Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation

doi:10.1016/j.jmst.2021.01.096

Abstract

Abstract:

To have an insight into the occurrence of inverse Hall-Petch relationship in ultrafine-grained (UFG) aluminum alloys produced by severe plastic deformation (SPD), ultra-SPD (i.e. inducing several ten thousand shear strains via high-pressure torsion, HPT) followed by aging is applied to an Al-La-Ce alloy. Average nanograin sizes of 40 and 80 nm are successfully achieved together with strain-induced Lomer-Cottrell dislocation lock formation and aging-induced semi-coherent Al11(La,Ce)3 precipitation. Analysis of hardening mechanisms in this alloy compared to SPD-processed pure aluminum with micrometer grain sizes, SPD-processed Al-based alloys with submicrometer grain sizes and ultra-SPD-processed Al-Ca alloy with nanograin sizes reveals the presence of two breaks in the Hall-Petch relationship. First, a positive up-break appears when the grain sizes decrease from micrometer to submicrometer which is due to extra hardening by solute-dislocation interactions. Second, a negative down-break and softening occur by decreasing the grain sizes from submicrometer to nanometer which is caused by weakening the dislocation hardening mechanism with minor contribution of the inverse Hall-Petch mechanism. Detailed analyses confirm that nanograin formation is not necessarily a solution for extra hardening of Al-based alloys and other accompanying strategies such as grain-boundary segregation and precipitation are required to overcome such a down-break and softening.

Key words: Aluminum-lanthanum-cerium alloys, Nanostructured alloys, Reverse Hall-Petch relationship, Precipitation hardening, Dislocation hardening

Abbas Mohammadi, Nariman A. Enikeev, Maxim Yu. Murashkin, Makoto Arita, Kaveh Edalati. Examination of inverse Hall-Petch relation in nanostructured aluminum alloys by ultra-severe plastic deformation[J]. J. Mater. Sci. Technol., 2021, 91: 78-89.

Figures/Tables 10

Table 1 Average grain size (dav), yield strength (σ) or one third of Vickers hardness (HV/3) and dislocation density (DDis) reported in literature [20,33,[40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]] for pure aluminum and Al-based alloys after processing by different SPD methods.

Material	Process	d_av (nm)	σ or HV/3 (MPa)	D_Dis (m^-2)	Refs.
Al (99.9999%)	HPT	20000	53		[33]
Al (99.999%)	HPT	4800	89		[33]
Al (99.999%)	ECAP	15000	72		[40]
Al (99.999%)	ECAP	1300	115		[41]
Al (99.99%)	HPT	1300	110		[33]
Al (99.99%)	ECAP	1300	135		[42]
Al (99.99%)	ECAP	2000	98		[43]
Al (99.99%)	ECAP	1000	120	1.8×10¹⁴	[44]
Al (99.99%)	ARB	1100	97	1.2×10¹³	[45]
Al (99.99%)	ARB	690	120	1.2×10¹³	[46]
Al (99.99%)	ARB	690	97	1.23×10¹³	[20]
A1050	ARB	600	260	1.33×10¹⁴	[47]
A1050	HPT	500	177	2-4×10¹⁴	[48]
A1050	HPT	600	173		[33]
A1100	HPT	400	285		[33]
A1100	ECAP	680	185		[49]
A1100	ARB	330	290		[50]
A1100	ARB	280	310	1.3×10¹⁴	[46]
A1100	ARB	260	302	2.2×10¹⁴	[51]
A2024	ARB	350	425	1.0×10¹⁵	[52]
A2024	ECAP	300	325		[49]
A3004	ECAP	290	370		[49]
A5083	ECAP	225	420		[49]
A6060	HPT	180	525		[53]
A6061	ECAP	400	380		[54]
A6061	ECAP	290	280		[49]
A6061	ECAP	280	327		[41]
A6061	HPT	200	510	2.6×10¹⁴	[55]
A6061	ARB	240	370		[56]
A6061	ARB	310	363		[57]
A6063	ECAP	500	255		[53]
A7075	ECAP	210	480		[49]
A8011	ARB	700	180		[58]
Al-1.1Mg (at%)	HPT	390	415		[59]
Al-3.3Mg (at%)	HPT	200	600		[59]
Al-5.5Mg (at%)	HPT	190	660		[59]
Al-8.8Mg (at%)	HPT	140	735		[59]
Al-1.3Ag (at%)	HPT	500	200		[59]
Al-3.0Ag (at%)	HPT	367	245		[59]
Al-5.9Ag (at%)	HPT	278	370		[59]
Al-1.7Cu (at%)	HPT	207	670		[59]
Al-2Fe (wt%)	HPT	300	440	< 1.1×10¹⁵	[60]
Al-2Fe (wt%)	HPT	150	590	< 2.6×10¹⁵	[60]
Al-2Fe (wt%)	HPT	140	655	< 3.3×10¹⁵	[60]
Al-0.2Zr (wt%)	ECAP	630	160		[61]
Al-3Mg (wt%)	ECAP	200	390	2.7×10¹⁵	[44]
Al-3Mg (wt%)	HPT	150	655		[62]
Al-1.6Fe-1Cu (wt%)	ECAP	370	190	3.22×10¹⁴	[63]
Al-3Cu-1Li (wt%)	HPT	130	700	2.78×10¹⁵	[64]

