ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy

doi:10.1016/j.jmst.2019.10.031

Abstract

Abstract:

In this work, the phase transformation sequence during the continuous heating process (3 °C/min) was investigated in a near β titanium alloy. The results show that the staring formation of ω phase is about 267 °C, and the ending precipitation temperature about 386 °C during the heating process. When the heating temperature is greater than 485 °C, there are no ω phase detected within the β matrix. Combined with the microstructural characterization, it is found that ω phase facilitates the nucleation of α phase nearby the ω/β interface and has a great effect on the refinement for α phase. As compared with the specimens directly aged, the specimens with ω-assisted refinement of α phase possess high tensile strength, but there is no yield stage detected on their stress-strain curve. Combined with the analyses of the fracture morphology, the specimens with ω-assisted refinement of α phase present a brittle fracture. This is mainly ascribed to its relatively lager width of grain boundaries and the absence of widmanstätten α precipitates.

Key words: Near β titanium alloy, Continuous heating, Phase transformation, ω-Assisted nucleation, α Precipitation, Tensile properties

Ruifeng Dong, Jinshan Li, Hongchao Kou, Jiangkun Fan, Yuhong Zhao, Hua Hou, Li Wu. ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy[J]. J. Mater. Sci. Technol., 2020, 44: 24-30.

Figures/Tables 11

Fig. 1. (a) Micrograph of the specimen after the solution treatment at 900 °C for 15 min and (b) the corresponding XRD pattern.

Fig. 2. (a) Dilatometry curve of the β-quenched specimen continuously heated at the rate of 3 °C/min and (b) the corresponding fraction of ω phase as a function of heating temperature.

Fig. 3. (a, c) Bright-field micrographs (the inset showing the selected-area electron diffraction (SAED) pattern in the [011]β zone axis) and (b, d) the corresponding dark-field micrographs of ω phase using the (0001)ω reflection indicated by the dotted circle in inset of specimens quenched at different heating temperatures: (a, b) 300 °C; (c, d) 350 °C.

Fig. 4. TEM observation of the specimen quenched at 400 °C: (a) the SAED pattern with the [011]β zone axis; (b) the corresponding key diagram of Fig. 4(a); (c) the dark-field micrograph taken by using the reflection circled by green circle in Fig. 4(a); (d) the high-resolution TEM (HRTEM) showing the co-existence of ω phase and α phase within β matrix; the corresponding fast Fouriter transform (FFT) diffractogram (e) and Inverse FFT lattice image (f) of Fig. 4(d).

Fig. 5. Micrographs of specimens after quenching at 600 °C: (a) SEM micrograph showing the overall microstructure; (b, c) the detailed TEM micrographs showing the intragranular α phase and the grain boundary α phase.

Table 1 Morphological parameters of α precipitates and the average width of GBs of specimens under different thermal conditions.

Heat treatment / Time	Average width of α_IG (nm)	Average length of α_IG (μm)	Volume fraction of α (%)	Average width of GBs (nm)
Continuously heating / ~192 min	84 ± 8	1.2 ± 0.5	62.3 ± 1.2	142.4 ± 5.6
Directly Aging / 192 min	282 ± 30	4.8 ± 0.3	61.5 ± 2.1	85.6 ± 7.8

Fig. 6. Schematic image illustrating the phase transformation sequence and the microstructure evolution during the continuous heating process.

Fig. 7. Tensile engineering stress-strain curves of the specimens under different thermal conditions at room temperature with the strain rate of 10-3 s-1.

Fig. 8. Fracture surface morphologies and the corresponding magnification micrographs of specimens under different thermal treatment conditions: (a, b) the as-solution treated specimens; (c, d) the directly aged specimens; (e, f) the continuously heated specimens.

Fig. 9. TEM images of the specimen directly aged at 600 °C for 192 min showing the (a) intragranular α precipitates and (b) the grain boundary α precipitates.

Fig. 10. Schematic diagram showing the crack propagation path of (a) the continuously heated specimens and (b) the directly aged specimens.

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