The Development And Thermal Stability Of Annealing Twins In Ultra-Pure Aluminium

Twinning is an important strengthening method in metal materials.For face-centered cubic metal materials,the high fraction of twins endow the materials with good properties,which improve the ductility and the resistance of intergranular corrosion,fatigue and creep.However,twins rarely appear in Al and Al alloys with high stacking fault energy,so it is difficult to be used to improve the strength of Al alloys.However,recent studies on ultra-pure aluminum show that there are high proportion of annealing twins in the annealing process.Therefore,it is of great significance to study the development conditions of high proportion annealing twins,development mechanism and thermal stability of annealing twins in ultra-pure aluminum.In this paper,ultra-pure aluminum,Al-1%Si alloy and Al-0.3%Cu alloy were used as research objects.After cold rolling and annealing,the microstructure evolution of the samples during annealing was systematically characterized by electron backscattering diffraction(EBSD).And the effects of different deformation and annealing processes on the development of annealing twins in ultra-pure aluminum and its alloys were studied.On the other hand,the annealing twins in ultra-pure aluminum were characterized by three-dimensional EBSD to study the effect of sample surface on the development of twins.At the same time,the thermal stability of annealing twins in ultra-pure aluminum and Al-0.3%Cu alloys was studied by in situ annealing to observe the dynamic evolution of annealing twins during annealing.The main conclusions of the research are as follows:(1)The medium deformation is beneficial to the development of annealing twins,and the fraction of twins reaches the maximum value of 20.53% when Al is cold rolled for 50% and annealed at 200? for 20 min.(2)The fraction of twins increases with the increase of recrystallized volume fraction.For ultra-pure aluminum with different deformation variables,the fraction peak value of twins is located in the sample with recrystallized volume fraction greater than 90%.During the process of grain growth in recrystallization region,the fraction of twins decreases gradually.(3)The surface of the materials is beneficial to the development of twins,butannealing twins are not only formed on the surface of ultra-pure aluminum,but also formed in the interior.And high fraction of annealing twins can also be developed in the interior of ultra-pure aluminum.(4)In the ultra-pure Al and Al-1%Si alloys with high purity matrix,the fraction of annealing twins is much higher than that of the Al-0.3%Cu alloy with higher solid solution elements in the matrix.The thermal stability of twins in ultra-pure Al is poor and gradually disappear in the process of grain growth.The existence of alloying elements Si and Cu can improve the thermal stability of annealing twins in aluminum alloys to a certain extent.