Achieving excellent strength-ductility synergy in twinned NiCoCr medium-entropy alloy via Al/Ta co-doping

doi:10.1016/j.jmst.2021.01.060

Abstract

Abstract:

Alloying is an effective strategy to tailor microstructure and mechanical properties of metallic materials to overcome the strength-ductility trade-off dilemma. In this work, we combined a novel alloy design principle, i.e. harvesting pronounced solid solution hardening (SSH) based on the misfit volumes engineering, and simultaneously, architecting the ductile matrix based on the valence electron concentrations (VEC) criterion, to fulfill an excellent strength-ductility synergy for the newly emerging high/medium-entropy alloys (HEAs/MEAs). Based on this strategy, Al/Ta co-doping within NiCoCr MEA leads to an efficient synthetic approach, that is minor Al/Ta co-doping not only renders significantly enhanced strength with notable SSH effect and ultrahigh strain-hardening capability, but also sharply refines grains and induces abnormal twinning behaviors of (NiCoCr)₉₂Al₆Ta₂ MEA. Compared with the partially twinned NiCoCr MEA, the yield strength (σ_y) and ultimate tensile strength (σ_UTS) of fully twinned Al/Ta-containing MEA were increased by ∼102 % to ∼600 MPa and ∼35 % to ∼1000 MPa, respectively, along with good ductility beyond 50 %. Different from the NiCoCr MEA with deformation twins (DTs)/stacking faults (SFs) dominated plasticity, the extraordinary strain-hardening capability of the solute-hardened (NiCoCr)₉₂Al₆Ta₂ MEA, deactivated deformation twinning, originates from the high density of dislocation walls, micro-bands and abundance of SFs. The abnormal twinning behaviors, i.e., prevalence of annealing twins (ATs) but absence of DTs in (NiCoCr)₉₂Al₆Ta₂ MEA, are explained in terms of the relaxation of grain boundaries (for ATs) and the twinning mechanism transition (for DTs), respectively.

Key words: Medium-entropy alloys, Mechanical properties, Solid solution hardening, Twinning behavior, Strength-ductility synergy

D.D. Zhang, H. Wang, J.Y. Zhang, H. Xue, G. Liu, J. Sun. Achieving excellent strength-ductility synergy in twinned NiCoCr medium-entropy alloy via Al/Ta co-doping[J]. J. Mater. Sci. Technol., 2021, 87: 184-195.

Figures/Tables 13

Fig. 1. Schematic of thermo-mechanical processing routes and the corresponding microstructure of the present Al6Ta2 MEA.

Fig. 2. The microstructural features of the annealed A6Ta2 MEA. (a) the OM image, showing fully twinned microstructure and the corresponding XRD pattern, showing randomly oriented grains of the fcc single-phase solid solution; (b) a TEM bright-field image of a twinned grain without dislocations, showing the coherent twin boundary (CTB) of AT (b1) and twinned relationship via the SAED pattern (b2); (c) the STEM/TEM image, showing the CTB and GB as indicated by arrows and confirming the chemical homogeneity of all elements from the EDS maps.

Fig. 3. The recrystallized microstructure of (a) the NiCoCr alloy and (b) the Al6Ta2 MEA, respectively. Insets in (a) and (b) showing their corresponding statistical results of the average grains size (including or excluding TBs).

Fig. 4. Typical deformation microstructures of the NiCoCr MEA strained to fracture. (a) The morphologies of hierarchical SFs and (b) the representative HRTEM images of SF bundles and the corresponding FFT pattern (the inset); (c) The morphologies of DTs and (d) the HRTEM image of DTs and the corresponding IFFT pattern (the inset).

Fig. 5. Typical internal deformation features of Al6Ta2 MEA strained to fracture. (a) Dislocation planar slip characteristics with zone axis of [110]; (b) The morphology of HDDWs; (c) The strain-induced micro-bands with high densities of dislocations; (d) The feature of SFs network and (e) a magnified image of the SFs associated with the IFFT pattern. (f) The feature twin-dislocation interactions showing dislocation transmit across the CTBs and abundant dislocations inside the annealing micro-twin.

Fig. 6. Mechanical response of the Al6Ta2 MEA in comparison with that of NiCoCr MEA recrystallized at 1150 ℃ for 3 min’ (a) representative engineering stress-strain curves of NiCoCr and Al6Ta2 MEAs; (b) The true stress-strain curves vs corresponding strain hardening rates of NiCoCr and Al6Ta2 MEA.

