Research Projects Supported by HKU's High Performance Computing Facilities
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Researcher:
Dr Alfonso Hing-wan Ngan, Department of Mechanical Engineering
Project Title:
Atomistic Simulation of 3-D Dislocation Processes
Project Description:
Crystalline materials deform by dislocation processes and so it is very important to understand how dislocations move at finite temperatures in a range of materials. This project aims at simulating the energetic pathways for 3-D dislocation processes in three different situations. These are i) lock formation in the L12-structured intermetallic Ni3A1 which is a high-temperature structural materials for aerospace applications, ii) homogeneous shear loop nucleation, and iii) kink-pair processes of screw dislocations ahead of crack tips in body-centred-cubic metals. These three are important dislocation processes. The common ground amongst them is that we plan to use the same atomistic simulation method, namely the ¡§nudged elastic band¡¨ method, to simulate their energetic pathways. i) above is related to the flow stress anomaly in Ni3A1 which makes superalloys maintain their strength at elevated temperatures. ii) is concerned with incipient plasticity in nano-scale mechanical contacts which is currently receiving a lot of attention in the materials science community. iii) is related to a fuller understanding of brittle-ductile transition in BCC metals.
Project Duration:
3 years from 1-9-2003 to 31-8-2006

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Project Significance:
The expected outcome is twofold. Apart from gaining a deeper understanding of each of the three dislocation processes and their associated macroscopic phenomena, this research should also help to establish the ¡§nudged elastic band¡¨ method as a promising research tool in computational materials science.
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Results Achieved:
Previous results include the following publications:

1. Zhang, H.F. and Ngan, A.H.W., ¡§Atomistic Simulation of Screw Dislocation Mobility Ahead of a Mode III Crack Tip in the BCC Structure¡¨, (1999), Scripta Mater., 41, pp. 737-742.

2. Ngan, A.H.W., ¡§Modelling Screw Dislocation Mobility in the Bulk and in the Vicinity of Crack Tips using a Peierls-Nabarro Approach¡¨, (2000), Key Engineering Materials, 177-180, pp. 135-140.

3. Ngan, A.H.W., ¡§Mobility of Screw Dislocations in BCC Crystals: A Review on Modelling Methods¡¨ (an invited review paper), (2000), Progress in Natural Science, 10, 721-729.

4. Wen, M. and Ngan, A.H.W., ¡§Atomistic Simulation of Kink-pairs of Screw Dislocations in BCC Fe¡¨, (2000), Acta Mater., 48, 4255-4265.

5. Ngan, A.H.W. and Wen, M., ¡§Atomistic Simulation of Energetics of Motion of Screw Dislocations in BCC Fe at Finite Temperatures¡¨, (2002), Computational Materials Science, 23, 139-145.

6. Ngan, A.H.W. and Wen, M., ¡§Dislocation Kink-Pair Energetics and Pencil Glide in Body-centered-cubic Crystals¡¨, (2001), Phys. Rev. Lett., 87, 75505.

7. Li, J., Ngan, A.H.W. and Gumbsch, P., ¡§Atomistic Modeling of Mechanical Behavior¡¨, (2003), invited article for Acta Materialia 50th Anniversary Special Issue, Acta Mater., 51, 5711-5742.

8. Ngan, A.H.W., Wen, M. and Woo, C.H., ¡§Atomistic Simulations of Paidar-Pope-Vitek Lock Formation in Ni3A1¡¨, accepted for publication in Computational Materials Science (Oct., 2003).

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Kink-pair configuration in screw superpartial dislocation in the intermetallic alloy Ni3A1.
Left: edge-on view along the slip plane. Right: 3-D view.
Remarks on the Use of High Performance Computing Cluster:
This project requires parallel computation, in which a string of replicas of the system are relaxed simultaneously and perpendicular to the ¡§reaction path¡¨.
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Email Address:
hwngan@hku.hk

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