Microstructure evolution of spray deposited and as-cast 2195 Al-Li alloys during homogenization

doi:10.1016/j.jmst.2020.12.028

Abstract

Abstract:

In this paper, a comparative study on the spray deposited and as-cast 2195 alloy was carried out to reveal their microstructure evolutions and differences during the homogenization process. The dissolution of the secondary particles and the diffusion of solute were studied based on microstructure characterization and kinetics analysis. The precipitation behavior of Al₃Zr dispersoids and its influence on recrystallization were investigated by using TEM and EBSD characterization. It was found that the large-size particles at triangular grain boundaries dissolve slower than the intragranular phases and other grain boundary phases. The required homogenization time depends on the dissolution processes of the large-size phases at grain boundaries. The size of grain boundary phases in the spray deposited alloy is much smaller than that in the as-cast alloy, so the homogenization time required for the spray deposited alloy is significantly shorter. Two-stage and ramp heating homogenization processes can promote the precipitation of Al₃Zr dispersoids in the two alloys. In the spray deposited alloy, the dispersoids tend to precipitate at the positions of the T₁ plates dissolved, which causes a non-uniform distribution and decreases the recrystallization resistance of the alloy. However, the distribution of the dispersoids in the as-cast alloy is more uniform after the homogenization, which brings a stronger inhibition on the recrystallization. According to the microstructural characterization and kinetics analysis results, it can be concluded that the homogenization with a slow ramp heating is suitable for the two 2195 alloys, and a shorter holding time can be used for spray deposited alloy, e.g. 12 h at 500 °C, while the holding time for the as-cast alloy is no less than 35 h at 500 °C.

Key words: Al-Li alloy, Homogenization, Dispersoids, Recrystallization, Diffusion kinetics

Yongxiao Wang, Xinwu Ma, Guoqun Zhao, Xiao Xu, Xiaoxue Chen, Cunsheng Zhang. Microstructure evolution of spray deposited and as-cast 2195 Al-Li alloys during homogenization[J]. J. Mater. Sci. Technol., 2021, 82: 161-178.

Figures/Tables 24

Table 1 Chemical compositions (wt.%) of the spray deposited and as-cast 2195 alloys.

Alloy type	Cu	Li	Mg	Ag	Zr	Mn	Si	Fe	Al
Spray deposited	3.72	1.06	0.44	0.31	0.11	<0.25	<0.1	<0.15	Bal.
As-cast	4.08	1.06	0.51	0.35	0.09	<0.25	<0.12	<0.15	Bal.
Standard	3.7-4.3	0.8-1.2	0.25-0.8	0.25-0.6	0.08-0.16	<0.25	<0.12	<0.15	Bal.

Fig. 1. Schematic diagram of the experimental route.

Table 2 Homogenization treatments for spray deposited and as-cast alloys 2195.

Alloy type	Treatment label	Treatment condition
Spray deposited alloy	NT_SP	No treatment
	ST_{SP(-2, 8, 12, 24)}	500 °C / 2 h, 8 h, 12 h, 24 h
	DT_SP	440 °C / 12 h + 500 °C / 24 h
	RHT_SP	Ramp heating with 1 °C/min and 500 °C / 24 h
As-cast alloy	NT_CA	No treatment
	ST_CA	500 °C / 24 h
	DT_CA	440 °C / 12 h + 500 °C / 24 h
	RHT_CA	Ramp heating with 1 °C/min and 500 °C / 24 h

Fig. 2. Sampling and observation method for TEM and EBSD tests.

Fig. 3. BSE images of (a) spray deposited alloy and (b) as-cast alloy; (c) XRD patterns of the two alloys; (d) DSC curves of the two alloys.

Fig. 4. BSE images and element distributions (a) of the spray deposited alloy and (b) the as-cast alloy.

Fig. 5. Evolution of secondary particles during homogenization of (a), (b) the spray deposited alloy and (c)?(f) the as-cast alloy.

Fig. 6. In-situ SEM images for revealing the transient evolution of secondary particles within the grains.

