稀土La在纳米晶(W,Ni,Fe,La)复合粉末烧结过程中的作用机理

Influencing Mechanism of Rare Earth La on the Nanocrystalline (W, Ni, Fe, La) Composite Powders during Sintering

采用溶胶-喷雾干燥法制备纳米晶(W,Ni,Fe,La)复合粉末,研究了粉末在烧结过程中La对抑制鼓泡和晶粒长大的作用机理,通过XRD与SEM分析了La在烧结过程中的相转变规律和在合金中的存在形式,讨论了稀土La对合金液相烧结过程中扩散的影响。结果表明:稀土La在纳米复合粉末中主要以La2WO6与La4W2O15的形式存在。经液相烧结后,稀土La主要以二次相颗粒的形式分布于粘结相中,生成高温下能稳定存在的La4W2O15相,该相对杂质元素Ca、O具有很好的亲和力,起到晶界净化和晶内净化的作用。同时二次相颗粒存在于粘结相中,抑制了W在粘结相中的扩散,降低了W在粘结相中的溶解度,使得液相烧结溶解-析出过程减慢,从而抑制液相烧结阶段的晶粒长大和鼓泡现象。

Nanocrystalline （W, Ni, Fe, La） composite powders were prepared by a multi-step process, consisting of sol-spray drying, calcination and subsequent hydrogen reduction process. The mechanism of the influence of La on grain growth and bubbling inhabitation during the liquid-phase sintering of the nanocrystalline （W, Ni, Fe, La） composite powders were studied. The phase transformation involved with La during the sintering of the composite powders was investigated. The distribution and existing form of La in the alloy were studied. The results show that the La2WO6 and La4W2O15 are formed in the synthesized nanocrystalline powders. In the sintered alloy, the rare-earth La is dispersed in the γ-（Ni, Fe） phase and converts into the La4W2O15 phase which can be stable at a high temperature. The La4W2O15 phase has a strong affinity with the impurities of Ca and O elements, playing the roles in purifying the grain boundaries and intracrystalline. The diffusion of W into the γ-（Ni, Fe） phase during liquid-phase sintering is inhibited and thus the solution of W in γ-（Ni, Fe） phase is significantly reduced by the second-phase particles, which slows down the solution-precipitation process and inhibits the grain growth as well as bubbling during liquid-phase sintering process.