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Science Bulletin: Dislocations Across Interphase Enable Plain Steel with High Strength-Ductility

May 12, 2021      Author:

Recently, Science Bulletin published the "Dislocations across interphase enable plain steel with high strength-ductility" research study of Professor Nailu Chen's group in cooperation with Professor Jian Lu's group (City University of Hong Kong). Professor Nailu Chen and Professor Jian Lu are the co-corresponding authors. Mr. Jiazhi Zhang, a PhD student under the Joint Ph.D. program offered by Shanghai Jiao Tong University and City University of Hong Kong, is the first author.

Low cost, high strength-ductility steel is always the major trends of steel development especially for automotive industry. However, raising strength often brings about loss of ductility, known as the strength-ductility trade-off. In the past, researchers usually balance strength and ductility through adding noble elements or complex processes but these will raise the cost.

This work reported a novel phenomenon of "dislocations across martensite/austenite interface" (DAMAI), which evidently improved the ductility of the "plain" quenching-partitioning-tempering (Q-P-T) martensitic steel. DAMAI effect was further verified by in-situ observation of transmission electron microscope (TEM) during dynamic tension (Fig.1 a to d) and molecular dynamics (MD) simulation (Fig.1 e to g), respectively. Based on the DAMAI effect, the authors proposed a new plainification strategy, namely, balancing both the volume fraction and mechanical stability of retained austenite to enhance DAMAI effect and reduce strain-induced martensitic transformation (SIMT), to reduce the cost significantly but enable plain steel with excellent product of strength and elongation. Under the guidance of this strategy, a high carbon Q-P-T steel was designed, exhibiting the highest cost performance (79.45 GPa%kg/$) compared with other high strength steels (Fig.1 h), and strength-ductility trade-off is solved only by increasing the low cost carbon content from low carbon to high carbon in Q-P-T steels, which is the century-long pursuit of researchers. The strategy is expected to provide an approach for developing other high strength-ductility martensitic steels.

Fig. 1. Verification of DAMAI effect by (a to d) in situ TEM observation and (e to g) MD simulation. (h) Properties of Q-P-T steels in comparison with other advanced high strength steels.

This work was supported by the National Natural Science Foundation of China (51771114 and 51371117), the National Key R&D Program of China (2017YFA0204403), the Key Projects of National Natural Science Foundation of China (51031001).

 

Source: School of Materials Science and Engineering, SJTU