Effects Of Rare Earth On Microstructure And Mechanical Properties Of Zn-25 Al-5 Mg-2.5Si Alloy

Zinc alloys containing higher aluminum (ZA alloys) are one of the widely used non-ferrous materials in recent decades because of their many advantages such as good casting and mechanical properties, excellent wear-resisting performance and lower manufacturing cost. Meanwhile, ZA alloys are environmentally friendly because the melting energy consumption for the alloys is much lower than for the copper alloys. ZA alloys have been used to substitute for some traditional alloys such as brass, bronze and babbitt metals to prepare some wear-resistant and anti-friction components.However, their application is partly restricted because of their strength and wear-resistance declining with increase of temperature when they are overloaded or under continuously working. In order to further improve the properties of ZA alloys at high temperature, this paper investigated the effects of rare earth(RE) elements on the microstructure and the strength at room temperature and high temperature (100℃and 180℃), which is significant for extending their application fields. Zn-25Al-5Mg-2.5Si-xRE alloys were prepared by adding different content of rare earth elements Gd, Y, Nd and Dy to Zn-25Al-5Mg-2.5Si alloys. Metallurgical microscopy, scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction were employed to investigate the effect of different rare earth elements on the microstructure, morphology and distribution of the phases of the Zn-25Al-5Mg-2.5Si-xRE alloys. Mechanical properties at room temperature and high temperature were tested and the optimum contents of rare earth elements were obtained.It was showed from the microstructure of the alloys that the additions of the appropriate amount of rare earth elements Gd, Y, Nd and Dy were capable of refining the as-casting microstructure and improving their morphology. Granular and massive compounds with higher melting point, hardness and stability were formed in the alloys as a result of the addition of rare earth elements. The compounds were dispersively distributed in the boundary and intracrystalline of the alloys. When 0.8wt% Gd or 0.8wt% Nd or 0.4wt% Y or 1.2wt% Dy was added to the alloy, the optimized grains refining effect on the alloys was obtained.XRD was used to analyze the phases composition in the alloys. The results showed that Al-RE compounds were preferentially formed as rare earth elements were added and then the redundant rare earth elements interacted with Zn to form Zn-RE compounds. Rare earth phases Al3Gd and GdZn12 were formed when Gd was introduced into Zn-25Al-5Mg-2.5Si alloy. Similarly, rare earth phases Al3Y and Y2Zn17, Al2Nd and NdZn2, AlDy and DyZn were also formed because of the addition of Y, Nd and Dy, respectively.It was revealed from the testing results of the mechanical properties of Zn-25Al-5Mg-2.5Si alloys adding RE that the higher hardness, tensile strength and plasticity at high temperature were achieved. The mechanical properties increased with increase of RE at first and then decreased later. When 0.8wt% Gd or 0.8wt% Nd or 0.4wt% Y or 1.2wt% Dy was added to the alloy, the maximum tensile strength was obtained.It is suggested that strength and hardness at the room temperature and high temperature increased and the plasticity of alloys were improved by adding rare earth elements Gd, Y, Nd and Dy mainly through the combined action of grain refining and precipitation strengthening.The comparison of the effect of four rare-earth elements on microstructure and properties of alloys showed that Dy was the most effective in alloy microstructure refinement and properties improvement, Gd, Nd and Y followed.SEM was used to characterize the fracture morphology of the alloys. It was showed that the fracture mode of the alloys without rare earth elements was brittle-fracture. With the increase of RE and the rise of temperature, the quantity of dimple fracture was increased, resulting in the improvement of plasticity of the alloys. However, when rare earth phases agglomerated in the matrix or grain boundary with adding excessive RE, plasticity of the alloys decreased.In this paper, the properties of non-ferrous alloys were remarkably improved with the addition of earth elements, which will extend the application fields of non-ferrous alloys and is significant for the economic development and resource utilization.