Phase transformation and structural evolution in a Ti-5at.% Al alloy induced by cold-rolling

doi:10.1016/j.jmst.2020.02.032

Abstract

Abstract:

The phase transformation, deformation mechanism and their correlation in a cold-rolled Ti-5at.%Al alloy were investigated. Two types of phase transformations from a hexagonal close-packed (HCP) structure to a face-centered cubic (FCC) structure were observed: the basal-type (B-type) with an orientation relationship of $<1\bar{2}\text{10}{{\text{}}_{\text{HCP}}}<1\bar{1}\text{0}{{\text{}}_{\text{FCC}}}$ and {0001}_HCP//{111}_FCC, and the prismatic-type (P-type) with an orientation relationship of $<1\bar{2}\text{10}{{\text{}}_{\text{HCP}}}<1\bar{1}\text{0}{{\text{}}_{\text{FCC}}}$ and ${{\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{0 }\!\!\}\!\!\text{ }}_{\text{HCP}}}\text{// }\!\!\{\!\!\text{ 110}{{\text{ }\!\!\}\!\!\text{ }}_{\text{FCC}}}$. The two types of transformations both accommodate the strain along the < c> axis of the HCP matrix. With the proceeding of deformation, different deformation mechanisms were activated in the FCC and the HCP structures, respectively, which led to a faster grain refinement rate in the FCC structure than in the HCP matrix. Deformation twins with zero macroscopic strain were prevalent in the FCC domains produced by the B-type transformation, while deformation twins with macroscopic strain were normally observed in the FCC domains produced by the P-type transformation. This is in accordance with the lattice mismatches produced during phase transformation. The easy occurrence of deformation twinning in the FCC structure contributed significantly to the grain refinement process. In addition, the interaction between neighboring FCC domains produced by the two types of phase transformations also accelerated the grain refinement process.

Key words: Ti-Al alloy, Deformation mechanism, Phase transformation, Grain refinement, Deformation twinning

Bingqiang Wei, Song Ni, Yong Liu, Xiaozhou Liao, Min Song. Phase transformation and structural evolution in a Ti-5at.% Al alloy induced by cold-rolling[J]. J. Mater. Sci. Technol., 2020, 49: 211-223.

Figures/Tables 13

Fig. 1. (a) A bright field TEM image of the un-deformed Ti-5at.%Al alloy, with the inset an SAED pattern under the [1 $\bar{2}$10] zone axis of the HCP matrix. (b) A bright-field TEM image of the R20 sample with FCC lamellas inside. (c) An SAED pattern taken from the area marked by the red dotted circle in (b), showing the HCP to FCC phase transformation with the OR of <1 $\bar{2}$10>HCP/???????/<1 $\bar{1}$0>FCC and {0001}HCP//{111}FCC. (d) An HRTEM image of an interface between the HCP and FCC phases, with the inset being a Fourier filtered image taken from the area marked by the square in Fig.1(d).

Fig. 2. (a) A bright-field TEM image of the R20 sample with another type of FCC lamella inside. (b) The SAED pattern taken from the interface of the HCP and FCC phases in (a), showing the HCP to FCC phase transformation with the OR of [0001]HCP//[001]FCC and (10$\bar{1}$0)HCP//(110)FCC. (c) An HRTEM image of the interface of the HCP and FCC phases, with the inset being a Fourier filtered image taken from the area marked by the square. (d) A bright-filed TEM image of a {101-2}<$\bar{1}$011> deformation twin, and the insets in the upper left and lower right corners are an SAED pattern and an HRTEM image of the {101-2}<$\bar{1}$011> twin, respectively.

Fig. 3. (a) A bright-field TEM image of the R40 sample. (b) An SAED pattern taken from the FCC area b in (a). (c) A dark-field TEM image using the reflection marked by the white arrow in (b). (d) An SAED pattern taken from the interface between the HCP and FCC area d in (a). (e) An SAED pattern taken from the HCP matrix area e in (a).

Fig. 4. (a) A bright-field TEM image of the HCP matrix in the R40 sample. (b, c) SAED patterns taken from the areas b and c marked by red dotted circles in (a), respectively.

Fig. 5. (a) A bright-field TEM image of the R60 sample for the HCP matrix, with the inset being an SAED pattern obtained from the area outlined by a red dotted circle. (b) A dark-field TEM image of (a) using the reflection marked by the red circle in the inset of (a). (c) A bright-field TEM image of the R60 sample for the FCC structure and the inset is a corresponding SAED pattern. (d) The grain size distribution of the FCC structure in the R60 sample.

Fig. 6. (a) An HRTEM image of zero macroscopic strain twins with the 9R structure. The inset is a Fourier filtered image taken from the area marked by the square. (b) Another HRTEM image of the 9R structure, with the inset being an FFT pattern taken from the area marked by the square. (c) An HRTEM image observed in a large area of the FCC structure in the R40 sample, and (d) an enlarged HRTEM image corresponding to the area marked by the square in (c).

Fig. 7. Schematics for the formation of ITBs and the 9R structure. (a) Schematics of the b1, b2, b3 partials for the HCP and FCC structures viewed from [0001]HCP and [111]FCC directions, respectively. (b) Schematic for the HCP to FCC transformation and formation of ITBs and the 9R structures under the action of b1, b2 and b3 partials.

Fig. 8. (a) A bright-field TEM image of the P-type phase transformation and (b) an HRTEM image taken from the area marked by the square in (a). (c) An HRTEM image of deformation twins with macroscopic strain observed in the FCC structure corresponding to the P-type phase transformation.

Fig. 9. (a,b) Atomic schematics of the lattice parameters and ORs for the two types of HCP to FCC phase transformations. (c,d) Fourier filtered HRTEM images of the interfaces of the HCP and the FCC phases for two types of phase transformations.

Fig. 10. (a) A bright-field TEM image containing both the B-type and the P-type phase transformations, with the inset being an SAED pattern taken from the area outlined by the white box. (b) An HRTEM image corresponding to the area outlined by the box in (a). (c) Another HRTEM image containing the two types of phase transformations. (d) Another HRTEM image in a large area of the FCC structure in the R40 sample.

Fig. 11. (a) A schematic of the experimental observed OR for the B-type phase trans-formation. (b,c) Schematics of the OR viewed along two other directions of the B typephase transformation.

Fig. 12. (a) A schematic of the experimental observed OR for the P-type phase trans-formation. (b,c) Schematics of the OR viewed along two other directions of the P typephase transformation.

Fig. 13. (a) An HRTEM image observed in a large area of the FCC structure. (b,c,d) are enlarged HRTEM images corresponding to the areas indicated by the numbers 1, 2, and 3, respectively.

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