Orientation Dependence of the Micro-Pillar Compression Strength in an Electron Beam Melted Ti-6Al-4V Alloy

doi:10.1007/s40195-020-01162-5

Abstract

Abstract:

An α/β two-phase Ti-6Al-4V alloy was fabricated by electron beam melting to obtain a basketweave structure. The orientation dependence of the mechanical properties of Ti-6Al-4V alloy was studied by micro-pillar compression and post-mortem transmission electron microscopy analysis. The results indicate that different grains have different mechanical responses, and the possible attributions were discussed. Besides the orientation effect, due to the limited volumes of micropillars, the size of the α phases, dispersion of the β phases, and the presence of the free dislocation path also affect the mechanical properties of the micropillars to a large extent. Although no direct link was discovered between the mechanical properties and the parent β orientations, this work provided a promising method to further study the anisotropic mechanical behavior in Ti-6Al-4V alloy.

Key words: Electron beam melting (EBM), Ti-6Al-4V, Micro-pillar compression, Orientation dependence, Mechanical properties

Jinsen Tian, Jiang Ma, Ming Yan, Zhuo Chen, Jun Shen, Jing Wu. Orientation Dependence of the Micro-Pillar Compression Strength in an Electron Beam Melted Ti-6Al-4V Alloy[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(4): 476-484.

Figures/Tables 10

Table 1 Chemical composition of the Ti64 powders (wt.%)

C	O	N	H	Fe	Al	V	Y	Others	Ti
0.01	0.09	0.02	0.003	0.18	6.42	4.13	0.001	<0.4	Balance

Fig. 1 a BSE image of the horizontal section showing the structure of EBM-fabricated Ti64 alloy; inset showing the schematic drawing of the sample section; b bright-field TEM image showing the overall microstructure and dislocations

Fig. 2 a EBSD orientation map of the horizontal section of the as-fabricated sample; only α phase is used for phase indexation. The parent β boundaries were indicated by white dashed line. b Pole figure of [0001] zone axis corresponding to the area labeled in a. The inverse pole figure indicates the normal directions of the transformed β grains, which is represented by small circles

Fig. 3 Stress-strain curves of the compressed micro-pillars corresponding to parent β grains in Fig. 2

Table 2 Compression strengths of the micro-pillars calculated from Fig. 4

Micro-pillars	0.2% yield strength, GPa	2% flow stress, GPa	10% flow stress, GPa	Minimum strength needed for dislocation activation*
A	1.07	1.32	1.67	0.83
B	0.97	1.19	1.51	0.84
C	0.92	1.12	1.37	0.81
D	0.86	1.06	1.30	0.92
E	0.88	1.08	1.37	0.92
F	0.97	1.18	1.46	0.91
G	0.95	1.19	1.49	0.81

Fig. 4 SEM images of the compressed pillars taken at stage tilt angle of 40°. The letters represent the pillars from corresponding grains

Fig. 5 TEM results from compressed pillar D: a STEM HAADF image showing the overview deformed structure; b a magnified STEM image showing a localized shear band; c, d TEM bright-field image and the corresponding selected area diffraction patterns of the shear band region

Fig. 6 Deformed morphologies from compressed pillar A: a STEM overview image showing the deformed structure, the inset showing the SEM image and the relative view direction for TEM observation; b magnified STEM image; c, d TEM bright-field images showing the dense dislocations in the α grains

Fig. 7 a, b STEM high angle annular dark field (HAADF) images from compressed pillar E showing large α variants, the dense dislocations appear as white lines and red arrows indicate the slip directions of few slip systems inside two large α variants; c, d STEM HAADF images from compressed pillar B revealing more dispersed β phase (indicated by blue boxes)

Fig. 8 a STEM image of deformed pillar B showing the interaction between β phase and dislocations in α phase. The dislocations appear as white lines and yellow arrows represent the slip directions of two groups of dislocations. The same region under different beam conditions is presented by the green box in b. The β phases were segmented by dislocations transmitted through, and the magenta arrows show the gaps between β segments caused by deformation

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