This research concerns the influence of microstructure on the ductile-brittle transition in lath martensitic steels. It specifically addresses the central role of the martensite "block" as the effective grain size in lath martensitic steel and the apparently very different influence of the block size on the strength and the cleavage fracture resistance. The research is technologically important since the high-strength steels now used or proposed for low-temperature, arctic or cryogenic service are lath martensitic steels. Technological advances in the properties and cost of these steels require understanding and exploiting the microstructural mechanisms that control low-temperature strength and toughness. The research is scientifically important since it promises to complete the emerging mechanistic theory of the microstructure-property relations that control the ductile-brittle transition in martensitic steels, a theory that has slowly developed through some forty years of international metallurgical research. The research will study a series of Fe-C, Fe-Ni and Fe-Mn steels with lath martensitic microstructures. Their grain, packet and block structures will be characterized, and modified using thermal treatments that are known to refine the effective block size. Appropriate strength and toughness tests will be done, and studied with high-resolution characterization techniques, including orientation imaging and profile fractography, to determine the influence of the block structure on the strength, toughness and cleavage fracture path. The results will provide new, probative information on the block structure of lath martensite and its specific influence on the strength and the ductile-brittle transition, suggesting new metallurgical approaches to design advanced alloys with superior properties.
NON-TECHNICAL SUMMARY: The use of high-strength steel in low-temperature, arctic or deep-sea environments is severely restricted by the danger of catastrophic failure through brittle fracture. The relevant metallurgical problem is the ductile-brittle transition; steels that are tough and ductile at high temperature become almost glassy brittle when used below their ductile-brittle transition temperature. To avoid the problem the designers of low-temperature structures and devices have used relatively low-strength steels, which compromises engineering efficiency, or highly alloyed steels, which raise cost. The ductile-brittle transition temperature of steel is strongly influenced by its internal structure ("microstructure") - the way its atoms are arranged on the micro- and nanoscale. Through some fifty years of directed research in many laboratories around the world metallurgists have come to understand many features of the microstructure of high strength steels and how that microstructure can be engineered to control the ductile-brittle transition. The present research project is intended to fill out that theoretical understanding by applying the best modern tools for microstructure characterization to steels that have been broken under controlled conditions. The results will provide new, probative information on the fundamental mechanisms that govern the brittle transition, suggesting new metallurgical approaches to design advanced alloys with superior properties for low-temperature, arctic or deep-sea service. The PI will continue to teach a freshman seminar course on the history of weaponry, which he has used previously to interest students in the field of Metallurgy. The PI will also encourage the involvement of women and underrepresented groups in his research projects, as he has done in the past.
