NSF-BSF:Influence of cohesion enhancing elements, impurities and hydrogen/deuterium at grain boundaries and heterophase interfaces on embrittlement of additive-manufactured steels

Non-Technical Summary:

Exposure of steels to hydrogen (1H) during manufacturing, storage or service may embrittle them, potentially resulting in catastrophic failures. Therefore, hydrogen embrittlement (HE) of steels is of concern in 1H energy systems, automobiles, aviation, marine applications, bridges, transportation infrastructure, and nuclear reactors. 1H concentrations in the atomic ppm range are sufficient to embrittle high-strength steels. 1H trapping at defects, grain boundaries, heterophase interfaces, and elastic stress-fields, affects its solubility, diffusivity, and the susceptibility of steels to HE.

Additively manufactured (AM) steels may be more susceptible to HE than their counterpart wrought steels due to different microstructures, porosity levels, and residual stress levels. The understanding of 1H ingress, local concentrations, trapping vs. mobile 1H, and its 3D spatial distributions, is important for chemical and microstructural design of next-generation, hydrogen-resistant steels.

The PIs are proposing a systematic study starting with the design of a novel precipitation-hardened stainless steel for selective laser melting; controlled powder synthesis and characterization; AM of steels by both powder-bed fusion SLM and directed energy deposition laser engineered net shaping; chemical, microstructural, and mechanical characterization of AM steels compared to their wrought counterpart steel, before and after 1H or 2D electrochemical charging. Scientific and technological strategies and alloy design principles will directly support the development of AM for industrial manufacturing.

The scientific and technological strategies and alloy design principles will support the development of AM for industrial manufacturing. Our projects will provide multi-dimensional training for students and postdocs, including processing-structure-properties relationships, the fundamental paradigm of materials science & engineering, physical metallurgy and alloy design, advanced atomic-level structural/chemical characterization, first-principles calculations, HE, mechanical properties evaluation and failure analysis.

Technical Summary:

The overarching scientific theme of the proposed research is to understand the effects of different elements, impurities and either 1H or 2D, on the cohesive energies of GBs and heterophase interfaces concerning HE in AM materials, focusing on PH stainless steels derived from the classical 17-4 PH steel. Wrought 17-4PH steel is included to provide a reference basis for comparison with the AM processed steels, i.e., QT17-4+.

This will enable us to better understand the influence of grain- and heterophase-boundary segregation of alloying elements, impurities, and 1H or 2D, and the effects of different processing conditions (e.g., heating rates) on the microstructures (GB characteristics of martensite lath and prior austenite grain-boundaries), GB cohesion, and susceptibility to HE. From this detailed understanding, we will develop mitigation strategies for embrittlement of AM and/or welded components and use alloy design principles to optimize interfacial cohesion of the defects critical for HE.

The fundamental data generated will pave the way to next-generation steel designs and support the industrial revolution created by the development of AM. The PIs will develop an online graduate-level course, Additive Manufacturing of Metallic Materials: Theory and Practice, for teaching at Tel Aviv University and Northwestern. Students will form groups of four and apply topics learned to a rapid manufacturing design project.

Students will gain significant experience with writing and presenting to a technical audience. At NUCAPT, the PIs will continue undergraduate student projects through the NSF-REU programs of the NSF-funded Materials Research Science and Engineering Center, SHyNE resource, programs aimed at women and underrepresented minorities, work-study and senior project students, national universities, national laboratories, and industry.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.