# A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures around Nanoindentation Imprints in a TiAl Alloy

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## Abstract

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## 1. Introduction

_{2}as a minor phase. The α

_{2}phase is hexagonal ($\frac{c}{a}=$ 0.8) with a DO19 structure, while the γ phase is tetragonal with a L1

_{0}structure close to cubic ($\frac{c}{a}=\frac{c}{b}=$ 1.02). Therefore, six order variants are possible. They can be visualized as generated by a 120° rotation around the $(1\text{}1\text{}1)$ plane normal [9].

_{2}[10,11].

_{0}structure [12]: along the $\langle \overline{1}\text{}1\text{}0]$-directions, there is only one kind of atom (Ti or Al). In this case, dislocations are called ordinary dislocations, and their Burgers vectors are $\frac{1}{2}\langle 1\text{}1\text{}0]$ types. Because Ti-atoms and Al-atoms interchange in $\langle 0\text{}1\text{}1]$-directions, the $\langle 1\text{}1\text{}\overline{2}]$ and the $\langle 1\text{}0\text{}1]$ dislocations are called superdislocations. These two types of superdislocations can undergo various dissociations into superpartials (i.e., partial dislocations with the associated planar faults). In addition, true twinning along $\frac{1}{6}\langle 1\text{}1\text{}\overline{2}]\{1\text{}1\text{}1\}$ occurs that does not alter the ordered L1

_{0}structure of the γ-TiAl. Because of the specific structure of the γ-TiAl, it is relatively easy to know the direction for either slip of ordinary dislocations or for true twinning when the slip/twin plane is known [12]. Note also that at RT twinning and then glide of ordinary dislocations are the easiest deformation modes [2,7,8]. In this manner, Kauffmann et al. suggested that increasing deformation leads to the nucleation of only a few new mechanical twins, since the dislocation movement becomes more dominant with increasing strain [8].

_{2}phase does not contribute to the deformation [6,12], evidence of prismatic slip $\langle 1\text{}\overline{2}\text{}1\text{}0\rangle \{1\text{}0\text{}\overline{1}\text{}0\}$, basal slip $\langle 1\text{}\overline{2}\text{}1\text{}0\rangle (0\text{}0\text{}0\text{}1)$, and pyramidal slip $\langle 1\text{}1\text{}\overline{2}\text{}\overline{6}\rangle \{1\text{}\overline{2}\text{}1\text{}1\}$ was reported [12].

## 2. Materials and Methods

## 3. Results

#### 3.1. Characterization of the Microstructure around the Regions of Interest

**g**were accessible by tilting and rotating the specimen: ${\mathit{g}}_{\mathbf{1}}=\text{}(1\text{}\overline{1}\text{}0)$, ${\mathit{g}}_{\mathbf{2}}=\text{}(1\text{}1\text{}\overline{1})$, ${\mathit{g}}_{\mathbf{3}}=\text{}(3\text{}\overline{1}\text{}\overline{1})$, ${\mathit{g}}_{\mathbf{4}}=\text{}(3\text{}\overline{3}\text{}1)$, ${\mathit{g}}_{\mathbf{5}}=\text{}(1\text{}3\text{}\overline{3})$, ${\mathit{g}}_{\mathbf{6}}=\text{}(1\text{}\overline{3}\text{}1)$, ${\mathit{g}}_{\mathbf{7}}=\text{}(4\text{}0\text{}\overline{2})$ (note that only the ECC image taken with

**g**is shown in Figure 2a). In such conditions, all defects are expected to be in contrast. Neither dislocation nor superdislocation are observed before deformation in Figure 2a. Only parallel linear contrasts (labelled NT) are clearly visible. In addition, they are aligned along the $~[2\text{}\overline{3}\text{}1]$ direction. Such BSE contrast is generally attributed to nano-twins (NTs) and is consistent with $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ as a true twin system [20,21]. After deformation (see Figure 2b), no dislocation is visible, but clearly identifiable changes are localized in the vicinity of the indent (Area 1 and Area 2 in Figure 2b). In Area 1, near the imprint, a $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ deformation NT was created. At the other side of the imprint (Area 2) NT5 extends along the $~[2\text{}\overline{3}\text{}1]$. Note that NT7 visible in Figure 2b comes from a neighbor imprint.

