形状记忆合金的弹性和马氏体相变的第一性原理研究

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	形状记忆合金的弹性和马氏体相变的第一性原理研究
	李春梅
学位类型	博士
导师	杨锐 ; 胡青苗
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	形状记忆合金马氏体相变弹性常数占位剪切模量形状记忆合金马氏体相变弹性常数占位剪切模量
其他摘要	" 本文主要运用第一原理Exact Muffin-tin orbital（EMTO）方法结合相干势近似（coherent potential approximation，CPA），研究了形状记忆合金Ni-Mn-Ga和In-Tl的弹性和相的稳定性。本文首先很好地验证了标准化学计量比Ni2MnGa和纯金属In在奥氏体和马氏体两相的物理性质，理论上获得的晶格常数和弹性常数与实验值及其他理论结果十分吻合。其次，通过对合金在不同原子占位情况下自由能的计算和比较，我们建构了非化学计量比Ni-Mn-Ga和Ni-Mn-Ga-X(X=Fe，Co和Cu)四元合金中的原子稳定占位构型，我们发现，并非在所有情况下过量组分的原子都要占据在 “正常” 位置上，也就是占据在不足组分的原子晶格位置上。例如：在Ga过量的Ni-Mn-Ga合金，Ga原子则优先占据在Mn原子晶格位置上；对于Co引入的四元合金，Co原子只倾向于占据在Ni原子晶格位置上；对于Fe加入Ga不足的合金，“正常” 占位和Fe占据在Ni（或Mn）原子晶格上，然后被取代的Ni（或Mn）原子占据在Ga晶格上的情况则具有同样的结构稳定性。基于建立起来的原子稳定占位构型和平衡状态下的基本参数，本文最后对合金化效应对In-Tl、Ni-Mn-Ga和Ni-Mn-Ga-X(X=Fe，Co和Cu)合金的0 K下弹性常数的影响做了计算研究，并且同时重点研究了立方相下四方剪切模量随成分的变化，结果发现，在In-Tl中，随着Tl含量的增加立方相剪切模量C′值逐渐增大而四方相C′值逐渐减小，在30% Tl附近，立方相C′由负数转变为正值。该理论结果验证了实验上马氏体转变温度TM随Tl含量增加而减小的趋势，并且在20%—30% Tl左右，In-Tl合金马氏体相变发生在室温附近；对于大多数Ni-Mn-Ga和Ni-Mn-Ga-X(X=Fe，Co和Cu)合金，随着平均每个合金原子的价电子数e/a的增加，剪切模量C′通常则逐渐减小，而TM的实验值逐渐升高。即：剪切模量C′可以看作是衡量这些形状记忆合金TM值大小的因子，通常较小的C′值对应着较高的TM值。然而，对于Ni2Mn1+xGa1-x和Ni2Mn1-xGaFex两个合金体系，他们则完全背离这种通常的变化趋势。进一步分析计算表明，这种背离很可能与合金中磁性原子特殊的磁相互作用有关。 " ; " In this thesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbital (EMTO) method in combination with coherent potential approximation (CPA). We first test the physical properties of standard stoichiometric Ni2MnGa and pure In with austenitc and martensitic phases, respectively, at 0 K. It is shown that their obtained theoretical lattice parameters and elastic constants agree well with the available theoretical and experimental data. Second, by comparing the theoretical free energy of the austenite and martensite, we determine the stable site occupation of the alloying atoms in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-X(X=Fe，Co和Cu) quaternary alloys. It is found that not all alloying atoms prefer to occupying the ‘‘normal’’ site, i.e., the deficient atoms’ site. For example, In Ga-rich Ni-Mn-Ga alloys, Ga atoms prefer to only occupying on Mn sublattice, no matter if Mn sublattice is occupied or not; For Co doped quaternary alloys, no matter which of atom among Ni, Mn, and Ga is deficient, Co always occupies on Ni sublattice; For Fe doped and Ga-deficient alloys, with the ‘‘normal’’ site occupation, as well as with Fe occupying on Ni (or Mn) sublattice and the replaced Ni (or Mn) atoms occupying Ga sublattice, the system has the same stability. Based on the theoretical determination of the stable site occupation and the equilibrium parameters, we finally investigate the alloying effect on the 0 K elastic properties of In-Tl、Ni-Mn-Ga and Ni-Mn-Ga-X(X=Fe, Co, and Cu) alloys, and at the same time, we put our emphasis on the investigation of the composition dependence of shear modulus C′=1/2(C11-C12). It is found that in In-Tl alloy, with the Tl addition, C′ in cubic phase gradually increases but it decreases in tetragonal phase, and around 30% Tl, C′ in cubic phase transits from negative to positive, which just confirms the experimental result that TM decreases with the Tl concentration increasing, and around 20%-30% Tl, the MT of In-Tl occurs around room temperature. For most of the off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-X(X=Fe, Co, and Cu) alloys, with the average valence electrons number per atom e/a increasing, shear modulus C′ generally decreases, whereas the experimental TM is enhanced. It is thus found in the present thesis that the shear modulus C′ could be seen as a factor of the TM of these shape memory alloys, generally a smaller C′ corresponds to a higher TM. However, Ni2Mn1+xGa1-x and Ni2Mn1-xGaFex two groups of alloys are exceptions, where the opposite trend of change is true, i.e., TM and C′ are directly proportional. By our furthermore calculation and analyzation, this deviation is ascribed to the special magnetic interaction between magnetic atoms in these alloys."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64435
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	李春梅. 形状记忆合金的弹性和马氏体相变的第一性原理研究[D]. 北京. 中国科学院金属研究所,2012.