The Compounds And Phase Equilibria In The Low Nd Side Of The Mg-zn-nd System

Although more and more attentions are payed to Mg-based alloys because of its special advantage of properties, the investigation on the alloying of Mg-based alloys is still backward in contrast to Fe-based and Al-based alloys. The alloying basically aims at probing effective strengthening phases and strengthening mechanisms. The attention to the Mg-Zn-Nd system, is just because the rare-earth elements can form more kinds of compounds with Zn and Mg, most of them have higher melting point, meantime, the Mg-based solid solution with Zn can be strengthened by the ageing strengthening. It is hopeful to produce the special strengthing effect. However, up to now, the phase diagrams of the Mg-Zn-Nd system can not further provide enough knowledge to guide on alloy design. Further, the previous research results in the low Nd side have the distinct difference, thus it is impossible to verify the equilibrium microstructure of alloys at the low Nd side.By means of conventional equilibrated alloy methods, the compositionsand structure of the ternary compounds, and the local phase equilibrium in the low Nd side of the Mg-Zn-Nd system have been researched though the microstructure observation, electron probe microanalysis,X-ray diffraction and electron diffraction of the alloys Mg49.5Zn49.5Nd1,Mg78Zn20Nd2,Mg62Zn34Nd4,Mg93Zn2Nd5,Mg75Zn20Nd5,Mg70Zn1 2Nd18,Mg60Zn30Ndl0,Mg70Zn28Nd2,Mg70Zn17Nd13,Mg75Znl7Nd8 and Mg67Zn25Nd8 treated at 350-200℃.It has been shown that the ternary compound T1 with the hexagonal structure, whose structure parameters are a=b=1.5nm, c=0.87nm, exists at the low Nd side in the Mg-Zn-Nd system. The composition range of the T1 phase are 28.1-32.0at% Mg,62.0-65.5at% Zn, 6.0-7.4at% Nd. The new ternary compounds T2 with C-centred orthorhombic crystal structure and T3 with fcc structure, exist at the low Nd side of the Mg-Zn-Nd system. The structure parameters of T2 phase are a=0.96nm, b=1.12nm,c=0.94nm, whose composition range are 60.2-63.5at% Mg,28.7-32.3at% Zn and 7.9-8.4at% Nd; the structure parameter of T3 phase is a=0.7nm, whose composition are about 31at% Mg,44.5at% Zn and 24.5at%Nd. The ternary compounds T1, T2 and T3 could exist stably at the temperature range of 350-200℃.The local phase equilibrium at the low Nd side of the Mg-Zn-Nd system at 350-200℃.have been investigated. The three-phase regions L+MgZn+T1,L+Mg+T1, Mg+T2+T3,Mg12Nd+T4+T5 and Mg+T5+T6 exist at 350℃in the isothermal section of the low Nd side in the Mg-Zn-Nd sysyem; and the three-phase regions Mg+T1+T2, Mg+Mg12Nd+T5 and Mg+T3+T6 maybe exist according to the phase relation.Below 300℃, no liquid exists in the Mg-Zn system, so the three-phase regions L+MgZn+T1 and L+Mg+T1 don't exist at 300℃and 200℃in the isothermal section of the low Nd side in the Mg-Zn-Nd sysyem,but the three-phase region Mg+MgZn+T1 exists at 300℃and 200℃instead.The composition of T4 are about 70.6at%Mg,8.7at% Zn and 20.7at% Nd; the composition of T5 are about 60at%Mg,18.3at% Zn and 21.7at% Nd; the composition of T6 are about 46.4at%Mg,29.5at% Zn and 24.1at% Nd.