David A. Korzekwa

Application of polycrystal plasticity to sheet forming

Abstract A methodology for including anisotropy in metal forming analyses is presented. A finite element formulation is developed for the analysis of the inhomogeneous macroscopic deformations. Anisotropic material properties are derived from a microscopic description based on polycrystal plasticity theory. Efficient computation of the microscopic variables is achieved through massive data parallel computations. A procedure is set forth for initialization of the microscopic state variables from experimental measurement of the metal texture. The feasibility of initializing (from experimental data) and evolving (through massive computations) a detailed microscopic description for a complex deformation process is demonstrated through a predictive simulation. The predicted location and height of ears in the hydroforming of aluminium sheets are in good agreement with experiment.

Dislocation substructure as a function of strain in a dual-phase steel

Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite.

Consideration of grain-size effect and kinetics in the plastic deformation of metal polycrystals

Abstract This work extends the constitutive model for the prediction of grain-size dependent hardening in f.c.c. polycrystalline metals proposed by Acharya and Beaudoin [1] (Grain-size effect in fcc viscoplastic polycrystals at moderate strains, 1999, in press) to include effects of temperature and strain rate dependence. A comparison is made between model predictions and compression data, taken at varying temperature and strain rate, for pure Ag having two different grain sizes. It is shown that an initial increase in yield stress and concomitant decrease in hardening rate for a fine-grained material, relative to a coarse-grained counterpart, can be captured through initialization of a state variable serving to describe stress response at prescribed reference conditions of temperature and strain rate. A grain-size dependence of hardening rate during parabolic (stage III) hardening is characterized by the evolution of net dislocation density in a finite element model of a polycrystal aggregate. Finally, observations from simulations of deformation of the polycrystal aggregate are introduced into an existing macroscopic constitutive model for metal plasticity based on the mechanical threshold.

Impression creep behavior of SiC particle-MoSi2 composites

Using a cylindrical indenter (or punch), the impression creep behavior of MoSi{sub 2}-SiC composites containing 0{endash}40{percent} SiC by volume, was characterized at 1000{endash}1200{degree}C, 258{endash}362 MPa punch pressure. Through finite element modeling, an equation that depends on the material stress exponent was derived that converts the stress distribution beneath the punch to an effective compressive stress. Using this relationship, direct comparisons were made between impression and compressive creep studies. Under certain conditions, compressive creep and impression creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the punch. However, rate-controlling mechanisms may be quite different under the two stressing conditions, in which case impression creep data should not be used to predict compressive creep behavior. The addition of SiC affects the impression creep behavior of MoSi{sub 2} in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi{sub 2} grains and by obstructing or altering both dislocation motion and grain boundary sliding. {copyright} {ital 1996 Materials Research Society.}

Creep behavior of MoSi{sub 2}-SiC composites

Using a cylindrical indenter, indentation creep behavior of hot pressed and HIPed MoSi{sub 2}-SiC composites containing 0--40% SiC by volume, was characterized at 1000--1200C, 258--362 MPa. Addition of SiC affects the creep behavior of MoSi{sub 2} in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi{sub 2} grains; by obstructing or altering both dislocation motion and grain boundary sliding; and by increasing the overall yield stress of the material. Comparisons are made between indentation and compressive creep studies. It is shown that under certain conditions, compressive creep and indentation creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the indenter.