Fig. 7. TEM images for revealing the edge of T1 phase: (a) bright-field image; (b) high-resolution image along the zone axis of <110>Al.

Fig. 8. TEM images of the spray deposited samples NTSP and STSP-24: (a) bright-field image of the sample NTSP; (b) corresponding SADP along <001>Al zone axis; (c) bright-field image of the sample STsp-24; (d) corresponding SADP along <110>Al zone axis; (e) EDS mapping of area I; (f) enlarged image of area II.

Fig. 9. TEM images of spray deposited samples DTSP and RHTSP: (a) bright field of the sample DTSP; (b) enlarged image of area I and corresponding SADP along <110>Al zone axis; (c) bright field of the sample RHTSP; (d) enlarged image of area II and corresponding SADP along <110>Al zone axis.

Fig. 10. TEM images of the as-cast samples NTCA and DTCA: (a) bright field of the sample NTCA; (b) enlarged image of area I and corresponding SADP along <110>Al zone axis; (c) EDS point analysis of the spherical phase in the sample NTCA; (d) bright field of the sample DTCA; (e) TEM image of DTCA samples at high magnification and corresponding SADP along <110>Al zone axis; (f) EDS point analysis of the spherical phase in sample DTCA.

Fig. 11. EBSD-derived images of the non-homogenized spray deposited sample NTSP after hot deformation and heat treatment: (a) IPF map of the sample after deformation and 1 h solution treatment; (b) corresponding GOS map; (c) IPF map of the sample subjected to another 30 min solution treatment.

Fig. 12. EBSD-derived images of the spray deposited samples after hot deformation and solution treatment: (a)-(d) GOS maps; (e)-(f) boundary misorientation distributions.

Fig. 13. EBSD-derived images of the spray deposited samples after hot deformation and solution treatment: (a) GOS map of DTSP sample; (b) grain boundary misorientation distribution of the DTSP sample; (c) GOS map of RHTSP sample; (d) grain boundary misorientation distribution of the RHTSP sample.

Fig. 14. EBSD-derived images of the as-cast sample NTCA after deformation and heat treatment: (a) IPF map of the sample after deformation and 1 h solution treatment; (b) corresponding GOS map; (c) IPF map of the sample subjected to another 30 min solution treatment.

Fig. 15. EBSD-derived images of the as-cast samples after hot deformation and solution treatment (including samples STCA-24, DTCA, and RHTCA): (a)-(c) GOS maps of the three samples; (d)-(e) corresponding grain boundary misorientation distributions.

Fig. 16. Schematic diagram of solute concentration distribution between T1 plates.

Fig. 17. (a) HRTEM image of the plate-like T1 phase in the spray deposited alloy (along <110>Al zone axis); (b) bright-field image for revealing the spacing of the T1 plates; (c) dissolution rate curves of intragranular plate-like phase; (d) diffusion kinetic curves of solute elements with grains; (e) BSE image of the spray deposited sample after treating at 430 °C for 15 min; (f) Cu element distribution in the corresponding area.

Fig. 18. (a) Schematic diagram of solute concentration distribution around the spherical phase; (b) dissolution rate curve of phase at grain boundary; (c) grain boundary phase size in the spray deposited alloy; (d) grain boundary phase size in the as-cast alloy.

Fig. 19. (a) Diffusion kinetics curves of solute at the grain boundary; (b) BSE image of the spray deposited sample after treating at 500 °C for 12 h; (c) and (d) Cu and Fe elements distribution in the corresponding area.

Fig. 20. TEM bright-field images of (a) the spray deposited sample STsp-24, (b) the spray deposited alloy, and (c) the as-cast alloy.

Fig. 21. SEI and IPF map of spray deposited sample NTSP after hot deformation and quenching: (a) SEI of the sample after electropolishing; (b) corresponding IPF map.

Fig. 22. (a) LoM map of the area corresponding to Fig. 11(a); (b) LoM map of the area corresponding to Fig. 11(c).

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