Study On The Microstructures And Properties Of Mg-Gd-Y-Nd-Zn Based Alloys Containing Long-period Stacking Ordered Phase

Recently, the long-period stacking ordered（LPSO） phases have been found in Mg-RE-Zn alloys（RE/Zn>1）. The Mg-RE-Zn based alloys with LPSO phase exhibit significantly superior ducility at room temperature（RT）, and the damping property can also be improved by the appropriate amount of LPSO phase. As a result, these alloys are promising magnesium alloys with high strength, high ducility as well as high damping.Generally, the RE elements are consumed by the formation of the LPSO phases, which discounts the precipitation strengthening, effect during further aging treatment. Compare precipitation strengthening with LPSO strengthening, precipitation strengthening is more effective, so they are restricted each other. Therefore, the key point of designing Mg alloys containing LPSO phase and showing high strength and ducility is the use of the LPSO phase to improve plasticity and also maintian high precipitation strengthening effect, the coexistence of RE elements involving in the formation of the LPSO phase and the RE elements which don’t participate in formation of the LPSO phase in the microstructure can achieve this gole. Numerous researches show that: Gd and Y elements will participate in the formation of the LPSO phase, while the Nd element is not involved in the formation of the LPSO phase, the amount of Nd elements can be compensated for the precipitation strengthening effect. As a kind of high strength and heat-resistant magnesium alloy, EW75 alloy has been applied in engineering, the strength of this alloy is high, but the plasticity of the alloy at the RT is very low, a large number of Y-riched particles will be released during hot deformation. Based on the above, the Mg-7Gd-3Y-1Nd-x Zn-0.5Zr（x=0.5, 1 and 2wt.%） alloys containing LPSO phase have been designed and studied, the microstructure and properties are systematically investigated.The microstructures of the three as-cast alloys are composed of equiaxed α-Mg matrix, eutectic phases in grain boundaries and needle-like stacking faults near grain boundaries.When the Zn content is 0.5wt.%,the discontinuous network eutectic phase is Mg₅（RE,Zn）, while the addition of the 1 and 2wt.% Zn lead to the skeletal-shaped eutectic phase（Mg,Zn）₃RE phase appear. TEM analyses indicate that the（Mg,Zn）₃RE and Mg₅（RE,Zn） phases have a face-centered cubic（f.c.c.） structure with lattice constants of 0.73 nm and 2.23 nm, the Zn content in（Mg,Zn）₃RE phase is higher than that in Mg₅（RE,Zn） phase. The new block-like 14H-LPSO phase exist in the alloy containing 2wt.%. With Zn content increasing, the needle-like stacking-faults are seen more and more clearly and the occurrence of the ordering of the RE and Zn elements in the stacking-faults.The microstructure evolutions during homogenization are studied systematically. Firstly, the signle stage homogenization is carried out at 505℃, 520℃ and 505℃. At 505℃ and 520℃ hear treatment for 48h: a lot of undissloved particles are observed in the alloy with 0.5wt.% Zn; some eutectic phases still exist in the alloy containing 1wt.%Zn; the coarse continuous mesh LPSO phase emerge in the alloy having 2wt.%Zn. At 535℃ for 2h, the phenomenon of over burning can be caused by（Mg,Zn）₃RE phase in the alloys with 1wt.% and 2wt.% Zn. Secondly, the double stage homogenization is investigated. In the first-stage homogenization at 480℃×24h,the parts of Mg₅（RE,Zn） phase dissolve into α-Mg matrix in the alloy with 0.5wt.% Zn; the（Mg,Zn）₃RE phase nearly all dissolve in the alloys containing 1wt.% and 2wt.% Zn,the microstructures show a little change after extending time, so the 480℃×24h is seclected as the first stage homogenization regime. The second stage homogenization is carried out at 520℃, 510℃ and 500℃ for 32 h. At 520℃, the phenomenon of over burning is seen in the alloy with 1wt.% and 2wt.%Zn; at 510℃, the Zn-riched phases and block-like LPSO phase coexist in grain boundaries; at 500℃,no other phase can be seen in the alloy with 0.5wt.%Zn and only block-like LPSO phase exist in the alloy with 1wt.% and 2wt.%Zn, so the optimum solution treatment condition for the three alloys is 480℃×24h+500℃×32h. The mechanism of the over burning is that Zn elements tend to gather phase at higher temperature, the diffusion of RE elements in LPSO phases lead to the LPSO phases transform to the Zn-riched phases, and then the Zn-riched phases over burn. According to the TEM analysis, it is known that the block-like LPSO phases in the alloy containing 1wt.% and 2wt.