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Recent years have seen extensive investigations on Fe-based alloys proposed as degradable biomaterials for temporary biomedical implants. Among all the Fe-based degradable biomaterials (DBMs), Fe-Mn-Si alloys are considered as promising candidates due to their excellent mechanical properties, good biocompatibility and degradability in a physiological environment. All the investigated Fe-based DBMs including Fe-Mn-Si alloys, however, present a limitation of low degradation rate. This inhibits the further application of Fe-Mn-Si alloys as DBMs. Powder metallurgy (PM) may provide a practical way to overcome the low degradation rate of the currently developed Fe-Mn-Si DBMs. This is because PM as a nearnet- shape technique presents the ability to synthesis porous products with controlled porosities. The porous structures in the PM Fe-Mn-Si alloys enable to provide more areas for degradation, and thereby improving the degradation rate. So far, Fe-Mn-Si alloys are traditionally fabricated by the cast/wrought route, and little work has been discussed in terms of the powder sintering behaviours of Fe-Mn-Si alloys. This is another motivation to explore the sintering behaviour of Fe-Mn-Si alloys. In this research, mechanical milling (MM) was employed to produce Fe-Mn-Si powder mixtures with homogeneous dispersion and fine microstructure, while the traditional press-and-sinter route was applied to densify the Fe-Mn- Si compacts. A series of Fe-28Mn-xSi alloys (wt.%, x = 0, 1, 2, 3, 4) produced via the PM route were compared in terms of degradation rate, densification and mechanical properties in order to identify the optimal Si content in the ternary Fe-Mn-Si DBMs. It was found that the degradation rate, densification and mechanical properties largely depended on the Si content in the sintered Fe-Mn-Si compacts. The degradation rate of the sintered Fe-Mn-Si alloys decreased with the increase of Si content. The density of the sintered Fe-Mn-Si alloys increased significantly with increasing Si content. With an increase in Si content, the tensile strength dramatically increased while modulus of elasticity increased only mildly. Based on the above results, the PM Fe-28Mn-3Si alloy is thought to be a good candidate for DBMs. Both MM and blended element (BE) Fe-28Mn-3Si powder mixtures were pressed and sintered at four different sintering temperatures (i.e., 1000, 1100, 1200 and 1300℃) for 3h. The density of the MM alloys increased drastically with the increase of sintering temperature. By contrast, no densification occurred in the BE alloys at all sintering temperatures. The phase constituents of the sintered BE alloys varied with the sintering temperatures. Predominant γ-austenite and minor ε-martensite were observed in the BE alloys sintered at temperatures≤1200 °C. However, Mn3Si intermetallic phase was identified in the BE alloy sintered at 1000 °C. Interestingly, only a single α-Fe phase is detected in the BE compacts when the sintering temperature increased to 1300°C. The MM alloys are comprised of duplex phases at all sintering temperatures: main γ-austenite along with ε-martensite. The pore size of MM alloys decreased significantly with increasing sintering temperature, while high sintering temperatures (≥ 1200°C) led to the formation of large pores (~150μm in size) in BE alloys. The mechanical properties of MM alloys, were remarkably higher than their BE counterparts. Mn depletion region (MDR) was identified on the surface of both the BE and MM alloys. The thickness of the MDR layer increased with the increase of sintering temperature. The Mn concentration increased parabolically with the increase of the distance from the surface (x=0) of the sintered alloys until stabilizing at ~ 27wt.%. The thickness of MDR layer in the sintered BE compacts was significantly larger than their MM counterparts. The sublimation of Mn in the sintered alloys was largely responsible for the weight loss of all the sintered compacts. The weight loss of all the sintered alloys increased significantly with the increase of sintering temperature. The weight loss of the MM alloys was much lower than their BE counterparts. Fe-28Mn-3Si alloys sintered at 1200℃ for various isothermal time (i.e., 0, 1h, 2h, 3h) were fabricated. The isothermal time dependence of densification, mechanical properties and Mn sublimation was studied. It was found that the densification and weight loss of the sintered Fe-Mn-Si alloys mainly occurred at the first hour of isothermal holding. The MDR region increased with the increase of isothermal time. It was also revealed that a long isothermal time (>1h) has little influence in the mechanical properties. The mechanical properties of the sintered Fe-28Mn-based samples as a function of immersion time in SBF solutions were evaluated. In addition, a preliminary biocompatibility test was performed for the sintered alloys. A severe deterioration in mechanical properties has been found in both Fe-Mn and Fe-Mn-Si alloys after being soaked for ≥ 45 days. The cell viability test demonstrated that both Si-free and Si-containing alloys had no inhibition on the proliferation of L929 cells. |
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