The Effects Of Zr And Hf Additions On Microstructure And High Temperature Mechanical Properties Of Al-Si-Mg Casting Alloy

Al-Si-Mg casting alloys have extensive applications in lightweight engine blocks and engine cylinder heads of modern vehicles,owing to an excellent combination of castability,high strength/weight ratio,good corrosion resistance and fatigue resistance.When the operating temperature is close to and above 200°C the utilized hardening?"and?'phases become unstable and transform into equilibrium phase?Mg₂Si phase?,finally gradually lose their strengthening efficacy,resulting in shortened service life of the material.Addition of trace Hf atoms in Al-7Si-0.3Mg cast alloy may form high temperature stable Si-Hf precipitates during solution treatment,which contributes to a improvement on high temperature creep resistance.Systematical study about this kind of high temperature stable phase in Al-7Si-0.3Mg based cast alloy could provide us a theoretical guidance and a technical support on design and development of high temperature aluminum alloys for automotive engine applications.The thesis had designed a series of Al-7Si-0.3Mg cast alloys with Zr and Hf additions.Multiple characterization and analysis techniques of Optical microscope?OM?,Focus Ion Beam and Scanning Electron Microscope dual system?FIB/SEM?,Transmission Electron Microscope?TEM?,High Resolution Transmission Electron Microscope?HRTEM?and Energy Disperse Spectroscopy?EDS?were employed to investigate the microstructure of Al-7Si-0.3Mg cast alloy with Zr and Hf additions.Vickers hardness measurement,tensile test,differential scanning calorimetry?DSC?,high temperature tensile test,high temperature low-cycle fatigue test and high temperature creep test were used to systematically study precipitation kinetics and high temperature mechanical properties.Combined with the First principle calculation method and the near coincident site lattice?NCS?model,we systematically analyze?1?the composition and morphology evolution of primary phases in Al-7Si-0.3Mg cast alloy with Zr and Hf additions;?2?the composition,structure and precipitation behavior of precipitates,where the emphasis is put on the morphology,sturcutral and formation mechanism of Al₃Zr precipitates,the orientation relationships between the precipitates and Al matrix and the formation mechanism of nanobelt-like Si-Zr/Hf/Zr+Hf precipitates;?3?the influence of Zr and Hf additions on the high temperature mechanical properties,where the emphasis is put on the interaction between nanobelt-like precipitates and Al matrix,the deformation mechanism of high temperature low-cycle fatigue and high temperature creep.The main conclusions can be obtained from the present research:The Al-7Si-0.3Mg alloy with additions of Zr/Hf displayed a complex microstructure with some typical phases.Some additional Zr and/or Hf-rich phases were formed,which were AlSiZr,AlSiHf,AlSiZrHf and Al₃?Zr,Hf?primary phases.The three-dimensional morphology of L1₂ structured Al₃?Zr,Hf?primary phase evolved from 10-polyhedron-like to compressed cube-like during solidification.The rectangle-like precipitates formed in as-solutionised Al-0.7Si-0.3Mg casting alloys with Zr and/or Hf addition are mainly L1₂ structured Al₃Zr phases,Si₂Hf and Si₂?Zr,Hf?phases.The nanobelt-like precipitates were almost the Si₂X?X=Zr,Hf?phases.The orientation relationship between nanobelt-like Si₂Zr phase and Al matrix is[011]_Al//[101]_p and?1-11?_Al//?0-20?_p,p is the Si₂Zr phase;The orientation relationship between nanobelt-like Si₂Hf/Si₂?Zr,Hf?phases and Al matrix is[011]_Al//[-101]_p and?100?_Al//?010?_p,?0-11?_Al//?101?_p,p is the Si₂Hf or Si₂?Zr,Hf?phase.The habit planes for the two orientation relationships are?1-11?_Aland?100?_Alplanes.High density non-uniform distribution of precipitates with different morphologies were formed in Al-0.7Si-0.3Mg-0.47 Hf and Al-0.7Si-0.3Mg-0.14Zr-0.44 Hf cast alloy after solution treatment at 560^? for 20 h.The rectangle-like and nanobelt-like precipitates were almost the Si₂Hf/Si₂?Zr,Hf?phases.The orientation relationship between Si₂Hf/Si₂?Zr,Hf?precipitate and Al matrix is[011]_Al//[101]_p and?100?_Al//?010?_p,?0-11?_Al//?101?_p,p is the Si₂Hf/Si₂?Zr,Hf?phase.Due to the density distribution of NCS,?100?_All planes are preferred as the habit plane for the nanobelt-like precipitate in Al matrix.The L1₂ structured Al₃Zr rectangle-like precipitate growed along the?100?_Aland?011?_Aldirections.The First principle calculation showed that the interfacial energy of?100?_Al/Al₃Zr interface is lower than that of?011?_Al/Al₃Zr interface,which means higher growing rate along?100?_Althan along?011?_Al.As a result,the L1₂ structured Al₃Zr showed a rectangle-like morphology.The habit plane of nano-like precipitate is?010?_P,which has lowest matching energy with Al matrix.As a result,the precipitate tends to grow along the?010?_p and form the nanobelt-like morphology.Al-0.7Si-0.3Mg-0.16 Zr and Al-0.7Si-0.3Mg-0.14Zr-0.44 Hf casting alloys have higher aging precipitation kinetics.The Al-0.7Si-0.3Mg-0.14Zr-0.44 Hf casting alloy has a better elongation at high temperature.All four alloys showed a cyclic softening at high temperature.High temperature dislocation climb over nano-sized precipitate and alloy overaging cause the alloy cyclic softening.The Al-0.7Si-0.3Mg-0.14Zr-0.44 Hf casting alloy showed the best low-cycle fatigue resistance.The Al-0.7Si-0.3Mg-0.16 Zr cast alloy showed the best creep resistance with the minimum creep rate of 3.17*10^-7s^-1measured in 300 ^? and 25 MPa.