Fig. 7. A comparison of the (a) yield strength and (b) ultimate tensile strength of the present solute-hardened Al6Ta2 MEA at ambient temperate with that of our NiCoCr MEA and the other reported homogenous, solid-solution and fcc HEAs/MEAs.

Fig. 8. Hall-Petch relationships of NiCoCr and Al6Ta2 MEAs. The squares and circles are present experimental data of σy against d for NiCoCr and Al6Ta2 alloys respectively, as well as the corresponding Hall-Petch plots, the triangles are the data of NiCoCr from the literature [32,[74], [75], [76]] and the fitting purple line is a fairly reliable Hall-Petch relationship of NiCoCr alloy based on the aggregated data of circles and triangles.

Table 1 The isotropic elastic constants for pure elements at room temperature, and literature data of atomic apparent volumes Vn of each element in fcc HEAs, the value of VTa was deduced from experiments on Ni-Ta binary alloy.

Elements	Fe	Co	Cr	Ni	Mn	Al	Ta	V
V_n (Å³)	12.09 [41]	11.12 [41]	12.27 [41]	10.94 [41]	12.60 [41]	14.0 [43]	17.35	13.91 [44]
G (GPa)	81 [80]	75 [80]	72 [80]	74 [80]	77 [45]	27 [80]	69 [80]	54 [80]
E (GPa)	211 [80]	209 [80]	245 [80]	196 [80]	198 [80]	71 [80]	177 [80]	147 [80]

Table 2 Material parameters at room temperature adopted in the SSH model and the predicted lattice friction stress σ0(th) for (NiCoCr)92Al6Ta2, (NiCoCr)98Ta2, (NiCoCr)94Al6, as well as various fcc HEAs/MEAs. The σ0(exp) extracted by deduced finite grain sizes polycrystalline data to infinite grain sizes.

	b (Å)	G (GPa)	υ	$\underset{n}{\mathop \sum }\,{{\text{c}}_{n}}\text{ }\!\!\Delta\!\!\text{ }V_{n}^{2}$(Å⁶)	ΔE_b (eV)	σ_0(th) (MPa)	σ_0(exp) (MPa)
(NiCoCr)₉₂Al₆Ta₂	2.529	83	0.34	1.21	1.80	431	405
(NiCoCr)₉₈Ta₂	2.537	86.8	0.34	1.02	1.72	366	—
(NiCoCr)₉₄Al₆	2.532 [32]	82 [32]	0.29 [32]	0.69	1.32	204	—
NiCoCr	2.517 [41]	87 [79]	0.30 [21]	0.35	1.14	136	135 [72] 143 [73] 158
NiCoFe	2.524 [41]	60 [79]	0.35 [21]	0.26	0.76	57	63 [81]
NiFeMn	2.557 [41]	73 [79]	0.24 [21]	0.48	0.98	97	119 [81]
NiCoV	2.546 [62]	72 [62]	0.334 [62]	1.71	1.69	392	383 [62]
NiCoFeMn	2.540 [41]	77 [79]	0.22 [21]	0.47	0.99	98	131 [79] 108 [81]
NiCoCrMn	2.538 [41]	78 [79]	0.25 [21]	0.51	1.08	123	131 [62]
NiCoCrFe	2.525 [41]	84 [79]	0.28 [21]	0.34	1.05	109	101 [79] 123 [81]
NiCoCrFeMn	2.545 [41]	80 [79]	0.26 [21]	0.43	1.06	112	125 [61] 130 [79]

Fig. 9. A comparison of (a) the absolute increment and (b) relative increment in the strength of the present Al6Ta2 MEA with reported (NiCoCr)100-xMx alloys.

Fig. 10. (a) A comparison of the experimental and theoretical results of lattice strength σ0 of the present and reported fcc HEAs/MEAs, and (b) The correlation between lattice friction stress σ0(th) and energy barrier ΔEb, showing the lattice strength increment (or SSH) caused by 6 at.% Al and 2 at.% Ta is about 68 and 230 MPa, respectively.

Fig. 11. The competitive mechanisms for deformation twinning of NiCoCr MEA (a) and Al6Ta2 MEA (b), respectively, showing their corresponding transition size regimes are about 11.7-34.5 μm and 9.5-29.5 μm, based on Eqs. (8) and (9) respectively.

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