Phase-Field Simulation And Experimental Studys Of Al-Li Alloys

With computer simulation and experiment study, aluminum-lithium alloys wereinvestigated in this paper. Based on microscopic phase-field model and continuousphase-field model, the kinetic mechanisms of precipitation ofδ' phase in Al-Li alloyswith various composition and interatomic interactions energy were simulated. Thecorresponding microstructure, fracture mechanism, and mechanical properties of thealloys were also obtained through experiment.It was found that, the precipitation ofδ' phase in the metastable region occursprimarily by non-classical nucleation and growth mechanism, and shows somecharacter of spinodal decomposition in some degree. Decomposition of the disorderphase in instable region occurs predominantly by spinodal decomposition mechanisms,and shows some character of non-classical nucleation mechanisms. The precipitationmechanism in transitional region shows mixed features by non-classical nucleationand spinodal decomposition. With increasing alloy composition, the unstabledecomposition mechanism begins to play the role and take dominant control. Withinthe whole intermediate range, including at the ordering and disordering instability line,the transition of the precipitation mechanisms is gradually. Change of the interatomicinteractions energy could make the transition of the precipitation mechanismsgradually too, no abrupt point. The increasing of the interatomic interactions energyleads to the spinodal decomposition mechanism play more important role.The decomposition always starts from a congruent ordering, which produces atransient nonstoichiometric ordered single-phase, then, it become equilibriumδ'phase. With increasing interatomic interactions energy, the boundary thickness ofordered phase become larger, the ratio of boundary thickness and radius of theordered phase decreases; the critical nucleation energy becomes smaller, and criticalnucleus radius keeps constant at first, and then becomes larger, the nucleation rateincreases firstly and then decreases, and accelerate the cluster. It was also found thetransitional region was affected by the interatomic interactions energy evidently. The sub-grain shape is equi-axial in the 2090 and 2090+Ce alloys. Theincubation period of T₁ phase is much longer than that ofδ' phase. The simulatingcalculation results ofδ' coarsening mechanism correspond well with the experimentalresults: the number of nucleus ofδ' phase decrease with time, while the diameter ofthem increases. The rare earth element Ce can accelerate the precipitation of T₁ andδ'phase. Also Ce can restrain the coarsening of the two phases and block the growth ofsubgrains. The evolution of microstructure in 2090+Ce alloys is relatively slower thanthat of 2090 alloys.Prolonging aging time and elevating aging temperature, the fracture mechanismchanges from dimple fracture to laminate fracture, and finally fracture along subgrainboundaries. Increasing pre-stretch, the fracture surface of the fracture toughnesssample is firstly characterized with dimple fracture, then short transverse delaminationfracture, finally quasi-cleavage fracture. Higher pre-stretch, aging temperature andlonger aging time will lead to decrease of ductile fracture and increase ofdelamination fracture and cleavage. Also the thickness of laminated layer willdecrease accordingly. The evolution of fracture mechanism in 2090+Ce alloys isslower than that in 2090 alloys. This is similar to the above microstructurecomparison between the two alloys.The microstructure, i.e. the size, number and distribution of T₁ andδ' phase, theintergranular phase, precipitate-free zone (PFZ), is the most important factor ofdetermining fracture mechanisms. There is a close relationship betweenmicrostructure and fracture mechanisms. The short transverse delamination fracture isdue to the weak bonding of grain during aging process. The first crack will begenerated in the center of the sample under 3-D tensile stress. The following crackswill appear at the position of the sample on one quarter, one eighthUnder different aging conditions, the strength of 2090+Ce alloys is comparableto that of 2090 alloys. With increasing aging time, the strength of the two alloysincreases and then decreases. However, the elongation of 2090 alloys will firstdecrease and then increase, while that 2090+Ce alloys will increase a little and then decrease, similar to their strength. Overall, the elongation of 2090+Ce alloys is alwayshigher than that of 2090 alloys for the same aging conditions. With higher pre-stretch,the fracture toughness decreases, and critical crack length decreases a little. For higheraging temperature, fracture toughness decrease more with longer aging time. Withlonger aging time, the fracture toughness decreases, critical crack length keepsconstant. Percentage of delamination will increase and thickness will decrease, withhigher pre-stretch, longer aging time, and higher aging temperature. Along with thepropagation of the main crack, the percentage of delamination is first increases andthen decreases. The percentage will reach a maximum value in the stable crackpropagation area. Whereas, the thickness of delamination is continuously decrease.