Laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy with significantly enhanced wear resistance

doi:10.1016/j.jmst.2021.09.029

Abstract

Abstract:

Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory high-entropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ∼100 µm-thick gradient nanostructured layer is dramatically refined from the original ∼200 µm to only ∼8 nm in the top surface layer. The microhardness is therefore gradually increased from ∼240 HV in matrix to ∼650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase body-centered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich body-centered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZrHf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.

Key words: Laser surface treatment, Refractory high-entropy alloy, Gradient nanostructure, Wear resistance, High-resolution transmission electron microscopy

Jiasi Luo, Wanting Sun, Ranxi Duan, Wenqing Yang, K.C. Chan, Fuzeng Ren, Xu-Sheng Yang. Laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy with significantly enhanced wear resistance[J]. J. Mater. Sci. Technol., 2022, 110: 43-56.

Figures/Tables 16

Fig. 1. A schematic illustration of LSR strategy.

Fig. 2. Phase and morphology of the as-cast and laser-treated TiZrHfTaNb RHEA. (a) EBSD inverse pole figure and (b) bright-field TEM image and corresponding SAED patterns (inset) of the as-cast TiZrHfTaNb RHEA. (c) Optical image for the morphology of laser-treated surface of TiZrHfTaNb RHEA. (d) Cross-sectional (ND-TD) SEM image of one typical surface laser-treated along the depth direction.

Fig. 3. Morphology of the cross-section (ND-LD) of laser treated TiZrHfTaNb RHEA. (a) Low-magnification observation (the boundary profile of the hardened layer is marked in white dashed line); (b) and (c) are magnified observation of the topmost region and bottom region of HZ in (a); (d) XRD patterns of the as-cast TiZrHfTaNb RHEA and the laser treated TiZrHfTaNb RHEA at different depths.

Fig. 4. Microstructure of the laser treated TiZrHfTaNb RHEA at the depth of 40-60 µm below the surface. (a) Bright-field TEM image, (b) the enlarged TEM image of rectangle area in (a), and (c) corresponding selected electron diffraction pattern. (d-f) Corresponding dark-field TEM images of the selected diffraction spots 1, 2 and 3 in (c), respectively.

Fig. 5. Phase and composition characterization at the depth of 40-60 µm below the surface. (a) HAADF-STEM image of secondary phase. (b-f) EDS elemental maps of Ti, Zr, Hf, Ta, and Nb, respectively. (g) HRTEM image obtained at the interface of the BCC matrix and HCP grains. (h, i) atomic Fourier-filtered images enraged from the squares marked in (g) and corresponding fast Fourier transform (FFT) patterns (insets), respectively.

Table 1. Quantitative EDS analysis of the region shown in Fig. 5(a).

Location of analysis	Element (at.%)
Location of analysis	Ti	Zr	Hf	Ta	Nb
Precipitates	17.4 ± 1.4	24.6 ± 2.2	24.4 ± 2.3	16.8 ± 1.4	16.8 ± 1.4
Phase boundary	19.8 ± 1.7	15.6 ± 1.2	12.6 ± 1.0	27.2 ± 2.3	24.8 ± 2.1
Matrix	20.3 ± 1.9	19.9 ± 1.7	19.8 ± 1.3	20.4 ± 1.6	19.6 ± 1.9

Fig. 6. TEM characterization of the region at the depth of 30-40 µm below the surface. (a) Bright-field TEM image and (b) corresponding SAED pattern. (c) HRTEM image of a representative region containing BCC, FCC and HCP phases. (d, e) FFT pattern and enlarged inverse FFT image of the square area marked in (c).

Fig. 7. Microstructure of the laser treated TiZrHfTaNb RHEA at the depth of 20-30 µm below the surface. (a) Bright-field TEM image at the depth of 20-30 µm, and corresponding (b, c) dark-field TEM image and grain size distribution; (d) HADDF-STEM image; (e-i) corresponding EDS elemental maps of Ti, Zr, Hf, Ta, and Nb, respectively. (j) HRTEM image of the including grains 2, 3 and 4. (k, l) IFFT and corresponding FFT (insets) images of the BCC grain (grain 4), and neighboring FCC - HCP grains (gain 2 and 3) in (j), respectively.

Table 2. Quantitative EDS analysis for the areas marked in Fig. 7(d).

Location of analysis	Crystal structure	Element (at.%)
Location of analysis	Crystal structure	Ti	Zr	Hf	Ta	Nb
1	BCC 1	44.9 ± 3.5	13.9 ± 1.1	8.8 ± 1.1	13.9 ± 1.3	18.5 ± 1.3
2	FCC	25.0 ± 2.1	24.7 ± 2.2	23.4 ± 2.3	13.1 ± 1.3	13.8 ± 1.4
3	HCP	17.9 ± 1.6	26.2 ± 2.1	26.1 ± 2.3	15.1 ± 1.3	14.7 ± 1.4
4	BCC 2	16.1 ± 1.3	12.8 ± 1.0	10.9 ± 1.1	29.9 ± 2.5	30.3 ± 2.8

Fig. 8. Microstructure of the laser treated TiZrHfTaNb RHEA near the topmost surface (< 10 µm). (a) TEM image with corresponding SAED pattern. (b) HRTEM image showing the NGs with extremely small grain size and (c) grain size distribution. (d) HRTEM image of the BCC and FCC NGs. (e, f) Atomic Fourier-filtered images of the BCC and FCC NGs showing the different crystalline defects.

