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Characterization and Modelling of dislocation-INterface Interactions in MetAllic Laminates at multiple scales

Periodic Reporting for period 1 - MINIMAL (Characterization and Modelling of dislocation-INterface Interactions in MetAllic Laminates at multiple scales)

Reporting period: 2018-05-01 to 2019-04-30

Nanolaminates and superlattices offer unique mechanical and physical properties due to the nanometer scale dimensions of the layers and the high density of interfaces. However, their deformation mechanisms are still the subject of scientific debate. MINIMAL has significantly contributed towards the fundamental understanding of their mechanical behavior by setting-up and performing novel in-situ micro- and nano-mechanical tests in Cu/Nb MNLs and InP twinning superlattices, both inside the SEM and TEM to observe the deformation mechanisms. The main scientific outcomes have been:

• The determination of the fracture mechanisms of Cu/Nb nanolaminates as a function of crack propagation direction.
• The discovery of dynamic strain ageing effects in Cu/Nb nanolaminates at 200ºC.
• The determination of the strength and fracture mechanisms of InP twinning superlattice nanowires.
- Scientific and/or technological achievements
1) The mechanisms of deformation and fracture in Cu/Nb MNLs were study by means of microtensile tests within the TEM in orientations parallel and perpendicular to the layers. Plastic deformation by confined layer slip (CLS) was found in both orientations for Cu and Nb layers and started in both orientations with a strain burst at an applied strain of ≈ 0.75%. Plastic deformation was rapidly localized in a thicker Cu layer in the specimens deformed in the perpendicular orientation. As a result, the strain hardening in this orientation was limited and failure was brittle and took place by the sudden propagation of crack in this layer at an applied strain of 4.5%. In the parallel orientation, both Cu and Nb nanolayers deformed homogeneously up to an applied strain of 8%. Localization of the deformation in one external Cu layers led to the formation of a crack but crack propagation was stopped at the Cu/Nb interfaces. Significant crack blunting and emission of dislocations took place before the crack could propagate through the Nb layers and failure took place by the coalescence of this crack with another crack coming from the opposite surface in a different plane through a shear crack at 45º. As a result, the Cu/Nb nanolaminate deformed parallel to the layers presented a ductile behaviour with failure strain of 22% and significant necking before fracture.
2) In-situ micropillar compression tests of Cu/Nb MNL micropillars were carried out at room temperature, 200 and 400. For the first time, dynamic strain ageing effects were observed at 200 ºC. Combined with focused-ion beam milling, the stress-driven elemental diffusion inside the shear bands was explored using electron microscopy and atom-probe tomography.
3) Taper-free InP twinning superlattice (TSL) nanowires with an average twin spacing of ~ 13 nm and an average diameter of 230 nm were grown along the zinc-blende close-packed [111] direction using metalorganic vapor phase epitaxy. The mechanical properties and fracture mechanisms of individual InP TSL nanowires in tension were ascertained by means of in-situ microtensile tests in a transmission electron microscope (TEM). The elastic modulus along the [111] orientation was 87 ± 17 GPa, while the fracture strain was 2.9 ± 0.3% at a strength of 2.50.5 GPa. Fracture was brittle in all cases and occurred by the propagation of a crack along the twin boundary interface. No evidence of inelastic deformation mechanisms was observed neither in the experimental stress-strain curve nor in the TEM images before fracture. MD simulations of the tensile deformation of untwinned and twinned InP nanowires also showed fracture triggered by the nucleation and propagation of a crack at one the twin boundaries, in agreement with the experimental observations. Thus the presence of twins limited the strength of InP nanowires.

- Conferences / activities attended for dissemination
1) 2019 MRS Spring Meeting & Exhibit, 22-26/04/2019, Phoenix, Arizona, US.
2) TMS Annual Meeting, 10-15/03/2019, San Antonio, Texas, US
3) 55th Annual Technical Meeting of the Society of Engineering Science (SES2018), Madrid, Spain.

- Project results in scientific publications
1) Z. Liu, M.A. Monclús, L.W. Yang, M. Castillo-Rodríguez, J.M. Molina-Aldareguía, J. Llorca, Tensile deformation and fracture mechanisms of Cu/Nb nanolaminates studied by in-situ TEM mechanical tests, Extreme Mechanics Letters 25 (2018) 60-65.
2) Z. Liu, I. Papadimitriou, M. Castillo-Rodríguez, C. Wang, G. Esteban-Manzanares, X. Yuan, H. Tan, J.M. Molina-Aldareguia, J. Llorca, Mechanical behavior of InP twinning superlattice nanowires, has been submitted to Nano Letters.
3) Z. Liu, M.A. Monclús, J. Snel, M. Castillo-Rodríguez, J. Llorca, J.M. Molina-Aldareguía. Anomalous strain-rate sensitivity of Cu/Nb nanolaminates at 200ºC, in preparation.

- Outreach activities
1) Research communication, 03-07/02/2019, Imperial College London and Brunel University London, London, UK.
2) Young Scholar Xiangjiang Forum, 14-17/10/2018, Central South University, Changsha, China.
3) The In-situ TEM Workshop, 15-17/05/2018, KNMF, Karlsruhe Institute Technology, Germany.
4) Atom-probe tomography experiments of the compressed Cu/Nb nanolaminate micropillars, 20-29/01/2019, KNMF, Karlsruhe Institute Technology, Germany.
- Contribution to the state of the art
1) The development of new FIB-assisted methodologies for fabricating nanoscale tensile specimens for testing in-situ in the TEM.
2) For the first time, the uniaxial tensile behavior Cu/Nb NMLs and InP twinned superlattice nanowires have been systematically and quantitatively studied in situ with a high-resolution CCD camera inside TEM.
3) Dynamic strain ageing effects, never observed before, have been found in Cu/Nb NMLs at 200ºC.

- Impact
1) The researcher has participated in several initiatives that have strengthen his career development, including i) training on the preparation and operation of state-of-the-art facilities, such as focused-ion beam (FIB)/SEM, in-situ TEM tensile testing, and in-situ SEM microcompression at IMDEA Materials Institute, ii) participation in the In-situ TEM Workshop at KNMF, Karlsruhe Institute Technology, Germany; iii) participation in the atom-probe tomography experiments with Dr. Torben Boll at KNMF, Karlsruhe Institute Technology, Germany.
2) This is a new research field for the researcher, which enhances his expertise. As a matter of fact, the researcher got a permanent position, associate professor, at Central South University, China. Future collaboration between IMDEA Materials Institute and Central South University will be bridged.
3) The project initiated some novel experimental methodologies on how to prepare nanoscale TEM tensile samples of advanced materials like MNLs and semiconductors.
4) The project results provide potential users with new experimental methodology on the in-situ SEM/TEM quantitative mechanical testing.