The thermal–elastic martensitic transformation from high-temperature Ni₂In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX (M = Ni or Co, and X = Ge or Si) alloys. However, the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear. In this work, the hexagonal–orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated. One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature (M _s). With the the sample cooing to M _s, the laths turn broader, indicating that the martensitic transformation starts from these pre-existing orthorhombic laths. Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal–orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the pre-existing orthorhombic laths. The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent. Therefore, the orthorhombic martensite can take these pre-existing laths as embryos and grow up. This work implies that the martensitic transformation in MnNiSi_1−x (CoNiGe) _x alloy is initiated by orthorhombic embryos.

中文翻译：

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MnNiSi1-x (CoNiGe) x 合金中预先存在的斜方晶胚诱导的六方-斜方马氏体转变

从高温Ni ₂ In型六方结构到低温TiNiSi型斜方结构的热弹性马氏体转变在Mn中得到了广泛的研究MX（中号= 镍或钴，并且X= Ge 或 Si) 合金。然而，斜方马氏体如何在六方母体中形核和生长的答案仍不清楚。在这项工作中，研究了 Co 和 Ge 共取代 MnNiSi 中的六方-斜方马氏体转变。人们可以在高于马氏体转变开始温度的温度下发现一些嵌入六方母体中的正交板条（中号 _）。随着样品冷却至中号 _{s 时}，板条变宽，表明马氏体转变从这些预先存在的斜方板条开始。微观结构观察表明，这些预先存在的斜方晶系板条并非源自六方-斜方晶格马氏体转变，因为六方母体中的掺杂元素与预先存在的斜方晶格板条中的掺杂元素的原子占据之间存在差异。唯象晶体学理论和实验研究证明，预先存在的斜方板条和生成的斜方马氏体与六方母体具有相同的晶体学关系。因此，斜方马氏体可以将这些预先存在的板条作为胚胎并长大。这项工作意味着 MnNiSi _{1−中的马氏体转变X} (钴镍锗) _X 合金是由斜方晶胚引发的。