活化剂对渗铬层结构和成分的影响

Effect of activators on the microstructure and composition of chromizing coatings in pack cementation method

采用粉末包埋法,分别以氯化铵、氯化钠、氯化镍为活化剂在镍基高温合金上于950℃进行了渗铬处理。扫描电镜和能谱分析结果表明,以氯化铵或氯化钠为活化剂得到的渗铬层以向外生长模式为主,但前者在与基体界面处夹杂氮/氧化物颗粒较多,同时含有高达约11.4%(质量分数)的Fe杂质,而后者在与基体界面处夹杂较少,渗铬层Fe含量仅为约1.4%(质量分数),远低于前者。以氯化镍为活化剂得到的渗铬层则以向内扩散为主,与基体无明显界面,但Ni及杂质Fe(19.7%(质量分数))含量最高。氯化钠表现出净化渗铬层成分的作用,而氯化镍具有增强与基体互扩散的作用,因此,活化剂种类对渗铬层成分及结构具有重要影响。

Chromizing of superalloys at 950 ℃ by pack cementation with three different activators of ammonia chloride,sodium chloride and nickel dichloride was investigated. For the ammonia chloride activated system,outward growth of the chromized coating was observed. The coating contained a relative high amount of Fe impurities of 11. 4wt%. Besides,a large amount of nitride / oxide impurities were embedded at the coating / substrate interface. For the sodium chloride activated system,similar outward growth manner of the coating was observed. But the Fe content decreased to about 1.4wt%. Meanwhile,less oxide impurities were observed at the coating / substrate interface. For the nickel dichloride activated system,the chromized coating grew inwardly. No apparent coating / substrate interface can be identified. And the coating contained much more Ni and Fe. These results indicate that sodium chloride is able to purify the chromized coating,while the nickel dichloride can enhance interdiffusion between the coating and substrate. Hence,activators play an essential role in optimizing the microstructure and composition of chromized coatings in pack cementation method.