Material	Process	d_av (nm)	σ or HV/3 (MPa)	D_Dis (m^-2)	Refs.
Al (99.9999%)	HPT	20000	53		[33]
Al (99.999%)	HPT	4800	89		[33]
Al (99.999%)	ECAP	15000	72		[40]
Al (99.999%)	ECAP	1300	115		[41]
Al (99.99%)	HPT	1300	110		[33]
Al (99.99%)	ECAP	1300	135		[42]
Al (99.99%)	ECAP	2000	98		[43]
Al (99.99%)	ECAP	1000	120	1.8×10¹⁴	[44]
Al (99.99%)	ARB	1100	97	1.2×10¹³	[45]
Al (99.99%)	ARB	690	120	1.2×10¹³	[46]
Al (99.99%)	ARB	690	97	1.23×10¹³	[20]
A1050	ARB	600	260	1.33×10¹⁴	[47]
A1050	HPT	500	177	2-4×10¹⁴	[48]
A1050	HPT	600	173		[33]
A1100	HPT	400	285		[33]
A1100	ECAP	680	185		[49]
A1100	ARB	330	290		[50]
A1100	ARB	280	310	1.3×10¹⁴	[46]
A1100	ARB	260	302	2.2×10¹⁴	[51]
A2024	ARB	350	425	1.0×10¹⁵	[52]
A2024	ECAP	300	325		[49]
A3004	ECAP	290	370		[49]
A5083	ECAP	225	420		[49]
A6060	HPT	180	525		[53]
A6061	ECAP	400	380		[54]
A6061	ECAP	290	280		[49]
A6061	ECAP	280	327		[41]
A6061	HPT	200	510	2.6×10¹⁴	[55]
A6061	ARB	240	370		[56]
A6061	ARB	310	363		[57]
A6063	ECAP	500	255		[53]
A7075	ECAP	210	480		[49]
A8011	ARB	700	180		[58]
Al-1.1Mg (at%)	HPT	390	415		[59]
Al-3.3Mg (at%)	HPT	200	600		[59]
Al-5.5Mg (at%)	HPT	190	660		[59]
Al-8.8Mg (at%)	HPT	140	735		[59]
Al-1.3Ag (at%)	HPT	500	200		[59]
Al-3.0Ag (at%)	HPT	367	245		[59]
Al-5.9Ag (at%)	HPT	278	370		[59]
Al-1.7Cu (at%)	HPT	207	670		[59]
Al-2Fe (wt%)	HPT	300	440	< 1.1×10¹⁵	[60]
Al-2Fe (wt%)	HPT	150	590	< 2.6×10¹⁵	[60]
Al-2Fe (wt%)	HPT	140	655	< 3.3×10¹⁵	[60]
Al-0.2Zr (wt%)	ECAP	630	160		[61]
Al-3Mg (wt%)	ECAP	200	390	2.7×10¹⁵	[44]
Al-3Mg (wt%)	HPT	150	655		[62]
Al-1.6Fe-1Cu (wt%)	ECAP	370	190	3.22×10¹⁴	[63]
Al-3Cu-1Li (wt%)	HPT	130	700	2.78×10¹⁵	[64]

