Mechanical Properties And Microstructures Of Al-Mg-Sc And Al-Mg-Sc-Zr Alloys

Five kinds of Al-Mg alloy containing different percents of Sc and Zr were prepared using ingot metallurgy processing. The room-temperature tensile mechanical properties of these alloys at different treatments (as-cast, hot-rolled, cold-rolled, annealed) were measured. The hardness at various annealing treatments and elevated-temperature tensile properties of Al-Mg-Sc alloy were also tested. The microstructures of these alloys at various treatments were evaluated using optical microscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of minor Sc and/or Zr on the mechanical properties and microstructures of Al-Mg, the recrystallization behaviour, superplasticity and cavity nudeation, cavity growth and the characteristics of fracture by cavitation in Al-Mg-Sc alloy during su-perplastic deformation have been investigated. The existing forms and acting mechanisms of minor Sc in Al-5Mg alloy, the action with combinations of minor Sc and Zr alloying, the mechanism of the recrystallizing nudeation and superplasu'c deformation of Al-Mg-Sc alloy were discussed. The results are as follows:Sc additions greater than the critical composition (0.4wt% ) were found to considerably improve the roam-temperature tensile properties of Al-Mg alloys, especially the strengthening effect of the addition of minor Sc and Zr to Al-Mg alloy is much more effective than that of Sc additions, the increment of Al-5Mg alloy stress strength is 80-150MPa.The tensile mechanical properties of Al-5Mg-0.2Sc-0.1Zr alloy cold-rolled sheet tempered at 130℃ for 3hr are extremely attractive combination of strength and elongation, the tensile strength σ_b, stress strength 00.2 and elongation 8 are 406MPa, 308MPa and 15%, respectively, which reaches the mechanical properties of the similar alloy reported by Russia. The stress strength of the alloy has 20% difference between rolling and transverse direction, whilst elongation is almost equal. The existence of (110) [112] rolled texture is the main reason of the difference.Minor Sc in the studied alloys mainly exists in form of Al₃SC, only a few of themin aluminium based solid solution. Primary AI3SC intennetallic particles that form during solidification when Sc additions are greater than the critical composition 0.4%, appear in a range of shapes in two-dimensional sections, based on squares and triangles, and were often found at the centre of aluminium grains and have clearly acted as heterogeneous nucleants. Whilst the addition of Sc and Zr to Al-Mg, Zr being dissolved in AI3SC, substituting for same of Sc atoms, formed A^CSc, Zr) dispersoid which is much more effective in inhibiting recrystallization and dispersoid strengthening.The addition of Sc to Al-Mg alloy leads to increase the recrystallizing temperature. The starting temperature point is 375"C, while the ending temperature point is 52013 in the recrystallization of Al-5Mg-0.2Sc alloy. The increase in recrystallizing temperature is caused by the pinning effect of highly dispersed AI3SC precipitates on dislocation and subgrain boundary. The recrystallizing process presents itself the nu-cleation mechanism of the alloy involving not only the subgrain coalescence but also the subgrain growth. Al-SMg-0. 4Sc alloy exhibits excellent superplasticity in a wide range of temperatures and initial strain rates. Its maximum elongation may be up to 396 % under elevated-temperature (520*C) deformation with initial strain rate 1.67 x lO^s"1. The TEM observation of the microstructural changes revealed that the superplasticity obtained was induced by the dynamic recrystallization which occured in the early stage of deformation. The whole deformation process can be divided into three stages of subgrain superplasticity, transition and fine grain superplasticity.The four processes will take place in sequence during superplastic deformation of Al-5Mg-0. 4Sc alloy, i. e. 0 dynamic recrystallization;? dislocations came from grain boundaries;? the motion of the dislocations within the grains is prohibited by the secondary phase Al3Sc as well as dislocations climb up and go over the dispersed particles;(D dislocations are annihilated in grain boundaries. The dominant mechanism of superplastic deformation is that grain boundary sliding process will be accompanied by intergrainular dislocation.Al-5Mg-0.4Sc alloy is sensitive to cavitation. The site of the cavity nucleation in the process of superplastic deformation of the alloy is intergranular particles and grain boundary triple junctions. The main cause for cavity nucleation is the high stress con-centration due to the grain boundary slip hindered at uneven grain boundary. Cavitygrowth behaviour of the alloy can be well described by the two-stage cavity growthmodel proposed in the present work.