The present work develops a novel unified approach to describe the crystal structure of orthorhombic martensite (α′′) in Ti alloys independent of chemical composition. By employing a straightforward yet highly instructive solid sphere model for the basic tetrahedral structural unit the crystal structures involved in the β ↔ α′′/α′ martensitic transformation are categorized into several intermediate configurations. Importantly, a new metric is introduced, δ, which unambiguously characterizes the atomic positions inside the orthorhombic unit cell depending on unit-cell geometry. Furthermore, the exclusive use of relative quantities to describe unit-cell geometry and atom positions renders the approach developed herein independent of alloy content. In this way, shortcomings of commonly suggested structural metrics for α′′ are eliminated. Subsequently, the novel methodology is applied to analyse and compare the crystal structure of α′′ across a broad range of Ti alloys based on experimentally measured unit-cell parameters. From this analysis it emerges that a large fraction of structural configurations along the b.c.c.–Cmcm–h.c.p. transformation path is not observed in quenched alloys. The threshold between the not-observed and the remaining well observed configurations is identified with an ideal Cmcm crystal structure, relative to which the experimentally found α′′ is compressed along its c axis.

中文翻译：

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钛合金斜方马氏体晶体结构的一般模型

目前的工作开发了一种新的统一方法来描述与化学成分无关的 Ti 合金中斜方马氏体 (α'') 的晶体结构。通过对基本四面体结构单元采用简单但非常有指导意义的固体球模型，β ↔ α''/α' 马氏体转变中涉及的晶体结构被分类为几种中间构型。重要的是，引入了一个新的度量，δ，它根据晶胞几何形状明确表征正交晶胞内的原子位置。此外，专门使用相对量来描述晶胞几何形状和原子位置使得本文开发的方法与合金含量无关。通过这种方式，消除了通常建议的 α'' 结构度量的缺点。随后，基于实验测量的晶胞参数，该新方法用于分析和比较各种 Ti 合金中 α'' 的晶体结构。从这个分析中可以看出，沿 bcc 的大部分结构配置 -在淬火合金中未观察到Cmcm -hcp 转变路径。未观察到的和剩余的良好观察到的配置之间的阈值被确定为理想的Cmcm晶体结构，相对于实验发现的 α'' 沿其c轴被压缩。