On the stability and formation of the a? and ? phases in Ti-Nb alloys upon cooling
On the stability and formation of the a? and ? phases in Ti-Nb alloys upon cooling
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关于a的稳定性和形成？
DOI：
10.1016/j.actamat.2023.119409
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发表时间：
2024
期刊：
影响因子：
9.4
通讯作者：
Talbot C
中科院分区：
文献类型：
--
作者：
Talbot C
基于Ti-Nb二元系的亚稳β-Ti合金可以表现出应力诱导的α″相转变，从而具有超弹性和形状记忆行为，但它们也容易形成六方ω相。ω的存在被广泛报道可以阻止α″转变，并以某些形式使材料变脆。大多数的研究都是针对ω相的情况，仅通过对ω相的观测来预测其稳定性。这种方法忽略了不同的潜在形成机制，并导致其真正稳定性的模糊画面。本文通过原位同步辐射X射线衍射，确定了一系列冷轧Ti-Nb合金中α″和ω相变随温度和成分变化的真实性质。在所研究的合金范围内，α″起始温度总是高于ω起始温度，但观察到两个相同时生长，这与以前的报道相矛盾。此外，发现所有合金中的ω起始温度均≤ 10 ℃，表明初始微观结构中存在的ω是亚稳的。这些观测结果与大量从现场数据中得出的关于ω相稳定性的结论相矛盾，并明确表明，单独对该相的观测不能用来确定其稳定性和相关的热力学温度ωs。
Metastable β-Ti alloys based on the binary Ti-Nb system can exhibit a stress induced transformation to the α″ phase allowing superelasticity and shape memory behaviours but they are also susceptible to the formation of the hexagonal ω phase. The presence of ω is widely reported to prevent the α″ transformation, and in certain forms embrittles the material. Most studies characterise the ω phaseex situand only observation of the ω phase is used to predict its stability. This approach ignores different potential formation mechanisms and results in an ambiguous picture of its true stability. Here, through the use ofin situsynchrotron X-ray diffraction we establish the true nature of the α″ and athermal ω phase transformations with respect to both temperature and composition in a series of cold rolled Ti-Nb alloys which initially contained both α″ and ω. Across the range of studied alloys, the α″ start temperature was always found to be above the ω start temperature but both phases were observed to grow simultaneously contradicting previous reports. In addition, the ω start temperature in all alloys was found to be ≤ 10˚C indicating that the ω present in the initial microstructures was metastable. These observations contradict a vast swathe of the conclusions drawn fromex situdata regarding the stability of the ω phase and categorically show that observation of this phase alone cannot be used to establish its stability and the relevant thermodynamic temperature,ωs.