Microstructure and Crystallography of α Phase Nucleated Dynamically during Thermo-Mechanical Treatments in Metastable β Titanium Alloy 

Jiangkun Fan (1, 2, 3) , Jinshan Li (3) , Yudong Zhang (1, 2) , Hongchao Kou (3) , Lionel Germain (2, 1) , Nathalie Siredey-Schwaller (1, 2) , Claude Esling (1, 2)

1 LEM3 - Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux
2 Labex DAMAS
3 State Key Laboratory of Solidification Processing

The a phase nucleated dynamically during the thermo-mechanical coupling process in titanium 8 alloy is really interesting but difficult to sufficiently ascertain. In the present work, the a phase 9 nucleation behavior, the orientation relationship between a/b as well as the phase transformation 10 kinetics during hot deformation of Ti-5553 alloy were investigated in-depth. Results reveal that the 11 "necklace" microstructure formed. The Burgers orientation relationship between a/b phases has 12 been destroyed gradually. The b ! a phase transformation is obviously retarded during the hot 13 compression due to the competitive effect of softening process (dynamic recovery/recrystallization). 14 These results could provide valuable reference for process optimization and the microstructural 15 evolution controlling. 16 1. Introduction Metastable b titanium alloys, such as Ti-10V-2Fe-3Al (Ti-17 1023), Ti-5Al-5Mo-5V-1Cr-1Fe (VT22), and Ti-5Al-5Mo-5V-18 3Cr (Ti-5553), are increasingly employed in the aerospace 19 industry because of their higher yield strength, better harden-20 ability, better fatigue, and crack propagation properties than a þ b 21 titanium alloys. [1,2] Microstructural development in these alloys 22 depends on the processing parameters, such as the type of 1 thermo-mechanical process, temperature, strain, strain rates, and 2 cooling rates. Moreover, metastable b titanium alloys are usually 3 sensitive to process parameters, even during isothermal process-4 ing. [3] Hence, significant effort to understand the microstructural 5 evolution of metastable b titanium alloys related to thermo-6 mechanical process have been done in the past decade. [4-7] 7 The b ! a phase transformation happens during the 8 thermo-mechanical processing below the b transus tempera-9 ture (T b). Generally, heterogeneous nucleation mechanisms 10 include grain boundary nucleation, dislocation nucleation, 11 and vacancy nucleation. Although much more defects are 12 introduced in microstructure during deformation, the a 13 precipitates nucleated mainly at the interface of two adjacent 14 b grains, triple junctions or quadruple points of b grains. The 15 reasons could be summarized as the following aspects: (i) the 16 higher interface energy could decrease the critical nucleation 17 energy; (ii) only part of interface of second phase need to be 18 reconstructed; (iii) the composition segregation is more 19 significant at the grain boundaries (GBs). In Ti-5553 alloy, 20 the dominant microstructural evolution mechanism during 21 hot deformation close to the T b is dynamic recovery (DRV) of 22 the b phase, [6,8] with a certain degree of the dynamic 23 recrystallization (DRX). [9] The a precipitates might nucleate 24 at these GBs preferentially including high angle grain 25 boundaries (HAGBs) and low angle grain boundaries 26 (LAGBs). Hence, dynamic stress induced phase transforma-27 tions (DSIPT) might happen during the thermo-mechanical 28 processing due to the more nucleation sites and accelerating

Domaines: Matériaux

Jiangkun Fan, Jinshan Li, Yudong Zhang, Hongchao Kou, Lionel Germain, et al.. Microstructure and Crystallography of α Phase Nucleated Dynamically during Thermo-Mechanical Treatments in Metastable β Titanium Alloy . Advanced Engineering Materials, 2017, 19 (7), pp.1600859. ⟨10.1002/adem.201600859⟩. ⟨hal-02483256⟩