Basic Investigation On The Phenomenon Of Severe Plastic Deformation Induced Re-dissolution Of Precipitated Phases At Low Temperature And Its Application

The re-dissolution of second phase particles during SPD is a new significant micro-phenomenon,which is of significance for developing a new heat-treatment technique of age hardening alloy.In order to investigate the re-dissolution phenomenon of precipitates and Its Application in Al alloy at low temperature during various SPD processes, the experimental materials such as Al-Cu,Al-Mg-Si and Al-Zn-Mg-Cu alloy and corresponding different tempered condition such as supersaturated solid solution,GP-zone,metastable phase(e.g.θ″,θ′)and equilibrium phase(e.g.θ)were employed in this paper.SPD was performed by means of Equal-channel Angular Pressing(ECAP), multi-axial compression(MAC)and Gleeble-1500 simulated tester.The evolution of properties and microstructures was characterized by hardness measurement,optical microscope,transmission electron microscopy, x-ray diffraction analysis and small-angle x-ray scattering(SAXS).The results suggested that the re-dissolution phenomenon of precipitates may occur during all different SPD of Al alloys.This phenomenon is a new microstrutures evolution during SPD,independent of deformation temperature.This indicates SPD has the same effect as heating as for re-dissolution of particles.Re-dissolution evolution of precipitates during ECAP and MAC may be devided into four stages: interaction of precipitates with dislocations,the change of morphology of precipitates,breaking of precipitates and re-dissolution of precipitated phases.Quite a lot of previous work presented a contradictory behavior of hardness variation due to re-dissolution of second phase during ECAP. Two kinds of severe plastic deformation such as ECAP and MAC of Al-Cu alloy were performed to understand this contradictory behavior in this work.The results suggested that in the equivilent strain range of 0.4 to 8.6 the behaviors of hardness variation coud be divided into three kinds. One is hardness of samples first increases to a peak value with increasing strain,and subsequently reduces to a minimum value with increasing strain,at last followed by a reincrease at the end of SPD.The second is the hardness of samples gradually increases to a saturated value with increasing strain,no reduction of hardness appears during SPD.The third one is the hardness of samples continuously increases with increasing strain,no saturated value of hardness appears in a quite large range of equivalent strain.It is suggested that the hardness variation is due to combined effect of strain hardening and starin softening caused by re-dissolution of precipitates during SPD of aged Al-Cu alloy.Different re-dissolution rate of precipitates in SPD is attributed to different interface structure between pricipitates and matrix(αphase),resulting in different behaviors of hardness evolution.In SPD deformation processing of Al-Cu alloys,the supersaturated solid solution formed after re-dissolution of precipitated phase induce by SPD will decompose once again to form new precipitates when the increment of strains achieves a certain degree in following deformation process.Under this condition,the precipitates sequence is not identical with the decomposition of conventional supersaturated solid solution formed by heating,metastable phase are restrained,forming immediately equilibrium phase.During re-aging,the supersaturated solid solution formed by SPD induced re-dissolution of precipitated phase will accelerate remarkably the process of re-aging,but its precipitation sequence is related to heating temperature,strain degree and grain size after deformation.When heating temperature can completely eliminable the micro-distortions of matrix lattices produced by deformation,the precipitation sequence is metastable phases→equilibrium phases.When heating temperature cannot enough remove the high lattice distortion and grain ultra-refinemen,metastable phases will be inhibited,the equilibriμm phases will be directly formed.During the re-aging of Al-Cu alloys in which the re-dissolution of precipitates has generated and supersaturated solid solution has produced after SPD,GP-zone and metastable phases which possess the coherent and semi-coherent relation to parent phase have no the possibility of formation in interior of matrix,because they will lead distortion energy to increase and result in necessarily the volume free energy to arise.A equilibrium phase which is free of coherent relation to parent phase may diminish the strain within matrix and lower the volume free energy.The re-dissolution rate of precipitated phases at low temperature effects immediately on the selection of procedure and technology of SPD of Al alloy.The research on this question has obtained that only when the strain attain to a definite quantity,the re-dissolution of precipitated phases can occur on matter what methods of SPD.All re-dissolution rate of each precipitated phase increases with increasing the strain.Under same deformation processing,the strain required by the re-dissolution of unlike precipitated phase is dissimilar because various precipitated phases have