Precipitation Behavior And Dispersion-strengthening Mechanism Of Nano-sized Dispersoids In Zirconium-Containing Aluminium Alloys

Al3Zr dispersoid has a broad prospect of application in developing heatresistant Al alloys.However,strengthening by Al3Zr has been limited by either low number density or low volume fraction.In addition,the interaction among multiple components is very complex in multi-component Al alloys during solidification/deformation/heat treatment,and it is very difficult to achieve a good combination in strengthening of Al3Zr with the intrinsic phase of each system.To solve thses problems,Al-Cu and Al-Mn systems were chosen in the present study,combining with thermal-dynamic/DFT calculations,SEM/TEM/EBSD/APT characterization techniques and tensile tests,to investigate the precipitation behavior and dispersion-strengthening mechanism of Al3Zr nanoparticles.The influencing factors of precipitation and phase equilibrium of systems were carefully considered,meanwhile the chemical compositions and homogenization/aging regimes were delicately designed.Resultantly,the optimized precipitation behavior of Al3Zr and synergistic hardening effect together with intrinsic phases have been achieved.Part ? elucidated the micro-alloying effect of Er and Zr on microstructure and mechanical properties in Al-Cu alloys.The addition of Er induced the formation of coarse Al8CuEr eutectic phase,which reduced subsequently precipitation hardening and inhibited the formation of Al3Er particles.In comparison,dense Al3(Er,Zr)dispersoids(1.33×1021m-3)formed in Al-Cu-Er-Zr alloy,which efficaciously facilitated heterogeneous nucleation of ?' during aging.The enhanced precipitation of ?' from heterogeneous nucleation can compensate the reduction of nucleation driving force due to the loss of Cu solute atoms resulted from Al8Cu4Er eutectic phase,thus increasing the yield strength.Part ? designed Al-2.5Cu-0.28Zr(wt.%)serie alloys based on phase diagram,and proposed a continuous heating regime to replace the traditional homogenization in order to obtain more Cu solution while inhibit the coarsening of Al3Zr.Furthermore,trace Si and Sn were added to promote the precipitation of Al3Zr and?',which mainly distributed at dendritic center and periphery,respectively.In this case,yield strength at RT was greatly improved from 72 MPa to 199 MPa while that at 250? can reach to 129 MPa,overwhelming Al-Cu(-Zr)reference alloys.In addition,the aggregation of fine Al3Zr along {002}Al habit plane influenced by Cu was discussed.Part ? systematically studied the recrystallization behavior of Al-2.5Cu0.28Zr(wt.%)series alloys during annealing.Al-Cu-Zr-Si-Sn-Mg(B3 alloy)exhibited superior recrystallization resistance in 300-450? range because of the dense Al3Zr dispersoids.Temporal evolution of tesile properties during 400? longtime aging was also studied.The results showed that the yield strength of B3 at different aging state are much higher than that of the reference alloys,e.g.80 MPa after 1000 h aging.HAADF-STEM characterization revealed the formation of antiphase-boundary(APB)along {100} plane within some Al3Zr dispersoids after 1000 h aging.In some cases,D023-structure can be observed between multiple APBs,indicating the formation of {100}-type APBs stands for the start of L12-D023 transformation.Ocassionally,the segregation of Cu on APB can be observed,resulting in the relaxation of local strain.DFT calculation proved that Cu atoms can spontaneously segregate to the pure Al layer between L12-and D023-stuctures.Part ? designed a new heat-resistant Al alloy with concurrent precipitation of nanoscale ?-Al(Mn,Fe)Si and Al3Zr dispersoids by adding 0.28 wt%Zr into a commercial 3003 alloy.During ramp-heating,?-dispersoids and Al3Zr precipitated at dendritic center and periphery sequencely and separately,resulting in higher density and better homogeneity compared with 3003 and Al-Zr alloys.The nucleation of Al3Zr on pre-formed ?-dispersoid was observed in dendritic periphery,and the crystallographic orientation relations between dispersoids and the matrix were indentified as:[001]Al//[001]Al3Zr//[532]? and {200}Al//{100}Al3Zr//{-352}?.The 3003-Zr alloy exhibited an excellent thermal stability at 400?.After 250h isothermal heating,the alloy has about the same room temperature yield strength as the peak-aged state.This is because that the strength increment from further precipitation of Al3Zr can compensate strength reduction caused by the coarsening of ?-dispersoid.Mechanism of controlling the coarsening of Al3Zr by means of decreasing the Si content in matrix via the growth of ?-dispersoid was summarized.The results of present study are of positive significance for the mass production of low-cost Zr-containing Al alloys.It can also serve as an effective method to tailor the precipitation behavior of Al3Zr dispersoids in the multicomponent Al systems and make full use of the synergistic hardening effect of Al3Zr together with intrinsic phases.