First-principles Study Of β" And β"/Al Interfaces In 6000-series Aluminum Alloys Adding Trace Amount Of Alloying Elements

6000-series aluminum alloys can be strengthened by heat treatments,which are widely used in fields containing automobiles,civil airplanes and buildings.To achieve excellent mechanical properties,solid solution treatment,quenching and aging treatment are often used.In addition,traces amount of alloying elements are also added into 6000-series aluminum alloys,which will affect precipitation of β” which is the main hardening phase in 6000-series aluminum alloys.To understand the behavior of these alloying elements and analyse the reason why the incorporation of Cu,Ge or Zn into β” makes the morphology of β” unchanged and the number density of β” changed,first-principles methods is used to calculate the geometry structure,the phase stability,the electronic properties of β” and the interfacial energy and strain energy of β”/Al interfaces before and after doping Cu,Ge or Zn.(1)Using first-principles methods,the change of geometry structure,phase stability,electronic properties of β” after adding Cu,Ge or Zn are calculated.The results indicate that,the volume of β” decreases after adding Cu or Zn,while increases after adding Ge;the reason why Cu,Ge or Zn can incorporate into β” is that the phase stability of β” improves after adding these elements,that is,the structure is more stable while Cu prefers to substitute Al atoms in β”,or large amount of Ge atoms incorporate into β”,or small amount of Zn atoms incorporate into β”;further analysis of electronic properties of β” after doping Cu,Ge or Zn shows,the phase stability of β” phases after doping those elements is closely related to the pseudo gap near the Fermi level;(2)By first-principles methods,the interfacial energy and strain energy of β”/Al interfaces before and after doping Cu,Ge or Zn are calculated.The results show that,the main factor influencing morphology of β” is the interfacial energy of β”/Al interfaces.No matter whether those elements are doped into alloys,β” phase will grow along the normal direction of the β”/Al interfaces whose interfacial energy is maximal,therefore the morphology of β” are all needlelike;When Cu,Ge or Zn incorporate into β”,the formation entropy of β”,the interfacial energy and strain energy of β”/Al interfaces are all changed,making critical nucleation energy of alloys adding those elements changed(The critical nucleation energy of alloys adding Zn increases a bit,while those of alloys adding Cu or Ge decrease distinctly).So,in alloys adding Cu or Ge,the number density of β” increases,and the size of β” decreases,leading to the hardness of alloys increasing,while in alloys adding Zn,the number density of β” decreases a little,and the size of β” increases a bit,leading to the hardness of alloys decreasing.