等通道转角挤压诱发Mg-Zn合金相变及其超塑性变形机制研究

In past years,Magnesium alloys have attracted increasing interest due to their potential as biomedcial implant materials. Magnesium alloy has higher strength than macromolecule polymer bioengineering materials such as polylactide,by which Magnesium alloy could be made into more kinds of sizes and specifications implant medical apparatus. Among Mg alloys, Mg-Zn alloy has a lower biological toxicity so that it is regarded as a more promising bioengineering material than other Mg alloys. However, for its poor ductility, it is difficulty to manufacture Mg alloy into delicate medical apparatus which obstruct the applications of Mg alloy in biology medical field. Relatively, superplastic forming has superiority in machining finy and delicate Mg alloy products.Thus, the development of superplastic Mg-Zn alloy is very important on the application of biology medical Mg alloy materials. Equal-channel angular pressing (ECAP) is the most typical type of severe plasticity deformation (SPD) technology, which introduce severe strain into materials and effectively refines microstructure, improves mechanical properties and develops superplasticity in magnesium alloys. besides these,in our former research on Mg-Zn-Al alloy, non-equilibrium phase Laves MgZn2 is found in ZA62 Mg alloy processed by multi-pass ECAP pressing , which indicate severe deformation could induce phase transition in Mg-Zn alloy.In this subject,Mg-Zn binery alloy will be processed by SPD methods such as ECAP to refine its microstructure and promote phases with high melting point to precipitateurthermore in order to develop its superplasticity, the forming mechanism and dynamic model of non-equilibrium phase in the procedure of SPD will be investigated.

近年来，镁合金以其在生物医用植入材料领域的潜力成为研究的热点，和高分子生物工程材料相比，镁合金具有更高的强度，可以用来制作更多尺寸和规格的植入器械。在镁合金中，镁-锌系合金具有生物毒性更低的优点，因此在生物医用镁合金的发展中受到更多的关注。然而塑性成形能力差的问题使得利用传统的塑性加工方法将镁合金制备成医用植入器械非常困难。相对来说，超塑性成形对于小型镁合金制件的加工具有独特的优势。因此，开发具有超塑性的Mg-Zn合金对于生物医用镁合金的应用具有重要的意义。以往对Mg-Zn-Al合金的研究发现，剧烈塑性变会引起中间相的析出，并使材料具有明显的超塑性，本项目拟在等通道挤压的基础上，对镁-锌二元合金进行剧烈塑性变形，以期获得具有细小显微组织，并导致具有高热稳定性的中间相析出。在此基础上制备出具有超塑性的镁锌合金，研究其中间相在剧烈塑性变形过程中的形成机制，热力学条件以及动力学模型。

Mg-6Zn合金中初始的组织为α-Mg和分布在晶界上的亚稳定的Mg7Zn3相。经过不同温度下多道次ECAP后发现，Mg-6Zn合金的显微组织中都有椭球状的中间相，椭球状析出物与基体有共格关系。析出相在晶界或者界隅上形核，也可以在晶内形核，在低温下变形时第二相尺寸在100nm-500nm之间，基体的晶粒均匀细小，大小在5微米左右，随着变形温度的升高，第二相的尺寸增大，晶粒尺寸也增加。但是这些中间相并不相同。低温下变形（160℃）ECAP会同时诱发Mg-6Zn合金中过渡相Mg4Zn7析出，随着变形道次的增加Mg4Zn7会逐步转变为Laves MgZn2相，6道次之后Mg4Zn7和MgZn2共存于镁合金中；在中温下变形（200℃）ECAP，4道次之后有MgZn2大量析出，同时有Mg4Zn7存在；在高温下ECAP（240℃），析出相主要是MgZn2。镁锌合金中，Mg4Zn7和MgZn2相都具有较高的稳定性，而MgZn2相的稳定性更高，这就导致镁合金中的Mg7Zn3相转变为稳定性更高的Mg4Zn7和MgZn2相，如果继续提高ECAP道次最终会转变为MgZn2相。.ECAP诱发镁锌合金中析出Mg4Zn7和MgZn相的机制是，经过固溶处理之后初始的晶界相附近形成过饱和固溶体区（SSSS），常规热挤压和ECAP之后，诱发大量位错和再结晶晶界形成，使得析出相在这些位置形核长大，通常会析出过渡相Mg4Zn7和MgZn2, 随着ECAP道次的增加Mg4Zn7逐渐转变为MgZn2。