Microstructure and mechanical properties of WMoTaNbTi refractory high-entropy alloys fabricated by selective electron beam melting

doi:10.1016/j.jmst.2021.07.041

Abstract

Abstract:

WMoTaNbTi RHEAs formed by SEBM with negative defocus distance were investigated. Four scanning speeds were applied, an electron beam with scanning speed at 2.5 m/s completely fused the premixed WMoTaNb alloyed powder and pure Ti powder. Significant vaporization of Nb and Ti elements happened during the formation of WMoTaNbTi RHEAs, however, the single BCC phase remains stable. Weakened solid-solute strengthening caused by elemental vaporization, dropping percentage of Nb and Ti solutes in the matrix as well as improved ductilizing effects with decreasing scanning speeds leads to falling microhardness and better local ductility. Microhardness of scanning speed at 4.0 m/s, 3.5 m/s, 3.0 m/s and 2.5 m/s is 578 ± 17 HV, 576 ± 12 HV, 573 ± 10 HV and 511 ± 2 HV, respectively. The as-deposited WMoTaNbTi RHEA formed at a scanning speed of 2.5 m/s displays ultimate strength of 1312 MPa.

Key words: Refractory high-entropy alloys, Selective electron beam melting, Solidification cracking, Porosity, Solid-solution strengthening

Bang Xiao, Wenpeng Jia, Huiping Tang, Jian Wang, Lian Zhou. Microstructure and mechanical properties of WMoTaNbTi refractory high-entropy alloys fabricated by selective electron beam melting[J]. J. Mater. Sci. Technol., 2022, 108: 54-63.

Figures/Tables 14

Fig. 1. Mixture of pre-alloyed WMoTaNb powder and pure titanium powder (a) and the enlarged morphology (b).

Fig. 2. (a) Schematic diagram of defocus distance and formed samples (XOY surface) with scanning speeds of (b) 4.0 m/s, (c) 3.5 m/s, (d) 3.0 m/s, (e) 2.5 m/s.

Table 1. Chemical composition (at.%) of as-deposited WMoTaNbTi RHEAs.

Scanning rate (m/s)	W	Mo	Ta	Nb	Ti
4.0	23.41	21.82	21.21	19.71	13.85
3.5	22.93	23.01	21.01	19.66	13.39
3.0	23.24	23.40	21.05	20.31	12.00
2.5	23.45	22.66	22.04	18.84	13.01

Fig. 3. Composition alteration of Nb and Ti in the as-deposited WMoTaNbTi RHEAs.

Fig. 4. X-ray diffraction patterns of the (a) WMoTaNb powder and (b) the as-deposited RHEAs.

Fig. 5. T-(fs)1/2 curves calculated by using data from Ref. [9].

Fig. 6. Microstructures of the as-polished XOZ surface (parallel to building direction): (a) 4.0 m/s, (b) 3.5 m/s, (c) 3.0 m/s and (d) 2.5 m/s.

Fig. 7. Optical microstructure of the as-etched XOZ surface of the WMoTaNbTi RHEAs: (a) 4.0 m/s, (b) 3.5 m/s, (c) 3.0 m/s, (d) 2.5 m/s.

Fig. 8. Elements distribution of XOZ as-polished surface of the WMoTaNbTi RHEAs: (a, b) 4.0 m/s, (c, d) 3.5 m/s, (e, f) 3.0 m/s, (g, h) 2.5 m/s.

Fig. 9. (a, b) Cracking initiation and feeding mechanism, (c) solute redistribution during solidification with complete diffusion in liquid and no diffusion in solid (CL(fS) and CS(fS) are solute concentrations changes with fS).

Fig. 10. Microhardness (a) and the hardness indentation of the WMoTaNbTi RHEAs at scanning speeds of (b) 4.0 m/s, (c) 3.5 m/s, (d) 3.0 m/s and (e) 2.5 m/s.

Fig. 11. Compressive engineering stress-strain curve (a) and fracture morphology (b) of the as-deposited WMoTaNbTi RHEA formed at the scanning speed of 2.5 m/s.

Fig. 12. Inverse pole figure map (a) and grain diameter distribution (b) of the as-deposited WMoTaNbTi RHEAs at the scanning speed of 2.5 m/s.

Fig. 13. Grain boundaries (a) and misorientation angle (b) of the as-deposited WMoTaNbTi RHEA at the scanning speed of 2.5 m/s.

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