Fig. 1. Microhardness plotted versus (a) shear strain and (b) electrical conductivity for Al-La-Ce alloy before and after HPT processing for various turns and after aging at different temperatures.

Fig. 2. XRD measurements and Rietveld analyses for Al-La-Ce alloy before and after HPT processing for various turns and after aging at 503 K. (a) Overall XRD profiles, (b) magnified view of (311) peak of Al, (c) lattice parameter of Al-based FCC phase plotted versus shear strain and (d) dislocation density and crystallite size of Al-based FCC phase plotted versus shear strain.

Fig. 3. (a) STEM bright-field image, (b) HAADF image, (c) EDS mapping for La, (d) EDS mapping for Ce, (e, h) TEM bright-field images, (f, i) TEM dark-field images and (g, j) SAED patterns taken from (a-d) overall microstructure, (e-g) eutectic region and (h-j) Al matrix for initial as-cast Al-La-Ce alloy.

Fig. 4. TEM bright-field images (left), dark-field images (right) and corresponding SAED patterns (center) for Al-La-Ce alloy processed by (a-c) HPT for N = 1000 and (d-f) HPT for N = 1000 followed by aging at 503 K.

Fig. 5. (a, e,i) STEM bright-field images, (b, f, j) HAADF images, (c, g) EDS mappings of La and (d, h) EDS mappings of Ce for Al-La-Ce alloy processed by (a-d) HPT for N = 1000 and (e-h) HPT for N = 1000 followed by aging at 503 K. Micrographs in (i) and (j) are magnified views of squared region in (e).

Fig. 6. TEM high-resolution images of Al-La-Ce alloy processed by HPT for N = 1000, where (c) and (d) are magnified views of squared regions indicated in (b). (a, b) distribution of Al grains and Al3RE11 (RE: La, Ce) particles, (c) semi-coherent Al/Al3RE11 boundary, and (d) Lomer-Cottrell dislocation lock within Al nanograin.

Fig. 7. TEM high-resolution images of Al-La-Ce alloy processed by HPT for N = 1000 and subsequently aged at 503 K, where (b), (d) and (f) are magnified views of squared regions indicated in (a), (c) and (e), respectively. (a) Al3RE11 (RE: La, Ce) particles at Al/Al boundary, (b) ordered structure of Al3RE11, (c) Al3RE11 particle at Al/Al/Al triple junction, (d) low-angle Al/Al boundary with corresponding geometrically-necessary dislocations denoted with T, (e, f) Al3RE11/Al3RE11 boundaries and (g) Al3RE11 precipitate within Al nanograin.

Fig. 8. Relationship between yield stress (σ) and one third of hardness (HV) with inverse root of average grain size (dav) for pure aluminum and Al-based alloys processed by SPD and for Al-La-Ce and Al-Ca processed by ultra-SPD and aging. Data for SPD processes were taken from Table 1 [20,33,[40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]] and data for ultra-SPD process of Al-Ca were taken from Ref. [15].

Table 2 Hardening by different mechanisms calculated using Eqs. (2-7) in comparison with experimentally-measured hardness.

	Calculated								Experimental	Difference
	σ₀ (MPa)	σ_GB (MPa)	σ_Dis (MPa)	σ_Pt (MPa)	σ_SS (MPa)	σ_Al (MPa)	σ (MPa)	HV(Hv)	HV(Hv)	HV(Hv, %)
Ultra-SPD	10	400	168	0	14	592	649	199	203	4 Hv, 2%
Aging	10	283	50	258	6	607	673	206	218	12 Hv, 5%

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