Showing papers on "Deformation (engineering) published in 2008"

Designing metallic glass matrix composites with high toughness and tensile ductility

Abstract: Metallic glasses have been the subject of intense scientific study since the 1960s, owing to their unique properties such as high strength, large elastic limit, high hardness, and amorphous microstructure. However, bulk metallic glasses have not been used in the high strength structural applications for which they have so much potential, owing to a highly localized failure mechanism that results in catastrophic failure during unconfined loading. In this thesis, bulk metallic glass matrix composites are designed with the combined benefits of high yield strengths and tensile ductility. This milestone is achieved by first investigating the length scale of the highly localized deformation, known as shear bands, that governs fracture in all metallic glasses. Under unconfined loading, a shear band grows to a certain length that is dependent on the fracture toughness of the glass before a crack nucleates and fracture occurs. Increasing the fracture toughness and ductility involves adding microstructural stabilization techniques that prevent shear bands from lengthening and promotes formation of multiple shear bands. To accomplish this, we develop in-situ formed bulk metallic glass matrix-composites with soft crystalline dendrites whose size and distribution are controlled through a novel semi-solid processing technique. The new alloys have a dramatically increased room-temperature ductility and a fracture toughness that appears to be similar to the toughest steels. Owing to their low modulus, the composites are therefore among the toughest known materials, a claim that has recently been confirmed independently by a fracture mechanics group. We extend our toughening strategy to a titanium-vanadium-based glass-dendrite composite system with density as low as 4.97 g/cm3. The new low-density composites rival the mechanical properties of the best structural crystalline Ti alloys. We demonstrate new processing techniques available in the highly toughened composites: room temperature cold rolling, work hardening, and thermoplastic forming. This thesis is a proven road map for developing metallic glass composites into real structural engineering materials.

Mechanical biocompatibilities of titanium alloys for biomedical applications.

Abstract: Young's modulus as well as tensile strength, ductility, fatigue life, fretting fatigue life, wear properties, functionalities, etc., should be adjusted to levels that are suitable for structural biomaterials used in implants that replace hard tissue. These factors may be collectively referred to as mechanical biocompatibilities. In this paper, the following are described with regard to biomedical applications of titanium alloys: the Young's modulus, wear properties, notch fatigue strength, fatigue behaviour on relation to ageing treatment, improvement of fatigue strength, fatigue crack propagation resistance and ductility by the deformation-induced martensitic transformation of the unstable beta phase, and multifunctional deformation behaviours of titanium alloys.

Elastic properties of chemically derived single graphene sheets.

Abstract: The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness (E = 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.

Microband-induced plasticity in a high Mn–Al–C light steel

Abstract: Room temperature tensile behavior of a high Mn–Al–C steel in the solid solution state was correlated to the microstructures developed during plastic deformation in order to clarify the dominant deformation mechanisms. The steel was fully austenitic with a fairly high stacking fault energy of ∼85 mJ/m 2 . The tensile behavior of the steel was manifested by an excellent combination of strength and ductility over 80,000 MPa% in association with continuous strain hardening to the high strain. In addition, the austenite phase was very stable during deformation. The high stacking fault energy and firm stability of austenite were attributed to the high Al content. In spite of the high stacking fault energy, deformed microstructures exhibited the planar glide characteristics, seemingly due to the glide plane softening effect. In the process of straining, the formation of crystallographic microbands and their intersections dominantly occurred. Microbands consisting of geometrically necessary dislocations led to the high total dislocation density state during deformation, resulting in continuous strain hardening. This microband-induced plasticity is to be the origin of the enhanced mechanical properties of the steel.

