The Mechanisms For The Precipitation Of Si In Al-1%Si Alloy And Its Effects On The Mechanical Properties

Aluminum alloys are among the most widely used materials in almost every aspect of industry due to the high specific strength,good electrical conductivity and excellent corrosion resistance.The superior properties of aluminum alloys are mostly derived from the second-phase precipitation.Generally,when the precipitate size is smaller and the density is higher,the performance of the alloys will be much better.Therefore,it is of the great significance to explore the mechanisms for the densely-distributed nanosize precipitation in aluminum alloys.In this paper,the key factors and the mechanisms for the homogenous nano-size precipitation of Si particles in Al-1%Si alloy have been investigated,which involves three parts: Firstly,the effect of the process parameters in the conventional thermal treatment on the precipitation were analyzed in terms of the precipitate size and distribution;secondly,both the thermal and athermal effects of electric current on the precipitation have been studied in two novel ageing treatments(with electric pulses and direct current,respectively);thirdly,the early-stage precipitation was directly observed by transmission electron microscopy(TEM),in which the electron beam could induce lattice defects within the sample and thereby promote the atom diffusion and clustering.Results of this study showed that:(1)With the conventional thermal treatment,the morphology and distribution of Si precipitates are essentially controlled by the concentration of vacancy and its clusters,which is not only dependent on the ageing temperature,but also conditioned by other parameters in solid-solution,quenching and pre-ageing treatment.Low temperature ageing could produce densely distributed fine Si particles with very long time and inefficiency;high temperature ageing could obtain densely distributed Si particles with short time,but could also cause the coarsening of precipitate.Therefore,a new ageing strategy consisted of two ageing steps was proposed,i.e.,with a rapid high-temperature ageing in the first place and a subsequent low-temperature ageing for a long time.In this way,densely distributed nano-size Si particles could be yielded with great efficiency.In addition to ageing,it is found that higher temperatures in solid-solution,faster quenching rate,lower temperatures for pre-ageing and the fast heating rate of ageing treatment can also improve the morphology and distribution of Si precipitates.This is due to the high concentration of excess vacancy retained within the sample could promote the diffusion and form large numbers of clusters(e.g.,vacancy clusters and vacancy-Si clusters)which would provide preferable nucleation sites for precipitation.However,for large-volume bulk materials,it is still quite difficult to get dense nanosize precipitation because the fast quenching/heating rate in the whole volume is hard to meet by the conventional thermal treatment/equipments.(2)Compared with the conventional thermal treatments,electric ageing treatment can produce much more homogenous and finer Si particles within short periods,where electric pulses treatment(EPT)is more efficient than the direct current.The enhanced precipitation by electric current can be explained from two aspects: firstly,the thermal effect(by joule heating)could rapidly heat up the sample,especially with high current density;secondly,the unique athermal effects also contribute to the rapid precipitation when the thermal effect was suppressed.The selective heating effect of electric current on defects or clusters could be the most likely athermal effect in this study.(3)The dynamic precipitation of Si solutes by electron beam irradiation in HRTEM revealed the critical role played by excess vacancy.The formation of the Si precipitates is attributed to the vacancy and dislocation loops generated by the incident electron beam,and to the initial high concentration of excess vacancy retained from quenching as well.It is also found that some precipitates is quite unstable to the extent that they could be redissolved into the matrix under observation.This structural instability is resulted from the non-equilibrium co-structure of Si solute with vacancy or dislocation loops.And the displacement damage caused by electron beam can also be a factor in the dissolution.The studies above revealed that the novel electric-current-assisted ageing treatment can provide extra modification of the precipitation process,and that electron beam irradiation can deepen our understanding of the early-stage precipitation mechanism,which may be beneficial to developing other novel ageing techniques.