The Microstructure And Mechanical Properties Evolution Of Al-3Li-1Cu-0.1Zr Alloy With Er Addition

In this thesis,the Er alloying Al-3Li-1Cu-0.1Zr-0.1Er alloy was studied to characterize the effect of the core-shell structure on the mechanical properties and the planar slip.The mechanisms of the low toughness in Al-Li alloy were also discussed.The effects of core-shell structure on the mechanical properties were verified by investigating the microstructure-properties relationships.The less effects of planar slip on the intergranular cracking and low toughness were proved.And the dominant effects of grain-boundary weakness in high Li content alloy on the intergranular cracking and low toughness were proposed.The specific results were introduced as follows.Al-3Li-1Cu-0.1Zr and Al-3Li-1Cu-0.1Zr-0.1Er alloys in the state of as-cast,hot rolling,solid solution and aging?36h,56h?were investigated by OM,XRD,TEM,SEM&EDS,respectively.The results show that the consecutive Cu and Zr rich phases are formed along the grain boundaries in the two as-cast alloys.After hot-rolling and solid solution,the amount of the consecutive Cu and Zr rich phases along the grain boundary decreases while a few discrete Cu and Zr rich precipitates remain in the matrix.Regarding of the effect of Er element,there are more dispersed and finer Al₃?Er,Zr,Li?formed in aging Al-3Li-1Cu-0.1Zr-0.1Er alloy.The thickness of the shell Al₃Li increases in the Al₃?Er,Zr,Li?particles with no obvious precipitate free zone when prolonging the aging time.What's more,the trace of the dislocations bypassing the Al₃?Er,Zr,Li?particles can also be observed in Al-3Li-1Cu-0.1Er-0.1Zr alloy.The mechanical properties of the two alloys were tested by the tensile machine.The results show that the hot rolling can improve the strength and the elongation.After solid solution treatment,the mechanical properties of the Al-3Li-1Cu-0.1Zr-0.1Er alloy show no obvious difference but the yield strength of the Al-3Li-1Cu-0.1Zr alloy decreases dramatically.Comparing the mechanical properties of the two alloy,the strength and work hardening rate increase obviously with 0.1wt.%Er addition,which also can impede the PLC phenomenon.However,the alloy with Er addition shows lower elongation.For example,the Al-3Li-1Cu-0.1Zr-0.1Er alloy aged for 56h possesses higher ultimate tensile strength of 306MPa,yield strength of 235MPa and a lower elongation of 9.4%.What's more,the hardness of the two alloys in solid solution and aging conditions were tested by the nanoindenter.The harness increases about 0.4GPa for the peak aged alloy with Er addition.Particularly,the hardness values near the grain boundaries of the two alloys in different conditions are almost the same near 1.5GPa.The fracutre of the two alloys after tensile test were investigated.The results show that there were many dimples in the fracture of the two alloys in solid solution condition.With the prolonging of the aging time,the fractographs show no obvious difference in Al-3Li-1Cu-0.1Zr alloy.Diversely,Al-3Li-1Cu-0.1Zr-0.1Er alloy revealed the propensity of intergranular fracture after aging for 36h and totally intergranular fracture after aging for 56h.In summary,the changes of mechanical properties of Er alloying Al-3Li-1Cu-0.1Zr alloy can be attributed to the formation of Al3?Er,Zr,Li?particles,which have good effect on improving the work hardening rate and impeding the PLC phenomenon.After verification by the strengthening mechanisms,the Al3?Er,Zr,Li?particles have been identified as the main strengthening particles for the Al-3Li-1Cu-0.1Zr-0.1Er alloy in Orowan mechanism,which can impede the movement of the dislocations and accelerate the uniformly moving of the dislocations preventing the planar slip.In terms of the intergranular fracture in Al-3Li-1Cu-0.1Zr-0.1Er alloy,the result of the nanoindenter can provide some explanation.The grain interior hardness of the peak aged Al-3Li-1Cu-0.1Zr-0.1Er alloy is near 1.9GPa,prominently higher than the grain boundary hardness,1.5GPa,which leads to the cracking near the grain boundaries.To study the poisonous effect of the Li content on the grain boundaries,the Al-1.3Li-3.2Cu-0.35Mg-0.1Zr alloy in the same process was chosen to compare with Al-3Li-1Cu-0.1Zr-0.1Er alloy.The result shows that the grain boundary harness of the Al-1.3Li-3.2Cu-0.35Mg-0.1Zr alloy is almost 2.0GPa,which is higher than that of Al-3Li-1Cu-0.1Zr-0.1Er alloy.What's more,the propensity of intergranular fracture in the Al-1.3Li-3.2Cu-0.35Mg-0.1Zr alloy is not as obvious as the Al-3Li-1Cu-0.1Zr-0.1Er.As a result,the grain boundaries may get weak leading to the intergranular fracture when the content of Li is increased dramatically.