Quasicontinuum multiscale modeling of the effect of rough surface on nanoindentation behavior

Roughness of surface has as an important influence on identifying the mechanical behavior and performance of crystalline metals. In this study, nanoindentation simulations are conducted by the two dimensional quasicontinuum method to determine the load–penetration response and the critical load associated with the onset of plasticity in rough surfaces of a face-centered cubic single crystal copper. The arithmetic roughness index, ranging between 2 and 13 Å, is used to specify the roughness of surface. Results of indentation with different roughnesses are in good agreement with previous studies for the indenter size of 10–140 Å. The resultant load–penetration scattering, which stems from the roughness, indicates different dislocation nucleation steps, different subsequent dislocations intervals and varying stiffness values of samples. It can be concluded that the surface roughness has a significant effect on the first dislocation emission because of the indenter position and surface interactions beneath it. Moreover, the critical penetration depth for the first dislocation emission increases by the increase of the contact area between the indenter and surface.

     

Superelasticity and stability of a shape memory alloy hexagonal honeycomb under in-plane compression

Nitinol (NiTi) shape memory alloy honeycombs, fabricated in low densities using a new brazing method [Grummon, D., Shaw, J., Foltz, J., 2006. Fabrication of cellular shape memory alloy materials by reactive eutectic brazing using niobium. Materials Science and Engineering A 438–440, 1113–1118], recently demonstrated enhanced shape memory and superelastic properties [Shaw, J. A., Grummon, D. S., Foltz, J., 2007b. Superelastic NiTi honeycombs: Fabrication and experiments. Smart Materials and Structures 16, S170–S178] by exploiting kinematic amplification of thin-walled deformations. The realization of such adaptive, light-weight cellular structures opens interesting possibilities for design and novel applications. This paper addresses the consequent need for design and simulation tools for engineers to make effective use of such structures by, as a first step, analyzing the multi-scale stability aspects of the superelastic behavior of a particular hexagonal, thin-walled, SMA honeycomb under in-plane compression. An in-depth parameter study is performed of the influence of different material laws on the behavior of honeycombs of finite and infinite extent with perfect and imperfect initial geometries. A finite element-based simulation is presented that credibly captures the behavior seen in experiments.     

Deformation behavior of tantalum and a tantalum tungsten alloy

A comparative study of the deformation behavior of tantalum and a tantalum 2.5 wt.% tungsten alloy is carried out. High strain-rate experimental data are used to develop phenomenological constitutive relations. The temperature and the strain-rate sensitivity of the flow stresses are compared. It is observed that although the flow stress for the Ta–2.5%W alloy is greater than that of Ta, the corresponding temperature and strain-rate sensitivity is less pronounced. Ta–2.5%W experiences a solid-solution softening, wherein the athermal stress component has increased, while the thermal component has decreased by the alloying.     

X-ray diffraction study of the fatigue behavior of a shot-peened aluminum-lithium alloy

The effect of different shot-peening treatments on the cyclic fluctuating bending fatigue behavior of a new aluminum-lithium alloy (the 2091) has been investigated. The residual-stresses in-depth profiles have been defined just after the shot peening using mainly the X-ray diffraction method. To reach a given depth inside the material, the surface layer was removed by electro-polishing. The acquired data had to be then corrected in order to account for the redistribution of the stresses during the polishing. Furthermore, this alloy contains up to 15-percent intermetallic precipitates. Moreover, the X-rays penetrate deeply inside the material (23 m). For these reasons, a special procedure, based on a self-consistent micromechanical scheme, has been developed to correct the acquired data. The stress profiles obtained by this procedure are compared to those obtained by other mechanical experimental methods and those calculated by a theoretical shot-peening method. Shot peening improves the life of the material but the surface residual stresses are partially released during the first cycles of fatigue and are then quickly stabilized.Paper was presented at the 1994 SEM International Conference on Residual Stresses (ICRS4) held in Baltimore, MD on June 8–10.     

Cyclic thermomechanical behavior of a polycrystalline pseudoelastic shape memory alloy

In this work, 3D finite element modeling is employed to examine the thermomechanical behavior of a polycrystalline Ni-Ti shape memory alloy in the pseudoelastic regime. It is shown that the tension-compression asymmetry during uniaxial cyclic loading is due to a preferred orientation of the crystallographic texture. In pure shear loading, the thermomechanical behavior exhibits symmetry in both senses of shear, due to the fiber texture of the specimen bar stock. It is also shown that the apparent strain rate-dependence is due to thermomechanical coupling with latent heat generation/absorption during phase transformation.     

