Relationship of particle stimulated nucleation, recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

doi:10.1016/j.jmst.2019.07.014

Abstract

Abstract:

The variations of coarse intermetallic particles in hot-extruded 7055 aluminum alloys with 0.041 wt% Fe and 0.024 wt% Si increasing to 0.272 wt% Fe and 0.134 wt% Si were investigated. The particle stimulated nucleation (PSN) behaviors for different kind of coarse particles were detailly analyzed by EBSD. Moreover, the effect of PSN responding to Fe and Si contents on recrystallization and tensile properties of 7055 alloys was evaluated. With increasing Fe and Si contents, the size and number density of coarse η/S particles are reduced, while the number densities of coarse Al₇Cu₂Fe and Mg₂Si particles are both increased and the coarse Al₇Cu₂Fe particles transform from rod-like to irregular. More PSN recrystallized grains with predominant orientations deviated from the extruded fiber textures are stimulated by the irregular Al₇Cu₂Fe and Mg₂Si particles, because a higher degree of local non-uniform deformation is produced. The rod-like Al₇Cu₂Fe particles cause the greatest degree of local non-uniform deformation owing to the largest aspect ratio, but the shape also restricts the area of particle deformation zone (PDZ) resulting in fewer PSN recrystallized grains. The irregular η/S particles give rise to the lowest degree of local non-uniform deformation and fewest PSN recrystallized grains with the major orientations close to the extruded fiber textures. Consequently, despite the number and size of coarse η/S particles are reduced, the proportion of high angle grain boundaries (HAGBs) is increased and the extruded fiber textures are weakened with Fe and Si contents increasing, because of the increased Al₇Cu₂Fe and Mg₂Si particles. The strength is slightly declined by the weakened <111>//ED (extrusion direction) fiber texture, while the elongation is reduced for a larger number of coarse particles and more HAGBs with higher Fe and Si contents.

Key words: 7055 aluminum alloy, Particles, Recrystallization, Texture, Mechanical properties

She Huan, Shu Da, Dong Anping, Wang Jun, Sun Baode, Lai Hongchang. Relationship of particle stimulated nucleation, recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys[J]. J. Mater. Sci. Technol., 2019, 35(11): 2570-2581.

Figures/Tables 14

Table 1 Actual chemical compositions of experimental alloys (wt%).

Alloy	Fe	Si	Zn	Mg	Cu	Mn	Cr	Ti	Zr	Al
1	0.041	0.024	8.25	2.29	2.17	0.0010	0	0.0006	0.12	Bal.
2	0.272	0.134	8.26	2.14	2.05	0.0012	0.0006	0.0023	0.15	Bal.

Fig. 1. BSE images of 7055 alloys with different Fe and Si contents: (a, b) as-cast microstructures of alloys 1 and 2; (c, d) homogenized microstructures of alloys 1 and 2; (e, f) aged microstructures of alloys 1 and 2.

Table 2 Undissolved coarse intermetallic particles in aged 7055 alloys with different Fe and Si contents.

Alloy	Undissolved Coarse Intermetallic Particles
Alloy	Chemical composition	Shape
1	η/S	Irregular
	Al₇Cu₂Fe	Rod-like
	Mg₂Si	Irregular
2	η/S	Irregular
	Al₇Cu₂Fe/Al₃Fe	Irregular
	Mg₂Si	Irregular

Fig. 2. Influences of Fe and Si contents on coarse intermetallic particles of aged 7055 alloys: (a) average length (l); (b) aspect ratio (λ); (c, d) number density distributions of alloys 1 and 2.

Fig. 3. EBSD patterns of the matrix surrounding coarse intermetallic particles in the longitudinal section of aged alloys: (a) η/S in alloy 1; (b) Al7Cu2Fe in alloy 1; (c) Al7Cu2Fe larger than 70 μm in alloy 1; (d) Al7Cu2Fe and Mg2Si in alloy 2.

Fig. 4. Area of PDZ around the particles in alloy 1 and alloy 2 (The scattered points come from the experiment data and the curves are fitted by Eqs. (2) and (3)).

Table 3 fzone and H0 of the four kinds of particles.

Coarse intermetallic particles	f_zone	H₀ (μm^-2)
Irregular η/S (alloy 1)	1.33	0.0113
Rod-like Al₇Cu₂Fe (alloy 1)	1.92	0.0167
Irregular Al₇Cu₂Fe (alloy 2)	1.86	0.0439
Irregular Mg₂Si (alloy 2)	1.73	0.0239

Fig. 5. Number and average diameter of grains in the PDZ of coarse intermetallic particles: (a) number of recrystallized grains (the scattered points come from the experiment data and the curves are fitted by Eqs. (5) and (6)); (b) average diameter of recrystallized grains; (c) number of subgrains; (d) average diameter of subgrains.

Fig. 6. Number of recrystallized grains with different orientation in the PDZ surrounding coarse intermetallic particles of different lengths: (a) <111>10°; (b) <100>15°; (c) <111>10°-25°; (d) <100>15°-30°; (e) R.

Fig. 7. (a, b) EBSD patterns colored with inverse pole figures in ED of longitudinal section for aged alloys 1 and 2 and (c, d) corresponding grain boundaries patterns of aged alloys 1 and 2.

Fig. 8. (a, b) Distribution of grain boundaries misorientation of alloys 1 and 2 and (c) effect of Fe and Si on number fraction of different misorientation grain boundaries.

Fig. 9. (a, b) Scattered inverse pole figures of alloys 1 and 2, (c) area fractions of <111>10° and <100>15° and (d) area fractions of <111>10°-25°, <100>15°-30° and random orientation.

Fig. 10. Areas of PSN grains deviated from the extruded fiber textures of coarse particles: (a) <111>10°-25°; (b) <100>15°-30°; (c) R; (d) <111>10°-25° + <100>15°-30° + R.

Fig. 11. Influence Fe and Si contents on the mechanical properties of aged 7055 alloys: (a) tensile stress-strain curves of alloys 1 and 2 loaded along extrusion direction; (b, c) SEM micrographs of tensile fracture surface for alloys 1 and 2.

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