_{1}#### 3.2. Microstructure Evolution of ROI2

## 4. Discussion

- At RT, twinning was observed to be the main deformation mechanism, in agreement with literature [2,7,8]. However, this runs contrary to Zambaldi et al., who prefer to suggest that ordinary dislocation glide is the main deformation mechanism at RT (without totally excluding twinning) from observations by atomic force microscopy around high-load (3000 µN) imprints [18].
- Deformation was observed to be localized near the indent.

- Under the indent, the $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ NT was formed.
- The stress concentration at the tip of the $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ NT nucleated ordinary $\pm \frac{1}{2}[1\text{}1\text{}0]$ dislocation loops gliding in the $(1\text{}\overline{1}\text{}1)$ planes. The dislocation loops formed an ellipsoid surrounding the NT, thus producing lines after projection on the observation plane.
- The elliptical area or B1 grew by adding successive dislocation loops at its extremity.
- B1 extended until it met an obstacle, such as the TB (for B2 for example).
- At the location where B2 intercepts the TB, a stress concentration appeared. It resulted in a local distortion of the boundary. Therefore, the TB seems to be a strong obstacle to the propagation of the deformation, and at higher load it may cause microcracking at its vicinity, as observed in References [18,25,26].

## 5. Conclusions

- At RT, twinning was observed to be the main deformation mechanism.
- Twinning was accommodated by ordinary dislocation mechanism, leading to the canalization of the deformation.
- TB could play the role of obstacle to the propagation of deformation to neighbor grains, leading to a stress concentration at the vicinity of the boundary. Therefore, the true twin seems to be one of the weak links explaining the poor ductility of γ-TiAl at RT.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**(

**a**) Electron backscattered diffraction (EBSD) orientation map of the zone of interest. (

**b**) Enhanced BSE image showing the microstructure before deformation. The nanoindentation array is localized in the white rectangle. (

**c**,

**d**) EBSD patterns corresponding to grains A and B.

**Figure 2.**Region of interest 1 (ROI1) for which the surface is close to $(4\text{}5\text{}7)$. (

**a**) Accurate electron channeling contrast imaging (aECCI) obtained with ${\mathit{g}}_{\mathbf{1}}=\text{}(1\text{}\overline{1}\text{}0)$ showing six $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ Nano-Twins (NTs) and the position of the imprint (transparent Berkovich imprint). The white arrows indicate the trace of the $\{111\}$ planes. (

**b**) Enhanced BSE image showing the 500 µN indent. Two areas (labelled Areas 1 and 2) have changed. The NT7 slightly visible in (

**b**) comes from a neighbor imprint.

**Figure 3.**ROI2, where the surface plane is near $(4\text{}5\text{}7)$ for twin A (left) and near $(0\text{}1\text{}3)$ for grain B. The TB corresponds to $[1\text{}1\text{}\overline{2}](1\text{}1\text{}1)$ the system. (

**a**) aECCI obtained with ${\mathit{g}}_{\mathbf{1}}=\text{}(1\text{}\overline{1}\text{}0)$ with the transparency position of the Berkovich imprint. The white arrows indicate the trace of the $\{1\text{}1\text{}1\}$ planes. (

**b**) Enhanced BSE image showing two buckling areas (labelled B1 and B2) clearly visible around the 500 µN indent. The blue arrow points to an NT and the yellow to dislocations. (

**c**) 3D schematic of B1 and B2.

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**MDPI and ACS Style**

Guitton, A.; Kriaa, H.; Bouzy, E.; Guyon, J.; Maloufi, N.
A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures around Nanoindentation Imprints in a TiAl Alloy. *Materials* **2018**, *11*, 305.
https://doi.org/10.3390/ma11020305

**AMA Style**

Guitton A, Kriaa H, Bouzy E, Guyon J, Maloufi N.
A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures around Nanoindentation Imprints in a TiAl Alloy. *Materials*. 2018; 11(2):305.
https://doi.org/10.3390/ma11020305

**Chicago/Turabian Style**

Guitton, Antoine, Hana Kriaa, Emmanuel Bouzy, Julien Guyon, and Nabila Maloufi.
2018. "A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures around Nanoindentation Imprints in a TiAl Alloy" *Materials* 11, no. 2: 305.
https://doi.org/10.3390/ma11020305