Mechanical behavior of titanium alloy Ti-6Al-4V with unprepared microstructure under jumpwise variations of the strain rate in the superplastic state

We present the results of extension tests with superplastic specimens made of structural titanium alloy with unprepared (coarse-grained) microstructure. The tests were performed at a constant temperature and constant or piecewise constant strain rate. It was shown that, in the case of a jumpwise decrease in the strain rate, the typical shape of the strain diagram depends on the test temperature. Some variations in the original microstructure are demonstrated.     

On the asymptotic behavior of a phase-field model for elastic phase transitions

We study the asymptotic behavior of a one-dimensional, dynamical model of solid-solid elastic transitions in which the phase is determined by an order parameter. The system is composed of two coupled evolution equations, the mechanical equation of elasticity which is hyperbolic and a parabolic equation in the order parameter. Due to the strong coupling and the lack of smoothing in the hyperbolic equation, the asymptotic behavior of solutions is difficult to determine using standard methods of gradient-like systems. However, we show that under suitable assumptions all solutions approach the equilibrium set weakly, while the phase field stabilizes strongly.     

Plastic behavior of a nickel-based alloy under monotonic-tension and low-cycle-fatigue loading

The plastic behavior of an annealed HASTELLOY^® C-22HS™ alloy, a face-centered cubic (FCC), nickel-based superalloy, was examined by in-situ neutron-diffraction measurements at room temperature. Both monotonic-tension and low-cycle-fatigue experiments were conducted. Monotonic-tension straining and cyclic-loading deformation were studied as a function of stress. The plastic behavior during deformation is discussed in light of the relationship between the stress and dislocation-density evolution. The calculated dislocation-density evolution within the alloy reflects the strain hardening and cyclic hardening/softening. Experimentally determined lattice strains are compared to verify the hardening mechanism at selected stress levels for tension and cyclic loadings. Combined with calculations of the dislocation densities, the neutron-diffraction experiments provide direct information about the strain and cyclic hardening of the alloy.     

Simulations of localized thermo-mechanical behavior in a NiTi shape memory alloy

Previous experiments have shown that stress-induced martensitic transformation in certain polycrystalline NiTi shape memory alloys can lead to strain localization and propagation phenomena when loaded in uniaxial tension. The number of nucleation events and kinetics of transformation fronts were found to be sensitive to the nature of the ambient media and imposed loading rate due to the release/absorption of latent heat and the material's inherent temperature sensitivity of the transformation stress. A special plasticity-based constitutive model used within a 3-D finite element framework has previously been shown to capture the isothermal, purely mechanical front features seen in experiments of thin uniaxial NiTi strips. This paper extends the approach to include the thermo-mechanical coupling of the material with its environment. The simulations successfully capture the nucleation and evolution of fronts and the corresponding temperature fields seen during the experiments.     

Segregation behavior of a ternary mixture of titanium dioxide particles in a pseudo two-dimensional fluidized bed

The segregation behavior of a mixture of silica-coated titanium dioxide(TiO2)particles of three different sizes in a pseudo two-dimensional fluidized bed was studied experimentally by the freeze-sieving method and numerically by the multi-fluid model(MFM).Three-dimensional computational fluid dynamics(CFD)simulations were carried out to evaluate the effects of the solid wall boundary conditions on particle segregation in terms of specularity and particle-wall restitution coefficients.The quantitative indexes of segregation tendency and segregation degree were used to determine the axial segregation of the mixture in triangular coordinates.The simulation results revealed that the axial segregation increased with the specularity coefficient,whereas the particle-wall restitution coefficient had a minor effect on axial segregation.Comparison of the simulation results with experimental data showed that the appropriate value of the specularity coefficient used in the CFD model depended on superficial gas velocity.The study of the effects of superficial gas velocity on segregation behavior demonstrated that the greatest segregation was obtained at minimum fluidization velocity and the segregation decreased as the gas velocity gradually increased.     

