Tensile properties and deformation behavior of an extra-low interstitial fine-grained powder metallurgy near alpha titanium alloy by recycling coarse pre-alloyed powder

doi:10.1016/j.jmst.2022.04.038

Abstract

Abstract:

An extra-low interstitial near alpha alloy Ti−3Al−2Zr−2Mo (wt%) was fabricated by hydrogenation and thermomechanical consolidation (TMC) of the coarse and spherical pre-alloyed powder with particle sizes of 60 to 270 μm. The coarse powder is a byproduct of pre-alloyed powder produced for selective laser and electron beam additive manufacturing. The TMC process involves powder compaction, fast sintering, in-situ dehydrogenation and an immediate hot extrusion to form a fully dense and fine-grained martensitic microstructure. Further dehydrogenation in vaccum at 700 °C converted the martensitic microstructure into an interwoven α/β microstructure which exhibited an improved yield strength, apparent necking and premature cracking at grain boundary α (α_GB) ribbons. A further annealing of 880 °C/1 h/AC led to the formation of a fine-grained α/β_t composite structure, which achieved an enhance ultimate tensile strength of 835 MPa and excellent tensile ductility of 16.0%. Analysis of the deformation behavior of the alloy in different states revealed that the α/β_t composite structures brought about an enhanced strain hardening capability by heterogeneous deformation effect of hard β_t and soft α-laths, which inhibited the formation of microcracks and consequently improved the coordinated deformation.

Key words: Near α-Ti alloy, Coarse pre-alloyed powder, Hydrogenation, Tensile properties, Deformation behaviour

H.R. Zhang, H.Z. Niu, M.C. Zang, Y.H. Zhang, S. Liu, D.L. Zhang. Tensile properties and deformation behavior of an extra-low interstitial fine-grained powder metallurgy near alpha titanium alloy by recycling coarse pre-alloyed powder[J]. J. Mater. Sci. Technol., 2022, 129: 96-107.

Figures/Tables 13

Fig. 1. (a) The hydrogenated pre-alloyed powder, (b) particle size distribution of the powder, (c) warm-pressed powder compacts and (d) fracture surface of an artificially broken powder compact.

Fig. 2. (a) Oxygen and hydrogen contents and (b) XRD profiles at different states.

Fig. 3. Microstructure of the as-extruded rod: (a) OM, (b) SEM and (c-e) TEM bright-field images. The selected area electron diffraction spots (SADS) of α' and δ-TiH2 were inserted in (c), (f-h) TEM dark-field images, and (i) the corresponding SADS of areas marked by yellow dotted circle.

Fig. 4. Microstructural characteristics of the PA-700 sample: (a) OM image, (b) and (c) SEM images, (d) all Euler map and (e) the historgram of α-Ti misorientation angles.

Fig. 5. Microstructural characteristics of the PA-880 sample: (a) OM image, (b) and (c) SEM images, (d) nano-hardness values at different indentation sites, (e) the representative load-displacement curves of α-laths and βt domains.

Fig. 6. (a) Tensile engineering stress-strain curves, (b) the related true stress-true strain curves and work hardening rate curves.

Fig. 7. The deformation characteristics near the fractures of (a-c) PA-700 sample, (d-f) PA-880 sample and (g-i) SPS-880 sample. (a), (d) And (g) surface slip traces, the inserted image in (a) showing one secondary crack near the fracture, (b), (e) and (h) microcracks and micropores on the longitudinal sections, (c), (f) and (i) fracture morphologies.

Fig. 8. Schematic illustration of the phase transition and microstructural evolution at the fast sintering & hot extrusion stage and vacuum dehydrogenation stage.

Fig. 9. EBSD results of the PA-700 sample: (a) a superposed image of IPF-X map of grain boundary α and band contrast (BC) map, (b) PFs of {110} β G1 and (0001) α GB1, scattered PF of β G2 is superposed on. (c) IPF-X map, the dotted black lines outline the prior β-Ti grains based on Burgers orientation relationships, (d) the corresponding PFs of {110} of prior β grain G6 and the superposed {110} pole density distributions of G3, G4, G5, G7, (e) a pie chart of frequencies of individual α-variants.

Fig. 10. The identified surface slip plane traces according to analysis of EBSD results of the PA-700 sample strained to 3.0%: (a) IPF-X map, (b) Kernel average misorientation (KAM) map, (c-f) IPF-X maps of individual α-variants superposed on a FSD image. The inserted PFs and unit cells on left bottom of the FSD image highlight the activated slip plane of individual α-variants.

Table 1. Corresponding SF values of individual α-variants in the prior β grain G1 of PA-700 sample shown in Fig. 10.

	(0001)<11-20>	{10-10}<11-20>	{10-11}<1-210>	{10-11}<11-2-3>	{11-22}<11-2-3>	SF (Basal/pris)	SF (pyra/pris)
1	0.12	0.01	0.09	0.49	0.49	-	-
2	0.13	0.45	0.41	0.47	0.39	0.29	0.91
3	0.32	0.39	0.49	0.36	0.29	0.82	1.26
4	0.33	0.37	0.49	0.34	0.26	0.89	1.33

Fig. 11. The identified surface slip plane traces of PA-880 sample strained to 6.0%: (a) IPF-X map, (b) KAM map, (c) and (d) IPF-X maps of individual α-variants superposed on the FSD image. The inserted 3D unit cells and PFs on the FSD image demonstrate the activated slip planes of individual α-variants, (e) differently oriented unit cells of adjacent β1 and β2 grains and the corresponding PFs of (0001) αGB.

Fig. 12. TEM bright-field (BF) dislocation micrographs of (a-c) PA-700 sample deformed to 3.0% strain and (d-f) PA-880 tensile fractured sample.

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