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Formation of Large Size Precipitate-Free Zones in β Annealing of the Near-β Ti-55531 Titanium Alloy

1
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
2
Shaanxi Key Laboratory of High-Performance Precision Forming Technology and Equipment, Northwestern Polytechnical University, Xi’an 710072, China
3
Lanzou Istitute of Physics, Chinese Academy of Space Technology, Lanzhou 730000, China
*
Author to whom correspondence should be addressed.
Metals 2019, 9(5), 544; https://doi.org/10.3390/met9050544
Received: 18 April 2019 / Revised: 4 May 2019 / Accepted: 6 May 2019 / Published: 10 May 2019
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Abstract

Large size (>10000 μm2) precipitate-free zones in the absence of microsegregation were observed in the near-β Ti-55531 titanium alloy after furnace cooling from high temperature and longtime annealing in the single-β phase field. To reveal the formation mechanism of the large size precipitate-free zone, continuous cooling and isothermal heat treatment were carried out to investigate the β-α phase transformation process. It was found that the large size precipitate free zone is attributed to the heterogeneous nucleation of α phase. The nucleation site evolves in three different modes: I-random nucleation inside the β grain, II-network nucleation inside the β grain and, III-heterogeneous nucleation on the precipitated α phase. Modes I and II lead to homogeneous transformed structure while Mode III results in the large size precipitate-free zone. Both modes II and III are promoted at high annealing temperature, rapid cooling above 600 °C or slow cooling below 600 °C. Mode II is common as it can minimize the strain energy in phase transformation. As a result, the formation of the large size precipitate-free zone is not deterministic. View Full-Text
Keywords: precipitate-free zone; phase transformation; heterogeneous nucleation; network structure; titanium alloys precipitate-free zone; phase transformation; heterogeneous nucleation; network structure; titanium alloys
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Jiang, X.; Shi, X.; Fan, X.; Li, Q. Formation of Large Size Precipitate-Free Zones in β Annealing of the Near-β Ti-55531 Titanium Alloy. Metals 2019, 9, 544.

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