Defect reconfiguration in a Ti–Al alloy via electroplasticity

Abstract

It has been known for decades that the application of pulsed direct current can significantly enhance the formability of metals. However, the detailed mechanisms of this effect have been difficult to separate from simple Joule heating. Here, we study the electroplastic deformation of Ti–Al (7 at.% Al), an alloy that is uniquely suited for uncoupling this behaviour because, contrary to most metals, it has inherently lower ductility at higher temperature. We find that during mechanical deformation, electropulsing enhances cross-slip, producing a wavy dislocation morphology, and enhances twinning, which is similar to what occurs during cryogenic deformation. As a consequence, dislocations are prevented from localizing into planar slip bands that would lead to the early failure of the alloy under tension. Our results demonstrate that this macroscopic electroplastic behaviour originates from defect-level microstructural reconfiguration that cannot be rationalized by simple Joule heating.

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Fig. 1: Mechanical and thermal measurements of the material at different conditions.
Fig. 2: Comparison of dislocation morphology of samples pre-deformed to 5% of engineering strain.
Fig. 3: Cryogenic deformation shows similar behaviour as pulsing deformation at ambient temperature.
Fig. 4: Characterization of the deformation twinning in the electrical-biasing deformed sample.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We gratefully acknowledge funding from the US Office of Naval Research under grant no. N00014-17-1-2283. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231.

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Authors

Contributions

S.Z. and A.M.M. proposed the experiments. S.Z., R.Z. and Y.C. performed the mechanical testing with electrical pulsing. S.Z. and R.Z. conducted the TEM and high-resolution STEM characterization of the resultant dislocation structures and twins. S.Z. conducted the EBSD. E.R. and D.C.C. estimated the magnetostriction and thermal diffusivity effects. R.Z., A.A. and X.L. assisted with the literature review. A.A. and M.A. provided the electrical free energy discussion. S.Z., J.W.M. and A.M.M. drafted the manuscript and all the authors contributed to the discussions. J.W.M. and A.M.M. supervised the project.

Corresponding author

Correspondence to Andrew M. Minor.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Dislocation morphology of Ti-7Al subjected to 2% of plastic strain using different loading conditions.

a, room temperature without pulsing shows planar dislocation configuration; (b) pulsing deformed showing wavy slip.

Extended Data Fig. 2 Planar dislocation configuration of materials subjected to continuous current.

a, Imaging condition near the basal plane and (b), imaging condition near the {11–23} plane. This Figure illustrates the dislocation structure deformed under continuous current (current density similar to the pulsing deformation reported in the main text). The temperature during deformation is measured to be around 200°C. It is shown clearly, in two distinctive imaging conditions (A-near the basal plane, and B-near the {11–23} plane), that the dislocation exhibits a planar configuration, indicating a lack of cross-slip and is very similar to the deformation at room temperature. This suggests that raising temperature at this range cannot trigger planar-to-wavy transition.

Extended Data Fig. 3 Dislocation configuration from the as fractured tensile specimen.

a, high density of wavy dislocation tangles in pulsing deformed sample; (b) Planar dislocation bands in sample deformed at room temperature (RT).

Extended Data Fig. 4 Comparison of twinning ability in the different deformation conditions.

EBSD grain boundary misorientation analysis shows that both electrical pulsing and cryogenic deformation temperature lead to an enhanced twinning ability for the Ti-7Al alloy. The field of view of the EBSD scans is 200 µm × 500 µm along the gauge section of the tensile samples.

Extended Data Figure 5 Inverse pole figure mapping of pulsing vs. no-pulsing deformed sample.

It shows virtually no twins in the RT-deformed sample (a), whereas noticeable {10–12} twins are observed in the pulsing-deformed sample.

Extended Data Fig. 6 Secondary electron fractographic micrographs.

a, tensile sample after pulsing deformation and (b) tensile sample without pulsing treatment.

Extended Data Fig. 7 Schematic illustration of dislocation configurations.

a, At room temperature, dislocation prefer to slip on the prismatic plane and cut through the obstacles (Red, solute atoms or short-range order clusters). b, The subsequent dislocations tend to follow the leading dislocation due to slip-plane softening, leading to a planar dislocation configuration similar to what is shown in Fig. 2 (a, b). Note that the obstacles will be destroyed after the passage of several dislocations as each dislocation causes a shear displacement. c, Upon applying a pulsed current, dislocation prefer to cross-slip (shown here on the first order pyramidal plane). d, This mechanism helps dislocations bypass the obstacles, leading to a non-planar dislocation configuration, similar to what is shown in Fig. 2 (c, d).

Extended Data Fig. 8 Detailed illustration of the dog-bone tensile specimen.

a, Schematic drawings of the geometry of the sample mounted in our loading fixture; (b) the CAD model detailed dimension of the dog-bone sample.

Supplementary information

Supplementary Information

Supplementary text.

Supplementary Video 1

Infrared video of the specimen subjected to pulsed and continuous currents.

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Zhao, S., Zhang, R., Chong, Y. et al. Defect reconfiguration in a Ti–Al alloy via electroplasticity. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-00817-z

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