Microstructure and mechanical properties of CxHf0.25NbTaW0.5 refractory high-entropy alloys at room and high temperatures

doi:10.1016/j.jmst.2021.05.015

Abstract

Abstract:

The microstructure and mechanical properties of as-cast and isothermally annealed C_xHf_0.25NbTaW_0.5 (x=0, 0.05, 0.15, 0.25) refractory high-entropy alloys (RHEAs) were studied. Both the as-cast and annealed RHEAs consisted of disordered body-centered cubic solid solution phase and metal carbide (MC) phase with a face-centered cubic crystal structure (Fm-3m space group). The primary carbides were enriched with Hf and C elements and tended to form lamellar eutectic-like microstructure in the interdendrites. The lamellar eutectic-like structure in the interdendrites would be formed from the decomposition of sub-carbide M₂C under the influence of Hf element. After isothermal annealing, slatted carbides were precipitated on the matrix, and the distribution became more uniform with high C content. The formation of carbides strongly influenced the mechanical properties both at room and high temperatures. The yield strength values of C_0.25Hf_0.25NbTaW_0.5 RHEA at 1473 and 1673 K were 792 and 749 MPa, respectively. The result had exceeded the high temperature mechanical properties of currently known RHEAs. Moreover, this RHEA exhibited high-temperature performance stability and excellent plasticity, exceeding 30 and 50% at room and elevated temperatures (above 1273 K), respectively. During thermal deformation, carbon-containing RHEAs obtained more severe work hardening than that of ACH0 RHEAs, and required greater dynamic recrystallization to achieve the dynamic equilibrium.

Key words: Refractory high entropy alloys, Mechanical properties, Microstructure, High temperature, Carbon content

Shiyu Wu, Dongxu Qiao, Haitao Zhang, Junwei Miao, Hongliang Zhao, Jun Wang, Yiping Lu, Tongmin Wang, Tingju Li. Microstructure and mechanical properties of C_xHf_0.25NbTaW_0.5 refractory high-entropy alloys at room and high temperatures[J]. J. Mater. Sci. Technol., 2022, 97: 229-238.

Figures/Tables 14

Fig. 1. XRD patterns of (a) as-cast specimens and (b) annealed specimens at 1673 K for 12 h.

Table 1 Lattice constants of BCC phase and FCC phase in CxHf0.25NbTaW0.5 RHEAs.

RHEAs	BCC(Å)	FCC(Å)
ACH0	3.2987	-
ACH5	3.2970	4.5656
ACH15	3.2904	4.5212
ACH25	3.2862	4.4889
ANH0	3.2958	-
ANH5	3.2928	4.6062
ANH15	3.2862	4.5959
ANH25	3.2795	4.5803
HfC*		4.638
NbC*		4.470
TaC*		4.455

Fig. 2. EPMA-BSE micrographs of the as-cast CxHf0.25NbTaW0.5: (a) ACH0 RHEA, (b) ACH5 RHEA, (c) ACH15 RHEA, and (d) ACH25 RHEA, where DR, ID, M, and C represent the dendrite, interdendritic, matrix, and carbide regions, respectively.

Table 2 Chemical compositions of the dendritic (DR), interdendritic (ID), matrix (M), and carbide (C) regions of the alloys measured by the EPMA-WDS (the atomic percentage).

RHEAs	Region	W	Hf	Nb	Ta	C
ACH0	DR	26.8	4.9	26.2	42.0	-
	ID	15.1	18.5	34.0	32.5	-
ACH5	DR	23.9	5.7	29.3	37.5	3.6
	ID	9.1	19.3	24.0	22.8	24.8
ACH15	M	23.9	5.8	30.2	37.5	2.5
	C	1.4	26.0	14.6	18.7	39.3
ACH25	M	25.8	4.3	29.0	38.9	1.9
	C	1.3	21.4	15.6	20.6	41.1
ANH0	DR	25.1	4.8	29.0	41.2	-
	ID	13.1	21.3	33.8	31.8	-
ANH5	DR	23.0	6.9	30.4	38.6	1.0
	ID	14.0	10.3	38.7	31.7	5.3
ANH15	M	23.4	5.2	29.9	37.7	3.8
	C	2.8	32.4	13.8	17.6	33.5
ANH25	M	25.0	4.5	27.1	37.5	5.9
	C	5.2	27.0	14.3	20.5	33.1

Fig. 3. EPMA-WDS mapping of Nb, Ta, W, Hf, and C of the ACH25 RHEA.

Fig. 4. EPMA-BSE microstructure of the annealed CxHf0.25NbTaW0.5 after etching: (a) ANH0 RHEA, (b) ANH5 RHEA, (c) ANH15 RHEA, and (d) ANH25 RHEA, where ID and M represent interdendrites and matrix, respectively.

Fig. 5. (a) TEM micrographs of annealed ANH25 RHEA; (b)-(c) SAED patterns corresponding to matrix and carbides regions in (a); (d) are HRTEM pictures of FCC-MC phases; (e) TEM-EDS elemental mappings of Hf, Nb, Ta, W, C

Fig. 6. Compressive stress-strain curves of CxHf0.25NbTaW0.5 at RT: (a) as-cast RHEAs, (b) annealed RHEAs

Table 3 Mechanical properties of CxHf0.25NbTaW0.5 RHEAs at RT.

RHEAs	Yield stress, MPa	Peak stress, MPa	Fracture strain, %	Hardness, HV
ACH0	1037	2568	42.7	403
ACH5	1192	2417	37.8	444
ACH10	1230	2700	41.7	460
ACH15	1265	2629	38.1	475
ACH20	1293	2631	36.9	490
ACH25	1317	2584	31.9	506
ANH0	942	1720	25.0	355
ANH5	1115	2044	36.7	423
ANH10	1194	2742	41.1	444
ANH15	1218	2646	39.7	469
ANH20	1279	2673	31.1	488
ANH25	1337	2799	31.0	514

Fig. 7. Compressive stress-strain curves of ACH0, ACH15, ACH25 at (a, b) 1273 K, (c, d)1473 K and (e, f) 1673 K.

Table 4 Mechanical properties of CxHf0.25NbTaW0.5 RHEAs at HT obtained via compressive stress-strain curves.

RHEAs	Temperature, K	Yield stress, MPa	Fracture strain, %
ACH0	1273	584	>50
	1473	577	>50
	1673	455	>50
ACH15	1273	768	>50
	1473	733	>50
	1673	639	>50
ACH25	1273	868	>50
	1473	792	>50
	1673	749	>50

Fig. 8. IPF maps (a and c) and distribution diagrams of recrystallized grains (b and d) of ACH0 (a and c) and ACH25 (b and d) specimens hot-compressed to 50% reduction at 1473K. Here D and R represent the volume fractions of deformed and recrystallized grains, respectively.

Fig. 9. (a) Temperature dependences of the yield strength of NbTaW0.5, ACH0 and ACH25 RHEAs, (b) the influence of volume fraction of the FCC carbide on the yield strength of CxHf0.25NbTaW0.5 RHEAs.

Fig. 10. Temperature dependence of the yield strength of CxHf0.25NbTaW0.5 RHEAs and some classic RHEAs (a) and the relationship between yield strength of CxHf0.25NbTaW0.5 RHEAs and some RHEAs at 1473 K and plasticity at 298 K (b).

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