First-principle Study Of Stacking Faults Of Hexagonal Close-packed Metal And Its Stability Under Strain Path Control

Hexagonal close-packed(HCP)metals,which have the densest way of atomic stacking,are widely used in industrial fields.For example,magnesium alloys have been used in electronic products,automobile and other industrial fields due to their excellent properties,such as lightweight,high specific strength,and high elastic modulus,etc.However,the magnesium alloy has poor plastic deformation due to the characteristic of hexagonal close-packed structure.For hafnium alloy,it is an ideal neutron absorber because of its considerable large thermal neutron capture cross section.Nevertheless,the vacancies and defects are easily generated in Hf alloys when exposed in irradiation,which causes the alloy to expand its volume and is prone to fracture,consequently to a reduced service life.In order to improve the ability of plastic deformation and service life of hexagonal close-packed metal,introducing dislocation or stacking faults into the metal alloy is considered as an effective method to strengthen the metal.In this thesis,there are two aspects will be studied to understand the stacking fault of hexagonal close-packed metal and its stability under strain path.First,the relationship is investigated between the general stacking faults energy(GSFE)of hexagonal close-packed metals and their plastic deformation.It shows that there is great difference of GSFEs of each slip plane in Mg,Ti,Zr,Hf.Detailed calculations also show that unlike Ti and Zr,the same group of Hf has lower I2 stacking faults energy on basal plane than SF1 on prismatic plane,which indicates that the preferred slip plane of Hf is basal plane rather than prismatic plane.Differential electron density of stacking faults also supports the GSFE results.Our prediction could be used to explain phase transformation of Hf from HCP to FCC through basal slipping under cold rolling.In the calculation of GSFE,it reveals that the in-plane relaxation is negligible in basal and prism I planes,while in prism II and pyramidal I and II planes,both in-plane and out-of-plane relaxation should be considered in the simulation to ensure the precision of the calculation.Second,effects of solid solute and strain path on the stability of twinning in Mg alloys were investigated.It is found that external strain could effectively tailor the stability of twins in Mg.The dependence of twinning energy on external strain,especially on paths of [10(?)2] and[10(?)1],becomes weak when there are solute atoms segregated into twinning boundary.Meanwhile,calculations also indicate that segregation of solute having its electron work function(EWF)either higher or lower than that of Mg can stabilize the twinning.It is of particular interest to notice that when solute elements respectively having higher and lower EWFs co-exist,the strong interactions between the solute elements may largely decrease the twinning energy and thus effectively pin the twins.It demonstrates that EWF is a promising indicator,which provides valuable information not only for solute selection towards maximized benefits from twinning but also for fundamental understanding of relevant electronic origin.