Study Of Precipitates In An Al-Si-Mg-Hf Alloy

Al-Si-Mg-based casting alloys mainly act as automotive aluminum engine in recent years,when the operating temperature close to and above 200°C the utilized hardening phases Mg2 Si become unstable and gradually lose their strengthening efficacy,finally resulting in shortened service life of the material.For high temperature stability of the precipitation strengthening phase in the material is a good idea to avoid this problem.Lots of researches focoused on the reactions between transitional metals(Ti,Zr,Hf,Sc et al.)and aluminum due to high temperature stablility Al3 M phase formation.Addition of traceHf atoms in the Al-Si-Mg with a high Si content,high temperature stable Si-Hf precipitates formed during heat treatment contribute to a remarkable improvement on high temperature creep.However,the intrinsic characteristic,formation mechanism and the effection on the matrix are still unclearly for the Si-Hf phase.This research could provide us a theoretical guidance and a technical support on design and development of high temperature aluminum alloys for automotive engine applications.Multiple characterization and analysis techniques including electron backscatter diffraction(EBSD),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 precipitates in Al-Si-Mg-Hf alloys.Combined with the First principle calculate method and the near coincident site lattice(NCS)model,we systematically analyze the compensation,structure and precipitation behavior of Si-Hf precipitates,where the emphasis is put on the distribution,morphology and formation mechanism of Si2Hf precipitates,the orientation relationships between Si2Hf precipitates and Al matrix and the formation mechanism of Si2Hf precipitates with different morphologies.The conclusions can be obtained from the present research:(1)High density precipitates with different morphologies(nanobelt-like,rectangle-like and square-like)were formed in Al-Mg-Si-Hf alloys after heat treatment at 560 oC for 20 h.The characterization triaxiality is more objective to represent the morphologies and distribution of precipitates,while it is discoverd the nanobelt-like precipitates have certain directions in this alloy by used of 3D-FIB technique.And we firstly combined the 3D-FIB and EBSD techniques to analyz the growing directions of nanobelt-like precipitates.Seven growth directions of nanobelt-like precipitates were calculated using matrix formulae and ensured seven different directions: <4 1 2>,<-6 5 4>,<-4 1-4>,<1 1 0>,<2 2 1>,<0 2 1>,<0 0 1>.(2)Study of the precipitates in Al-Si-Mg-Hf alloy after heat treatment at 560 oC for short time(10 and 20 mins)and long time(20h).The precipitates are identified to be Si3Hf phase with an L12 structure which is coherent with Al matrix after heat 20 min treatment.The lattice parameter be calculated and opyimized by the first principle calculation method and resulting in a=3.970?.During the heat treating time,two atoms in the opposite face are replaced byHf-atoms and formed Si2Hf2 structure with a tetragonal structure,and then the Si2Hf2 phase would transform to a tasition sturecture between Si2Hf2 and Si2Hf phase.Finally,the formation of equilibrium Si2Hf phase with orthorhombic structure happens.The evolution of precipitates would be doscribed as: Si3Hf ?Si2Hf2?transition?Si2Hf.(3)Combined the HADDF-STEM atomic resolution imaging and high resolution EDS techniques the Si2Hf precipitates with different morphologies were further analyzed in Al-Si-Mg-Hf alloy after heat treatment at 560 oC for 20 h.The rectangle-like and square-like precipitates are mainly the(Si2-xAlx)Hf phases,where some Si atoms can be instead by Al atoms.The nanobelt-like precipitates were almost the Si2Hf phases because of few or even none of Si atoms can be substituted.Si has eight sites in Si2Hf unit cell structure,while the Si6 and Si8 sites(Si6 and Si8 share an equivalent site)is the most favorable sites substituted by Al in the orthorhombic Si2Hf phase as revealed by First-principle calculations.(4)The orientation relationships(ORs)between Al matrix and nanobelt-like or rectangle-like Si2Hf precipitates in the Al–Si–Mg–Hf alloy after heat treatment at 560 oC for 20 h were investigatedFour ORs between nanobelt-like precipitates and Al matrix were identified as:OR 1:(100)Al //(010)p,(0-11)Al //(101)p and [011]Al // [-101]p;OR 2:(11-1)Al //(010)p and [011]Al // [-101]p;OR 3:(12-1)Al //(010)p,(101)Al //(110)p and [1-1-1]Al // [001]p;OR 4:(-1-11)Al //(010)p and [112]Al // [-101]p.It is found that(100)Al(11-1)Al,(12-1)Al and(-1-11)Al planes are preferred as the habit plane(HP)for the corresponding OR1,OR2,OR3 and OR4,respectively.All HPs are parallel to the(010)p plane of Si2Hf precipitates.Four ORs between rectangle-like precipitates and Al matris could be expressed as:OR 1: [112]Al // [010]p and(11-1)Al from about 3.21 o to(200)p;OR 2: [012]Al // [010]p and(200)Al //(002)p;OR 3: [-113]Al // [010]p and(2-42)Al from about 6.77 o to(20-2)p;OR 4: [-113]Al // [3-1-3]p and(220)Al //(130)p.The habit planes were identified to be(112)Al,(012)Al and(-113)Al corresponding to OR1,OR2 and OR3,respectively.And all the three ORs have the(010)p plane as the common habit plane of precipitates.Because of the obeseved direction is not parallel or vertical the facets of the fourth precipitate,the HP could not be confirmed in the fourth OR.(5)The density distribution of NCS for different ORs between nanobelt-like precipitates and Al matrix were simulated by the free PTCLab software.During four ORs,there was only highest NCS density plane when the atoms were matched between nanobelt-like phases and Al matrix,and the lowest interfacial energy in the habit plane results in the precipitates growing preferentially in the(010)p plane.However,there is not a good matching plane of(010)p in Si2Hf phase duo to a angle exsited between the good matching of rectangle-like phases and Al matrix.In the other word,all orientations of rectangle-llike precipitates share the requirement that common(010)p habit plane but may differ by a rotation of up to an uncertain angle about [010]p axis between the preferred orientation and growing direction.That resulted in a relative interfacial energy in the major side facts,which limited the precipitates growing along the preferred plane,so the precipitates grow as a rectangle-like morphology.The orientation relationship between precipitates and matrix would be effected by defects during nucleation of precipitates.There are lots of defects like vancacies,dislocations and interfaces etc.in the Al-Mg-Si-Hf.The good matching plane deviation between recatangle-like Si2Hf precipitates and matrix caused by nucleation in association with pre-existing defects,that resulting in the heterogeneous morphologies.