INVESTIGATION OF THE MICROSTRUCTURE
AND TEXTURE OF A Ti-3Al-2.5V SHEET DURING
COLD UNIDIRECTIONAL-ROLLING PROCESS

Mélaine Tournay, J.S. Lecomte, Christophe Schuman, Y. D. Zhang, Eric Fleury

► To cite this version:

Mélaine Tournay, J.S. Lecomte, Christophe Schuman, Y. D. Zhang, Eric Fleury. INVESTIGATION OF THE MICROSTRUCTURE AND TEXTURE OF A Ti-3Al-2.5V SHEET DURING COLD UNIDIRECTIONAL-ROLLING PROCESS. Ti-2023-World Titanium Conference 2023, Jun 2023, Edinburgh, United Kingdom, IOM3, Jun 2023, Edinbourg, Ecosse, United Kingdom. 5 p., 10.7490/f1000research.1119885.1. http://hal-05337255

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# INVESTIGATION OF THE MICROSTRUCTURE AND TEXTURE OF A Ti-3Al-2.5V SHEET DURING COLD UNIDIRECTIONAL-ROLLING PROCESS

Mélaine Tournay¹, Jean-Sébastien Lecomte¹, Christophe Schuman¹, Yudong Zhang¹, Éric Fleury¹

1 Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux (LEM3), CNRS UMR 7239, Université de Lorraine, F-57045 Metz, France

This study investigates a Ti-3Al-2.5V sheet exhibiting an inhomogeneous microstructure and a strong transverse texture with <c>-axes in the transverse direction (TD). Cold unidirectional-rolling is carried out in the rolling direction (RD-rolling) and in the transverse direction (TD-rolling) to deformation thickness reduction ratio of 5% to 75%. The microstructure and texture evolutions are studied by Electron-Back-Scattered Diffraction (EBSD). By progressively increasing the thickness reduction ratio during TD-rolling, the <c>-axes is gradually brought from the TD towards the normal direction (ND), which eventually results in a basal texture. In contrast, during RD-rolling, the <c>-axes remained inclined by 30° from the ND towards the TD. Tensile twins {1012} are only observed during TD-rolling inside macrozones composed of small grains (< 2 µm) with a high density of low angle grain boundaries (LAGBs). The annealing carried out after rolling process leads to homogeneous grain size distribution for RD- and TD-rolling with a considerable decrease of LAGBs as the rolling thickness reduction is increased.

**Keywords:** Titanium, rolling, twinning, texture, EBSD

## 1. Introduction

Ti-3Al-2.5V (ASTM grade 9) is a near alpha titanium alloy used in aircraft industries due to its excellent cold workability. The strength of this alloy is intermediate between commercially pure titanium (CP-Ti) and $\alpha+\beta$ titanium alloys such as the Ti-6Al-4V alloy [1]. The industrial sheet-forming processes, such as hot and/or cold rolling, involve strong plastic deformation leading to a pronounced texture that results in a high mechanical anisotropy [2]. Additional heat treatments improve the mechanical properties but do not affect the orientations distribution, which conserved the anisotropic behaviour of the Ti-3Al-2.5V [3]. A prior control of the microstructure in terms of grain size, texture, and distribution of low angle grain boundaries (LAGBs) could help to homogenize the mechanical behaviour of this alloy, which has the significant advantage in its ability to undergo cold deformation. The understanding of deformation mechanisms taking place in titanium is necessary to predict the texture changes during rolling deformation. Twinning activity has been reported to be a dominant deformation mechanism during cold rolling deformation in titanium and its alloy [4, 6]. The aim of this study is to obtain a better understanding of the microstructure and texture evolution during cold deformation by unidirectional rolling with a subsequent annealing.

## 2. Material and experimental

The material used in this study is a Ti-3Al-2.5V sheet of 2 mm thickness purchased from TIMET. RD, TD and ND represent the rolling direction, transverse direction, and normal direction, respectively, of the initial state. 4 specimens of 100 mm x 70 mm are

unidirectionally cold-rolled in the RD (RD-rolling) and 4 specimens of 70 mm x 100 mm are unidirectionally cold-rolled in the TD (TD-rolling). Specimens are rolled down to the final thickness reduction ratios of 5%, 25%, 50% and 75% for both RD- and TD-rolling. The samples are then annealed at 875°C for 3 hours. The samples are polished manually with SiC paper using grit sizes from 15 µm to 5 µm and then electropolished for 30 s in a solution of 80% methanol and 20% perchloric acid at 30V. Electron backscatter diffraction (EBSD) is used to examine the microstructures and texture evolution after rolling and after annealing. EBSD scans are performed on an In-lens Schottky Plus field emission gun scanning electron microscope (JSM-F100) equipped with a CMOS (Complementary Metal-Oxide-Semiconductor) EBSD detector (Oxford Instruments). The acceleration voltage is 15kV, the working distance is 15 mm and the samples are tilted at 70°. The measurement step size is 0.15 µm or 0.08 µm. EBSD measurements are performed on the TD-ND section for RD-rolling and the RD-ND section for TD-rolling. The textures of the 75%-rolled samples are analysed by XRD after rolling to increase the measurement volume with respect to EBSD measurement. The grain size distribution, grain boundary characters and twinning activity are analysed based on the EBSD measurements using the MTEX v5.9 toolbox for MATLAB® [7]. Orientations are represented in pole figures on the RD-TD plane of the initial reference system.

