| Al-Cu-Li-Mg alloys are one of the most promising structural materials used in airspace applications,due to their excellent combination of low density,high stiffness,high strength,high fatigue crack growth resistance and high damage tolerance.Al-Cu-Li-Mg alloys are typical heat treatable alloys,and are mainly strengthened by fine precipitates formed upon artificial ageing treatment,such as Ti-precipitates,?'-precipitates,?'-precipitates,S-precipitates and GPB zones.Although the crystalline structures of abovementioned precipitates have already been determined,the structures of precursors and the evolution scenario at early-stage ageing condition remains to be established.The competition among these precipitates,and the effects of alloy elements on the precipitation have not been adequately understood.Combining atomic-resolution HAADF-STEM imaging and first-principles calculations,this dissertation conducted a systematic study on the ageing precipitation behaviors of Al-Cu-Li-Mg alloys.The atomic structures and crystallography characteristics of all precipitates and their precursors have been determined,in order to understand the complex precipitation behavior,and the effects of solute elements such as Cu,Li,and Mg on the precipitation in Al-Cu-Li-Mg alloys.The main contents and conclusions of this dissertation are summarized as below:(1)Two types of S-precipitates can be observed according to their orientation relationships(OR)with respect to Al-matrix.Type ? precipitates follow the classical OR as:(001)s//(021)Al,[100]s//[100]Al and typically grow into lath-like morphology with habit plane(001)s//(021)Al.Type ? precipitates follow the common axis[100]s//[100]Al but deviate a small rotation from the classical OR(001)s//(021)Al range from 2 to 6°.The interfaces of type ? are comprised of(001)s//(021)Al habit plane and minor interface of(001)s modified by GPB zone units.The interfaces of type ? are comprised of(011)s//(035)Al habit plane,(021)s//(014)Al habit plane,and structural ledges.The atom kinds and positions at interfaces of S-precipitates may be modified to maintain better atom matching,thus the interfacial energy is reduced.(2)Driven by interfacial energy decreament and better connecting of two type ?precipitates,interfaces of(001)s,(021)s and(011)s are formed in type ? precipitates,leading to the rotation of type ?.The varied rotation angle range from 2° to 6° is attributed to:(i)the interfaces and ORs of type ? are always under evolution during its growth process;(ii)(011)s//(035)Al habit plane,(021)s//(014)ai habit plane,and structural ledges are observed in well-developed,independent type II,and each interfaces correspond to a certain OR.(3)Ti-precipitates,?'/GP/?' and ?'/?'/?' composite precipitates and GPB zone variants are the main precipitates in AI-4.2Cu-1.1Li-0.3Mg alloy(low Li-content),while 8'-precipitates and GPB zone variants are the main precipitates in Al-3.0Cu-1.7Li-0.9Mg alloy(high Li-content).The formation of Ti-precipitates,?'-precipitates and S-precipitates have been significantly depressed in the high Li-content alloy,due to the rapid occurring of ordered ?'-precipitates.Introduction of pre-stretch before the artificial ageing treatment can promote the nucleation of Ti-precipitates and S-precipitates while depress the formation of 8'-precipitates and GPB zone variants.(4)Employing atomic-resolution HAADF-STEM imaging and first-principles calculations,the interfaces structures of ?'/GP/?' and ?'/?'/?' composite precipitates have been determined and refined.Regardless of the interfaces in ?'/GP/?' or in ?'/?'/?'composite precipitates,their interface structures are similarly featured:the interfacial Li-atoms in the ?'-sides are required to occupy the second nearest neighbor sites of the interfacial Cu-atoms in GP-zone-side or ?'-side.First principles calculations confirm that the proposed interface models for ?'/GP/?' or in ?'/?'/?' composite precipitates are energetically favorable,and indicate that the interfacial energy of the ?'/Al interfaces can significantly be reduced by forming the Al/?'/?'/?'/Al interfaces.In ?'/GP/?'composite precipitates,the two sideward ?'-precipitates have an "anti-phase"relationship,whereas in ?'/?'/?' composite precipitates,they may have an "in-phase" or an "anti-phase" relationships,depending on the number of Cu layers contained in the inward ?'-precipitates.An "anti-phase" ?'/?'/?' composite precipitate can transform into an "in-phase" one when the Cu layers in the ?'-portion change from even to odd number,and vice versa.This interfacial structure switching between the two sideward ?'-portions constrains the inward ?'-portion from rapid coarsening,leading to a much finer?'-precipitates microstructure in the 3rd generation Al-Cu-Li alloys with relatively low Li-content.(5)It is found that Li can take part in the formation of GPB zone variants and S-precipitates in Al-Cu-Li-Mg alloys.GPB zone variants is a quasi-two-dimensional precipitates with needle-like morphology.GPB zone variants have significantly modified structures compared with conventional GPB zones in non-Li Al-Cu-Mg alloys,due to a featured Li-segregation at their interfaces with the matrix and possible Li-replacement of Mg atoms in the structure.GPB zone variants will transformed into S-precipitates after long term thermal exposure.The stability of S-precipitates can be improved due to preferential segregation of Li atoms on their interfaces. |
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