Effect of Rare Earths on Hot Cracking Resistant Property of Mg-Al Alloys

The effect of rare earths （RE） ranging from 0.1% to 1.2%（mass fraction） on hot cracking resistant property of Mg-Al alloys was investigated. The results show that hot cracking resistant property of Mg-Al alloys remarkably declines with an increase of RE addition. The causes of the decline are as follows： First, grain coarsening of Mg-Al alloys caused by RE addition reduces the fracture strain required for hot crack initiation. Second, RE reduces the eutectic microstructure of Mg- Al alloys, and as a result, shortens the time that the feeding channel remains open, making it difficult to feed the alloy. Furthermore, RE elevates the eutectic reaction temperature, which leads to the decrease in the strength of the interdendritic liquid film at the late stage of solidification. Third, when a-Mg dendrites form continuous skeletons, the interdendritic Al11 RE3 phase tends to block the feeding channels and increases the difficulty of feeding. Last, the shrinkage ratio discrepancy between Al11RE3 phases and α-Mg matrix is prone to cause shrinkage stress and promote hot crack initiation.

Grain Coarsening Behavior of Mg-Al Alloys with Mischmetal Addition

Small addition of mischmetal （MM） into aluminum alloys can lead to grain refinement. However, it is still uncertain whether the same effect applies to Mg-Al alloys. This work indicated that small amount of mischmetal addition ranging from 0.1% to 1.2% （mass fraction） did not cause grain refinement in Mg-Al alloys. On the contrary, they tended to coarsen the grains. When added into Mg-Al alloys, MM reacted preferentially with Al to form Al11 MM3 phase. As Al11 MM3 phase mainly distributed within α-Mg grains than at grain boundaries, it had little effect in restricting grain growth. In addition, MM reacted with Al8（Mn, Fe）5 or ε-AlMn particles to form Al-MM-Mn compounds, thus it reduced the amount of heterogeneous nuclei in the melt and resulted in remarkable grain coarsening.

Improvement of corrosion resistance in NaOH solution and glass forming ability of as-cast Mg-based bulk metallic glasses by microalloying

The influences of the addition of Ag on the glass forming ability (GFA) and corrosion behavior were investigated in the Mg-Ni-based alloy system by X-ray diffraction (XRD) and electrochemical polarization in 0.1 mol/L NaOH solution.Results shows that the GFA of the Mg-Ni-based BMGs can be improved dramatically by the addition of an appropriate amount of Ag;and the addition element Ag can improve the corrosion resistance of Mg-Ni-based bulk metallic glass.The large difference in atomic size and large negative mixing enthalpy in alloy system can contribute to the high GFA.The addition element Ag improves the forming speed and the stability of the passive film,which is helpful to decrease the passivation current density and to improve the corrosion resistance of Mg-Ni-based bulk metallic glass.

Microstructure and properties of Mg-Al binary alloys

The effects of different amounts of added Al, ranging from 1 % to 9 %, on the microstructure and properties of Mg-Al binary alloys were investigated. The results showed that when the amount of added Al is less than 5%, the grain size of the Mg-Al binary alloys decreases dramatically from 3 097 μm to 151 μm with increasing addition of Al. Further addition of Al up to 9% makes the grain size decrease slowly to 111 μm. The α-Mg dendrite arms are also refined. Increasing the amount of added Al decreases the hot cracking susceptibility of the Mg-Al binary alloys remarkably, and enhances the micro-hardness of the α-Mg matrix.

Stress induced magnetic field abnormality

Residual stresses in ferromagnetic material affect the direction and structure of domains and generate magnetic field abnormality on the surface. In the formation of stress induced magnetic field, the influence of geomagnetic field is unclear. Residual stress specimen was produced by tight matching of a round ring and a peg. The magnetic fields of contrast specimens, which were produced in geomagnetic field or in shielding geomagnetic field, were inspected with 8 mm lift-off. The results show that mean amplitude of magnetic field of the specimen produced in geomagnetic field is 0.85% larger than that of specimen produced in shielding geomagnetic field. So the formation of stress induced magnetic field abnormality above the surface of inspected ferromagnetic material geomagnetic field gives little contribution.