Fundamentals of Deformation in gamma - gamma prime - delta Ni-Base Superalloys
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Nickel-base superalloys are the materials of choice for structures used in the “hot section” of modern gas turbines engines, and are likely to remain so for the foreseeable future. As the fundamental understanding pertaining to the physical metallurgy of g-g¢ Ni-base alloys matures, increasingly stringent intrinsic limitations must be overcome via changes in alloying or processing to produce modest improvements in properties. More often than not, the modest improvements in properties also accompany compromises in manufacturing, environmental resistance and cost. Additionally, the incremental improvements in mechanical properties or temperature capability are often outweighed by the compromises that are required for the insertion of the new alloy or process. For this reason, this study is aimed at investigating and developing a revolutionary new class of structural materials for high temperature applications. The innovative alloy system is based on the Ni-Al-Nb pseudo-ternary eutectic g-g'-d. In addition to utilizing the ordered Ni3Al g' precipitate as a strengthening phase, this alloy also contains large volume fractions of a primary intermetallic Ni3Nb d phase dispersed both intragranularly as well as along the grain boundaries. Compared to commercial polycrystalline g-g' Ni-base superalloys, preliminary results indicate that the novel g-g'-d possess higher specific strengths, significantly improved temperature capability and are amenable for low cost manufacturing techniques.
Deformation of Investment Cast NiTi Shape Memory Alloys
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NiTi alloys are part of a unique class of materials that exhibit a strong coupling between their mechanical behavior and temperature. Unlike conventional materials that deform permanently when subjected to stresses that exceed the yield strength, these adaptive materials have been demonstrated to exhibit a shape memory effect that enables them to recover and restore otherwise irreversible plastic strains when subjected to an external temperature field. The underlying mechanism associated with this novel behavior has been attributed to the unique solid-state martensitic transformations occur within the underlying crystal structure of NiTi alloys. Martensitic transformations are diffusionless, displacive and athermal transformations that result in a distortion in the crystal lattice. For equiatomic or nearly equiatomic NiTi alloys, the equilibrium parent phase at elevated temperatures is a face-centered cubic (FCC) austenite. Upon cooling of the alloy, the austenitic crystal structure begins to transform into a low symmetry, monoclinic martensitic structure at a critical martensite start, Ms, temperature. Unlike other crystalline solids in which dislocation activity is dominant during plastic deformation, deformation of this low symmetry, martensitic structure leads to the alignment of the martensitic structures via “detwinning” or glide of twin boundaries into oriented configurations. It is this microscopic alignment of the martensitic structure that enables these NiTi alloys to accommodate macroscopic strains without the movement or multiplication of dislocations. Upon subsequent heating of the deformed or detwinned material to a temperature above the austenite transformation temperature, Af, the oriented martensitic structure reverts back to the original austenitic structure. Since the transformation from martensite to austenite only involves small atomic displacements relative to the lattice size, this enables the material to revert back to its original reference configuration. This study aims to quantify and better understand how processing parameters affect the bulk transformation behavior of investment cast NiTi structures using Digital Image Correlation (DIC) techniques.
Mechanisms of Grain Refinement during Hot Deformation
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Novel methods of grain size refinement have been given a great deal of attention in recent years by those looking to further enhance material properties. Ultra-fine grained (UFG) materials exhibit enhanced strength, wear resistance and superplastic behavior, without the substantial reduction in fracture toughness and ductility associated with many other methods of enhancing these properties. The benefits of UFG materials stem from a high fraction of grain boundaries, which serve to limit dislocation mobility and lead to strengthening according to the Hall-Petch relation. These potential benefits have led many to investigate various methods of producing UFG materials such as severe plastic deformation. This technique involves grain refinement via recrystallization under a combination of intense plastic strain and elevated temperature. Such methods are ideal for their ability to produce large-scale bulk UFG parts at lower relative costs. Substantial grain refinement has shown to be achievable by these methods, provided sufficiently high plastic strains are achieved. Since deformation temperature is also a key factor, as it controls both the recrystallization and grain growth kinetics, deformation processing at relatively low temperatures (below that typically used in conventional forming) can yield a fine recrystallized grain size with very little subsequent growth, due to the differences in low-temperature deformation kinetics found in many alloy systems.
Grain Boundary Engineering of Ni-Base Superalloys
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Recent advances in the development of electron back-scatter diffraction (EBSD) characterization tools have enabled materials scientists and engineers to develop a significantly better understanding of the role of crystallographic texture and grain boundary character on the properties of virtually all classes of materials. Grain boundary engineering or controlling the distribution and relative fractions of twin and other high CSL boundaries with S <29 has been an active topic of research. Pertaining to Ni-base superalloys, various studies have reported improvements in creep resistance, intragranular SCC and fatigue crack growth resistance associated with having large fractions of special grain boundaries that break up the interconnectivity of the pre-existing random grain boundary network in polycrystalline Ni-base superalloys. The associated benefits stemming from GBE have been successfully demonstrated on commercial cast/wrought and powder processed Ni-base superalloys such as IN600, IN718, IN718 Plus, René 104, and RR1000. Despite some successes in achieving radically improved properties via GBE, these advances have not yet been fully realized in physically large bulk structures. Existing practice for GBE utilizes multiple iterations of cold rolling to 5 to 20% strain followed by annealing. Since each iteration of deformation and annealing imparts a modest increase in the fraction of twin and special grain boundaries, multiple iterations are required to achieve a sufficiently high fraction (~50%) of special grain boundaries that result in the improved properties. This GBE approach therefore is not suitable for the fabrication of large, complex structures and leads to added manufacturing lead time and cost. We are working on the development of a revolutionary process for cost effective GBE of bulk Ni-base superalloy structures and components.
High Temperature Nano- and Micro- Indentation
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Advances in instrumented indentation have enabled the development of a powerful characterization tool capable of probing the mechanical properties of micro- and nano-scale features within a microstructure. For this technique, load-depth data is recorded during indentation and used to extract micromechanical properties, such as Young’s modulus, hardness, strain hardening coefficient and indentation stress-strain. In addition, this technique can be adapted to conduct micro-compression tests on miniaturized samples that provide site-specific mechanical properties which correspond to specific microstructural features. The recent development of high temperature instrumented indentation may be particularly useful in screening the preliminary properties of novel structural materials. As applied to NiTi SMAs, an instrumented Vickers microindentation technique has been effectively used to demonstrate the influence of crystallographic grain orientation and Ti3Ni4 precipitates sizes on the associated deformation mechanisms. In addition, recovery of micro- and nano-indents of NiTi SMAs after heating has also been well documented. It was found that the penetration depth imposed by spherical indenters can almost be fully recovered upon heating, while only a fraction of the resultant deformation induced by a pyramidal indenter can be recovered. These recent studies clearly show the successful application of microindentation techniques to quantify the mechanical properties and shape recovery of NiTi alloys.
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