Low-Temperature Superplastic Deformation Behavior of Bimodal Microstructure of Friction Stir Processed Ti-6Al-4V Alloy

doi:10.1007/s40195-025-01878-2

Abstract

Abstract:

For a long time, the conventional superplastic forming temperature for Ti alloys is generally too high (~ 900-920 °C), which leads to too long production cycles, heavy surface oxidation, and property reduction. In this study, an ultrafine bimodal microstructure, consisting of ultrafine equiaxed microstructure (0.66 μm) and 43.3% lamellar microstructure, was achieved in the Ti-6Al-4V alloy by friction stir processing (FSP). The low-temperature superplastic behavior and deformation mechanism of the FSP Ti-6Al-4V alloy were investigated at temperatures of 550-675 °C and strain rates ranging from 1 × 10⁻⁴ to 3 × 10⁻³ s⁻¹. The FSP alloy exhibited superplastic elongations of > 200% at the temperature range from 550 to 650 °C, and an optimal superplastic elongation of 611% was achieved at 625 °C and 1 × 10⁻⁴ s⁻¹. This is the first time to report the low-temperature superplasticity of the bimodal microstructure in Ti alloys. Grain boundary sliding was identified as the dominant deformation mechanism, which was effectively accommodated by the comprehensive effect of dislocation-induced β phase precipitation and dynamic spheroidization of the lamellar structure. This study provides a novel insight into the low-temperature superplastic deformation behavior of the bimodal microstructure.

Key words: Titanium alloys, Friction stir processing, Superplasticity, Bimodal microstructure, Spheroidization

H. Q. Dai, N. Li, L. H. Wu, J. Wang, P. Xue, F. C. Liu, D. R. Ni, B. L. Xiao, Z. Y. Ma. Low-Temperature Superplastic Deformation Behavior of Bimodal Microstructure of Friction Stir Processed Ti-6Al-4V Alloy[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(9): 1559-1569.

Figures/Tables 7

Fig. 1 a Schematic illustration of the shape and size of the tensile sample, b schematic diagram of the superplastic tensile fracture sample

Fig. 2 a Cross-sectional OM image of Ti-6Al-4V alloy by FSP; b SEM image of BM; c SEM image of SZ

Fig. 3 Microstructures of the BM: a inverse pole figure (IPF) map; b misorientation angle distribution map; c phase map; d TEM map

Fig. 4 Microstructures of the SZ: a IPF map; b misorientation angle distribution map; c phase map; d TEM map

Fig. 5 Variation in a elongation and b flow stress with initial strain rate at testing temperatures of 600-650 °C for FSP specimens; c true stress-strain curves of the FSP specimens at 625 °C; d initial tensile specimen and superplastic tensile fractured specimens at 1 × 10-4 s−1 at different temperatures

Fig. 6 EBSD maps of superplastic tensile test samples in different regions at 625 °C and 1 × 10-4 s−1: a superplastic tensile fractured specimens; b1-b3 phase maps; c1-c3 distribution of grain boundary misorientation angles; d1-d3 KAM maps

Fig. 7 EBSD maps of the samples at 625 °C, 1 × 10-4 s−1 and different strain states (0%, 50%, 100%, 200%); a1-d1 phase maps; a2-d2 distribution of grain boundary misorientation angles; a3-d3 KAM maps

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