Suresh Babu

Phase Field Simulations of Autocatalytic Formation of Alpha Lamellar Colonies in Ti-6Al-4V

We present phase field simulations incorporating contributions due to chemical free energy, anisotropic interfacial energy, and elastic energy due to transformation strain, to demonstrate the nucleation and growth of multiple variants of alpha from undercooled beta in Ti-6Al-4V under isothermal conditions. A new composite nucleation seeding approach is used within the phase field simulations to demonstrate that the presence of a pre-existing strain field can cause the nucleation of specific crystallographic variants of alpha based on minimization of local elastic strain energy. Under conditions where specific combinations of elastic strains exist, for example in the vicinity of one or more pre-existing alpha variants, the nucleation of a new alpha variant is followed by the successive nucleation of the same variant in the form of a lamellar colony by an autocatalytic mechanism. At a given thermodynamic undercooling, the colony structure was favored at a nucleation rate that was low enough to allow sufficient growth of previously nucleated variants before another nucleus formed in their vicinity. Basket weave morphology was formed at higher nucleation rates where multiple nuclei variants grew almost simultaneously under evolving strain fields of several adjacent nuclei.

Atom-Probe Field Ion Microscopy

More than fifty years have passed since Muller and his coworkers invented the atom-probe field ion microscope (APFIM) in 1968 (Muller et al., 1968). The APFIM was originally developed as a tool for surface science; however, from the emerging stage of the technique, physical metallurgists realized that its use could solve many critical problems on the microstructures of metallic materials, in particular steels. By the 1980s, several researchers started applying the atom probe (AP) technique for microstructural characterizations of various metallic materials. However, the recent successful implementation of pulsed laser to assist field evaporation for 3DAP analyses expanded the application areas to a wide variety of materials including semiconductors, insulator thin films, and even to bulk insulating ceramics. In addition, the development of site specific specimen preparation method using the focused ion beam (FIB) technique made it possible to prepare tips from all types of materials, including powder, thin films, and devices. The aim of this chapter is to provide the basic principles of the 3DAP technique for those who are about to start using this technique for microstructure characterization of materials including metals and alloys, ceramics and semiconductors.

Progress in the Processing and Understanding of Alloy 718 Fabricated Through Powder Bed Additive Manufacturing Processes

This paper reviews currently available information on the processing and understanding of Alloy 718 fabricated through powder bed additive manufacturing processes, specifically selective laser melting, electron beam melting, and binder jet additive manufacturing. In each instance, the microstructures formed exhibit attributes unique to the process used. Through post-processing, these materials are capable of achieving property behaviors similar to that of the long utilized wrought material. While AM processes are complex, computational modeling has been successfully applied to capture the heat and mass transfer, microstructure evolution, and constitutive response of the material.

Development of Novel Functionally Graded Transition Joints for Improving the Creep Strength of Dissimilar Metal Welds in Nuclear Applications

Dissimilar Metal Welds (DMWs) between 2.25Cr-1Mo steel and Alloy 800 are required to fabricate the Very High Temperature Reactor (VHTR) that will be used to generate nuclear power. Experience has demonstrated that failures of such DMWs can occur prematurely well below the expected service life of either base metal. As described in a recent review article written by the PI, premature failure is caused by the sharp change in composition, microstructure, and mechanical properties that occur across the fusion line of the weld. Recent research completed by the PI and co-PIs has demonstrated that these factors can be eliminated with Graded Transition Joints (GTJs) that are made with a gradual change in composition from the 2.25Cr-1Mo steel to Alloy 800. With this approach, the GTJ is placed between the two alloys so that similar welds can be made to each alloy, thus eliminating the DMW that is prone to failure. Research is now needed to advance these concepts for application to welds required in the VHTR. Thus, the primary objectives of this research are to: 1) Develop design and processing methods for fabricating novel GTJs that eliminate failures that occur due to enhanced carbon diffusion and high thermal stresses, 2) Establish a database of creep life and improved creep life models of GTJs that are supported by new techniques for measuring localized strain in the presence of composition and microstructure gradients, and 3) Apply this information to initiate code acceptance by the American Society of Mechanical Engineers (ASME) for use of GTJs in the VHTR application. This research will benefit both the nuclear and fossil power generation industries by avoiding premature DMW failures that have plagued each industry for more than 40 years. The research will directly address the needs described in Advanced Structural Materials, Dissimilar Transition Weld Issues for High Temperature Reactors. The collaborative project capitalizes on recent advances made by each organization in the field of dissimilar metal joining. The team will first conduct high temperature tests on candidate GTJs that were recently developed by the PI. The tests will be conducted with new strain measurement techniques (recently developed by the co-PIs) that are critical for understanding the performance of the joints. These results will then be used to develop refined creep models of strain localization for the GTJs. Methods will also be developed for further optimization of the GTJs by minimizing carbon diffusion and thermal stresses that arise due to CTE mismatch. The results from these activities will then be combined and applied to manufacture and test optimized GTJs. The GTJs will be made with the dual wire gas tungsten arc welding (DWGTAW) process that has already been demonstrated for this purpose and is easily scalable to mass fabrication. The results will produce a database of high temperature mechanical properties and a creep constitutive model for GTJs that will pave the way for eventual ASME code acceptance that is required for used of GTJs in the VHTR application.