Variant Selection in Grain Boundary Nucleation of Upper Bainite

The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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Mechanical Properties,Dislocation Density and Grain Structure of Ultrafine-Grained Aluminum and Aluminum-Magnesium Alloys

The kind and amount of alloying elements strongly affects the formation of ultrafine-grained microstructures. Aluminum alloys with different amounts of the alloying element magnesium, and a commercially pure aluminum alloy, have been investigated in order to evaluate how the obtained microstructures will affect the mechanical properties. X-ray profile analysis has been used to determine grain size and dislocation density. With increasing amounts of alloying elements, a smaller grain size and a higher dislocation density after severe plastic deformation (SPD) are obtained, which lead to higher hardness and improved fatigue properties. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application”, which occurred September 25-27, 2006 in Kloster Irsee, Germany.

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Severe Plastic Deformation of Difficult-to-Deform Materials at Near-Ambient Temperatures

Plane-strain machining can be used to impart large plastic strains in alloys that are difficult to deform by other severe plastic deformation (SPD) processes. By cutting at low speeds, the heating caused by friction with the tool can be reduced to insignificant levels. The utility of this approach for characterizing microstructure development in SPD is demonstrated using a variety of commercial alloys that exhibit different deformation behaviors and strengthening mechanisms, including CP-titanium, aluminum alloy 6061-T6, nickel-base superalloy IN-718, and pearlitic plain-carbon steel. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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Grain Boundary Curvature in a Model Ni-Based Superalloy

The local grain boundary (GB) curvature in a model Ni-based superalloy was measured experimentally using Dehoff’s tangent count method. The results show that, in materials containing significant amounts of second-phase particles, the curvature parameter, κ, which relates the mean local curvature to the grain size, can adopt far lower values than have been reported previously. It is also shown that the value of κ is not a constant, as is usually assumed, but instead varies both with the volume fraction of second-phase particles and with the holding time during high-temperature annealing. The lowest values for κ were obtained for high particle volume fractions and long annealing times. Because the local boundary curvature constitutes the driving force for grain growth, these observations could help to explain grain growth phenomena in heavily pinned systems.

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Effects of Deformation Twinning on Energy Dissipation in High Rate Deformed Zirconium

The posit that deformation twinning can result in energy storage is examined by measuring the temperature increase of zirconium during adiabatic compression at high strain rates and correlating the response with the resulting microstructure. After examining the underlying assumptions of homogeneous deformation via microscopy and numerical modeling, it is determined that the occurrence of twinning does not correlate with significant energy storage relative to dissipation, and the appearance of storage may be caused by inhomogeneity in the deformation. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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Enhanced Corrosion Resistance of Stainless Steel Carburized at Low Temperature

The pitting corrosion resistance of surface-modified 316L austenitic stainless steel and N08367 (a “superaustenitic” stainless steel) were evaluated in 0.6 M NaCl solutions and compared to untreated samples of the same materials. The surface modification process used to treat the surfaces was a low-temperature carburization technology termed “low-temperature colossal supersaturation” (LTCSS). The process typically produces surface carbon concentrations of ~15 at. pct without the formation of carbides. The pitting potential of the LTCSS-treated 316L stainless steel in the NaCl solution substantially increased compared to untreated 316L stainless steel, while the pitting behavior of the LTCSS-treated N08367 was unchanged compared to the untreated alloy. This article is based on a presentation given at the “International Conference on Surface Hardening of Stainless Steels,” which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS.

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Poisson Effects on X-Ray Diffraction Patterns in Low-Temperature-Carburized Austenitic Stainless Steel

Austenitic stainless steel was carburized at low temperature to generate a hard surface layer. X-ray diffractometry (XRD) revealed that this “case” contained an expanded fcc lattice and significant residual stresses due to the interstitial carbon. The XRD patterns also exhibit consistent variations with crystallographic orientation. Using published elastic constants for austenitic stainless steel and appropriate approximations for the XRD elastic constants, the XRD peak position variations can be accounted for by orientation-dependent Poisson effects due to biaxial residual stresses. The XRD patterns of specimens containing either compressive or tensile residual stresses were consistent with this hypothesis. This article is based on a presentation given at the “International Conference on Surface Hardening of Stainless Steels,” which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS.

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Effect of Low Temperature on Fatigue Life and Cyclic Stress-Strain Response of Ultrafine-Grained Copper

Fatigue life curves and cyclic stress-strain curves of ultrafine-grained (UFG) copper of purity 99.9 pct produced by equal-channel angular pressing (ECAP) were determined under stress control at room temperature (RT) and at a temperature of 173 K. The obtained curves were compared to the corresponding curves obtained on conventional-grain (CG) copper. At both temperatures, the lifetime of UFG copper is longer than that of CG copper. The S-N curve of UFG copper is temperature dependent, while its cyclic stress-strain curve is temperature insensitive. To explain this temperature effect, two mechanisms of cyclic plastic deformation were proposed: the temperature-independent bulk dislocation mechanism taking place in the entire loaded volume and the temperature-dependent localized mechanism consisting of cooperative grain boundary (GB) sliding along the shear plane of the last ECAP pass taking place in the surface layer and leading to formation of surface fatigue markings. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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Maximizing the Glass Fraction in Iron-Based High Velocity Oxy-Fuel Coatings

Developing iron-based coatings, from glass forming alloys such as SAM2X5, which exhibit outstanding corrosion performance superior to nickel-based alloys, results in particular challenges. This is because the resulting corrosion performance of the coating depends on a complex inter-relationship between the intrinsic properties including coating chemistry with its resulting protective oxide layer, the extrinsic properties related to the macrostructure with its defects resulting from the spray process, and the microstructure where one key factor is the total level of microstructural refinement achieved. As the microstructural scale is reduced, it becomes increasingly difficult for the electrochemical system to initiate electrochemical attack. Metallic glasses, which can be considered “angstrom” scaled materials, represent the ultimate in microstructural uniformity. In this article, the influence of the feedstock powder structure on the resulting glass content in the coating will be explored, because maximizing the glass percentage is one key factor in improving corrosion performance. This article is based on a presentation given in the symposium entitled “Iron-Based Amorphous Metals: An Important Family of High-Performance Corrosion-Resistant Materials,” which occurred during the MS&T meeting, September 16–20, 2007, in Detroit, Michigan, under the auspices of The American Ceramics Society (ACerS), The Association for Iron and Steel Technology (AIST), ASM International, and TMS.

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Simulation of Nucleation of Proeutectoid Ferrite at Austenite Grain Boundaries during Continuous Cooling

The nucleation kinetics of proeutectoid ferrite during continuous cooling in three Fe-C-Mn-Si steels, measured in-situ by three-dimensional X-ray diffraction microscope, are compared with numerical simulation that takes into account differences in the activation energy of nucleation among grain boundary faces, edges, and corners. The essential feature of ferrite nucleation in the 0.21 pct C steel, i.e., nucleation occurred just below Ae₃ and ceased at a small undercooling, is reproduced taking into account the site consumption, primarily at grain corners and overlap of solute diffusion fields in the grain boundary region or the matrix and assuming a very small or almost null activation energy of nucleation. In the 0.35 and 0.45 pct C steels, small activation energy, as reported by Offerman et al., was not unequivocally obtained because ferrite nucleation occurred at considerably large undercoolings, even below the paraequilibrium Ae₃ in these steels. The increasing rate of the observed particle number with decreasing temperature is considerably smaller than calculation. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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