Microstructural Control of Nanoscale Solute Clusters and Precipitates in Ferritic and Martensitic Low-alloy Steels by Associate Professor Yongjie Zhang

NTU MSE Seminar Hosted by Nanyang Assistant Professor Eason Y.-S. Chen

Abstract

Low-alloy steels can be effectively strengthened by nanoscale precipitates and solute clusters while maintaining good ductility and toughness. This presentation focuses on two representative strengthening features: alloy carbides formed by interphase precipitation during ferrite transformation and carbon-related nanostructures formed during martensite tempering.

In the first part, the formation behavior of interphase precipitation in microalloyed low-carbon steels will be discussed. Quantitative analyses revealed that interphase precipitation is promoted by reduced interfacial coherency associated with deviation from the exact Kurdjumov-Sachs orientation relationship. A larger precipitation driving force, controlled by transformation temperature and alloying addition, leads to a finer dispersion of alloy carbides, whereas ferrite growth rate shows only a minor influence within the investigated range. The refined carbide dispersion contributes directly to higher ferrite hardness.

In the second part, low-temperature tempering behavior of high-carbon martensitic steels will be introduced. During tempering, carbon segregation/clustering, metastable carbide precipitation, retained austenite decomposition, and cementite precipitation occur sequentially. These reactions were investigated using in-situ neutron diffraction, TEM, and 3DAP, with particular attention to alloying effects on tempering kinetics and microstructural evolution.
Overall, this presentation highlights how nanoscale precipitation and clustering formed in low-alloy steels can be understood and controlled.

Biography

Associate Professor Yongjie Zhang
Institute for Materials Research (IMR)
Tohoku University, Japan

Dr. Yongjie Zhang is an associate professor at Institute for Materials Research (IMR), Tohoku University, Japan. His research interests cover various fundamental aspects of microstructural control in steels, including nano-precipitation and strengthening, alloying effects on transformation kinetics, solute segregation at grain boundary, etc. During his Ph.D course, he was recruited as the Research Fellowship for Young Scientists (DC1) by the Japan Society for the Promotion of Science (JSPS). He has authored/co-authored around 50 refereed papers, and received multiple academic awards such as the Young Researcher Award by the Japan Institute of Metals and Materials (JIM) and the Iron and Steel Institute of Japan (ISIJ), respectively.