磁场、环境耦合作用下Fe-Ga合金应力腐蚀的规律及机理

Considerable attention has been paid to the new set of magnetostrictive Fe-Ga alloy recently by developers because of its attractive combination of disable performance. Fe-Ga alloys exhibit bout 200~400ppm saturated magnetostrictive strain, which lies between traditional magnetostrictive strain materials, about 20ppm for Fe and Manganese-Zinc ferrite ceramics, and super magnetostrictive strain material, such as above 5.2% for Ni2MnGa. However, the study of its environmental performance reliability and damage mechanism is much backward when compared with development, even few people was involved. There are three main factors that may do damage to magnetostrictive Fe-Ga alloy during its spanning service life: magnetic field, stress and environmental factors such as hydrogen and humid air. The purpose of this proposal is to figure out the influence of magnetic field to fracture toughness of Fe-Ga alloy and related dynamics process, the role of magnetic field playing in hydrogen transport and chemical reaction process, the effect of magnetic field causing on the two competitive process (metal passivation and passive film rupture) which determines the susceptibility of stress corrosion cracking. Based on the above results, the further aims of this project are to analyze the damage behavior and mechanism for Fe-Ga alloy under the coupling effect of magnetic field, stress and environmental factors, and to demonstrate the key controlling factors of how the magnetic field influence the corrosion process and electrochemical process for magnetostrictive Fe-Ga alloy.

Fe-Ga合金是近十年发展起来的一种令人兴奋的具有良好综合性能的新型磁致伸缩材料，广泛吸引了磁性研究工作者和应用器件开发者的高度关注。其饱和磁致伸缩应变约200-400ppm，介于传统磁致伸缩材料(约20ppm)和稀土超磁致伸缩材料(如NiMnGa，5.2%)之间。但是材料的应力腐蚀及氢致开裂等环境服役性能损伤研究几乎无人涉及。本文针对磁场、应力和环境(如湿空气、氢)等因素，对Fe-Ga合金应力腐蚀性能进行研究：即在首先弄清其在典型环境中应力腐蚀主控机制、弄清磁场对合金断裂韧性有无影响及影响动力学、弄清磁场对氢在材料中的扩散输运及化学反应过程、弄清磁场对这种磁致伸缩材料在典型环境中钝化和膜破裂两个相互竞争过程的影响的基础上，确定磁场影响Fe-Ga磁致伸缩材料应力腐蚀电化学过程的关键控制因素，明确磁场、应力、环境因素耦合作用下Fe-Ga合金应力腐蚀的规律及机理。

铁镓合金是介于传统磁致伸缩材料和巨磁致伸缩材料之间的一种新型磁致伸缩材料，已经用于试制新型水声换能器，由于其较低的饱和磁场和良好的综合力学性能，近年受到国内外学者的广泛关注。目前对Fe-Ga合金的研究主要集中在其磁致伸缩性能和微结构等方面，对其服役性能的研究鲜见报道。正是在此背景下，本项目研究了Fe-Ga合金在模拟海水中的腐蚀和应力腐蚀行为，不仅对了解这种新型材料的服役性能有重要的实用价值，同时对了解磁致伸缩材料的应力腐蚀机理有重要的理论意义。.本项目采用恒载荷试验研究了其在模拟海水中的滞后开裂，发现Fe-Ga合金在低载荷下能够发生滞后断裂，即能够发生应力腐蚀。铸态Fe85Ga15发生应力腐蚀的门槛应力为0.34b。进一步研究发现，恒载荷断裂时间随阴极极化而升高，随阳极极化而降低，表明其应力腐蚀机制为阳极溶解型。慢应变速率拉伸显示，当应变速率在10-5/s和10-7/s之间时，强度和延伸率均降低，并且在510-7/s附近应力腐蚀敏感性有极大值。 采用恒位移加载方式原位观察了Fe-Ga合金预裂纹的扩展，发现由于腐蚀造成裂纹尖端应力松弛，因而预裂纹在NaCl溶液中不发生滞后扩展。 采用浸泡失重法和慢应变速率拉伸研究了磁场的影响。发现由于磁场梯度可以移除腐蚀坑底部的顺磁性物质如FeCl2等，因此平行磁场可以减轻Fe-Ga合金在NaCl溶液中的腐蚀。不同方向磁场对Fe-Ga合金在NaCl溶液中的应力腐蚀敏感性的影响不同：横向磁场基本不产生影响，法向磁场降低其应力腐蚀敏感性，而轴向磁场却明显抑制其应力腐蚀。轴向磁场在试样表面微裂纹内产生指向裂尖的场梯度力，阻碍Fe2+离开裂尖，从而减缓Fe的进一步溶解。 电化学充氢结合原位观察确定了Fe-Ga合金产生氢鼓泡的临界氢浓度约为0.21ppm。利用压痕方法观察了氢对塑性区的影响，发现Fe-Ga合金试样表面的压痕塑性区和压痕下方的塑性区在充氢前后均无明显变化，表明氢对Fe-Ga合金的局部塑性变形没有明显影响。这可能与该材料自身的本征脆性有关。.