Papers by Author: S. Lee Semiatin

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Authors: Gilles Damamme, David Piot, Frank Montheillet, S. Lee Semiatin
Abstract: A simple mesoscale model was developed for discontinuous dynamic recrystallization. The material is described on a grain scale as a set of (variable) spherical grains. Each grain is characterized by two internal variables: its diameter and dislocation density (assumed homogeneous within the grain). Each grain is then considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain boundary migration equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a dislocation-density evolution equation, such as the Yoshie–Laasraoui–Jonas or Kocks–Mecking relationship, involving strain hardening and dynamic recovery, and (iii) an equation governing the total number of grains in the system due to the nucleation of new grains. The model can be used to predict transient and steady-state flow stresses, recrystallized fractions, and grain-size distributions. A method to fit the model coefficients is also described. The application of the model to pure Ni is presented.
Authors: H.S. Lee, Won Kyu Bang, D.W. Kim, S. Lee Semiatin, Young Won Chang
Authors: S. Lee Semiatin, Gordon A. Sargent
Abstract: The low-temperature superplastic flow behavior of two lots of Ti-6Al-4V sheet with an ultrafine microstructure was modeled. One lot (Sheet A) had an equiaxed-alpha starting microstructure; the flow stress/flow hardening exhibited by this material was explained on the basis of the Bird-Mukherjee-Dorn constitutive equation. The other material (Sheet B), having a mixed equiaxed- and remnant-lamellar alpha microstructure, underwent flow softening, flow hardening, or steady-state flow depending on test temperature and strain rate. These behaviors were interpreted in the context of a dynamic spheroidization model. The apparent flow softening at the end of all of the flow curves was explained using a simple flow-localization model.
Authors: S. Lee Semiatin, Donald S. Weaver, Robert L. Goetz, J.P. Thomas, Todd J. Turner
Authors: Jeoung Han Kim, S. Lee Semiatin, Chong Soo Lee
Abstract: The high temperature deformation mechanisms of two phase a+b alloy and a near-a alloy were investigated, and compared within the framework of inelastic-deformation theory. For this purpose, load-relaxation tests were conducted on the two alloys at temperatures of 750~900°C. The flow stress-vs.-strain rate curves for both alloys were well fit with inelastic deformation equations describing dislocation glide and grain boundary sliding. The amount of grain boundary sliding resistance was higher in the near-a alloy rather than the two phase a+b alloy due to difficulty in stress relaxation at triple junction region.
Authors: J.E. Park, J.B. Jeon, S. Lee Semiatin, Chong Soo Lee, Young Won Chang
Abstract: Textures developed during hot rolling process may affect the high temperature deformation behaviors of Ti alloys, but their relation has not been well understood or quantitatively analyzed yet. A series of load relaxation and creep tests for hot rolled Ti-6Al-4V alloy has been conducted in this work to clarify the effect of textures on the deformation behaviors of the alloy under 700 °C and the result was analyzed by using an internal variables approach. The internal strength σ* was found to vary significantly by the textures, but not by the temperature change, while the texture effect was found to decrease at higher temperatures.
Authors: Gennady A. Salishchev, S.V. Zerebtsov, S.Yu. Mironov, S. Lee Semiatin
Abstract: Microstructure evolution in alpha-beta Ti-64 alloy samples with lamellar structure deformed to a height reduction of 70% at temperatures between 450 and 800°C has been investigated. The deformation led to a distinctly globularized structure of α- and β-phase in the whole temperature interval. The dependence of globular grain size on deformation temperature is of a linear character up to the temperature of warm deformation at which formation of an SMC structure takes place. Continuous recrystallization was observed in the α-and β-phases. Different types of defects responsible for splitting of α-lamellae such as low and high angle boundaries, shear bands and twins were found. An investigation of boundary misorientation spectra in the α-and β-phases deformed to different strains at 550 and 800°C was carried out. Typical boundary misorientation spectra for single phase metals with the same lattice were obtained. The boundary misorientation spectrum depends weakly on strain and deformation temperature. The results of this study show the importance of transformation of semi-coherent interphase boundaries to non-coherent ones for globularization of lamellar microstructures.
Authors: Jeoung Han Kim, S. Lee Semiatin, Chong Soo Lee
Authors: Frank Montheillet, S. Girard, Christophe Desrayaud, S. Lee Semiatin, J. Le Coze
Abstract: The present work deals with the influence of niobium in solid solution on the dynamic recrystallization of pure nickel. High-purity nickel and two model nickel-niobium alloys were deformed to large strains via torsion at temperatures between 800 and 1000°C. Niobium additions considerably increased the flow stress, while they lowered the strain-rate sensitivity and increased the apparent activation energy. EBSD of the steady-state microstructures revealed strong grain refinement. Substructure development was favored, whereas thermal twinning was reduced by niobium. More generally, discontinuous recrystallization kinetics were considerably decreased.
Authors: A.K. Ghosh, Dong Ho Bae, S. Lee Semiatin
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