A Research On The Grain Refinement And Aging Precipitates Modifying Of The Magnesium-Aluminium Casting Alloys

Mg-Al alloys,which draw lots of attention from materials researchers,are the most commonly-used magnesium alloys.However,some key issues have been unsolved for many years,among which grain refinement and discontinuous precipitates modification are the two most important ones.Since Al and Zr have very strong binding ability to form very stable compounds,it is not accessable to refine Mg-Al as-cast alloys by adding Zr.Besides,Mg-Al alloys have good potential to be strengthed by aging treatment,however,among the aging precipitates,the discontinuous precipitates,which result in bad ductility and non-ideal mechanical properties,play the dominate role.In this thesis,the effects of Sm and Cu on the grain refinement and aging behavior of Mg-Al alloys were investigated.?1?The effects of Sm on the grain refinement of Mg-Al-Sm alloys were investigated.It was found that Sm addition to Mg-Al alloys has poisoning effect to the nucleating potency of the Al-C-O?or Al-Fe-C-O?particles,which leads to grain coarsening when Al₂Sm partilces are not present.According to the E2EM calculations,Al₂Sm partilces have good potential to act as heterogenous nulcei of?-Mg,hence,Mg-Al-Sm alloys tend to be refined when Al₂Sm partilces are generate during the solidification.?2?It was found that,in the Mg-3Al-2Sm alloy,the fraction of operative Al₂Sm particles,which act as heterogenous nuclei during the solidification,is much higher than other alloys such as AZ91 alloy or Al-TiB₂ alloy.Unsing the most widely accepted nucleation selection theory,“The Intherdependence Theory”,the author revealed the nucleation selecting process in the Mg-3Al-2Sm alloys,and pointed that the main reason for the above phernomenon is the absence of the“Nucleation Free Zone”.?3?The effects of Cu on the grain refinement,aging behavior and corrosion resistance property of the Mg-9Al alloy were investigated.More than 1%-Cu can refine the Mg-9Al alloy,and less such as 0.2%-Cu leads to grain coarsening but was proved to be able to improve the continuous precipitation and depress the discontinuous precipitation.?4?To understand the reason for the improving effect of Cu on the continuous precipitation in the Mg-9Al alloy,1st-principle calculation and modeling based on classical“Nucleation-Growth”theory were performed.It was shown that the addition of Cu to Mg-Al alloy can enhance the diffusion ability of Al atoms,leading to the improvement of the nucleation of the continuous precipitates.And the improvement of the continuous precipitation is the direct cause of the depression of the discontinuous precipitation.