Quasipatterns versus superlattices resulting from the superposition of two hexagonal patterns

We present a short review of the experimental observations and mechanisms related to the generation of quasipatterns and superlattices by the Faraday instability with two-frequency forcing. We show how two-frequency forcing makes possible triad interactions that generate hexagonal patterns, twelvefold quasipatterns or superlattices that consist of two hexagonal patterns rotated by an angle α relative to each other. We then consider which patterns could be observed when α does not belong to the set of prescribed values that give rise to periodic superlattices. Using the Swift–Hohenberg equation as a model, we find that quasipattern solutions exist for nearly all values of α. However, these quasipatterns have not been observed in experiments with the Faraday instability for $\alpha \ne \mathrm{\pi }/6$. We discuss possible reasons and mention a simpler framework that could give some hint about this problem.     

Rate and thermal sensitivities of unstable transformation behavior in a shape memory alloy

A special plasticity-based constitutive model with an up–down–up flow rule used within a finite element framework has previously been shown to simulate the inhomogeneous nature and the thermo-mechanical coupling of stress-induced transformation seen in a NiTi shape memory alloy. This paper continues this numerical study by investigating the trends of localized nucleation and propagation phenomena for a wider range of loading rates and ambient thermal conditions. Local self-heating (due to latent heat of transformation), the inherent Clausius–Clapeyron relation (sensitivity of the material's transformation stress with temperature), the size of the specimen's nucleation barriers, the loading rate, and the nature of the ambient environment all interact to create a variety of mechanical responses and transformation kinetics. The number of transformation fronts is shown to increase dramatically from a few fronts under nearly isothermal conditions to numerous fronts under nearly adiabatic conditions. A non-dimensional film coefficient and non-dimensional conductivity are identified to be the major players in the range of responses observed. It is shown that the non-dimensional film coefficient generally determines the overall temperature response, and therefore force–displacement response, of a transforming specimen; whereas, the non-dimensional conductivity is the more important player in determining the number of nucleations, and therefore the number of transformation fronts, that may occur.     

Analysis of phase transformation from austenite to martensite in NiTi alloy strips under uniaxial tension

Phase transformation from austenite to martensite in NiTi alloy strips under the uniaxial tension has been observed in experiments and numerically simulated as a localized deformation.This work presents an analysis using the theory of phase transfor- mation.The jump of deformation gradient across the interface between two phases and the Maxwell relation are considered.Governing equations for the phase transformation are derived.The analysis is reduced to finding the minimum value of the loading at which the governing equations have a unique,real and physically acceptable solution.The equa- tions are solved numerically and it is verified that the unique solution exists definitely. The Maxwell stress,the stresses and strains inside both anstenite and martensite phases, and the transformation-front orientation angle are determined to be in reasonably good agreement with experimental observations.     

Crystal-plasticity finite-element analysis of inelastic behavior during unloading in a magnesium alloy sheet

A crystal-plasticity finite-element analysis of the loading-unloading process under uniaxial tension of a rolled magnesium alloy sheet was carried out, and the mechanism of the inelastic response during unloading was examined, focusing on the effects of basal and nonbasal slip systems. The prismatic and basal slip systems were mainly activated during loading, but the activation of the prismatic slip systems was more dominant. Thus the overall stress level during loading was determined primarily by the prismatic slip systems. The prismatic slip systems were hardly activated during unloading because the stress level was of course lower than that during loading. On the other hand, because the strength of the basal slip systems was much lower than that of the prismatic slip systems, the basal slip systems would be easily activated under the stress level during unloading in the opposite direction when their Schmid’s resolved shear stresses changed signs because of the inhomogeneity of the material. These results indicated that one explanation for the inelastic behavior during unloading was that the basal slip systems were primarily activated owing to their low strengths compared to that of the prismatic slip systems. Numerical tests using the sheets with random orientations and with the more pronounced texture were conducted to further examine the mechanism.     

Solidification of binary alloy in a finned enclosure from the bottom

This paper presents experimental findings on the phenomenon of solidification of a binary alloy in a finned enclosure using aqueous ammonium chloride solution. Solidification experiments are carried out over a wide range of initial composition of binary alloy solution from hypoeutectic to hypereutectic concentration ranging from 8, 16 and 24% of ammonium chloride are discussed. An interesting “snowing” phenomenon is observed for the hypereutectic concentration in a finned enclosure.