Fig. 9. (a) Grain size and microhardness varying along the depth away from the surface of the laser treated TiZrHfTaNb RHEA. (b) A Hall-Petch plot of calculated yield stress for GNS TiZrHfTaNb RHEA in this work comparing with that of cold rolled (CR) TiZrHfTaNb (Refs. [8] and [45]), HPT-processed TiZrHfTaNb (Ref. [34]), spark-plasma-sintered (SPS) MoNbTaTiV (Ref. [44]), cast WNbMoTaV (Ref. [7]), CR TiZrHfNb (Ref. [46]), powder metallurgy (PM) NbTaTiV (Ref. [47]), HPT TiZrHfNb (Ref. [48]), Al-containing RHEAs (Ref. [49]), HPT-processed CoCrFeNiMn (Ref. [29]) and CoCrFeNiMnTi0.1 (Ref. [30]), cyclic dynamic torsion (CTD) processed Al0.1CoCrFeNi (Ref. [19]), rotationally accelerated shot peening (RASP) treated Co21.5Cr21.5Fe21.5Mn21.5Ni14 (Ref. [20]), and surface mechanical attrition treated (SMAT) FeCoNiCrMn (Ref. [50]).

Fig. 10. Comparisons of dry-sliding wear performance between the laser surface treated and as-cast TiZrHfTaNb RHEA. (a) The COFs as a function of sliding time of as-cast and laser-treated specimens under different normal loads (16N, 24N and 32N). (b) The wear rates of as-cast and laser-treated specimens under different normal loads (16N, 24N and 32N). (c) and (d) showing 2D cross-sectional profile along the dotted line (y direction, TD) in the corresponding inset. The insets in (c) and (d) display the 3D profile of wear tracks of as-cast and laser-treated specimens under the normal load of 24N, respectively. (e) and (f) are 3D profiles of worn surface for as-cast alloy and laser-treated alloy under the normal load of 24N, respectively.

Table 3. Wear characteristics of the alloys upon dry sliding against Si3N4 at room temperature.

Specimen	Normal load (N)	COFs	Wear rate (mm³/m·N)	Wear track roughness/Sa (µm)
As-cast	16	0.29 ± 0.01	3.41 ( ± 0.25) × 10^-5	1.90 ± 0.15
	24	0.30 ± 0.01	3.64 ( ± 0.30) × 10^-5	1.97 ± 0.16
	32	0.30 ± 0.01	3.83 ( ± 0.29) × 10^-5	2.46 ± 0.19
Laser- treated	16	0.32 ± 0.02	2.21 ( ± 0.17) × 10^-6	0.18 ± 0.07
	24	0.33 ± 0.01	2.38 ( ± 0.20) × 10^-6	0.47 ± 0.17
	32	0.34 ± 0.02	2.94 ( ± 0.22) × 10^-6	0.84 ± 0.20

Fig. 11. Surface morphology and composition of the alloys after dry sliding against Si3N4 under 24N at room temperature. (a) and (d) are SEM images of the worn surfaces of as-cast alloy and laser-treated alloy, respectively; the arrows indicate the sliding directions; (b) and (e) are high-magnification SEM images of the selected region in (a) and (d), respectively; (c) and (f) are EDS elemental maps of the worn surfaces of as-cast alloy and laser-treated alloy, respectively.

Fig. 12. Schematic diagram of microstructural evolution processes of TiZrHfTaNb RHEA induced by LSR. (a) Coarse BCC grain with multiple dislocations inside; (b) grains get refined, and HCP structured phases occur in the interior and grain boundaries of BCC grains; (c) FCC structured phases occur and grains get further refined, forming continuous network-like microstructures; (d) grains are continually refined and get disconnected; (e) grains are finally refined to several nanometers.

Table 4. Wear performance comparison between TiZrHfTaNb RHEA in this work and other RHEAs from reported literature.

Specimen	Wear condition			COFs	Wear rate	Refs.
Specimen	Counter-body	Normal load (N)	Sliding speed (m/s)	Empty Cell	(mm³/(N·m))	Empty Cell
Laser-treated TiZrHfTaNb	6 mm Si₃N₄	16, 24, 32	0.01	0.32-0.34	(2.21-2.94) × 10^-6	This work
As-cast TiZrHfTaNb				0.29-0.30	(3.41-3.83) × 10^-5
723 K-annealed TiZrHfTaNb	6 mm Si₃N₄	5	0.035, 0.35	0.46-0.65	(0.5-2.5) × 10^-4	[60]
723 K-annealed TiZrHfNb	6 mm Si₃N₄	5	0.035, 0.35	0.42-0.62	(1.7-2.2) × 10^-4	[60]
As-cast	6 mm Al₂O₃	5	0.1	∼ 0.5	0.83	[61]
MoNbTaVW	6 mm 100Cr₆			∼ 0.7	0.21-0.23
As-cast	6 mm Al₂O₃	5	0.1	∼0.72	0.13	[62]
MoNbTaTiZr	6 mm 100Cr₆			∼ 0.6	1.5 × 10^-2
730 K-annealed HfTaTiVZr	3 mm Si₃N₄	50	0.035	∼0.25	∼ 10^-4	[63]
730 K-annealed TaTiVWZr	3 mm Si₃N₄	50	0.035	∼0.32	∼ 10^-4
1200 °C-annealed HfNbTiZr	600 nm 90 °C Cone-spherical diamond tip (Nanoscratch)	0.01-1	(0.01, 0.1, 1) × 10^-6	∼ 0.16	1.28 × 10^-2	[64]

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