## 3. Results and discussion

### 3.1 As-received state
(a)

<figure><img src="image_2.png" /><figcaption>Figure 1 Sample at as-received state (a) EBSD inverse pole figure (IPF) map (b) (0001) pole figure (c) low angle grain boundaries (LAGBs) distribution</figcaption></figure>

<figure><img src="image_3.png" /></figure>

(b)

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

Figure 1 shows the initial microstructure and orientation of Ti-3Al-2.5V. The microstructure can be divided into two groups of grains as shown in Figure 1a. On one side, macrozones are observed with an elongated morphology in the RD and being characterised by subgrains with an average size of 2 µm and a specific orientation (red colour) corresponding to the <0001> direction parallel to the TD. On the other side, there are grains with a more equiaxed morphology that are spread out with random orientations. The (0001) pole figure of the α-phase indicates that the strong basal poles are oriented towards the TD, meaning that for most grains (red subgrains), their <c>-axes are parallel to the TD (Figure 1b). The misorientation angle distribution shows 63% of low angle grain boundaries (LAGBs) defined as the misorientation angle inferior to 10° (Figure 1c). This high density of LAGBs are mainly present in the macrozones at the as-received state as shown with the grey lines in Figure 1a.

## 3.2 Microstructure and texture evolution during rolling

Figure 2 indicates that the direction of rolling (RD or TD) influences the texture evolution as indicated by the (0001) pole figures of the α-phase in the 25%-, 50%- and 75%- RD- and TD-rolled samples. During the RD-rolling, for the 25%- and 50%-rolled samples, the maxima of pole densities is slightly inclined to 50°-70° from the ND to the TD as shown in Figures 2a and 2c.

For the 75%-rolled sample, the maxima of pole densities is stabilized at 30° around the ND from the TD and from the RD (Figure 2e). During the TD-rolling, for the 25%-rolled sample, intense poles are inclined to 30° around the ND as shown in Figure 2b. The poles are then intensified and move close to the ND (less than 10° around the ND) with the increase of thickness reduction from 50% to 75% as shown in Figures 2d and 2f. Thus, as the rolling thickness reduction increases, the <c>-axes are tilted until 30° to the ND during the RD-rolling, whereas the <c>-axes are almost parallel to the ND during the TD-rolling. Moreover, the (0001) poles of the TD-rolling are more intense than those from the RD-rolling as shown with the values of the maximum intensity of the poles (3.8 for 75% RD-rolling and 8.8 for 75% TD-rolling). The difference in texture evolution and the difference in poles intensities show that the orientation of the grains is difficult to be reoriented during the RD-rolling, while it is more easily done during the TD-rolling. However, it is important to notice that the {0001} poles appear at 30° from the ND to the RD are observed for all the RD- and TD-rolled samples after certain deformation amounts.

The 5%- RD- and TD-rolled samples are analysed by EBSD for microstructural observations of the deformed state as shown in Figure 3a and Figure 3b.
<figure><img src="image_6.png" /><figcaption>Figure 3), represent the orientation of the red subgrains,</figcaption></figure>

Twinning activity is observed for TD-rolled sample (Figure 3b) with on average 12% of {1012} extension twins (blue regions crossing the red grains) and 3% of {1121} extension twins whereas no twinning activity is observed for the RD-rolled sample (Figure 3a). The {1012} extension twins are activated in grains with <c>-axes oriented in the TD which corresponds to the macrozone areas. It is known that the grains sizes have a significant effect on twinning activity in pure titanium with more twin activation and growth for larger grains [8, 9]. Nevertheless, twinning has been reported in grain with diameter size of 4 µm and 6 µm [10, 11]. Figure 3b shows that each {1012} extension twin crosses several subgrains without deviation, so the twinning activation occurred in areas with grain diameter larger than 4 µm. Due to the LAGBs between the red subgrains (lower than 2°), the macrozone is considered as a single grain orientation for twin growth. Figure 3b and Figure 3d show that noticeable orientations are influencing the maxima of pole density in pole figures during RD- and TD-rolling. Firstly, the red circles (marked by A in

