Al-Nd Intermetallic Phase Stability And Its Effects On Properties Of Mg-Al-Nd Alloys

Mg-Al-RE(AE)alloys have been a hot spot in the field of heat-resistant magnesium alloys.But the creep resistance of the Mg-Al-RE alloys is far from satisfactory because of certain factors related to microstructures and there are conflicting views among different researchers.Therefore,microstructural stability is a crucial but controversial issue for Mg-Al-RE alloys.Under this background,it is important to study the thermal stability of Al-RE intermetallic phase with individual RE element.In this work,Mg-4Al-4Nd-0.2Mn(AlNd44,wt.%)alloy and Mg-6Al-3Nd-0.2Mn(AlNd63,wt.%)alloy with individual RE element Nd were prepared by high-pressure die-casting(HPDC).The microstructure,microstructural stability and its effects on tensile properties,creep resistance and corrosion behavior were investigated.Characterization was performed by light microscopy,X-ray diffraction,scanning and transmission electron microscopy.In the first part,this study mainly investigated Al-Nd intermetallic phase stability and its effects on mechanical properties and corrosion resistance of HPDC AlNd44 alloy.Microstructural stability near grain boundary plays an extremely key role in creep resistance of fine-grained HPDC Mg alloys.The detailed microstructure evolution and phase transformation occurred in the process of annealing in HPDC AlNd44 alloy and the property changes resulted from microstructure evolution were presented.The results show that the HPDC AlNd44 alloy has two intermetallic phases,i.e.,acicular Al11Nd3 and particulate Al2Nd,which are distributed at grain/dendritic boundary regions and form reticular structure.According to the quantitative X-ray diffraction phase analysis,the content of Al11Nd3 and Al2Nd phases is 2.5 wt.%and 1.8 wt.%,respectively.Both the two phases appear to be thermally stable at 473 K.While the acicular Al11Nd3 phase has a tendency to decompose at temperature up to 573 K,and would decompose to particulate Al2Nd phase thoroughly at 673 K.Moreover,Al2Nd phase would also partly decompose at temperature above 573 K.The alloy in as-cast state,with an intermetallic skeleton at grain/dendritic boundary regions,exhibits relatively high tensile properties and corrosion resistance.In contrast,the alloy after annealing at 673 K has the worst tensile properties and corrosion resistance due to the obvious phase transition and microstructure change.Tensile creep behavior of the high-pressure die-cast AlNd44 alloy under stress of 70 MPa and at 473 K and changes in microstructure during creep process were investigated.The discontinuous Mg17Al12 precipitation and coarsening of Al-Nd intermetallic compounds were observed in the micro structure of the alloy after creep test.For the alloy studied,grain boundary disintegration is suggested to be a possible factor for the deterioration of creep resistance at elevated temperatures.The microstructure and mechanical properties of die-cast AlNd63 alloy in as-cast conditions and after long-time annealing were investigated.In as-cast condition the AlNd63 alloy was composed of a-Mg,Al2Nd intermetallic phase and a small amount of Mg +Mg17Al12 divorced eutectic.During annealing at 473 K,the Mg17Al12 secondary precipitation process took place in grain boundaries enriched in aluminium during solidification.The obtained results show a significant influence of Mg7Al12 precipitation on tensile properties.The alloy after annealing at 473 K has better mechanical performance compared with alloys in as-cast state and after annealing at 673 K.