Research On Microstructures And Properties Of Rapidly Solidified Mg-Zn Based Magnesium Alloy

Magnesium alloys have great potentials to be used as structural materials in the fields of electronic, automobile and airspace industries mainly because of their low densities, high specific strength, good dimension stability, electromagnetism shield and etc. As one of the precipitation strengthening alloys, the Mg-Zn alloy exhibits moderate strength, good plasticity and corrosion resistance. However, its wide crystallization temperature range leads to the poor casting property and the difficulty in grain refinement. Thus, the Mg-Zn alloy can not be used as industrial castings or forgings, and its applications is remarkably limited. Rapidly solidification (RS) is an effective method to prepare high performance materials, characteristics of grain refinement, microstructural homogeneity, solid solubility extension and the formation of non-equilibrium phases. The problems in the Mg-Zn alloy as mentioned above are desirable to be resolved by rapid solidification processing. In the present work, the combination of rapid solidification processing and alloying has been adopted to develop the Mg-Zn based alloys with uniform fine-scale dispersions of thermal stable intermetallic precipitates in order to further improve the high-temperature performance of the alloys.In the present dissertation, the atomization-twin rolls quenching technology has been utilized to successfully prepare the RS Mg-Zn based alloys in the form of flakes, including the RS Mg-Zn, Mg-Zn-Ca, Mg-Zn-Ce and Mg-Zn-Ca-RE (Ce and La) alloys. The effects of the sole Ca and RE additions and the combined additions on the microstructures, phase compositions, thermal stability and isochronal age-hardening behaviors of the RS Mg-Zn alloy have been systematically investigated. On the basis of the study mentioned above, the rapidly solidification/powder metallurgy (RS/PM) Mg-Zn based alloys in the form of rods have been prepared by hot extrusion. These alloys can be divided in two series: (1) RS/PM Mg-Zn-Ca system alloys, mainly including Mg-6Zn-5Ca and Mg-6Zn-5Ca-RE (3Ce and 0.5La) alloys; (2) RS/PM Mg-Zn-Ce system alloys, mainly including Mg-6Zn-5Ce and Mg-6Zn-5Ce-1.5Ca alloys. The microstructures, mechanical properties at room and elevated temperatures and creep resistance have been investigated. The main results are listed as follows: (1) Microstructures and properties of the RS Mg-Zn based alloy flakesThe Ca addition improves the microstructures and properties of the RS Mg-Zn alloy remarkably. With the increase of Ca, the Mg51Zn20 phase with a low melting point in the alloys are gradually replaced by the Ca2Mg6Zn3 and Mg2Ca phases, which have relatively high melting points. Therefore, the thermal stability of the alloys increases gradually with the increment of Ca. With the Ca content higher than 1.5wt.%, the alloys are characteristics of the notably refined microstructures and the remarkably higher volume fractions of dispersions. The minimal grain size of the alloys is about 3～5μm. For the RS Mg-6Zn-5Ca alloy, the isochronal age-hardening behavior of the alloy is distinct and the maximum hardness is 120.8±4.5Hv, mainly due to the precipitation of fine and dispersed Ca2Mg6Zn3 within the grains. In addition, the Mg2Ca and Ca2Mg6Zn3 phases with relatively larger sizes are observed at the grain boundary. All of them are beneficial for the alloy to maintain a relatively high hardness at high temperatures.The Ce addition is beneficial for the refinement of the microstructures and remarkable improvement of the thermal stability of the alloy. With the increase of Ce, the microstructures are gradually refined and the volume fractions of dispersions are increased remarkably. The minimal grain size of the alloys is about 4～7μm. The stable intermetallic compound i.e. the Mg-Zn-Ce ternary phase (T phase) with a high melting point(470～500℃) is formed in the RS Mg-Zn-Ce alloys at the expense of the Mg51Zn20 phase and thus the thermal stability of the alloys is enhanced. However, the isochronal age-hardening behavior of the alloys gradually decreases with the increase of Ce. The hardness of the RS Mg-6Zn-5Ce alloy is much higher than the other Mg-Zn-Ce alloys at the range of RT to 400℃and the highest hardness of the alloy is 91.5±7Hv, mainly caused by fine microstructure and the precipitation of dispersed precipitates. In the RS Mg-6Zn-5Ce alloy, the atomic percentage of the T phase is 80.8at.