| Zn-Mn alloy has great potential in degradable bone implants due to its good degradation behavior and excellent osteogenic ability.However,due to the low solid solubility of Mn in Zn,the alloyed Mn content of Zn-Mn alloy prepared by conventional process needs to be strictly limited at room temperature,otherwise a large amount of alloying will easily lead to the formation of brittle second phase,thus affecting the mechanical properties of the alloy.Therefore,in this paper,we propose to prepare non-equilibrium Zn-Mn supersaturated solid solution powders by utilizing the non-equilibrium properties of mechanical alloying(MA)and forcibly increasing the solid solubility limit of Mn in Zn.Then,the non-equilibrium structure was maintained through the rapid solidification characteristics of over-selective laser melting to prepare a supersaturated Zn-Mn alloy,thereby improving the mechanical properties of the alloy.The main research contents and innovations of this paper are as follows:The effect of ball milling parameters on the properties of Zn-Mn alloy powders during MA process was studied.During MA process,zn-Mn prealloy powder will undergo a series of phase transitions under the action of external energy.The ball milling time is too long,the rotating speed is too high,the energy absorbed by powder is too high,the welding between powder is serious,and the unbalanced meeting generated in MA process changes to stable phase.With short milling time,low rotational speed and less energy absorption,the powder can maintain better crystal structure integrity,but is not conducive to the diffusion of solute atoms.At the same time,the proper amount of process control agent(PCA)can effectively hinder the welding between powders and improve the powder yield.Finally,a set of optimal milling parameters were obtained:milling time 15 h,milling speed 300 r/min,PCA 2 wt.%anhydrous ethanolThe effect of different Mn content on Zn-Mn supersaturated solid solution powder and its supersaturated solid solution mechanism were studied.It is found that the non-equilibrium and coercivity of MA can greatly improve the solution limit of Mn in the matrix Zn lattice at room temperature,and Zn-Mn supersaturated solid solution can be prepared.And with the increase of Mn content,the content of supersaturated solid solution in Zn-Mn alloy powder increases,the embedding phenomenon between powder particles increases,and the size of powder particles increases.The results of the thermodynamic and kinetic analyses show that the expansion of Mn solid solubility in Zn lattice is due to the existence of high-density dislocation and the increase of grain boundary energy.The maintenance mechanism of Zn-Mn supersaturated solid solution during SLM forming was studied.The extremely high cooling rate of SLM process(10~6～10~8 K/s)will increase the solute distribution coefficient at solid-liquid interface,which leads to a large number of Mn atoms dissolved in the molten pool and dissolved in the matrix Zn lattice,thus maintaining the supersaturated solution of Mn in the matrix Zn lattice.As a result,the XRD peak(101)of Zn in the Zn-Mn alloy prepared by SLM is only slightly offset compared with the Supersaturated solid solution powder.The mechanical properties and strengthening mechanism of supersaturated Zn-Mn alloy processed by MA and SLM were studied.It is found that the supersaturated solid solution of Mn in the alloy sample and the refinement of the alloy grain increase the hindrance to dislocation movement and cause dislocation accumulation.A higher stress level is required to make dislocation move again,thus improving the mechanical properties of the alloy.When Mn content is 15 wt.%,the compressive yield strength of Zn-15Mn alloy with MA(178.16±11.51 MPa)and microhardness(90.69±6.53 HV)are significantly improved compared with Zn-15Mn alloy without MA.In addition,electrochemical and in vitro cell tests of Zn-Mn alloy showed that Zn-Mn alloy showed a moderate degradation rate and good cellular compatibility. |
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