CAREER: Unraveling Secondary Phase-Dislocation Interactions to Overcome Embrittlement in Ultrahigh-Strength Martensitic Steels Under Dynamic Loading

NON-TECHNICAL SUMMARY: Ultrahigh-strength steels are among the strongest of materials used in defense, construction, transportation, and energy infrastructure applications. However, they can become unexpectedly brittle after specific heat treatments or when subjected to high-rate or dynamic impact conditions (e.g. crash events and high-speed forming). This brittleness limits their broader use and often forces manufacturers to use conservative, energy-intensive processing strategies. Unfortunately, the origins of this brittleness in particular materials and processing conditions remain poorly understood, limiting opportunities to improve existing materials and unlock new generations of ultrahigh-strength steels. This project is (i) establishing a fundamental understanding of why ultrahigh-strength steels lose toughness during high-rate impact conditions and (ii) developing economic and energy-efficient pathways for designing high-strength high-toughness steels. By combining modern computational tools, accelerated experimentation/characterization, and high-rate mechanical testing, this research is revealing how microscale features within these steels influence crack formation and fracture during dynamic loading. These insights are enabling accelerated discovery and deployment of ultrahigh-strength steels that can achieve exceptional strength and superior toughness. The project outcomes are directly benefitting the U.S. by strengthening the scientific foundations of structural materials used in critical infrastructure, transportation safety, and national defense. Improved understanding of the processing-fracture property relationships are reducing reliance on energy-intensive processing, lowering manufacturing costs, and enhancing the durability and safety of engineered systems. Integrated with the research, the educational program is (iii) strengthening workforce development by connecting traditional metallurgy with modern computational approaches, equipping students with skills increasingly needed across industry, national laboratories, and academia. Undergraduate and graduate students are receiving targeted training and mentorship involving both national laboratories and steel manufacturers. These related activities are preparing the next generation of materials engineers with expertise in modern computational and experimental tools, while advancing steel science and technologies that accelerate innovation and enhance U.S. competitiveness in manufacturing and critical material systems. TECHNICAL SUMMARY: Ultrahigh-strength martensitic steels represent the upper bound of strength among commercial structural materials, with critical applications across defense, construction, transportation, and energy infrastructure. A key barrier to broader deployment is tempered martensite embrittlement (TME), which limits fracture resistance under dynamic loading conditions, e.g. during crash events and high-rate manufacturing. Success in resolving the metallurgical origins of TME will expand the performance envelope of ultrahigh-strength steels, enabling step-change advances across their varied applications. This research is: (i) establishing a fundamental understanding of secondary phase-defect-crack interactions and their role in dynamic deformation and TME; while also (ii) developing a design framework that integrates accelerated experimentation, characterization, and data-driven computational methods to guide alloying and processing strategies for ultrahigh-strength steels with enhanced dynamic performance. Recent advances in atomic-scale, site-specific characterization, and dynamic testing with in-situ imaging now offer a powerful opportunity to revisit and potentially resolve enduring debates on embrittlement mechanisms, thereby forming the basis of this project’s approach to resolve TME and dynamic shear localization. The outcomes of this project are establishing materials design principles that overcome the trade-off between stren NSF Award ID: 2543792 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT,01002930DB NSF RESEARCH & RELATED ACTIVIT,01003031DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Lawrence Cho | Institution: Colorado School of Mines, GOLDEN, CO | Award Amount: $435,983 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2543792 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2543792.html