%Mg,12.4at.%Zn and 6.8at.%Ce,and the (Zn/Ce)at is about 2:1.With the addition of RE (Ce and La) in the RS Mg-6Zn-5Ca alloy, a Mg-Zn-RE phase with a few Ca (about 2～3at.%) is formed in the alloy, which is shortened as the T′phase. The melting point of the T′phase is close to that of the T phase, but much higher than that of the Ca2Mg6Zn3 phase. Moreover, with the increase of RE in the Mg-6Zn-5Ca alloy, a little of RE is detected in the Ca2Mg6Zn3 phase and leads to the improvement of the thermal stability of the Ca2Mg6Zn3 phase. Both the′T phase a nd the Ca2Mg6Zn3 phase containing RE are beneficial to the improvement of the thermal stability of the RS Mg-Zn-Ca-RE alloys. The Mg-6Zn-5Ca-3Ce alloy exhibits an obvious isochronal age-hardening behavior and the highest hardness of the alloy is 162.4±5.5Hv, mainly derived from fine and dispersed Ca2Mg6Zn3 within the grains. Moreover, the T′phase, Mg12Ce and Mg2Ca are detected at the grain boundaries, which contributes to the enhancement of the hardness. (2) Microstructures, strength at room temperature (RT) and high temperatures and creep resistance of the RS/PM Mg-Zn based alloy rodsThe microstructures of the alloys are sharply refined and the volume fractions of dispersions are increased remarkably in the hot-extrusion state, but the kinds of the mostly phases are not changed. For the RS/PM Mg-Zn-Ca system, the mechanical properties of the alloys at RT and elevated temperatures can be improved by the sequential additions of 5wt.%Ca and RE (Ce and La) in the RS/PM Mg-Zn alloy. With the addition of 5wt.%Ca in the Mg-Zn alloy, some relatively stable Ca2Mg6Zn3 and high-melting Mg2Ca phases which are found at the grain boundaries lead to the effective strengthening of the grain boundaries. Thus, the compressive strength of the alloy at 200℃is remarkably enhanced from 40.7MPa (Mg-6Zn) to 202.3MPa (Mg-6Zn-5Ca) with Ca additions. The minimum creep rate of the latter one is about 134 folds lower than that of the former (200℃/50MPa). With the further addition of RE in the Mg-Zn-Ca alloy, there are some′T phases with a higher melting point at the grain boundaries besides Mg2Ca phases. Moreover, the RE addition is beneficial to the improvement of the thermal stability of the Ca2Mg6Zn3 phase. Therefore, the high-temperature strength (200℃) of the alloy is further enhanced up to 234.0MPa (Mg-6Zn-5Ca-3Ce alloy) and 244.6MPa (Mg-6Zn-5Ca-3Ce-0.5La alloy), respectively. The minimum creep rate of the latter two alloys is about 2 and 4 folds lower than that of the former (175℃/50MPa).For the RS/PM Mg-Zn-Ce system alloys, the mechanical properties of the alloys at RT and elevated temperatures can be increased by the sequential addition of Ce and Ca in the RS/PM Mg-Zn alloy. With the addition of 5wt.%Ce in the Mg-Zn alloy, a lot of fine and dispersed T phases are formed within the grains and at the grain boundaries. So the compressive strength of RS/PM Mg-6Zn-5Ce alloy at 200℃is enhanced up to 225.9MPa. The minimum creep rate of the alloy is about 1075 folds lower than that of Mg-6Zn alloy (200℃/50MPa). The creep resistance of the RS/PM Mg-6Zn-5Ce alloy is much higher than that of the RS/PM Mg-6Zn-5Ca alloy due to the higher thermal stability of the T phase than the Ca2Mg6Zn3 phase. With the further addition of Ca in the Mg-6Zn-5Ce alloy, a lot of the Mg-Zn-Ce phases in the different shapes with the (Zn/Ce)at about 1.5:1 and containing a few of Ca (about 1at.%) are observed at the grain boundaries. So it can be inferred that the enhanced high-temperature strength and creep resistance of the alloy may be resulted from the dissovement of the Ca in the Mg-Zn-Ce phase. The high-temperature (200℃) strength of the alloy is up to 258.2MPa and the minimum creep rate of the alloy is about 1/2 of that of the Mg-6Zn-5Ce alloy (200℃/90MPa).