which are still oriented with the <c>-axes in the TD after rolling. Secondly, orange squares, marked by B in Figure 3, are related to grains with an orange colour on the EBSD TD IPF (inverse pole figure) micrograph. The maximum of pole density, represented with the black background in Figure 3c and 3d, is overlapped with the orange squares at 30° to 50° from the ND to the TD. This specific orientation is still a maximum in pole density at higher

thickness reduction ratio as shown in Figure 2, meaning that the <c>-axes are mostly inclined at 30° to 50° from the ND to the TD after rolling. Thirdly, the blue circles highlight the orientation of the {1012} extension twins during TD-rolling (C marks in Figure 3d). Blue circles are oriented at 30° from the ND to the RD. The presence of poles at this position is observable in the rolled texture to a greater or lesser degree (Figure 2). This suggests that the {1012} twins are activated for a low deformation thickness reduction during TD-rolling, whereas it is activated at larger deformation during the RD-rolling. The texture evolution is mainly represented

<figure><img src="image_7.png" /></figure>
by two specific orientations: the orange grains with <c>-axes inclined between 30° and 50° from the ND to the TD and the {1012} twins with <c>-axes at 30° from the ND along the RD.

### 3.3 Microstructure and texture evolution after annealing

Figure 4 shows the (0001) pole figures of the α-phase after annealing for the 75%-rolled samples. The maximum of pole density remains inclined at 30° from the ND to the TD for the RD-rolling (Figure 4a) whereas it is fully centred in the ND for the TD-rolling, meaning that the <c>-axes are parallel to the ND (Figure 4b). This texture, called basal texture, enables the planar isotropy of the Ti-3Al-2.5V alloy sheet. The texture component of the {1012} twins is no longer observable at 30° from the ND to the RD. The grain orientation spread (GOS) is calculated with EBSD measurements for 25%-, 50%-, and 75%- RD- and TD-rolled sample after annealing treatment, to evaluate the recrystallized-state of the microstructure. The grains with GOS values higher than 1° are considered as non-recrystallized. Results give an average of 0.07% of grains with GOS value higher than 1°, meaning that the microstructures are fully recrystallized after annealing.

Figure 5a and Figure 5b show the EBSD orientation map of the 25%- and 75%- TD-rolled samples after annealing. As for the initial state, the amount of LAGBs is directly correlated to the macrozones. The amount of LAGBs decreases from 63% in the as-received state to 31%, 8.7% and 6.4% for the 25%-, 50%- and 75%-rolled samples after annealing, respectively. Figure 5a (25%-rolled and annealed) shows the presence of macrozones with equiaxed grains and LAGBs (grey and red lines) whereas no macrozones are observed for the 75%-rolled

(Figure 5b). There is no major difference between RD-

and TD-rolled and annealed in the LAGB data, meaning that the presence of macrozone is not related to the texture achieved after rolling. The grain size distribution is calculated as shown in Figure 5c. A minimum diameter of 0.9 µm and a minimum grain area of 0.5 µm² are chosen for analysis. The two rolling processes (RD- and TD-rolling) are equivalent in terms of grain size distribution if compared at the same thickness reduction ratio as shown Figure 5c. The average grain size of the 25%-rolled samples is 4.5 ± 2.7 µm, whereas it is 6.8 ± 4.2 µm for 50%-rolling. The 75%-rolled and annealed samples are similar to the 50%-rolled and annealed ones with an average grain size of 7.0 ± 4.0 µm, meaning that the grain growth does not progress after 50% of rolling.

## 4 Conclusion

The evolutions of texture and microstructure during unidirectional RD- and TD-rolling of a Ti-3Al-2.5V sheet has been investigated. Based on the results, the following conclusions are established:

- The strong transverse anisotropy at the as-received state is correlated to the presence of the macrozones characterized by a high density of LAGBs (subgrains <2 µm) and a specific orientation with the <c>-axes along the TD.

- A common rolling-texture with the <c>-axes oriented at 30° from the ND to the TD is achieved for unidirectional RD-rolling.

- A centred-texture with the <c>-axes along the ND is achieved for the TD-rolling due to the activation of {1012} extension twins at small deformation.

- After annealing, the microstructure is fully recrystallized with equiaxed grains. The

<figure><img src="image_8.png" /><figcaption>Figure 5 EBSD IPF maps after TD-rolling and annealing (3h at 875°C) for (a) 25%- (b) 75%-rolled sample and (c) grain size distribution after RD- and TD-rolling of 25%, 50% and 75% and annealing</figcaption></figure>
presence of macrozones decreases during the grain growth as the rolling thickness reduction increases.

## 5 Acknowledgments

This work was supported by the ANR AFoTi project, grant ANR-20-CE08-0020-02 of the French Agence Nationale de la Recherche

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