Exact Characterization Of The Microstructure And Its Relationship With Properties In Al-Cu-Mg-(Si) Alloys

As a precipitation-strengthened aluminum alloy,Al-Cu-Mg alloy can be strengthened by various types of precipitates,such as S phase,GPB(Guinier-Preston-Bagaryatsky)zone and??phase by adjusting alloy compositions and heat treatments,leading to high specific strength,good corrosion resistance and excellent creep resistance.The structure and evolution of the precipitate are directly related to the macroscopic performance.Characterizing the crystal structure and interface structure of the precipitates accurately,and further exploring the internal relationship between the precipitation behaviors,the alloy composition,the processing technology and macroscopic performance would help to achieve high-efficiency control of the alloy microstructure,thereby improving the macroscopic performance.As the main precipitates of Al-Cu-Mg alloy with a low Cu/Mg atomic ratio,S phase and GPB zone have been the focus of research,especially the structure issues and the competition relationship under the dislocation,which would be more complicated with the participation of trace elements Si.To tackle this issue,the spherical aberration-corrected electron microscopy,first-principles calculation,image simulation,and hardness test were used to study the effect of 6%cold-rolled pre-deformation on the precipitation behavior and hardening behavior during the subsequent artificial aging process for Al-3.0Cu-1.8Mg(wt.%)alloy(Al Cu Mg alloy)and Al-3.0Cu-1.8Mg-0.5Si(wt.%)alloy(Al Cu Mg Si alloy).It was found that the 6%pre-deformation has different effects on the microstructure and macroscopic properties of the two alloys since the trace element Si participates in nucleation during the aging process.There are two forms of Si atoms participating in the nucleation.One is to form the so-called Si-modified GPB zone,which is GPB zone riched in Si atoms,to improve the stability of the GPB zone;the other is to promote the precipitation of Si-containing precipitates such as the C phase,Q?phase or their structural units,together with S phase to form a continuously distributed coarse precipitate,which is named zig-zag composite precipitate.The precipitation of coarse zig-zag composite precipitate and the reduction of Si-modified GPB zone influenced by 6%cold rolling pre-deformation reduced the peak hardness and tensile strength of Al Cu Mg Si alloy.In addition,a two-dimensional rectangular phase with a fixed periodic structure was first discovered in the deformed Si-containing alloy.The structure was accurately identified using atomic-resolution TEM imaging and first-principles calculations.It was found that the increased amount of pre-deformation can improve the hardness and strength for the peak-aged Al Cu Mg Si alloy with a small deformation.The methods of electron backscatter diffraction,X-ray diffraction and transmission electron microscopy were used to quantitatively calculate the contribution to strength of the grain boundary strengthening,dislocation strengthening,and precipitation strengthening from the aspects of grain structure,dislocation density and precipitation behavior.It was found that the increase in the amount of deformation increase s the peak strength of aged Al Cu Mg Si alloy results from the higher density of dislocation and the precipitates with a more uniform distribution and a finer size.Lager deformation amount can certainly increase the hardness and strength of the peak-aged Al Cu Mg Si alloy,but it is inevitably accompanied by a certain degree of plasticity reduction.Pre-aging treatment such as natural aging or artificial aging prior to 6%pre-deformation can increase the strength without loss of the plasticity.It has been found that the pre-aging prior to deformation can increase the nucleation rate of Si-modified GPB zone,and in turn decrease the amount of unfavorable zig-zag composite precipitates,leading to the increase in the mechanical properties.The systematical study of the precipitation behavior and its relationship with mechanical properties of Al-Cu-Mg-Si alloy under different process conditions,and deeply understanding the various combinations of solid solution atoms would provide an important reference value for the development of new alloys and new processes.As the main precipitate of Al-Cu-Mg alloys with a high Cu/Mg atomic ratio,the??phase has attracted extensive attention in the field of metal materials and calculations about the interface issues.In addition,a variant of??phase,??_II phase,has been observed in some Al Cu Mg alloys.The orientation relationship between??_II phase and matrix is{001}_???//{110}_?,<100>_???//<001>_?,which is different from that of??phase,finally leading to the different interface combinations.In-depth exploration of the interface issues about??phase and??_II phase is helpful to understand the relationship between the phase interface and the morphology of the precipitate,the difficulty of nucleation and the evolution of the precipitate sequence.Several Al-Cu-Mg alloys with high Cu/Mg atomic ratios and different Mn contents were taken as research objects to tackle the interface issues of the??phase and??_II phase,using the advanced atomic imaging technology and first-principles calculations.By comparing with the conventional??phase,the rationality of??_II phase's existance can be derived from the calculated data such as the formation enthalpy,strain energy,and interface energy,which also explains why the??phase is plate-shaped and the??_II phase is rod-shaped.Furthermore,it was found that the precipitation and growth of??_II phase can be promoted by the addition of trace element Mn and higher aging temperature.