Metallographic Study on Nucleation of Martensite in Ti-Ni Shape Memory Alloys

1.对几种成分接近Ti-40 Ni-10 Cu（at%）的合金中马氏体的形核和长大进行了原位电子显微镜观察，这些合金在室温附近由B2母相热弹性转变为正交马氏体。我们发现马氏体在某些应力集中的地方形核，如位错缠结、表面台阶等。我们也尝试了高分辨电子显微镜实验，虽然获得了相变后马氏体的高分辨图像，但由于相变引起的图像漂移，至今未能成功地原位观察到形核.用光学显微镜和电子显微镜研究了Ti-Ni-Fe和Ti-Ni-Al形状记忆合金中R相的转变和自适应机制。利用电视摄像机成功地用录像带记录了转化事件的序列。我们发现，R-相变发生的形核和生长的异质方式。形核发生在应力集中的地方，如基体-夹杂物界面和位错。R相可以从单个位错的应力场形核。并对自适应机制进行了探讨.用会聚束电子衍射法测得R相的空间群为P31 m。然而，这一结果似乎有些模糊，因为观测在几何上受到相当大的限制。本研究的目的是研究R相的晶体结构。为了观察室温下的R相，我们选择Ti-47.8Ni-1.5Fe（at%）合金，并进行固溶处理（1273 K，3.6ks.，I.Q.）。通过电子显微镜获得了单一变体R-相中各个带轴的衍射图案。对P3模型和P31 m模型的衍射图进行了强度比较。观测结果都支持P3空间群而不是P31 m空间群。

1. In situ electron microscope observations were made on the nucleation and growth of martensite in a few alloys near the composition Ti-40Ni-10Cu(at%) which transform thermoelastically from a B2 parent to an orthorhombic martensite near room temperature. We found that martensite nucleate at certain stress concentrated places, e.g. dislocation tangles, surface steps. We alsotried high resolution electron microscopy experiments, but were so far no success in observing in situ the nucleation due to the image drift caused by the transformation, although we obtained high resolution image of martensite after the transformation.2. Transformation and self-accommodation mechanisms of the R-phase in Ti-Ni-Fe and Ti-Ni-Al shape memory alloys were studied by means of optical and electron microscope observations. Sequences of transformation events were recorded successfully with video tapes using a TV camera. We found that the R-phase transformation takes place by nucleation and growth in a heterogeneous manner. Nucleation takes place at stress concentrated places, e.g. matrixinclusion interfaces and dislocation. The R-phase can nucleate from the stress field of a single dislocation. Self-accommodation mechanism was also discussed.3. The space group of the R-phase was reported to be P31m by means of convergent beam electron diffraction. However, there seemed to be some ambiguity about this result, because observations were rather limited geometrically. The purpose of this study is to investigate the crystal structure of the R-phase. To observe the R-phase at room temperature, we chose Ti-47.8Ni-1.5Fe(at%) alloy and made a solution treatment(1273K, 3.6ks., I.Q.). Diffraction patterns from various zone axes in single variant R-phase were obtained by electron microscopy. Comparison of the intensities in diffraction patters were made between P3 model and P31m model. The observation results all support the space group P3 rather than P31m.