Microstructure Evolution Of Aluminum Alloys During Aging Processes

Aluminum and aluminium alloys have been widely used in our life as the second prime metal after steel due to their good electric and thermal conductivity and well mechanical formability properties and so on. Among them, AlMg based alloys have good corrosion resistance, particularly good resistance to electrochemical corrosion. AlAg alloy is often used for the study of phase transition using transmission electron microscopy imaging, due to the similar atomic radius but large atomic weight difference of Al and Ag. In these two alloys, the positron affinities of solute atom Mg or Ag are both stronger than that of the Al host atoms. This makes positron annihilation to be very appropriate method for the study of defect evolution and precipitate formation during heat treatment and aging process.In this paper, we used positron annihilation spectroscopy as the primary method to study the micro structure evolution of Mg during artificial aging of quenched and heavily deformed AlMg based alloys and the microstructure evolution of Ag during artificial aging of quenched AlAg alloys as well as temperature dependence of positron capture of solute cluster at low temperature, the main conclusions are listed as follows:1. After quenching at550℃of AA5052, AA5754and AA5083alloys, the average positron annihilation lifetime of alloy sample during natural aging process has no change, i.e. there is no natural aging effect of AlMg based alloys. During180℃artificial aging of quenched AA5083, positron lifetime of the alloy decreased from193ps of quenched state to nearly170ps of annealed in a short aging time followed by continue increment. After600min artificial aging, positron lifetime of alloy is208ps and keeps constant. Positron annihilation S parameter during artificial aging coincides well with positron lifetime evolution. Result of ratio curve indicates that intensity of Ag signal of alloy declines to minimum when positron lifetime comes to the smallest value, after that the intensity of Ag signal grows with positron lifetime increases. Result of Vickers microhardness measurement shows that the initial microhardness value of quenched AA5083is about71Hv and keeps constant in120min artificial aging at180℃; after400min artificial aging the microhardness increases to maximum77Hv and decreases to a final value of74Hv. There is only Mn-rich particles occurs in as-quenched AA5083alloy from TEM measurement; solute clusters present after400min artificial aging where the Vickers microhardness has the maximum value; the solute clusters coarsened after50h artificial aging. The conlusion is that the vacancy-Mg complex formed during quenching resolved and Mg clusters formed which capture positrons during180℃artificial aging.2. After severly deformed, there are large number density of dislocations in AA5083which causes positrons annihilated in dislocations almostly which gives a single positron lifetime of222ps. With2h isochronal aging performance at100℃,150℃and200℃, X ray diffraction peak broadening, positron annihilation lifetime and Vickers microhardness of the alloy decrease all compares to the value of deformation state. After isochronal aging at250℃with2h, X ray diffraction peak broadening, positron annihilation lifetime and Vickers microhardness show a distinct variation and the values are close to that of annealed AA5083alloy. Temperature dependence of the effective positron trapping rate of defect in AA5083alloy calculated from positron annihilation S parameters coincides well with the temperature dependence of Vickers microhardness evolution during isochronal aging performance. The results indicate that during100-200℃isochronal aging, the major evolution is microstructure variation of dislocations and positrons are annihilated in dislocations almostly; while after isochronal aging at250℃and higher temperatures, subgrains formed during severely deformation are coarsened with dislocations vanishing and positrons are mostly annihilated in grains. Positron annihilation lifetime of the deformed alloy with180℃isothermal aging with50h is the same as the value of alloy with5min isothermal aging. While with30min isothermal aging performance at250℃, positron annihilation lifetime of the deformed alloy begins to decrease and the value comes to nearly the same with that of annealed sample after2h aging. The result shows that elevating the temperature is more effective than prolonging the aging time on providing energy to dislocations for overcome the obstacles.3. After quenching performance at550℃of three variants of AlAg alloys with lwt%,5wt%and15wt%solute concentration, the positron annihilation lifetime and S parameter of the three alloys in natural aging process show fast decrease in the initial aging time and finally comes to constant values. Result of coincidence Doppler broadening ratio curve shows Ag signal of the sample after24h natural aging process which means there are positrons annihilated with electrons of Ag atom, and the higher solute concentration of the alloy the stronger Ag signal of the ratio curve. Result of in situ S-W parameter curve of the three alloys indicates that with solute concentration growing and aging time prolonging in the natural aging process, S-W parameter of the three alloys moves from the value near S-W parameter of pure Al to that of pure Ag. And the results indicated that solute clusters are fastly formed in the natural aging process. Positron lifetime and S parameter show continue decrement in artificial aging at140℃of the three alloys firstly natural aged with24h due to solute clusters coarsened and more and more positrons annihilated in them, meanwhile the intensity of Ag signal of ration curve becomes more and more strong. With artificial aging at200℃and250℃for24natural aged1%alloy, the values of positron lifetime and S parameter of the sample during aging process are nearly the same as those of annealed sample and Ag signal of ratio curve disappears, which indicates that solute clusters formed during natural aging dissolved and most of positrons are annihilated with electrons of Al atom. While for24natural aged5%and15%alloys, positron lifetime and S parameter show decrease firstly followed by increase in200℃artificial aging process, and the intensity of Ag signal of ratio curve shows increase firstly followed by decrease which is opposite to variation of positron lifetime and S parameter. This is may because of the evolution of η-GP phase to ε-GP phase precipitates or variation of GP precipitates to semicoherent γ’(Ag2Al) phase precipitates in200artificial aging process. For natural aged24h5wt%and15wt%alloys, positron lifetime and S parameter during artificial aging at250℃show monotonic increase, while intensity of Ag signal of ratio curve shows monotonic decrease, which are due to fast transform of GP zones to incoherent γ phase precipitates and positrons are annihilated in interfaces during250℃artificial aging performance. Result of temperature dependence of solute clusters capture positrons at low temperature indicates that solute clusters in AlAg alloys can act as shallow positron trapps and their ability of capture positron increase with temperature decrease. The binding energies of solute clusters to positron in the three alloys with natural aging performance from simulation of positron annihilation S parameters of experiment results based on positron trapping model and relationship of detrapping rate and trapping rate are in the range of14-18meV, while the positron binding energy of solute clusters of140℃artificial aged1wt%alloy is about50meV, which coincides well with positron binding energy of shallow trapps in prevenient researches.