Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement

doi:10.1016/j.jmst.2018.04.018

Abstract

Abstract:

Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl₃ and TiB₂ in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl₃ particles and then gradually expanded, which resulted in continuous refinement of TiAl₃ particles. In addition, micro-distribution uniformity of TiB₂ particles was improved due to the impingement of TiAl₃ particles to TiB₂ clusters during deformation. Excessively large sizes of TiAl₃ particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl₃ particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18-22 μm for TiAl₃ particles was suggested.

Key words: Equal channel angular pressing process, Secondary phases, Refinement mechanism, Grain refinement effect

Hong Yu, Ning Wang, Renguo Guan, Di Tie, Zheng Li, Yanan An, Yang Zhang. Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement[J]. J. Mater. Sci. Technol., 2018, 34(12): 2297-2306.

Figures/Tables 16

Table 1 Composition (wt%) of the Al-5Ti-1B alloy ingots.

Element	Ti	B	Fe	Si	V	Al
Weight Percent (%)	4.82	1.04	0.133	0.132	0.008	Others

Fig 1. Microstructure of Al-5Ti-1B alloy after ECAP: (a) as-cast, (b) 3 passes, (c) 6 passes, (d) 9 passes.

Fig. 2. Variation in average size of TiAl3 versus ECAP passes.

Fig. 3. Variation in average aspect ratio of TiAl3 versus ECAP passes.

Fig. 4. Distribution of TiAl3 particles in Al-5Ti-1B alloy: (a) as-cast, (b) 3 passes, (c) 6 passes, (d) 9 passes.

Fig. 5. Microstructure of TiB2 in as-cast and multi-pass ECAP processed Al-5Ti-1B alloy: (a) as-cast, (b) 1 pass, (c) 5 passes, (d) 10 passes.

Fig. 6. Average size of TiB2 particles in Al-5Ti-1B alloy versus different ECAP passes.

Fig. 7. Distribution of TiB2 particles in Al-5Ti-1B alloy: (a) as-cast, (b) 10 passes.

Fig. 8. Microstructure of pure Al versus the addition of Al-5Ti-1B alloy treated by multi-pass ECAP: (a) non-refined, (b) refined by as-cast Al-5Ti-1B alloy, (c) the addition of Al-5Ti-1B alloy with 2 ECAP passes, (d) the addition of Al-5Ti-1B alloy with 4 ECAP passes, (e) the addition of Al-5Ti-1B alloy with 6 ECAP passes, (f) the addition of Al-5Ti-1B alloy with 8 ECAP passes.

Fig. 9. The final average grain size of pure Al refined by Al-5Ti-1B alloy processed with ECAP prior to addition to the pure Al versus: (a) different ECAP passes and, (b) different average sizes of TiAl3 particles.

Fig. 10. Micro-cracks on the surface of TiAl3 in Al-5Ti-1B alloy processed by ECAP for 10 passes: (a) micro-cracks on the surface of TiAl3 are circled by solid line; the merged cracks are indicated by dotted line, (b) initial micro-cracks are within the circle; the dotted lines point out the growing micro-cracks have not yet merged.

Fig. 11. SEM images of the morphology for Al-5Ti-1B alloy after 10 ECAP passes: (a) internal micro-cracks in TiAl3, (b) micro-cracks expansion, (c) fracture of TiAl3, (d) schematic illustration of cracks formation in TiAl3: the appearance, growth of micro-cracks and as well as the refinement of TiAl3.

Fig. 12. SEM image of the morphology for Al-5Ti-1B alloy after 1 ECAP pass (partial magnification).

Fig. 13. (a) SEM image of morphology for as-cast Al-5Ti-1B alloy and (b) enlarged image of the frame part in (a).

Fig. 14. TiAl3 particles inserted in TiB2 clusters during their movements.

Fig. 15. The average size of TiAl3 particles and the average grain size of pure Al, versus different ECAP passes (the refining time is 5 min).

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