Formation Mechanism of Precipitation-Free Zone in TB18 Alloy Structure

Abstract:

TB18 alloy bars were used as the research object, and the formation mechanism of the precipitation-free zone (PFZ) was investigated through microstructure characterization, composition analysis, and phase structure analysis. Results show that after solution treatment of 870 °C×120 min+aging treatment of 525 °C×240 min, discontinuous white bright spot-like pure-β PFZs exist in the middle region of the cross-section of TB18 alloy bars. During the isothermal aging, the α? phase is preferentially precipitated and grows at the β grain boundaries. With the prolongation of aging time, PFZs are decreased to a certain extent but still remain. Composition analysis reveals that in PFZs, Mo content is relatively higher, Nb content is relatively lower, and Ti-Nb clusters exist. During solution treatment, the metastable β phase decomposes into β′′ phase which is rich in β-stabilizing elements and β′ phase which lacks β-stabilizing elements. The β′ phase can serve as a nucleation substrate for the α phase, whereas the regions rich in β-stabilizing elements and Ti-Nb clusters will inhibit the precipitation of α phase, ultimately leading to the formation of PFZs. This study provides a theoretical basis for the process optimization of TB18 alloy.