Ni-Mn-Ga-Ti鐵磁形狀記憶合金的相穩定性和磁性能的第一性原理計算

Ni-Mn-Ga-Ti鐵磁形狀記憶合金的相穩定性和磁性能的第一性原理計算Phase stability and magnetic properties of Ni-Mn-Ga-Ti ferromagnetic shape memory alloys form first-principles calculations

以Ni-Mn-Ga合金為代表的鐵磁形狀記憶合金是新型的智能材料，具有高達10 %的磁誘導應變以及kHz級別的響應頻率。適量Ti摻雜可以析出Ni3Ti第二相而顯著提高壓縮強度和延展性，從而改善該類合金的高脆性，提高可加工性能從而加速實用化進程。本文通過第一性原理計算系統地研究了摻雜Ti含量對Ni8Mn4-xGa4Tix (x=0, 0.5, 1, 1.5和2)鐵磁形狀記憶合金相穩定性和磁性能的影響。根據能量最低原理，摻雜的Ti組元優先占據Ni2MnGa合金中的Mn陣點。隨著Ti含量的增加，順磁奧氏體與鐵磁奧氏體相的總能之差減小從本質上導致了實驗觀察到的合金居里溫度TC的降低。隨著Ti含量的逐漸增加，費米面以下自旋向上總電子態密度逐漸降低，而自旋向下的部分幾乎不變，導致自旋向上與自旋向下的電子數之差減小，這是Ti含量增加而合金總磁矩降低的本質原因。本文的計算結果在指導實驗中的成分設計和開發新型磁控形狀記憶合金方面具有重要意義。

In the past two decades, ferromagnetic shape memory alloys (FSMAs) Ni-Mn-Ga had attracted great attention due to their large magnetic field-induced strain (MFIS) and high response frequency. Recent investigations showed that Ti doping can significantly enhance the compressive strength and improve the ductility of Ni-Mn-Ga alloys attributed to the proper amount of Ni3Ti precipitate with appropriate ageing treatment. Therefore, the main purpose of the present paper is to explore the influence of the Ti addition on crystal structure, phase stability, magnetic properties and electronic structures of the Ni8Mn4-xGa4Tix (x=0, 0.5, 1,1.5 and 2) alloys by first-principles calculations, aiming at providing the theoretical data and directions for developing high performance FSMAs in this alloy system. The formation energy results indicate that the doped Ti preferentially occupies the Mn sites in Ni2MnGa alloy. With the increase of Ti content, the optimized lattice parameter of the ferromagnetic austenite increases regularly. For the martensitic phase, the lattice parameter a increases while c decreases, leading to a decreased c/a ratio. The paramagnetic and ferromagnetic austenitic phases both become stable because their Eform gradually decreases with the increasing amount of Ti. The experimentally reported decrease in the Curie temperature with increasing Ti content is derived from the decrease of the total energy difference between the paramagnetic and the ferromagnetic austenite. The total magnetic moment is mainly contributed by Mn, while the magnetic moments of Ga and Ti are nearly zero. The total magnetic moment decreases notably when Mn is gradually substituted by Ti because the atomic magnetic moment of Ti is much less than that of Mn, which is in fair consistent with the experimental observations. The intensity of up-spin total DOS decreased dramatically with the increase of the Ti content; whereas the change of the down-spin part below EF is not obvious. This feature gives rise to the decrease of the total magnetic moments in these alloys. The results of present paper are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.

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