Co、Pd掺杂对NiTiNb记忆合金相变影响的第一原理研究

Influencing of Co,Pd addition on Martensitic Transformation of NiTiNb Shape Memory Alloy: A First-Principle Study

基于第一原理计算方法,通过比较Co、Pd合金化前、后NiTiNb合金能态、电子、态密度等微观结构变化,表征和评判了Co、Pd掺杂对NiTiNb合金相变的影响,结果表明:Co元素在NiTi和NiTi(Nb)相取代Ni位的结合能和形成焓小于其取代Ti位,表明Co元素将以取代Ni位形式固溶,Mulliken布居和电子密度计算显示,Co固溶后将与最近邻Ti原子形成强键作用,导致Co周围Ti原子活性降低,Ni/Ti原子比升高,所以Co合金化后NiTiNb合金Ms反而降低。对于Pd元素,虽然其取代Ti位的形成焓略小于取代Ni位,但NiTi(Pd→Ni)和NiTi(Nb)(Pd→Ni)的结合能均小于Pd取代Ti位,同时Pd→Ni位后固溶相的Fermi能级处电子数减少,Pd成键峰峰值更高、峰型更集中,所以Pd以置换Ni位更稳定,Ni/Ti原子比降低,NiTiNb合金Ms升高。

Using first-principles pseudo-potential plane wave method, the formation enthalpy AH, the binding energy AE and the electronic structure of the NiTiNb shape memory alloy doped with Co, Pd elements have been systematically calculated and analyzed carefully. The results show that the AE and AH of NiTi（Co→Ni） and NiTi（Nb, Co→Ni） models are smaller than those of NiTi（Co→Ti） and NiTi（Nb, Co→Ti）, which means the alloying element Co will replace Ni atom in NiTiNb alloy. However, the Mulliken population and electronic density calculations show that the Co will cohere the nearest neighbor Ti atoms to be confined by strong bonds after solid solution, resulting in the Ni/Ti ratio increasing instead of decreasing by Co→Ni reaction process, then the Ms of NiTiNb（Co） is decreased. To Pd element addition, although the AH of NiTi（Pd→Ti） and NiTi（Nb, Pd→Ti） crystals are bit smaller than that of NiTi（Pd→Ni） and NiTi（Nb, Pd→Ni）, the AE of the Pd replacing Ni models are much smaller than those of Pd replacing Ti. And the Mulliken population results show that some strong metallic bonds appear between Pd and Ti atoms in Pd→Ni solid solution reaction. So the Ms of NiTiNb（Pd） alloy will increase by the addition of Ni/Ti ratio for Pd taking the place of Ni. Then it can be seen, the bond strength between alloying elements and Ni or Ti atoms is the other micro-factor influencing on the ratio of Ni/Ti to the Ms.