Papers by Author: S. Lee Semiatin

Abstract: Multidirectional forging has been developed to produce an ultrafine-grain (UFG) microstructure in the two-phase titanium alloy Ti-6Al-4V. A microstructure with a grain size of 135 nm was attained, enabling low-temperature superplasticity (LTSP) at 550°C. A total elongation of 1000% and strain-rate-sensitivity coefficient m=0.47 were obtained at the optimal strain rate of 2×10^-4 s^-1. Important features of the microstructure and superplastic behavior of the alloy are summarized in the present work. It is shown that microstructure evolution during low-temperature deformation plays a key role in superplastic flow behavior.

Abstract: The recrystallization behavior of cold-rolled, commercial-purity titanium was studied experimentally and with Monte-Carlo (MC) modeling. Utilization of EBSD-OIM as input for MC modeling resulted in realistic predictions of recrystallization kinetics, microstructure and texture, which were in good agreement with experimental results. MC modeling of recrystallization kinetics predicted that the non-uniform stored energy distribution, heterogeneous nucleation of recrystallization and recovery in combination leads to a negative deviation from linear JMAK kinetics. It was found that concurrent recovery that takes place during recrystallization is an important process that controls both the overall recrystallization kinetics and the deviation of linear JMAK kinetics. On the other hand, the non-uniformly distributed stored energy itself has little effect on the negative deviation from JMAK kinetics but intensifies the deviation when heterogeneous nucleation is combined. Modeling results also revealed that heterogeneous nucleation of recrystallized grains and their early impingement in local areas of high deformation are essential for producing a log-normal distribution of grain size and a typical recrystallization texture of rolled titanium.

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.

Abstract: This experimental work deals with the influence of niobium additions to high purity nickel on dynamic recrystallization behavior during hot working. Various high-purity alloys were prepared (unalloyed Ni and Ni–0.01, 0.1, 1 and 10 wt % Nb) and deformed to high strains by hot torsion tests to characterize the rheological behavior within the range 800 – 1000°C at strain rates of 0.03, 0.1 and 0.3 s–1. Niobium additions strongly increased the flow stress. To quantify such behavior, the strain-hardening parameter h and dynamic-recovery parameter r in the Yoshie-Laasraoui-Jonas constitutive equation were determined from the initial part of the experimental stress-strain curves (i.e., at strains before the stress peak) in which dynamic recrystallization does not alter the mechanical behavior. A table showing the variation of h and r as a function of strain rate, temperature, and niobium content was compiled and used to fit a simple empirical model for predicting h and r from the deformation conditions and alloy composition. In addition, microstructures were determined by optical metallography and SEM/EBSD. Based on this work, it appears that niobium additions noticeably refine the steady-state grain size by considerably decreasing the kinetics of dynamic recrystallization in nickel.

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.

Abstract: A simple analytical model is proposed for estimating grain boundary mobility during dynamic recrystallization in metallic alloys. The combined effects of solutes (solute drag) and second phase particles (Zener pinning) on mobility are considered. The approach is based on (and is consistent with) a recently published mesoscale model of discontinuous dynamic recrystallization. The dependence of grain boundary mobility on solute concentration and particle size is summarized in the form of two-dimensional maps.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V (VT6) and near-beta Ti-5Al-5Mo-5V-1Cr-1Fe (VT22) titanium alloys during uniaxial compression at 600°C to a high strain of 70% was studied. The plastic-flow response for both alloys is characterized by successive stages of strain hardening, flow softening, and steady-state flow. During compression the lamellae spheroidized to produce a partially globular microstructure. Globularization in VT6 is associated with the loss of the initial Burgers-type coherency between the alpha and beta phases and the subsequent individual deformation of each phase. The misorientations of boundaries increase to the high-angle range by means of the accumulation of lattice dislocations. In VT22 alloy the alpha phase evolves similar to that in VT6 alloy, while in the beta phase mainly low-angle boundaries are observed even after 70 pct. reduction.

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.

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.