Balasubramaniam Radhakrishnan

Numerical analysis of GTA welding process with emphasis on post-solidification phase transformation effects on residual stresses

The objective of this work was to analyze the residual stress state in spot welds made in an HY-100 steel disk by an autogenous gas tungsten arc (GTA) welding process. An uncoupled thermal-mechanical finite element (FE) model was developed that took into account the effects of liquid-to-solid and solid-state phase transformations. Effects of variations in mechanical properties due to solid-state phase transformations on residual stresses in the weld were studied. Extensive experimental testing was carried out to determine the mechanical properties of HY-100 steel. The residual stresses in the disk with the spot weld were measured by a neutron diffraction (ND) technique. The FE results are in good agreement with the ND measurements. The results show that the volumetric changes associated with the austenite to martensite phase transformation in HY-100 steel significantly affect residual stresses in the weld fusion zone and the heat affected zone.

Modeling the kinetics and microstructural evolution during static recrystallization : Monte Carlo simulation of recrystallization

The kinetics of microstructure and texture evolution during static recrystallization of a cold-rolled and annealed f.c.c. material is simulated by coupling a finite element model of microstructural deformation with a Monte Carlo simulation of recrystallization. The salient features of the simulations include a nucleation model for recrystallization based on subgrain growth and the modeling of simultaneous recovery during recrystallization. The simulation results quantify the effects of non-uniform stored energy distribution and orientation gradients present in the cold-worked microstructure on recovery by subgrain growth, and hence on the spatial distribution of nuclei and their orientations. The growth of these recrystallized nuclei in the presence of continued recovery of the substructure has been simulated for initial cold-work levels of e=0.7 and 1.1 obtained by plane strain compression. The simulations are shown to be potentially capable of capturing the formation and evolution of cube texture commonly observed in cold-rolled and annealed f.c.c. materials.

Simulation of curvature-driven grain growth by using a modified monte carlo algorithm

The Monte Carlo (MC) algorithm that currently exists in the literature for simulating curvature-driven grain growth has been modified. The modified algorithm results in an acceleration of the simulated grain growth and an early estimate of the grain growth exponent that is close to the theoretical value of 0.5. The upper limit of grain size distributions obtained with the new algorithm is significantly lower than that obtained with the old, because the new algorithm eliminates grain coalescence during grain growth. The log-normal function provides an excellent fit to the grain size distribution data obtained with the new algorithm, after taking into account the anisotropy in grain boundary energy.

Large Discrete Resistance Jump at Grain Boundary in Copper Nanowire

Copper is the current interconnect metal of choice in integrated circuits. As interconnect dimensions decrease, the resistivity of copper increases dramatically because of electron scattering from surfaces, impurities, and grain boundaries (GBs) and threatens to stymie continued device scaling. Lacking direct measurements of individual scattering sources, understanding of the relative importance of these scattering mechanisms has largely relied on semiempirical modeling. Here we present the first ever attempt to measure and calculate individual GB resistances in copper nanowires with a one-to-one correspondence to the GB structure. Large resistance jumps are directly measured at the random GBs with a value far greater than at coincidence GBs and first-principles calculations. The high resistivity of the random GB appears to be intrinsic, arising from the scaling of electron mean free path with the size of the lattice relaxation region. The striking impact of random GB scattering adds vital information for understanding nanoscale conductors.

Finite element simulations of cold deformation at the mesoscale

Abstract The deformation of polycrystalline aggregates is modeled using the finite element method. Explicit discretization at the single crystal level is employed to study the inhomogeneous deformations of individual crystals. Plastic deformation by crystallographic slip is treated using a constitutive model based on crystal plasticity. The formulation is used to predict the non-uniform nature of strain hardening and texture evolution in the crystals subjected to plane strain compression. The capability of the simulations to capture the inhomogeneous deformation of individual grains during plastic deformation of polycrystals is demonstrated. The hardness and orientation values of elements from the same grain evolve to different final values due to local inhomogeneities and interactions with neighbors. The simulations provide a means to obtain quantitative information on the inhomogeneous distributions of stored energy and orientations among the different crystals comprising the microstructure.