%Zn are all 14H-type.The morphology evolution of 14H-LPSO phase during heat treatment is investigated. At high temperature, the increasing solid solubility of RE in α-Mg matrix and the easy to gather on the grain boundaries of Zn element is benefit for the formation of the block-like 14H-LPSO phase on grain boundaries; when the temperature is getting down, the decreasing solid solubility of RE in α-Mg matrix and the diffusion of Zn element into α-Mg matrix lead to form the needle-like 14H-LPSO phase in α-Mg martix. The heat treatment are carried out at constant temperature for different time suggests that the formation mechanism of the 14H-LPSO phase is a discontinuous removal solution process, the grain boundaries become preferential nucleation sites at first, then grow into the α-Mg matrix in the form of needle-like, the the needle-like14H-LPSO gradually decrease its length after growing a certain stage, the block-like 14H-LPSO phase forms in the end.The microstructure controlling is researched systematically after homogenization. The needle-like 14H-LPSO phase are observed by the furnace cooling to different temperatures followed homogenization. The decreasing temperature or prolonging time leads to increasing the needle-like 14H-LPSO phase into α-Mg. With the needle-like 14H-LPSO phase increasing, the precipitation strengthening effect is reducing, the strength of the alloy is decreasing and plasticity is enhancing. The existence of the needle-like 14H-LPSO phase leads to a decline the damping property in the low strain amplitude（10-5¹0-4）, but increase the damping property in the high strain amplitude（>10-4）. The block-like 14H-LPSO phase can decompose at low temperature, the diffusion of Zn and RE atoms after 14H-LPSO decomposing is not synchronized, which results in Zn atoms diffusion into α-Mg martix rapidly, but the RE atoms are entrapped in grain boundaries.The effect of 14H-LPSO phase on the hot compression deformation behavior is studied. The higher hardness of the14H-LPSO phase than α-Mg matrix results in the higher deformation resistance, the deformation activation energy is 290.39 k J/mol. The block-like 14H-LPSO phase can play a role in inducing dynamic recrystallization and dynamic recrystallization activation energy is only 29.609 k J/mol. The flow stress is decreased when the needle-like 14H-LPSO phase exists in the microstructure before deformation, the needle-like 14H-LPSO phase mainly appears within the deformed grain, and the block-like 14H-LPSO phase is mainly distributed in the dynamic recrystallization zone after deformation.The microstructure and mechanical properties of the as-extruded alloys are studied. With increasing Zn content, the proportion of dynamic recrystallization grains drops, but the proportion of deformed grains increases, and the needle-like 14H-LPSO phase distributs in the interior of deformed grains, the texture of the three alloys are all（0001） basal texture, the density of（0001） pole is 42.698, 3.577 and 4.935. The mechanical properties at room temperature suggest that with increasing Zn content, the UTS are 320 MPa, 339 MPa, 346 MPa, the YS are 243 MPa, 268 MPa,280MPa and the elongation are 11%, 13%,15%, respectively. The pre-precipitating needle-like 14H-LPSO phase before extrution in the alloy with 1wt.%Zn shows that the proportion of deformed grains increases and the texture is {101?0}α, the UTS, YS and elongation is 341 MPa, 284 MPa, and 22%。The effects of aging treatment after extrusion were studied. With increasing Zn content, precipitation strengthening effect gradually weakening. The tensile tests at RT reveal that with increasing Zn content, the UTS are 415 MPa, 400 MPa, 390 MPa, the YS are 346 MPa,351MPa,346 MPa and the elongation are 6%, 12%,9.5%, respectively. The tensile tests at 300℃ demonstrate that with increasing Zn content, the UTS are 297 MPa, 279 MPa, 272 MPa, the elongation are 14%, 35%,42%, respectively. The existence of needle-like LPSO phase in the alloy with 1wt.%Zn has little effect on the strength, but it can significantly improve plasticity at RT, and the plasticity is still 15% after peak-aged.The damping properties of the as-extruded alloy after peak-aging are studied systemically.The damping strain spectrum at RT shows that: when strain amplitude between 10-5 and 10-4, Q^-1_0.5wt.%Zn>Q^-1_2wt.%Zn>Q^-1_1wt.%Zn; when strain amplitude above 10-4, Q^-1_1wt.%Zn>Q^-1_2wt.%Zn>Q^-1_0.5wt.%Zn. The damping strain spectrum at 300 ℃ suggests that: when strain amplitude between 10-5 and 10-4, Q^-1_2wt.%Zn>Q^-1_1wt.%Zn>Q^-1_0.5wt.%Zn; when strain amplitude above 10-4, Q^-1_1wt.%Zn>Q^-1_2wt.%Zn>Q^-1（0.5wt.%Zn）. The damping temperature spectrum reveals that: the dislocation damping peak and grain boundary damping peak are observed, and with increasing Zn content, the intensity of the grain boundary damping peak is gradually weakened.The precipitates were studied by TEM. The < 11（2|-）0 > α demonstrates that the stacking fault can be observed after peak-ageing, the serious lattice distortion appears near the stacking fault.The [0001] zone axis shows that the precipitate is β′ and a new rod phase is found adjacent to the β′, the RE content in the new rod phase is higher than that in the β′ phase, and the rod phase can be subdivided into a plurality of mutually parallel structure unit, and each structure unit is composed of six atom on close-hexagonal plane with each other.