Mg-Al系合金熔体碳质孕育晶核的“免毒化”效应及机制研究

Mg-Al alloys are important structural metallic materials with low density. Grain refinement is a key technology to improve the mechanical properties and carbon inoculation is an effective approach to reduce the grain size. However, there still exists the problem that the carbonaceous nuclei are easily poisoned by Fe(Mn). We found that Ca(Sr) can prevent such poisoning and demonstrate poisoning-free effect in some cases. The exisitence of Ca(Sr) is beneficial to the formation and stability of the nucleating particles with dual-phased strucutre of Al-Fe(Mn)→Al-C. These particles can act as potent nuclei for Mg grains. However, the influence mechanism of Ca(Sr) in the foramtion and evolution of poisoing-free nuclei is still unclear. This project aims to solve the poisoning problem of Fe(Mn) in Mg-Al alloys inoculated by carbon. The Mg-Al melt containing Fe(Mn) trace elements will be modified by the addition of carbon and Ca(Sr). The detailed structure of carbonaceous nuclei and the interface characteristic will be studied by TEM and HRTEM. The mechanism for the interacting effects of Fe(Mn) and Ca(Sr) on the carbonaceous nuclei structure and the evolution of their interfaces with Mg matrix will be deeply investigated. The physical and chemical characteristics of the interfaces between carbonaceous nucleus and Mg matrix, and some related interfaces will be investigated by first-principles study. The formation Gibbs free energy of these dual-phased particles with poisoning-free effect will be calculated based on the adsorbing thermodynamics theory. The detailed mechanism for the “poisoning-free” effects of Ca(Sr) on the carbonaceous nuclei and their high stability will be disclosed from the viewpoint of structure and energy. The relations between solidification features of the carbon-inoculated Mg-Al melt and the nucleating activity of inoculating nuclei will be systematiclly investigated by computer-aided cooling curve analysis system. This research will lay theoretical foundation for the “poisoning-free” effect of the carbonaceous nuclei resulting from Fe(Mn) in carbon-inoculated Mg-Al alloy. The achievement of this project is of significance to develop grain refiners for magnesium alloys and speed up the application of carbon inoculation technique in the magnesium industry.

Mg-Al系合金是重要的轻质金属结构材料，晶粒细化可显著改善其力学性能，其中碳质孕育细化最为有效，但其晶核易遭Fe(Mn)“毒化”问题仍未有效解决。我们前期研究发现Ca(Sr)可使碳质晶核免遭Fe(Mn)“毒化”，促进Al-Fe(Mn)→Al-C双相有效晶核生成且具有高的结构稳定性，但其作用机制尚未深入揭示。本课题拟深入揭示双相晶核的精细结构特征，探索Fe(Mn)和Ca(Sr)在双相晶核生成及界面结构演变过程中的作用规律。基于第一性原理计算和吸附热力学基本理论，深入揭示Ca(Sr)促进“免毒化”双相晶核生成和稳定性的热力学条件和动力学机制，从结构和能量角度探索Ca(Sr)实现碳质孕育晶核“免毒化”与结构高稳定的内在机制。系统研究碳质晶核活性和凝固特性之间的关系规律。本研究可为Mg-Al系合金碳质晶核“免毒化”奠定一定的理论基础，对镁合金晶粒细化剂开发及碳质孕育技术推广具有参考价值。

基于碳质孕育实现Mg-Al系合金晶粒细化是提高其力学性能的有效手段，但碳质孕育晶核易遭Fe(Mn)“毒化”是影响细化效果稳定性和推广应用的主要瓶颈。研究发现Ca(Sr)可使碳质晶核免遭Fe(Mn)“毒化”，项目系统研了究工艺条件和晶核“免毒化”以及熔体凝固特性之间的关系，紧紧围绕Ca(Sr)在高稳定性“免毒化”晶核的生成与演变过程、晶核结构特征和稳定化热力学机制开展研究工作。. 碳和Ca(Sr)复合孕育Mg-Al熔体具有优异的协同细化作用，未发生明显的孕育衰退。Fe(Mn)合金化对复合孕育细化效果无明显影响，Ca(Cr)可有效抑制Fe(Mn)对碳质孕育细化效果和孕育衰退的不利影响。Fe和Mn对碳质晶核结构特征的影响存在差异，除均存在Al-C异质晶核外，含Fe熔体中还存在Al-C包覆Al-Fe相的具有双相结构特征的有效晶核，而含Mn熔体中发现在Al-Mn相表面存在非晶碳膜。基于相互依存理论构建Ca含量、成分过冷、晶核尺度和晶粒尺寸间关系的协同细化模型，Ca可降低有效晶核尺度，促进更多颗粒作为有效晶核。构建热力学模型分析并揭示了两种类型颗粒形成的热力学条件和竞争机制。热力学计算结果表明，双相颗粒和Al-C团簇颗粒均能原位自发形成，且孕育保温时间的延长，吸附形成双相颗粒比团聚长大的趋势更为明显。理论计算与分析结果与实验观测结果吻合较好。Ca(Sr)的加入降低了形成双相颗粒的阻力，促进了双相颗粒的形成。. 首次发现碳质孕育合金中Al8Mn5表面吸附非晶C层，该非晶层可显著降低了α-Mg与Al8Mn5间的错配度，从而有利于形核。提出一种碳质孕育新机制：Al8Mn5→非晶碳层→α-Mg的“双相”异质形核机制。第一性原理计算表明Ca吸附可提高Mg在碳质晶核表面吸附的吸附能，并可促进Mg朝表面持续吸附，从而促进形核。孕育熔体凝固热分析表明形核过冷度与合金的晶粒细化效果成正相关，形核过冷度值越大，晶粒细化效果越显著。.研究证实碳和Ca(Sr)复合可高效细化Mg-Al合金，并实现碳质晶核“免毒化”，基于热力学深入揭示Ca(Sr)对晶核生成和结构稳定性的内在机制，发现晶核活性与形核过冷度成正相关。研究结果对镁合金晶粒细化剂设计与开发，碳质孕育技术推广和孕育细化效果在线判断具有具有积极的指导意义和参考价值。