbon-Inoculation

Magnesium（Mg）-3% aluminum（Al）（in weight） alloy was modified by carbon（C） inoculation combining with strontium（Sr）.The effects of trace 0.1% iron（Fe） addition and operating sequence of carboninoculation and Fe addition on the grain size of Mg-3%Al alloy were studied.The results reveal that the Sr addition could effectively suppress grain-coarsening resulted from the inclusion of Fe in the carboninoculated Mg-Al alloy.Sr addition could contribute to the formation of the duplex-phase particles that Al-C-rich phases coated on Al-Fe or Al-C-Fe-rich phases,regardless of the Fe addition sequence.These duplex-phase particles should be the potent substrates for α-Mg grains.Consequently,Sr addition could effectively subsidize the inhibiting effect of Fe on grain refinement and the active nuclei were maintained.In other words,Sr plays a counter role in the poisoning effect of Fe on the microstructure of Mg-3%Al alloy.

Drag Effects of Solute and Second Phase Distributions on the Grain Growth Kinetics of Pre-Extruded Mg-6Zn Alloy

In order to control the grain size during hot forming,grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated.Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator.The mean grain size Dg of each annealed specimen was measured by the quantitative metallography technique.The grain growth kinetics of the Mg-6Zn alloy annealed at 473-623 K was obtained as Dg^4- Dg0^4=2.25 ×10^11 exp（-95450）by the least square linear regression method.The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration.Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc.This indicates that second phase pinning effect plays an important role in microstructure refinement.