different interface between precipitate and matrix.For Al-Cu alloy, the strain needed by the re-dissolution ofθ″-phase with coherent relation to matrix is minimal,that ofθ′-phase with semi-coherent relation to matrix is second,and that ofθ-phase without coherent rotation to matrix is maximal.The strain required by re-dissolution is also unlike for the phases with similar interface under various SPD methods.The evolution of efficacious size and volume fraction ofθ′-phase in ECAP and MAC processes were quantitatively investigated using a small angle X-ray diffraction.The size and volume fraction all fall with an increase in the strains of two SPD processing,but the lowering of size and volume fraction of precipitated phases forθ′-phase is rapider in MAC deformation than in ECAP deformation.Here the experimental results indicated that the strain rates have a effect on the re-dissolution rate of precipitated.The strain needed by the re-dissolution rate of three precipitated phase in Al-Cu alloy is smaller at high strain rate than at low strain rate in double-axial compression.On the basis of a great deal of observation and examination data,the authors considered that the thermal driving force of SPD induced re-dissolution at low temperature was the enhancement of strain and interface energies of precipitates.The re-dissolution phenomenon of Al-Cu alloy at low temperature caused by SPD is a spontaneous process when the increment of volume free energy of precipitated phases is larger than that of matrix."deformation free energy hypothesis"and"A critical nuclei size hypothesis"are provided to interpret the relationship between the evolution of hardness and the re-dissolution of precipitates during SPD at room temperature,the difference between The re-dissolving of precipitated phases by SPD and Heating retrogression was analyzed according to the experiment results.SPD induced re-dissolution of precipitated phases at low temperature is new important micro-phenomenon,and is a solution process under preserving simultaneously the SPD micro-structures.Therefore the parameters of size,shape and distribution of precipitated phases during following aging after SPD are different with that during conventional aging.If this phenomenon is utilized to strain processing and heat-treatment,strain strengthening and dispersion strengthening will attain to give fully play,and a new SPD and heat-treatment techniques with high properties of alloys or simple working technologies will been developed.We have do the research on that the phenomenon of re-dissolution of precipitated phases at low temperature effects on the refinement and thermo-stability of grains in MAC deformation of Al-Cu alloy.It was obtained in the investigation that the rate of grain refinement of solid solutioned specimen is higher than that of specimen containing precipitated phases,and that of specimen containingθ′-phases is slightly faster than that of specimen containingθ-phases.The plasticity and toughness of ultra-finegrained structures may been improved inasmuch as the decrease of piled-up dislocations density and the abundant deformation of cellular structure resulted from the re-dissolution of precipitates in supersaturate solid solution and two phases alloy.Consequently,the re-dissolution of precipitates in SPD deformation is conducive to acquire a homogeneous ultra-fine equiaxed grains with high-angle boundaries,and leads the comprehensive mechanical properties of materials to reach a new level.When ultra-finegrained Al-Cu alloy produced by SPD are annealed at a temperature below 200℃,the grain size and high-angle boundary percentage are basically unchanging,it is only when annealing temperature surpasses 250℃that grain size and high-angle boundary percentage are all increased.When annealing temperature is below 200℃, supersaturated solid solution which has not occurred to decompose will precipitate into the granular particles of equilibriumθphase at grain boundary because the lattice distortion of matrix led by SPD still exists.It is only when the annealing temperature is high 250℃that the fine directional distribution of metastable phases(θ″,θ′)will form in the interior of grains inasmuch as the micro-distortion of matrix lattices has be completely eliminated.Along with SPD techniques move towards industrial application from experimental room,acquiring the aluminum alloys with excellent comprehensive mechanical property will promote the utilization of ultra-fine grained Al alloys.The tendency of applied basic research of SPD induced re-dissolving phenomenon of precipitates at low temperature is to utilize the combination of stress-treatment and heat-treatment which produces ultrafine equiaxed grains separated by high-angle boundaries and the re-dissolution of precipitated phases and re-decomposition of supersaturated solid solution at re-aging after SPD, achieving the optimum matching of microstructure under severe plastic deformation with aging microstructure,and yielding a new technologies of strain and aging attained to optimum combination of mechanical properties.On this basis,stress-heat treatment which possesses a independent intellectual property right in our country will been developed.