| As precipitation strengthening Mg alloys, Mg-Y-Nd alloys have high strengths at room temperature, excellent heat resistance at elevated temperature and good biocompatibility. Therefore, Mg-Y-Nd alloys have been used in aerospace and aeroplane, automobile and biomedical materials fields. However, Mg-Y-Nd alloys show poor ductility due to their intrinsic HCP crystal structure, which limits the formability and industrial applications to some extent. Friction stir processing（FSP） is a novel severe plastic deformation technique for microstructure refinement, which have great potential to produce metal materials with high ductility through grain refinement. With the help of cooling equipments for hindering the grain growth during FSP, submerged friction stir processing（SFSP）, i.e. FSP conducted underwater, has been developed most recently.In this study, cast Mg-Y-Nd alloy was selected as base metal and subjected to normal FSP（NFSP） and SFSP. Microstructure evolution during FSP, effect of aging treatment on mechanical properties of NFSP specimens, superplastic behavior of SFSP specimens, corrosion of FSP specimens in simulated body fluid in vitro were investigated. The aims are（a） to provide key technologies for enhancing the mechanical properties and improving the formability of Mg-Y-Nd alloys;（b） to provide experimental basics for preparing biomedical materials by FSP.Firstly, Mg-Y-Nd cast alloy is subjected to different processing parameters by NFSP and SFSP, respectively. After FSP, the microstructure of Mg-Y-Nd alloy is significantly refined and the mechanical properties are greatly enhanced. Compared with SFSP specimen, the width of stirred zone（SZ） and the average grain size in NFSP specimen are both increased, which are attributed to much more heat input during NFSP. In addition, band structures are shown in NFSP specimen. Due to the cooling effect by water, much finer-grained Mg-Y-Nd alloy is obtained after SFSP, and the comprehensive mechanical properties of SFSP specimen significantly increase. The optimal processing parameter underwater is 600 rpm /60 mm/min.Secondly, according to the thermal histories, solution and precipitation behaviors of Mg-Y-Nd casting during NFSP are investigated. The NFSP specimens are subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 54 h for 150 ℃ and 30 h for 180 ℃. Strengths of the peak-aged specimens are further increased, which is attributed to the large quantity of β′′ and β1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased, since more crack nucleation sites are provided with the increased precipitates. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of NFSP and peak-aged specimens are all mixed ductile-brittle fracture mode.Thirdly, microstructure evolution and superplastic behavior of SFSP specimen are investigated in the temperature ranges of 683-758 K and the strain rate ranges from 1×10-1 to 4×10(-40 s-1. Due to the fine-grained and thermal-stable microstructure, the maximum elongation of 967% is obtained at 733 K and 3×10-3 s-1; excellent high strain rate superplacticity of 900% was achieved at 758 K and 2×10-2 s-1. Moreover, elongations are all above 500% within a wide high temperature range of 708-758 K at 1×10-1 s-1. Because of the good deformation compatibility, cavities are easily formed at the grain boundaries instead of the interface between particles and matrix. Grain boundary sliding accommodated by lattice diffusion is the dominated deformation mechanism during superplastic deformation, and the specimens failed through cavity coalescence.At last, the effect of FSP on corrosion behavior of Mg-Y-Nd casting in simulated body fluid（SBF） is investigated. The electrochemical and immersion corrosion tests both indicate that the corrosion resistance of NFSP and SFSP specimens is improved compared to the cast counterpart. Micro-galvanic corrosion is the dominated corrosion behavior for as-cast and FSP specimens. Due to the coarse microstructure, the separation of grains and second phases leads to more underneath α-Mg matrix exposed in SBF, resulting in seriously localized corrosion morphology is shown in as-cast specimen. On the other hand, the uniform corrosion morphology is observed on NFSP and SFSP specimens with fine-grained and homogeneous microstructure. In addition, after immersion in SBF for 5 days, the maximum tensile load for as-cast specimen is significantly decreases to 20% of the original value. While the maximum tensile load of NFSP and SFSP specimens is still 23% and 29% of the original value after immersion in SBF for 12 days, respectively, which is attributed to the improved corrosion resistance and uniform corrosion behavior after FSP. |
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