Effect Of Quasicrystal Master Alloy, Nd And Yb On The Microstructure And Mechanical Properties Of ZK60 Alloy

Quasicrystal is a kind of phase with long-range quasi-periods translational order and non-crystallographic rotational symmetry solid state ordered, which is different from crystal and non-crystal. However, quasicrystal can not be directly used as structural application materials due to its innate high hardness and brittleness. Whereas, quasicrystals, with their unusual quasi-periodic lattice structure, have unique properties such as low interfacial energy, high hardness, low coefficient of friction, high corrosion resistance and excellent heat-resistant etc, which quite favors for its application as a strengthening phase in toughness matrix materials. The mechanical properties of magnesium alloy at ambient and elevated temperature can be obviously improved by using quasicrystal as reinforced phase, which exhibit high research value and great promising application future.As well known, ZK60 magnesium alloy has the highest mechanical properties among all the Mg alloys, but the poor castability, severe hot crack tendency and obvious shrinkage restrict it to be used extensively due to wide solidification range and low eutectic temperature. This paper is based on the idea of quasicrystal strengthening magnesium-based composite materials and the Mg-Zn-Nd (MZN) master alloy containing spherical quasicrystal preparaed in author's labtoratory was added into ZK60 alloy melt using the method of external addition. The effect of Mg-Zn-Nd quasicrystal master alloy on the microstructure and mechanical properties of ZK60 alloy was studied using OM, XRD, SEM and EDS. The results show that:(1) The solidification microstructure of ZK60-based quasicrystal reinforced composites (ZK60 alloy with various additions of MZN master alloy) consists ofα-Mg matrix, MgZn phase, MgZn2 phase and I phase (Mg4oZn55Nd5). Much finer grain size of a-Mg matrix is achieved and morphology of MgZn phase transforms from quasi-continuous network into discontinuous network after the addition of MZN master alloy into ZK60 alloy.(2) Both the ultimate tensile strengths (UTS) and tensile yield strength (TYS) at room temperature (RT) of ZK60 alloy are obviously improved after adding MZN master alloy. While the addition is up to 4.0 wt.%, the value of UTS and TYS at RT of ZK60 alloy reaches the peak value,256.7MPa and 178.5MPa, which increases by 17.8% and 24.1%, respectively. While the addition amount reaches 1.0 wt.%, the maximum elongation of composites is 17.8%, which is 42.4% higher than that of ZK60 master alloy. The elongation decreases with MZN master alloy amounts further increased, the minimum elongation is obtained for ZK60-5.0 wt.% MZN that is 13.6%, which is still about 8.8% higher than that of ZK60 master alloy.(3) The variation trend of impact toughness is first increased and then decreased with an increase in the volume fraction of MZN master alloy. While MZN addition is up to 4.0 wt.%, the impact toughness value reaches peak value, 41.1 J/cm2, which is 19.1% higher than that of master alloy. The macro-hardness value of ZK60 alloy increases with an increase in the volume fraction of MZN master alloy. While MZN addition is 5.0 wt.%, the macro-hardness value reaches the highest points,66.2 HB, which is 13.4% higher than that of ZK60 master alloy.In addition, the effect of Nd, Yb on the microstructure and mechanical properties of ZK60 alloy was researched because of rare earth can purify magnesium alloy and improve casting microstructure, castability, and room temperature, high temperature tensile properties of magnesium alloy, and also has an important effect on the microstructure and precipitated phase of ZK60 alloy. What's more, in order to contrast that with strengthening effect of ZK60 magnesium alloy with Mg-Zn-Nd quasicrystal master alloy, so the pure Nd element was choosed. The results show that:(1) The microstructure of ZK60 alloy with the addition of 0.2Nd or 1.5Yb is evidently refined respectively, Mg-Zn-Nd and Mg-Zn-Yb ternary rare earth phases are formed at grain boundaries and distributed nearby grain boundaries. However, Mg-Zn-Nd-Yb quaternary rare earth phase is formed and distributed in shape of network in ZK60 alloy with combination of 0.2Nd and 1.5Yb addition, and the lamellar eutectics are somehow thickened.(2) After T4 solution treatment, the eutectic structure of ZK60 alloy is fully dissolved into matrix while there are still some undissolved compounds at grain boundaries in ZK60-0.2Nd, ZK60-1.5Yb and ZK60-0.2Nd-1.5Yb alloy. Moreover, solution treatment has little effect on the morphology and distribution of second phase of ZK60-0.2Nd and ZK60-1.5Yb alloy, but excluded ZK60-1.5Yb alloy. The mechanical property of ZK60-1.5Yb alloy is remarkably improved due to Mg5g.32Zn32.24Yb9.44 spherical particles formed after T4 solution treatment. In addition, it can be concluded that ZK60-1.5Yb alloy is quite suit to solution treatment.(3) The mechanical properties of ZK60-0.2Nd and ZK60-1.5Yb alloy at ambient temperature are superior to ZK60 alloy, but ZK60-0.2Nd-1.5Yb alloy is inferior to ZK60 alloy. At the elevated temperature, the UTS of ZK60 alloy is remarkably decreased and have poor performance compared to the other alloys, but the alloy with combination of 0.2Nd and 1.5Yb addition has a bad performance in elongation.(4) The strengthening effects of ZK60 magnesium alloy are compared with Mg-Zn-Nd quasicrystal master alloy and pure Nd element, the results show that the mechanical properties of ZK60 magnesium alloy adding Mg-Zn-Nd quasicrystal master alloy are higher than that of adding Nd.