Min Zha

Morphology and size control of octahedral and cubic primary Mg2Si in an Mg–Si system by regulating Sr contents

In the present study, primary Mg2Si with different morphologies and sizes over a regime of 5–70 μm have been prepared. By precisely controlling the content of Sr in Mg–4Si alloys, we have obtained octahedral and cubic primary Mg2Si of different average size. Morphologies of primary Mg2Si were identified though field-emission scanning electron microscopy (FESEM) in three-dimensional space. The modifier Sr plays an important role in determining the morphologies of primary Mg2Si, which can be transformed from equiaxed-dendrite to octahedron and finally to a cube shape with increasing Sr content. The 2D nucleation growth pattern of primary Mg2Si modified by Sr element has been revealed for the first time. An adsorption model has been applied to explain the influence of Sr on the growth kinetics of Mg2Si. It is proposed that Sr atoms preferentially adsorbing on {100} facets of Mg2Si crystal is the key reason for the morphology evolution. Our study demonstrates an effective, one-step and cheap method to control the morphologies and sizes of primary Mg2Si crystals in metallic melts, which is critical to achieve industrial application of light alloys with high strength and toughness.

Microstructure, hardness evolution and thermal stability of binary Al-7Mg alloy processed by ECAP with intermediate annealing

Abstract A binary Al-7Mg alloy was processed by equal channel angular pressing (ECAP) at room temperature via route Bc, combined with intermediate annealing. After 6 passes, a high hardness of 218 HV is achieved. Transmission electron microscopy (TEM) observations demonstrate that ECAP leads to a significant grain refinement and ultrafine grains down to 100-200 nm are developed after 5 or 6 passes. X-ray diffraction (XRD) analysis indicates that the major part of Mg atoms are in solid solution in the deformed material, and the possible strengthening effect of Mg solute atom clusters or precipitates is neglected. The high hardness of the 6 pass-treated materials comes mainly from grain boundary strengthening, which contributes about 41% to the total strength, while dislocations and Mg solid solution contribute about 24% and 35% to the remaining strength, respectively. Also, the thermal stability of this severely deformed material was investigated by hardness measurements. The material is relatively stable when annealed at a temperature lower than 250 °C, while annealing at 300 °C leads to a rapid softening of the material.

Influence of melt superheating on microstructures of Mg-3.5Si-1Al alloys

The influence of melt superheating treatment on the microstructures of Mg-3.5Si-1Al alloys unmodified and modified with 0.2% Sr-Sb (mass fraction) was investigated. The results show that when the melt superheating temperature increases from 750 to 900℃, the average size of primary Mg2Si in the unmodified alloys decreases progressively from about 27 to about 19μm, while that in Sr-Sb-modified alloys. As refined considerably from about 14 to about 7μm when the temperature increases from 750 to 850℃, and then slightly increases to about 9μm with temperature further increasing to 900℃, which might be attributed to the burning loss of Sr and Sb in melts. However, the superheating temperature only has a slight effect on the morphologies of both primary and eutectic Mg2Si phases in unmodified and Sr-Sb-modified alloys.

Morphology evolution of primary Mg2Si in Al–20Mg2Si–0.1Ca alloys prepared with various solidification cooling rates

The morphology evolution of primary Mg2Si in the Al–20Mg2Si–0.1Ca alloy was investigated by a mould temperature-controlled solidification technique at various solidification cooling rates. It is found that by lowering the pre-set mould temperature, i.e. increasing the cooling rates, the influence of external factors, i.e. the adsorption of Ca atoms on {100} facets, on the morphology evolution of primary Mg2Si can be strengthened, leading to primary Mg2Si with varying morphologies. With the decrease of mould temperature from 600 to 400 °C and then to 200 °C and finally to room temperature, primary Mg2Si transformed from a mixture of an equiaxed-dendrite and an octahedron to a truncated octahedron and then to a truncated cube and finally to a cube; in contrast, primary Mg2Si still remained dendritic in the Ca-free Al–20Mg2Si alloy. Moreover, the skeleton-type growth process of the truncated cubic primary Mg2Si in the modified alloy was identified, where growth steps with some hillocks were observed for the first time. Our study offers a simple and low-cost method to control the morphology and size of primary Mg2Si crystals in the Al–Mg–Si alloy, which is beneficial to the design of lightweight Al–Mg–Si alloys with high strength and toughness.

Refinement and modification of primary Mg2Si in an Al–20Mg2Si alloy by a combined addition of yttrium and antimony

The complex modification of primary Mg2Si in an Al–20Mg2Si alloy by simultaneous addition of yttrium (Y) and antimony (Sb) was investigated in the present work. It was found that the combined addition of 0.5 wt% Y–Sb had a more significant modifying effect than the single addition of an equivalent amount of Y or Sb. After modification, coarse Mg2Si dendrites were changed into fine polyhedra with an average size decreasing from 86 to less than 18 μm. It has been demonstrated that Sb participated in the formation of Mg3Sb2, which acts as a heterogeneous nucleus of primary Mg2Si and hence significantly refines their sizes. Meanwhile, Y could obviously adsorb and poison the preferred growth along the directions of primary Mg2Si crystals, and hence not only changed their final morphology to a truncated octahedron but also reduced their sizes. Furthermore, the skeleton-type growth process of the truncated octahedral primary Mg2Si in the Al–20Mg2Si alloy co-modified by 0.5 wt% Y–Sb was also revealed. Our study provides a new insight into the design of more efficient modifiers by the combined addition of a refiner (e.g. Sb) and a growth inhibitor (e.g. Y), which is critical to tailor new light-weight alloys with high strength and toughness.

Influence of Mg Content, Grain Size and Strain Rate on Mechanical Properties and DSA Behavior of Al-Mg Alloys Processed by ECAP and Annealing

Ultrafine-grained (UFG) binary Al-xMg (x=1, 5 and 7 wt %) alloys were processed by equal channel angular pressing (ECAP) at room temperature via route Bc combined with inter-pass annealing. The effects of Mg content, grain size and strain rate on mechanical properties and dynamic strain aging (DSA) behaviour of the Al-Mg alloys upon tensile testing at room temperature were studied. An increase in Mg content from 5 to 7 wt % leads to a pronounced increase in strength and uniform elongation in both the as-homogenized and as-ECAP Al-Mg alloys. Thereby, the Al-7Mg alloy, either prior to or after ECAP processing, possess significantly higher strength and comparable or even higher uniform elongation than the more dilute Al-Mg alloys. However, the as-ECAP Al-Mg alloys exhibit significantly higher strength but little work hardening and hence rather limited uniform elongation. In general, decreasing grain size leads to significant increase in strength while dramatic decrease in ductility. Moreover, DSA serration amplitudes increase with reducing grain size in the micrometer range. However, the UFG Al-Mg alloys exhibit much less DSA effect than the micrometer scaled grain size counterparts, i.e. probably due to the high dislocation densities and special grain boundary features in the UFG materials. Also, the Al-Mg alloys, especially those with a UFG structure, exhibit higher strength and ductility at lower strain rate than at higher strain rate, due mainly to enhanced DSA effect and hence work hardening at a lower strain rate.