Refined microstructure and enhanced mechanical properties of AlCrFe2Ni2 medium entropy alloy produced via laser remelting

doi:10.1016/j.jmst.2021.05.033

Abstract

Abstract:

A Co-free as-cast AlCrFe₂Ni₂ medium entropy alloy (MEA) with multi-phases was remelted by fiber laser in this study. The effect of laser remelting on the microstructure, phase distribution and mechanical properties was investigated by characterizing the as-cast and the remelted AlCrFe₂Ni₂ alloy. The laser remelting process resulted in a significant decrease of grain size from about 780 μm to 58.89 μm (longitudinal section) and 15.87 μm (transverse section) and an increase of hardness from 4.72 ± 0.293 GPa to 6.40 ± 0.147 GPa (longitudinal section) and 7.55 ± 0.360 GPa (transverse section). It was also found that the long side plate-like microstructure composed of FCC phase, ordered B2 phase and disordered BCC phase in the as-cast alloy was transformed into nano-size weave-like microstructure consisting of alternating ordered B2 and disordered BCC phases. The mechanical properties were evaluated by the derived stress-strain relationship obtained from nano-indentation tests data. The results showed that the yield stress increased from 661.9 MPa to 1347.6 MPa (longitudinal section) and 1647.2 MPa (transverse section) after remelting. The individual contribution of four potential strengthening mechanisms to the yield strength of the remelted alloy was quantitatively evaluated, including grain boundary strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening. The calculation results indicated that dislocation and precipitation are dominant strengthening mechanisms in the laser remelted MEA.

Key words: Medium entropy alloy, Laser remelting, Microstructure, Nano-indentation, Strengthening mechanism

Tianyi Han, Yong Liu, Mingqing Liao, Danni Yang, Nan Qu, Zhonghong Lai, Jingchuan Zhu. Refined microstructure and enhanced mechanical properties of AlCrFe2Ni2 medium entropy alloy produced via laser remelting[J]. J. Mater. Sci. Technol., 2022, 99: 18-27.

Figures/Tables 14

Fig. 1. (a) XRD patterns of the as-cast and the remelted AlCrFe2Ni2 alloys; (b) enlarge image of the peaks of remelted sample at (1 1 0).

Fig. 2. (a) Morphological image of transverse section of the remelted fusion zone and the base metal for the AlCrFe2Ni2 alloy; (b) grain size distribution analysis of the as-cast alloy.

Fig. 3. EBSD images of the laser remelted samples (a) IPF Y map of longitudinal section; (b) IPF Y map of transverse section; (c) phase distribution map of longitudinal section; (d) phase distribution map of transverse section.

Fig. 4. Grain size distribution of two cross-sections of the remelted MEA (a) longitudinal section; (b) transverse section.

Fig. 5. SEM images of the (a) as-cast MEA and (b) remelted MEA; insets of (a) and (b) are SEM images at a higher magnification.

Fig. 6. TEM micrographs of the as-cast AlCrFe2Ni2 alloy. (a) HAADF image of representative phases; (b) The elemental distribution mapped with STEM; The corresponding SAED patterns of (c) FeNiCr-rich FCC phase (d) AlNi-rich ordered B2 phase (e) FeCr-rich BCC phase.

Table 1 Elemental distribution analysis of AlCrFe2Ni2 alloy.

	Chemical composition (at.%)
	Al	Cr	Fe	Ni
Nominal	16.7	16.7	33.3	33.3
As-cast	19.1	16.2	32.7	32.0
Remelted	17.5	16.0	33.7	32.8

Fig. 7. TEM micrographs of the remelted AlCrFe2Ni2 alloy. (a) HAADF images of the microstructure in remelted MEA. The inset is the magnified HAADF image showing the nano-size phases; (b) The elemental distribution mapped with STEM of the identical region marked in (a); (c) HRTEM showing the interface between B2 and BCC phases; The relative FFT patterns of (d) AlNi-rich ordered B2 phase and (e) FeCr-rich BCC phase.

Table 2 Chemical compositions of each phase in as-cast and remelted AlCrFe2Ni2 alloys.

Phase	Sample	Chemical composition (at.%)
Phase	Sample	Al	Cr	Fe	Ni
B2	As-cast	27.90	2.50	14.48	55.12
B2	Remelted	26.36	4.72	19.88	49.04
BCC	As-cast	0.86	43.44	51.52	4.18
BCC	Remelted	3.52	28.43	49.17	18.89
FCC	As-cast	4.25	23.87	48.35	23.53
FCC	Remelted	/	/	/	/

Fig. 8. (a) Hardness obtained from nano-indentation tests; (b) Load-displacement curves of as-cast and two cross-sections of the remelted MEA.

Table 3 Parameters of constitutive equations calculated from dimensionless functions.

	E(GPa)	E/σ_y	ν	n	σ_y(MPa)
As-cast	216.5	327.0887	0.3	0.2574	661.9
Longitudinal section	153.1	113.6094	0.3	0.3296	1347.6
Transverse section	204.7	124.2715	0.3	0.3054	1647.2

Fig. 9. (a) Calculated stress-strain curves of the as-cast and two cross-sections of the laser remelted MEA samples; (b) Room-temperature tensile stress-strain curve of the as-cast MEA.

Fig. 10. (a) KAM map of transverse section of the remelted MEA; (b) Local misorientation difference frequency.

Fig. 11. Size of the precipitate distribution measured by TEM images.

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