Size effects on the martensitic phase transformation of NiTi nanograins
Size effects on the martensitic phase transformation of NiTi nanograins
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DOI：
10.1016/j.jmps.2006.06.006
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发表时间：
2007-02
影响因子：
5.3
通讯作者：
T. Waitz;T. Antretter;F. Fischer;N. Simha;H. Karnthaler
中科院分区：
文献类型：
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作者：
T. Waitz;T. Antretter;F. Fischer;N. Simha;H. Karnthaler
透射电子显微镜（TEM）分析表明，纳米晶NiTi的马氏体相变是通过形成原子级孪晶进行的。尺寸小于约50nm的晶粒即使在大的过冷度下也不会转变为马氏体。对这些现象进行了系统的研究，阐明了晶粒尺寸对相变能垒的影响。在实验的基础上，将纳米晶粒模拟为含有（001）复合孪晶马氏体的球形夹杂物。将夹杂物的变形应变分解为剪切本征应变和法向本征应变，便于分析计算剪切和法向应变能与晶粒尺寸、孪晶层宽度和弹性性能的关系。对于特殊情况，应力计算采用解析法，否则采用数值法。从孪晶层到孪晶层交替的剪切应力集中在晶界处，导致对应变能的贡献与夹杂物的表面积成比例，而由转变应变的法向分量和温度依赖的化学自由能引起的应变能与夹杂物的体积成比例。在纳米晶粒中，计算这些不同的能量贡献，从而可以预测临界晶粒尺寸，低于该临界晶粒尺寸，马氏体相变变得不可能。最后，原子级孪晶的实验结果可以解释的分析计算，占的剪切应变和孪晶界能的孪晶带形态的马氏体纳米晶粒的转化相反的贡献。
The analysis of nanocrystalline NiTi by transmission electron microscopy (TEM) shows that the martensitic transformation proceeds by the formation of atomic-scale twins. Grains of a size less than about 50nm do not transform to martensite even upon large undercooling. A systematic investigation of these phenomena was carried out elucidating the influence of the grain size on the energy barrier of the transformation. Based on the experiment, nanograins were modeled as spherical inclusions containing (001) compound twinned martensite. Decomposition of the transformation strains of the inclusions into a shear eigenstrain and a normal eigenstrain facilitates the analytical calculation of shear and normal strain energies in dependence of grain size, twin layer width and elastic properties. Stresses were computed analytically for special cases, otherwise numerically. The shear stresses that alternate from twin layer to twin layer are concentrated at the grain boundaries causing a contribution to the strain energy scaling with the surface area of the inclusion, whereas the strain energy induced by the normal components of the transformation strain and the temperature dependent chemical free energy scale with the volume of the inclusion. In the nanograins these different energy contributions were calculated which allow to predict a critical grain size below which the martensitic transformation becomes unlikely. Finally, the experimental result of the atomic-scale twinning can be explained by analytical calculations that account for the transformation-opposing contributions of the shear strain and the twin boundary energy of the twin-banded morphology of martensitic nanograins.