| Mg-Zn-based alloy is one of the most widely used high-strength deformed Mg alloys,among which Zr is added in Mg-Zn-based alloy to form Mg-Zn-Zr-based(ZK-based)alloys.ZK60 is the typical ZK-based alloy and has the highest specific strength among the known materials,which has excellent precipitation strengthening effects.The main strengthening phases of the Mg-Zn-based alloy are β1’ and β2’ phases.However,compared with the Al alloy,the strength of commercial Mg-Zn-based alloy is still relatively low.So,it is of great significance to study the precipitation strengthening mechanism of Mg-Zn-based alloys.In this paper,we use the first-principles calculation method based on the density functional theory to study the interfacial properties constructed with the Laves-MgZn2 of β1’ and β2’ phases and α-Mg metrix,and explore the precipitation strengthening mechanism in the Mg-Zn-based alloy to guide the experimental research and production applications.We firstly study the interfaces composed of HCP-Mg’s and MgZn2’s crystal planes of low Miller indices,which include the coherent and semi-coherent interfaces.The coherent interface of β1’ is[11(?)0]MgZn2//[0001]α,(0001)MgZn2//(1120)a,recorded as OR1-I.The coherent interface of β2’ is[11(?)0]MgZn2//[1010]α,(0001)Mgzn2//(0001)α,recorded as OR2-I.Here we consider two semi-coherent interfaces of β1’,OR1’-I:[0001]MgZn2//[11(?)0]α,(1120)MgZn2//(0001)α and OR1"-I:[11(?)0]MgZn2//[0001]α,(1100)MgZn2//(1100)α;and one semi-coherent interface of β2’,OR2’-I:[0001]MgZn2//[0001]α,(1010)MgZn2//(1120)a.Due to the complexity of the atomic structure of MgZn2,we systematically investigate the lattice mismatch,the average number of chemical bonds of interfacial atoms,interfacial energy,strain energy and formation energy of each interfacial structure,as well as the effect of vacancy defects upon the interfacial stability.We find that the most stable coherent interface of β1’/α-Mg is OR1-I6,the interfacial energy of which is lower than that of the most stable OR2-I1 of β2’/α-Mg.The reason is that atomic average bonds of ORl-I6 are more than those of OR2-I1,while the effect of the change of chemical bond energy at the interface is very slight.Most interfaces’formation energies of OR2-I are lower than those of OR1-I and the formation energy of OR2-I1 is lower than that of OR1-I6.Although β1’ and β2’ phases have the same MgZn2 crystal structure,the formation energy of the β2’ phase is lower than that of the β1’ phase because of the lower interfacial formation energy of β2’.Therefore,the β1’ phases will eventually transform into β2’ phases to reach the case of overaging,which explains the fact that the β1’ phase gradually transforms into the β2’ phase during the heat treatment in the precipitation sequence of the Mg-Zn-based alloy.Two most stable semi-coherent interfaces of β1’/α-Mg are OR1’-I4 and OR1"-I2,respectively,and the most stable semi-coherent interface of β2’/α-Mg is OR2’-I6.For the semi-coherent interfaces,the large mismatch,significant structural deformation and obvious atomic reconstruction lead the bond lengths of the interfaces and their distributions to change greatly compared with those inside the crystal.The chemical bond number of interfacial atoms cannot reflect the interfacial energy any more,and the change of bond energy caused by structural deformation becomes the main aspect influencing the interfacial energy of the semi-coherent interface.The strain energy of the interfacial slab approximately has a linear relationship with the atom number of MgZn2 in the slab.In addition,we research the effect of the vacancy defects at the interface on the interface forming and stability of the five most stable interfaces.The results show that even if the formation energy of some semi-coherent interfaces is lower than that of coherent interfaces,the existance of vacancy defects makes the formation energy and interfacial energy of OR1-I6 and OR1"-I2 decrease greatly and especially for OR1-I6,its formation and interfacial energies are much lower than others’.While the formation energies of OR1’-I4,OR2-I1 and OR2’-I6 change less relatively.Therefore,in the early aging with abundant vacancy defects,the β1’ phase is more stable,which explains that the β1’ phase precedes to precipitate in the aging progress.With the progress of heat treatment and the decreaseing of vacancy defects,the formation of β2’ phases is dominant gradually in the overaging stage.Studying the interfaces of precipitates is the key to understand the stability of β1’and β2’ phases.This paper reveals the relative stability of Pi’ and β2’ through the study of Laves phase interfaces,which is very important to understand the formation of precipitates,and also plays a basic role in understanding the strengthening mechanism of Mg-Zn-based alloys. |
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