Study On Precipitated Phase Of Aged Binary Mg-Ce Alloy

Magnesium(Mg)alloys have attracted widespread attention for applications in automobile and aerospace industries due to their low density,high specific strength,excellent castability and other favorable attributes.The addition of rare earth(RE)elements into ?-Mg matrix can improve significantly the hardness,strength and creep resistance of alloys,which mainly depends on solid solution and precipitation strengthening mechanisms.By means of an aberration-corrected high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),precipitates and precipitation sequence of the aged Mg-3.0 wt.%Ce alloy were characterized,and the ? domain structures formed during the phase transition from ?1(Mg3Ce)to ?(Mg12Ce)were discussed particularly in this paper.Regarding the presence or absence of ?" precipitate in Mg-RE binary alloy,it is found that the ?"precipitate can be formed at the early aging of Mg-Ce binary alloy and has a D019 structure and a composition of Mg3Ce.The full precipitation sequence is concluded as:SSSS(supersaturated solid solution)?GP(GuinierPreston)zones??"??1??.In the age hardening response curve,the second peak occurred at the traditional over-aging stage,which is related to the structural evolution from ?1 to ? precipitates.The structural transformation from ?1 phase with a face-centered cubic lattice to ? phase with a tetragonal lattice mainly occurred on the {111}?1 planes,where the formed ? lattices exhibit two categories of domain structures,namely rotational and translational domains.The rotational domain is composed of three ? domain structures(?RA,?RB and ?RC),which are related by a 120° rotation with respect to each other around the<111>?1 axis of their?1 parent phase.The transitional domain contains four ? domain structures(?TA,?TB,?TC and ?TD),among which any two differ by a vector of 1/6<112>?1.The initial nucleation of the translational domain originates from the four sets of sublattices with the same orientation in the {111}?1 crystallographic planes of ?1 parent phase.It is deduced theoretically that there are twenty-four ? domain structures during the structural evolution from ?1 to ? precipitates.However,with the interfacial misfit considered,only one-third of domain structures can grow up and eventually forms ? ribbon.Furthermore,a majority of ? ribbons overlap partially ?1 plate,which is beneficial to relax interfacial stress among ?,?1 and?-Mg matrix(?/?/?1)and regulate effectively interfacial misfit of a/p and ?/?1.In this paper,the proposed multiple ? domains and the relaxed ?/?/?1 interfaces can not only reasonably explain the second peak in the age hardening response curve,but also provide a certain theoretical basis for improving the performance of the traditional over-aging stage of Mg-RE alloys.