IMSE presents the North American Materials Colloquium Series:
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Third generation advanced high strength steels (AHSS) were developed as automotive structural materials capable of enhancing vehicle fuel efficiency and crash-worthiness. Recently, the use of zinc (Zn) coated AHSS has focused attention on Zn-assisted liquid metal embrittlement (LME). LME is represented by the premature “loss” in the ductility of a solid metal in intimate contact with another liquid metal. The embrittlement is activated by “intergranular” penetration of the liquid into the solid substrate, driven by the reduction in the energy of grain boundary interfaces. The simultaneous action of liquid and tensile stress is considered critical to trigger LME. Classical LME literature recognizes that strong metals are LME-sensitive; this trend has also been noted in the case of Zn-assisted LME of steels, i.e. AHSS are considerably more LME-sensitive compared to conventional (lower strength) mild steels. In addition to the high strength, AHSS have complex multiphase microstructures and rich alloy compositions, and it is possible that both these variables “independently” affect LME susceptibility. Because the alloy composition, microstructure, and properties often vary simultaneously, it has been difficult to identify the precise origin of LME behavior.
Our research focusses on high temperature tension tests using simulated resistance spot weld thermomechanical cycles, to determine the critical factors affecting LME behavior of AHSS. The steel alloys employed for our experiments are laboratory-produced with careful variations in the microstructure and alloying concentrations. Our initial experiments have revealed that LME development in AHSS could be accelerated by the presence of specific alloying elements (such as silicon (Si)) and/or certain phase boundaries (such as austenite/austenite boundaries). Thus, both microstructure and alloying elements can distinctly influence Zn-LME sensitivity of AHSS. The long-term goal of this research is to tailor the processing strategies for developing “LME-resistant” AHSS.
Diptak Bhattacharya is a Ph.D. student in the Advanced Steel Processing and Products Research Center (ASPPRC) in the Colorado School of Mines. Diptak graduated with B.Tech (Metallurgy) from the Indian Institute of Engineering, Science and Technology, Shibpur in 2013.
He worked as a Manager in the Products Technology Group of Tata Steel Limited from 2013-2017. He started his Ph.D. program at the Colorado School of Mines in the Fall of 2017 and is working with Professor John Speer on liquid metal embrittlement of advanced high strength steels.