Study Of Microstructure And Properties Of Ultra-high Strength Al-Zn-Mg-Cu-Zr Alloys Containing Sc

The thermal section at 430℃of the Al-4at.%Mg-Sc-Zr system in the Al-rich range and the thermal section at 450℃of the Al-0.30wt.%Sc-Zn-Cu system in the Al-rich range were determined by means of equilibrium alloys.Nine kinds of Al-Zn-Cu-Mg-Zr alloys with different content of Sc,Zn,Cu,were prepared.The as-cast microstrcture of Al-Zn-Mg-Cu-Zr alloys containing different Sc,was studied by using optical microscopy(OM),transmission electron microscopy(TEM),and scanning electron microscopy(SEM),as well as the precipitation of secondary Al₃(Sc,Zr)particles of the as-cast C-2^# alloy,the effect of minor Sc and different heat-treatment on microstructure of Al-Zn-Mg-Cu-Zr alloy and recrystallization behaviour of C-2^# alloy.The mechanism of the recrystallizating nucleation of C-2^# alloy is discussed. Several conclusions can be summarized as follows:(1).There are one single-phase field(α(Al)),two double-phase fields(α(Al)+ Al₃(Sc,Zr),α(Al)+ Al₃(Zr,Sc))and one triple-phase field (α(Al)+Al₃(Sc,Zr)+Al₃(Zr,Sc))in the isothermal section of the Al-4at.%Mg-Sc-Zr system in the Al-rich range at 430℃determined by means of equilibrium alloys.It is established that the phase Al₃(Sc,Zr) with L1₂ structure and Al₃(Zr,Sc)with DO₂₃structure are in equilibrium with theα-solid solution.The maximum solid solubility of Sc in Al₃Zr is about 20at.%Sc and the maximum one of Zr in Al₃Sc is about 50at.%Zr.(2).There are two double-phase fields(α(Al)+Al₃Sc,α(Al)+W) and one triple-phase field(α(Al)+Al₃Sc+W)in the isothermal section of the Al-0.30wt.%Sc-Zn-Cu system in the Al-rich range at 450℃determined by means of equilibrium alloys.(3).During solidification of Al-Zn-Mg-Cu-Zr alloy with Sc,the cubic precipitate with 75.05at.%Al,15.59at.%Sc and 6.80at.%Zr,were formed from the melting alloy by using the energy spectrum analysis.The Al/(Sc+Zr)atom percent accords with that of Al₃(Sc,Zr)partical.The cubic can be recognized as primary Al₃(Sc,Zr)combining with the result of X-ray analysis.Theα(Al)can nucleate and grow from the primary Al₃(Sc,Zr)as it has the same crystal structure asα(Al)and its lattice parameter is close to that ofα(Al).The primary Al₃(Sc,Zr)could also refineα(Al)grain size.(4).During solidification of Al-Zn-Mg-Cu-Zr alloy with Sc, metastable phase Al₃Zr with L1₂ structure first formed,then the Sc diffused to the Al₃Zr precipitates during the reaction of L+Al₃Zr→α(Al)+Al₃(Sc,Zr),replacing parts of Zr in Al₃Zr precipitates to form the primary Al₃(Sc,Zr)with L1₂ structure.The Sc,Zr incubic Al₃(Sc,Zr)precipitates distributed interleavingly with the Zr-rich layer in the core.The contrast is light layer alternating with shade one.(5).Bean-like secondary Al₃(Sc,Zr)precipitates formed from the C-2^# as-cast alloyα(Al)matrix after 450℃×2h.The secondary Al₃(Sc,Zr) precipitates did not grow up after 450℃×32h.Its diameter is 16～23 nm. It was observed that the Al₃(Sc,Zr)was completely coherent withα(Al) matrix by use of HRTEM.(6).The main strengthening mechanisms of Al-Zn-Mg-Cu-Zr alloys with Sc are as follows:firstly,grain refining strengthening.The A-2^#, B-2^#,B-3^#,C-2^#,C-3^# and C-4^# alloys with Sc were refined by primary Al₃(Sc,Zr)precipitates formed during solidifying.Secondly, sub-structure strengthening.The secondary Al₃(Sc,Zr)precipitates in Al-Zn-Mg-Cu-Zr alloys with Sc can pin disclocations and make crystiform forming from tangled dislocation during plastic deformation to be sub-grain boundry consisting of two-dimensional dislocation network during solution heat-treatment.Thirdly,precipitation strengthening.The secondary Al₃(Sc,Zr)precipitates and stress field interacted with the dislocations.Interface energe increased with surface increasing.When the dislocations cut the secondary Al₃(Sc,Zr)precipitates,that caused deformation around Al₃(Sc,Zr)which,blocked dislocation movement, then the alloy was strengthened.When extended dislocation cut secondary Al₃(Sc,Zr)precipitates,energy would increase because of the width of dislocation changing.And the width of dislocation would change because of the stacking fault energy of Al₃(Sc,Zr)precipitates being different fromα(Al)matrix.The dislocation jog who formed during screw dislocation cut Al₃(Sc,Zr)precipitates reject movement of dislocation.These make the effect of strengthening of the alloys with Sc increased.(7).The mechanical properties of A-2^#,B-2^#,B-3^#,C-2^#,C-3^# and C-4^# alloys with Sc at different heat-treatment were distinctly higher than these of A-1^#,B-1^# and C-1^# alloys without Sc.The mechanical properties of the alloys with Sc increased when the Zn and Cu content raised respectively.There is no W(AlCuSc)phase in the Al-Zn-Mg-Cu-Sc-Zr alloy with 2.50wt.%Cu.After strengthening solution treatment and T6, the tensile strength,elongation and electric conductivity of C-2^# alloy are 765.2MPa,8.1%and 33.4%IACS,respectively.The C-2^# alloy with 8.90wt.%Zn,2.45wt.%Cu,0.20wt.%Sc and 0.16wt.%Zr is prospective ultra-high Al alloy.(8).The amount of dislocation of the cold-rolled C-2^# alloy increased dramatically,those dislocations formed three-dimensional crystiform whose wall made of complicatied dislocation tangles.After 375℃×1h annealing,the width of dislocation tangles in the C-2^# cold-rolled alloy started to release partly.Enormous complicated tangled dislocation atmosphere and lots of dispersed precipitates were observed.The secondary Al₃(Sc,Zr)precipitates pining dislocation were observed,too. After 400℃×1h annealing,these tangles began to lighten.After 425℃×1h annealing,cell wall of dislocation tangles became thin,dislocation rearrangement formed dislocation wall and regular dislocation network, all of these phenomena indicated the C-2^# cold-rolled alloy was in process of recovery.Through 450℃×1h,the number of dislocation inα(Al)was greatly reduced,near linear dialocation wall become clearly more and sub-grain started to form.After 475℃×1h annealing,the dislocation became even more less and sub-grain was merging and growing,and recrystallization process started.(9).The starting and ending recrystallization temperature of C-2^# cold-rolled alloy is 475℃and 525℃which was obtained by measuring micro-hardness and optical microscopy at different temperature for 1h.(10).In the course of recrystallization annealing,the refined and dispersal secondary Al₃(Sc,Zr)is coherent withα(Al)matrix,which could strongly pin the complex tangled dislocation ofα(Al)tri-dimension grain and sub-structure boundary during dislocation configuration chang, so that the dislocation movement of yield C-2^# alloy,migration of sub-grain boundary and sub-grain merging and growing were suffocated during recovery,which stunted the nucleation of recrystallization and heighten the temperature of recrystallization.