The story started in early 2015, Dr. Pham thought that if we could use 3D printing to construct crystal-like macro-structures of metallic glasses, we could solve the brittle issue of metallic glasses while maintaining the high strength of metallic glasses. After joining Imperial College London as an independent research fellow in Engineering Alloys, Dr. Pham realised the post-yielding causes architected macro-lattices with single orientation to collapse catastrophically. As a metallurgist, he was right away to think of employing the strengthening mechanisms observed in metallic alloys to solve this post-yield collapse problem. In late 2016, Prof. Todd (University of Sheffield) and Dr. Pham had a brainstorm that led to grant a proposal to seek external support from EPSRC. Unfortunately, the proposal was not successful. Although there were compliments in the novelty of this approach, reviewers were right in raising a main concern that there had not been any previous studies to support this adventurous approach. Despite of this unsuccessful outcome, Dr. Pham kept working and collaborating with Prof. Todd on the project. Dr. Pham also presented this idea to Prof. Dunne (Imperial) and Prof. Rollett (Carnegie Mellon University) and Prof. Peter Haynes (Head of Materials department, Imperial), and received tremendous support and mentoring in pursuing this novel approach.
I started my PhD study in January 2017 to work on the experimental element of this approach. We were struggling to find CAD tools to realise our design ideas in the beginning of my PhD study. However, we made a breakthrough in designing models that mimicked the polygrain microstructure of crystals (each domain that mimics a crystal grain was named a meta-grain). We then moved on to design precipitate-inspired, and finally multiphase-inspired architected materials. We were very excited to see the similar behaviour between shear bands in twinned meta-grains and dislocation slip in twinned bi-crystals. We had a great moment of joy when observing that the yield strength and flow stress of polycrystal-like architected materials are reversely proportional to the size of meta-grain, very similar to the Hall-Petch relationship found in metallurgy. We subsequently demonstrated that the other key strengthening mechanisms such as precipitation and multiphase hardening are also applicable to crystal-inspired architected materials. In Oct 2017, we were excited to welcome Jedsada joining our group for a PhD study. Jedsada filled an important missing puzzle – Simulation of architected materials to obtain insights into the deformation and collapse of lattice struts. Despite difficulties associated with tremendously computational costs, Jedsada brilliantly simulated the deformation behaviour of twinned meta-grains and Kresling lattices, providing many insights into our experimental observations.
In conclusion, we presented a transformative way to enable the employment of the strengthening mechanisms found in physical metallurgy by 3D printing to design lightweight and damage-tolerant architected materials. Many opportunities offered by this approach are remained to be explored. Throughout the whole course of this study, we have encountered many difficulties due to the adventurous nature of this approach. This study would not have been possible without tremendous support from Prof. Rollett, Prof. Dunne and Prof. Haynes. We also want to thank Dr. Holdsworth (Empa), Dr. Gourlay and Dr. Hooper (Imperial) and students (Tom Walton who assisted the project during his summer project funded by Imperial’s Undergraduate Research Opportunities Programme http://www.imperial.ac.uk/urop) for their helps.