Simulations of deformation and recrystallization of single crystals of aluminium containing hard particles

The deformation of a single crystal of aluminium in the Goss orientation {011}100 containing a coarse particle of silicon was modelled by using a finite-element (FE) code based on the crystal plasticity approach. The simulations clearly captured the heterogeneous deformation of the aluminium matrix, resulting in a region of high deformation in the vicinity of the hard particle, surrounded by a region where the amount of deformation was significantly lower. The evolution of the corresponding deformation substructure during annealing was simulated using a Monte Carlo technique. The simulations clearly demonstrated the discontinuous evolution of the subgrains in the deformation zone to form recrystallization nuclei around the hard particle, and the subsequent growth of these nuclei to consume the matrix region around the particle. For plane strain compression up to ezz = -0.4 that was used in this study, the deformation texture components near the particle consisted of rotations up to 20° from the initial Goss orientation about the transverse direction. Recrystallization simulations captured the formation and growth of nuclei from the deformation heterogeneities existing near the hard particle and predicted a significant strengthening of the orientations present in the particle deformation zone. The simulation results are shown to capture many of the experimentally observed features of deformation and recrystallization textures in aluminium single crystals containing coarse particles of silicon.

Monte Carlo simulation of grain boundary pinning in the weld heat-affected zone

A methodology for obtaining a one-to-one correlation between Monte Carlo (MC) and real parameters of grain size and time is described. Using the methodology, and the MC grain growth algorithm, the grain structure in the weld heat-affected zone (HAZ) of a 0.5 Mo-Cr-V steel has been simulated. The simulations clearly show that the kinetics of grain growth can be retarded by the presence of steep temperature gradients in the weld HAZ. Additional pinning due to the formation of grain boundary liquid near the solidus temperature has also been simulated. It is shown that in order to accurately predict the observed grain size in the weld HAZ of the 0.5Cr-Mo-V steel, the retardation in growth kinetics due to temperature gradients as well as liquid pinning should be considered.

Mesoscale Modeling of Microstructure and Texture Evolution During Deformation Processing of Metals

The application of the finite element method to model the deformation of metals at the mesoscale to study the microstructure and texture evolution is described. The finite element discretization is applied directly to the various grains, and crystal plasticity is used as the constitutive basis to model the plastic deformation by crystallographic slip, and to evolve the slip system strength and crystal lattice orientation of the material. Applications of the methodology to detailed studies of the non-uniform deformations of individual grains, and effects of grain interactions on the distributions of deformation and stress in the microstructure are discussed.

Monte Carlo simulation of deformation substructure evolution during recrystallization

Recently, a microstructure and texture evolution model was presented for static recrystallization by coupling a Monte Carlo (MC) simulation of recrystallization with a finite element (FE) simulation of microstructural deformation based on crystal plasticity. The crystal plasticity model provided a quantitative description of the orientation distributions in the deformed microstructure. In this paper, a new model for recrystallization is presented where recrystallization is modeled as a one-step, heterogeneous subgrain growth process. The simulations indicate how existing prior high angle boundaries as well as the high angle boundaries produced by the deformation process migrate to sweep through the deformed regions to produce typical recrystallized microstructures and kinetics. The simulations also capture the heterogeneous growth of subgrains induced by long-range orientation gradients present in the deformation substructure. The simulations are used to evaluate the recrystallization kinetics, microstructure and texture evolution for deformed fcc polycrystals.

Kinetics of grain growth in the weld heat-affected zone of alloy 718

Grain-boundary liquation occurs in the weld heat-affected zone (HAZ) of the Ni-base superalloy 718 at locations where the peak temperatures are greater than about 1200 ‡C. The evolution of the grain structure at these HAZ locations depends upon the interaction between the grains and the grain-boundary liquid. The evolution of grain structure in the presence of grain-boundary liquid was simulated by subjecting samples to controlled thermal cycles using resistance heating. A measurement of grain size as a function of isothermal hold at two peak temperatures of 1200 ‡C and 1227 ‡C indicated that in alloy 718, the kinetics of grain growth depended upon the prior thermal history of the alloy. In the solution-treated alloy, the presence of grain-boundary liquid did not arrest grain growth at either peak temperature. In the homogenized and aged alloy, a grain refinement was observed at the peak temperature of 1227 ‡C, while an arrest of grain growth was observed at a peak temperature of 1200‡C. Liquid film migration (LFM) and subgrain coalescence, either acting alone or simultaneously, are shown to explain most of the observed microstructural phenomena and the kinetics of grain growth in the alloy.