| 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 α-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 β domain structures formed during the phase transition from β1(Mg3Ce)to β(Mg12Ce)were discussed particularly in this paper.Regarding the presence or absence of β" precipitate in Mg-RE binary alloy,it is found that the β"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→β"→β1→β.In the age hardening response curve,the second peak occurred at the traditional over-aging stage,which is related to the structural evolution from β1 to β precipitates.The structural transformation from β1 phase with a face-centered cubic lattice to β phase with a tetragonal lattice mainly occurred on the {111}β1 planes,where the formed β lattices exhibit two categories of domain structures,namely rotational and translational domains.The rotational domain is composed of three β domain structures(βRA,βRB and βRC),which are related by a 120° rotation with respect to each other around the<111>β1 axis of theirβ1 parent phase.The transitional domain contains four β domain structures(βTA,βTB,βTC and βTD),among which any two differ by a vector of 1/6<112>β1.The initial nucleation of the translational domain originates from the four sets of sublattices with the same orientation in the {111}β1 crystallographic planes of β1 parent phase.It is deduced theoretically that there are twenty-four β domain structures during the structural evolution from β1 to β precipitates.However,with the interfacial misfit considered,only one-third of domain structures can grow up and eventually forms β ribbon.Furthermore,a majority of β ribbons overlap partially β1 plate,which is beneficial to relax interfacial stress among β,β1 andα-Mg matrix(α/β/β1)and regulate effectively interfacial misfit of a/p and β/β1.In this paper,the proposed multiple β domains and the relaxed α/β/β1 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. |
