| The effects of Er addition on the microstructure, mechanical property and corrosion behavior of Mg-3Nd-0.2Zn-0.4Zr(No.l NZ30K) were investigated by optical microscopy(OM), transmission electron microscopy (TEM), scanning electron microscope (SEM), energy-dispersive spectroscopy(EDS), mechanical properties test, X-ray diffraction (XRD), immersion test and potentiodynamic polarization, which indicate that the influential mechanism of Er on the microstructure, mechanical property and corrosion property of NZ30K alloy. The main results are as follows:(1) Trace Er has great effect on grain refinement on the microstructure of NZ30K-Er alloy(No.2) with addition of0.5wt.%Er. Small white particles including Er also were dispersed in the grain boundaries besides being dissolved into Mg matrix. And a small part of Er was dissolved into the eutectic compounds (Mg12Nd), which improves the thermostability of NZ30K-Er alloy.(2) The addition of Er doesn’t change the type of precipitated phase in Mg-Nd-Zn-Zr alloy’s peak-aged process. But the alloy with Er (number2)reaches the peak-aged hardness sooner than that without Er(number1) with more uniform fine precipitates, so its aging strengthening effect is better with Er. The reason the two alloys reached their peak-aged hardness at200℃is because of the precipitation of many uniform and homogeneously distributed fine β" phase in the Mg matrix. β" phase has a strengthening effect on Mg matrix and it has hcp DO19structure. and its oriented relationships with Mg matrix is [2423]β"//[2423]α-Mg,,(1010)β"//(1010)α-Mg.(3) Good mechanical properties were obtained in the NZ30K-Er alloy in the room and elevated temperature, its ultimate tensile strength(UTS), yield strength(YS) and elongation(E.I.) reaches283.4MPa,170.2MPa and4%respectively. As Er is added to NZ30K-Er alloy, its UTS and YS increases while its E.I. is almost at the same.NZ30K-Er alloy also has good mechanical properties at250℃, its UTS gets to201MPa and E.I reaches10.56%.(4) There are some rugged cleavage plates, tear ridges and a few dimples in the room temperature fracture surface images of NZ30K-Er alloy; The particles in the dimples mainly include Er, Nd. This shows the alloy with Er has a good plasticity. In the room temperature, the fracture modes of both alloys after age-treatment are majorly quasi-cleavage. In the elevated temperature, the fracture surface images of No.1alloy’s T6state is similar with the previous images in the room temperature, its fracture mode is still majorly quasi-cleavage; while there are lots of dimples which are more and deeper than before in the fracture surface images of No.2alloy’s T6state and its fracture mode is ductile fracture primarily(5) Heat treatment influenced the morphologies of corrosion products of NZ30K alloy. The corrosion product was identified mainly as Mg(OH)2by XRD. For F sample, the corrosion product was composed of fluffy particles. For T4sample, the product was compact and uniform with sandwich-shaped particles, while the surface of T6sample was made up of compact and homogeneously distributed band-shaped fine particles. Corrosion rates are different remarkably due to different compactness of corrosion products film and different types, quantities and distribution of precipitation phase. The corrosion resistance rates of different states on No.2alloy were2T6>2T4>2F.Comparing with the corrosion resistance of the two alloys in the same condition, we get to know that the relationships of the corrosion resistance rates between the two alloys are1F>2F,1T4>2T4,1T6<2T6.(6) Through full analysis, the comprehensive properties of No.2alloy’s T6state is the best. It has great mechanical properties in room and elevated temperature, meanwhile, it also has an excellent corrosion resistance property. Immersed in the5wt.%NaCl solution, its corrosion current density is only8.012μA/cm2and its corrosion weight loss rate is just0.243mg/(cm2*day) which is better than AZ91DMg alloy. |
PDF Full Text Request