Effect Of Dy On The Microstructures And Mechanical Properties Of Mg-Y-Nd Alloys

Magnesium alloys have been widely used in the automotive, electronics and other fields due to their high specific strength and high specific stiffness. However, their poor performance at the high temperature has seriously limited their wide applications in aerospace industry. Rare earth(RE) elements can effectively improve the properties of magnesium alloy, especially the high temperature mechanical properties and high temperature creep properties. Therefore, the magnesium alloy containing RE elements has currently become one of the hottest spots in the research field of magnesium alloys. In this thesis, the Dy was added into the WE43 alloy to form Mg-4Y-3Nd-x Dy alloys. The effect of Dy addition on microstructure mophologies, phases as well as mechanical properties at room temperature and elevated temperatures were investigated.The as-cast microstructures of Mg-4Y-3Nd-x Dy-0.5Zr alloys are composed of primary α-Mg, bone-like eutectic Mg5 RE phases distributed at grain boundaries(f.c.c, α=2.2-2.3nm), and isolated cuboid-shaped RE17Mg3(f.c.c, α=0.5-0.6nm) phases distributed near the euteic phases and within grains. Dy addition exerts no effect on the microstructures of the as-cast alloy, but has a weak effect on the lattice paramers. With the increase of Dy element, the volume fraction and size of bone-like eutectic phases increase, and morphologies become more complicated. During solidification, Nd is partially replaced by Dy in bone-like eutectic phase, while the concentration of Y remains the same. For the cuoid-shaped RE17Mg3 phases, the concentration ratio of Dy to Y is increased with the increase of Dy addition, while the concentration of Nd keeps unchanged. Nd is mainly distributed in bone-like Mg5 RE phases, whereas Y and Dy are slightly enriched at grain boundaries and within bone-like Mg5 RE phases.The solution treatment of Mg-4Y-3Nd-x Dy-0.5Zr(x = 0, 1, 2, 3, 5) alloys was carried out at different temperatures. For the case of 500 ℃ solution treatment, the bone-like Mg5 RE phases keeps stable. When the solution temperatures are increased to 525℃ and 550℃, the bone-like Mg5 RE phases are dissolutioned quickly. The cuoid-shaped RE17Mg3 phases are stable under 525℃. Nd in the cuoid-shaped RE17Mg3 diffuses gradually into the matrix until its concentration is the same as that in matrix. For the case of 550℃ solution treatment, the cuoid-shaped RE17Mg3 phases are dissolutioned gradually and the grain size increases rapidly due to the fading of their pinning effect on grain boundaries.Investigations on ageing hardening and precipitated behaviors of Mg-4Y-3Nd-x Dy-0.5Zr alloys show that the Dy addition has an obvious effect on ageing hardening. The effect is decreased with the increase of ageing temperatures. With the addition of Dy element into the base alloy, the hardness significantly increases, the time to the peak ageing shortens obviously, and the precipiated phases becomes finer. When aged at 250℃, the precipiate sequence in Mg-4Y-3Nd alloy is ?", ?′, ????Dy added into the base alloy leads to nucleation and precipitation of ?′ phase, which contributes to the shortened time to peak ageing. The analysis on the driving force of precipitate processes between the Mg-4Y-3Nd-0.5Zr and Mg-4Y-3Nd-2Dy-0.5Zr alloys indicates that the WN43?G and WND432?G is-85J/mol, and-101J/mol, respectively. The low driving force for Mg-4Y-3Nd-2Dy-0.5Zr alloy promotes the nucleation and direct precipitation of ?′ phases from the matrix.The mechanical results of Mg-4Y-3Nd-x Dy-0.5Zr(x = 0, 1, 2, 3, 5) alloys indicate that Dy can enhance the tensile strength and yield strengh, especially at elevated temperatures. The tensile strength and yield strength of the Mg-4Y-3Nd-3Dy-0.5Zr alloys are 43.2% and 37.6% higher than those of Mg-4Y-3Nd-0.5Zr alloy at 300℃, respectively. Due to their relatively low concentration, the addition of Dy has little effect on creep-resistance, especially at low testing temperatures. The stress exponent is 2.3, and the creep mechanism is basal slip and pyramidal slip for 250 ℃ creep tests of Mg-4Y-3Nd-2Dy-0.5Zr alloy. The creep activation energy of Mg-4Y-3Nd-2Dy-0.5Zr alloy is 94 k J/mol and 195 k J/mol, for the 40 MPa and 80 MPa creep stress, respectively. Fine and isolated cuboid-shaped RE17Mg3 phases have a strengthening effect on the alloy, weaker than the solid solution strengthening effect resulted from the solid solution of RE elements into the matrix.