| In this dissertation, the research results on the precipitates in the Mg-Gd-Nd system are presented.By means of high resolution transmission electron microscopy (HREM), morphology, structural and compositional characteristics of the precipitates in the Mg-15.4Gd-1.6Nd alloy, prepared by induction melting method and aged at 200℃, have been studied and the related precipitation behaviors are discussed. In addition, the formation and growth of precipitate free zones (PFZ) along low-angle grain boundaries(GBs) have been investigated.The precipitation reactions in the Mg-15.4Gd-1.6Nd alloy have been investged and a four–stage precipitation sequence has been clarified : Mg(SSSS)→β"( D019)→βˊ(cbco)→β1(fcc)→β(fcc).At the early stage of age hardening, two kinds of local ordered structures are identified. One can be regard as the D019 structure and another is mono-layered D019 structure. On the whole, the D019-like diffraction pattern observed in this stage actually is the result of multiple diffraction from local areas with different extent of ordering. With the progress of age-hardening, two types of precipitates, which exhibit platelet and irregular globular morphologies respectively, are observed at the peak-aging stage. The platelet precipitate, which is lying along one of {1120}αplanes, has a transition structure and exhibits ragged morphology, while the irregular globular precipitates, formed along {1010}αplanes, has an orthorhombic structure with lattice parameters of a=0.64nm,b=2.223nm,c=0.521nm. Both platelet and irregular globular precipitate decomposed at the over-aging stage. Instead, another platelet precipitate with definite morphology, that is so-calledβ1, was observed to form along {1010}α. Theβ1 phase, which is in between the metastableβˊand equilibriumβin the precipitation sequences, has a f.c.c structure with lattice parameter of a=0.74 nm. The orientation relationship between theβ1 and the matrix was found to be [110]β1 //[ 0001]α,{111}β1 //{1120}α. On the other hand, someβ1 was observed to adjoinβcoherently, indicating that aβ1-to-βphase transformation is undergoing at the over-aging stage. The composition ofβ1 is determined to be about Mg87.2Gd10.6Nd2.2 by EDX. The equilibrium precipitate phaseβ, having an f.c.c structure with lattice parameter of a=2.223 nm, is formed at the final stage. Its composition is determined to be about Mg84.9Y12.5Nd2.6 by energy dispersive X-ray (EDX) analysis EDX. The Nd-content is significantly lower than the Gd-content in the platelet-shaped precipitate, the ratio of Gd:Nd is near 5:1.No evidence was found of the presence of theβ" orβˊ in the Mg-Gd-Nd during ageing at 300℃and a precipitation sequence of Mg(SSSS)→β1(fcc)→β(fcc) is suggested for this alloy.A rapid hardening response has been noted in the Mg-Gd-Nd alloy during ageing at 200℃and this has been attributed to the rapid formation and high density distribution of fineβˊand plate transition precipitates having D019 superstructure.The formation and growth of precipitate free zones (PFZ) along low-angle grain boundaries (GBs) also have been investigated for the Mg83Gd15.4Nd1.6 alloy prepared by induction melting method following by aging at 200℃. By means of TEM observations and nano-electron-probe EDS analysis, we found that the formation of a PFZ is mainly controlled by vacancy depletion along GBs at the earlier stage of aging. With increasing aging time, the equilibrium precipitates formed along GBs coarsens, and growth of PFZ turns to be affected by the process of solute depletion near GBs .The width of PFZ increases with the solute concentration inside PFZ decreasing. When the solute concentration within PFZ is close to the corresponding equilibrium concentration in the matrix at a defined aging temperature, the width of PFZ will reach its final value—275nm～300nm. Through a simplified model of PFZ which we built, the volume diffusion coefficient of rare earth elements (Gd+Nd) in the Mg matrix is estimated to be about 0.998×10-20m2/s.
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