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Titanium is an important structural metal. Titanium alloy has the advantages of light weight, high specific strength, strong corrosion resistance, high heat resistance, etc., and is widely used in aerospace and biomedical fields.
Recently, a team led by Professor Yu Qian from the Research Group of Professor Zhang Ze from the Center for Electron Microscopy of Zhejiang University and Professor Maen, Professor Zhang Wei and Professor Long Qing of Xi’an Jiaotong University collaborated with Dr. Chen from Pennsylvania State University to explore the microstructure of titanium at high temperatures. Evolution. By using cutting-edge in-situ and multi-scale electron microscopy techniques, synchrotron radiation and computational simulations, researchers have jointly conducted an in-depth study of the α-β transformation mechanism in titanium-molybdenum alloys. They found a significant difference between this transformation process and the process described in the classical nucleation theory. Their research results were published in the November 26th issue of "Nature Materials".
The phases in alloys are usually homogeneous components with the same aggregation state, crystal structure and properties. Different phases have different characteristics due to their structure and composition. In material design, we can make full use of their complementary advantages to optimize the overall performance of the material.
Titanium-based alloys are usually a mixture of low-temperature hexagonal close-packed (hcp) α phase and high-temperature body-centered cubic (bcc) β phase. The combination of two-phase is adjusted by alloying of β-stable (such as Mo, Nb, Ta) and α-stable (such as O, Al, La) elements. Phase transition can be simply understood as the transition from one phase to another caused by external stimuli.
"To achieve precise control of the phase structure, we need to understand the true process of phase transition and its basic laws," said Professor Yu.
The classic nucleation and growth mechanism starts from the nucleation of a new phase, which has the same crystal structure and composition as the final equilibrium product phase. However, this simple picture cannot explain the phase transition paths in many alloy systems. Generally, one or more intermediate states that are significantly different from the product phase in terms of composition, crystal structure, and chemical sequence precede the evolution into the product phase. The intermediate state is metastable. As it develops, its thermodynamic properties will also change, including its interface energy with the surrounding environment. Recent reports have shown that there is non-classical nucleation during the formation of inorganic nanoparticles, protein crystallization 16 and diamond crystal nucleation from solution, and the crystallization of amorphous tungsten carbide, but the atomic details of complex reactions in metallurgical systems are still difficult. fathom.
Yu Qian et al. Using titanium-molybdenum alloy as a representative system, direct and time-resolved experimental observations of non-classical nucleation-mediated phase transitions were performed with sub-ngström resolution. They found a nano-scale and chemically ordered superstructure in the alpha phase matrix; its composition, chemical sequence and crystal structure were found to be different from the parent phase and product phase, but it triggered lower and lower conversions. It is the energy barrier of β phase. When the molybdenum concentration in the superstructure exceeds the critical value, the latter phase transition can be performed immediately by vibration switching.
"This is the first time this non-classical solid-state phase transition has been reported in a titanium-based alloy," said Professor Yu. "Our research can be used as a major example of combining in-situ and atomic-scale experiments with computer simulations. Therefore, it opens the door to the study of solid-state phase transitions in other alloys." "More information on crystal defects: Xiaoqian Fu et al., Atomic-scale observations of non-classical nucleation-mediated phase transitions in titanium alloys, Natural Materials (2021). DOI: 10.1038/s41563-021-01144-7 Journal information: Nature Materials
Citation provided by Zhejiang University: Titanium alloy phase transition observed on the atomic scale (December 13, 2021) Retrieved on December 13, 2021 from https://phys.org/news/2021-12-phase-titanium- alloys-atomic-scale .html This document is protected by copyright. Except for any fair transaction for private learning or research purposes, no part may be copied without written permission. The content is for reference only.
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