Effects Of Micro-alloying And Accumulative Roll Bonding On Microstructure And Mechanical Properties Of Al-2Li Based Materials

Aluminum-lithium(Al-Li)alloys have been considered to be the most ideal lightweight and high-strength structural material for the aerospace industry in the 21st century due to the low density and high Young's modulus.The primary strategies to improve the mechanical properties of 3^rdgeneration Al-Li alloys include decreasing Li content and increasing Cu content,meanwhile,adding a variety of other alloying elements.These strategies inevitably weaken the two most important advantages of low density and high modulus in Al-Li alloys.Consequently,the improvement of mechanical properties of Al-Li alloys must be based on the premise without reducing the two major advantages.In this research,the composition design was based on Al-2Li-5Mg(wt.%)alloy with the lowest density and higher modulus among the current commercial Al-Li alloys.0.2 wt.%Sc and 0.2 wt.%Zr were added in Al-2Li-5Mg alloy.The effect of Al₃(Sc,Zr)precipitates with different size on microstructure and mechanical properties of as-cast and peak-aging Al-2Li-5Mg alloys was systematically investigated.According to the results,in order to avoid the formation of coarse primary phases and further promote the precipitation of nano-scale composite particles,Al-2Li-0.1Sc-0.1Zr-0.1Yb-0.1Er alloy was fabricated to only precipitate the core-shell Al₃(Li,X)composite precipitates.The coarsening kinetics and strengthening mechanism of core-shell Al₃(Li,X)composite particles were explored.Subsequently,under the guiding ideology of compositional“plainification”,the ultrafine-grained binary Al-2Li alloy was prepared by accumulative roll bonding(ARB)utilizing the dislocations strengthening and grain refinement.Due to the high strength but low strain hardening ability of ultrafine grained Al-2Li alloy,a series of optimization strategies were proposed to further simultaneously improve the strain hardening and strength.Finally,the near-nanostructured B₄C/Al-2Li laminated composite with high modulus,moderate strength and ductility was fabricated by combining ARB process and B₄C reinforcement.Micro-scale primary Al₃(Sc,Zr)phase is formed in as-cast Al-2Li-5Mg alloy with the additions of 0.2 wt.%Sc and 0.2 wt.%Zr.The primary Al₃(Sc,Zr)particle can significantly refine grain size of as-cast alloy,however,it brings about detrimental influence on the mechanical properties of as-cast alloy.At the same time,the formation of primary Al₃(Sc,Zr)particles consumes the concentration of Sc and Zr in the matrix.In the subsequent solution treatment,the nano-scale secondary Al₃(Sc,Zr)particles are precipitated and the coarse primary Al₃(Sc,Zr)particles are difficult to be dissolved.The bean-like secondary Al₃(Sc,Zr)phase has the same L1₂structure as Al₃Li phase.Therefore,Al₃Li phases can heterogeneously nucleate on the secondary Al₃(Sc,Zr)precipitates during aging treatment,resulting in the formation of Al₃Li/Al₃(Sc,Zr)composite precipitates.Al₃Li/Al₃(Sc,Zr)composite precipitates significantly improve the age hardening ability of Al-2Li-5Mg alloy,triggering the improvement of mechanical properties of peak-aging Al-2Li-5Mg alloy.The core-shell Al₃(Li,Yb,Er,Sc,Zr)composite particles are precipitated in Al-2Li-0.1Sc-0.1Zr-0.1Yb-0.1Er alloy through two-stage isothermal aging:the core is(Yb,Er,Sc,and Zr)-rich formed at 300?24 h and the shell is Li-rich formed at the subsequent low-temperature aging stage(150^oC).By investigating coarsening kinetics of core-shell Al₃(Li,Yb,Er,Sc,Zr)composite precipitates at 300?24 h+150?0-144 h,the core-shell precipitates is controlled by interface reaction.Meanwhile,the main strengthening mechanism of core-shell Al₃(Li,Yb,Er,Sc,Zr)composite precipitates is determined as Orowan strengthening mechanism,indicating that the formation of core-shell Al₃(Li,Yb,Er,Sc,Zr)composite precipitates can effectively pin the movement of dislocations,thereby changing the feature that Al₃Li phase is easily sheared by dislocations.After 5 cycles of ARB process at room temperature,the ultrafine-grained Al-2Li alloy is fabricated.Although the ultimate tensile strength(UTS)of alloy is improved to 300 MPa,the alloy possesses the extremely low strain hardening during the tensile testing with only 2.8%of uniform elongation.Therefore,a series of strategies are proposed to further improve the strain hardening and strength of ultrafine grained Al-2Li alloy.Among them,the introduction of Al-2Li-0.1Zr layer in the ultrafine grained Al-2Li alloy is the most effective method.In5-cycle ARB processed Al-2Li/Al-2Li-0.1Zr alloy,due to the coupling effect of nano-precipitate and a bimodal size distribution,the strain hardening and strength of ultrafine grained Al-2Li alloy are significantly improved.The UTS and uniform elongation are significantly increased to 377 MPa and 8.4%,respectively.Moreover,the proportion of uniform elongation in total elongation is also significantly improved to 78.5%,and the proportion is much larger than that of coarse-grained Al-2Li alloy.Combining B₄C particle reinforcement and ARB process,B₄C/Al-2Li laminated composite is prepared.The uniform distribution of B₄C particles and the bonding of layer interfaces are gradually improved with the increase of ARB cycles,leading to the decrease of porosity in B₄C/Al-2Li composite.Therefore,the strength and ductility of B₄C/Al-2Li composite are increased with the increase of ARB cycles.Finally,the high modulus(87 GPa),moderate strength(UTS of 387 MPa)and plasticity(total elongation of 7.8%)of B₄C/Al-2Li composite with near nanostructure is produced after 8 cycles of ARB process.