The Investigation Of The Precipitation Of Al-5.0Cu-(0.3 Sc) Alloys

As heat-treatable aluminum alloys,Al-Cu alloys have attracted wide attention due to their attractive properties of low density and high strength.The objective of this work is to investigate the precipitation behavior of Al-5.0 Cu-(0.3 Sc)(wt.%)aging at different temperatures(from 150^oC to 250^oC),particularly to explore the precipitation scenario of??-phase processed at high temperatures,examined by scanning electron microscopy(SEM),high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),energy spectrum(EDS)analysis method and the first-principles calculations.In addition,detailed analysis was conducted to understand the relationship between microstructures and mechanical properties of alloys studied by micro hardness test and room temperature tensile test,as well as the Sc microalloying effect on Al-Cu alloys.The following main conclusions are drawn:(1)Upon aging at elevated temperatures(about 200 ^oC or above),the??-phase precipitate in Al-Cu alloys actually has its own high-temperature precursors named as??_HTP-phase.In the alloy samples at 200 ^oC,there are two types of precursors for the??-phase,i.e.??-phase and??_HTP-phase,and consequently two parallel precipitation scenarios coexist:(i)SSSS??????,and(ii)SSSS???_HTP???.Only??_HTPphase is precipitated at 250^oC to act as the precursors of the??phase.the“high temperature”evolution scenario of??-phase can occur in a genetic manner without changes in precipitate morphology and interfacial structure,and consequently the??phase forms“bulk(??)-like structure”interfaces,that is(100)_??HTP-Cu-Al plane structure.(2)There are different ways for Sc to exist in Al-Cu alloys.First of all,Al₃Sc dispersions are formed during the process of solidification or heat treatment such as homogenization,or solid solution.Then,Sc atoms are solutionized in Al matrix,laying the foundation for subsequent aging treatment.At last,during the crystallization of bulk,it is easy to form W phase,which is hardly to be eliminated and thus negatively affecting the subsequent aging process.(3)The addition of minor Sc slows down the aging response during the low-temperature aging process and delays the evolution rate of the precipitation of Al-Cu alloys.The number of GPII-regions precipitated in Al-Cu-Sc alloys at 150 ^oC is less than that in Al-Cu alloys while the number of??phases formed at 180 ^oC is small compared to Al-Cu alloys and there are no??phases in Al-Cu-Sc alloys.This is why the age hardening ability of Al-Cu-Sc alloys is weakened relative to that of Al-Cu alloys.(4)Sc addition strongly suppresses the formation of??_HTPphases during the high-temperature aging process of Al-Cu alloy and thereby changing the precipitation pathways of??phases in Al-Cu-Sc alloys.Aging at 200^oC,the precipitation scenario is SSSS??????while at 250 ^oC,the??phase also has its own high-temperature precursors called Pre-??phase,and the evolution path is SSSS?Pre-?????,where the thickness of??phases is only 2 C_??and it is difficult for??phases in Al-Cu-Sc alloys to coarsen in the subsequent aging process,thereby improving the high-temperature strength of alloys,which means that Sc-added alloys could be applied at high temperatures.