Finite deformation thermo-mechanical behavior of thermally induced shape memory polymers

Abstract: Shape memory polymers (SMPs) are polymers that can demonstrate programmable shape memory effects. Typically, an SMP is pre-deformed from an initial shape to a deformed shape by applying a mechanical load at the temperature TH>Tg. It will maintain this deformed shape after subsequently lowering the temperature to TL Tg, where the initial shape is recovered. In this paper, the finite deformation thermo-mechanical behaviors of amorphous SMPs are experimentally investigated. Based on the experimental observations and an understanding of the underlying physical mechanism of the shape memory behavior, a three-dimensional (3D) constitutive model is developed to describe the finite deformation thermo-mechanical response of SMPs. The model in this paper has been implemented into an ABAQUS user material subroutine (UMAT) for finite element analysis, and numerical simulations of the thermo-mechanical experiments verify the efficiency of the model. This model will serve as a modeling tool for the design of more complicated SMP-based structures and devices.

Size effect on strength and strain hardening of small-scale [111] nickel compression pillars

Abstract: This study investigates uniaxial compression behavior of focused ion beam (FIB) manufactured [1 1 1] nickel (Ni) small-scale pillars, ranging in diameter from approximately 25 μm to below 200 nm, in order to examine the effect of crystallographic orientation on the mechanical properties. This study is unique from other micro-pillar studies in that the [1 1 1] orientation has a considerably lower Schmid factor, and has multiple slip systems available. The [1 1 1] Ni pillars show a strong increase in yield stress and work hardening with decreasing diameter. The relationship between yield stress and diameter (σy ∝ d−0.69) matches well with previous small-scale pillar studies. Strain hardening, which has been inconsistently observed in other micro-pillar studies, is found to be a function of both diameter and orientation. Although the precise mechanism for hardening is unknown, transmission electron microscopy reveals dislocations throughout the pillar and into the base material suggesting that dislocation interactions and deformation below the pillar play a role in the observed strain hardening. Furthermore, a slight crystallographic rotation of the pillar is observed likely contributing to the observed mechanical properties. By exploring the role of crystallography on the plastic deformation behavior, this study provides additional insight into the nature of the size effect.

Fundamental differences in mechanical behavior between two types of crystals at the nanoscale.

Abstract: We present differences in the mechanical behavior of nanoscale gold and molybdenum single crystals. A significant strength increase is observed as the size is reduced to 100 nm. Both nanocrystals exhibit discrete strain bursts during plastic deformation. We postulate that they arise from significant differences in the dislocation behavior. Dislocation starvation is the predominant mechanism of plasticity in nanoscale fcc crystals, while junction formation and hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed as a function of size and compared with stochastic models.

Room temperature formability of a magnesium AZ31 alloy: Examining the role of texture on the deformation mechanisms

Abstract: The deformation behavior, texture and microstructure evolution of six sample types of a commercial magnesium alloy AZ31 with different processing histories were investigated during plane strain compression at room temperature using a channel-die device. Although all the samples were deformed under the same conditions, i.e. temperature and strain rate, the initial state of the samples prior to deformation was responsible for the final texture and microstructure. Stress–strain curves showed a maximum ductility of 28% for the sample with a hot rolling history. EBSD analysis was carried out to give a better insight into the operating deformation mechanisms. Besides the expected { 1 0 1 ¯ 2 } -tensile twinning, { 1 0 1 ¯ 1 } -compression twinning and { 1 0 1 ¯ 1 } − { 1 0 1 ¯ 2 } -double twinning were also observed in some specimens and were correlated to microcrack formation, which caused an early shear failure.

Estimation of lattice strain in nanocrystalline silver from X-ray diffraction line broadening

Abstract: The lattice strain contribution to the X-ray diffraction line broadening in nanocrystalline silver samples with an average crystallite size of about 50 nm is studied using Williamson-Hall analysis assuming uniform deformation, uniform deformation stress and uniform deformation energy density models. It is observed that the anisotropy of the crystallite should be taken into account, while separating the strain and particle size contributions to line broadening. Uniform deformation energy density model is found to model the lattice strain appropriately. The lattice strain estimated from the interplanar spacing data are compared with that estimated using uniform-energy density model. The lattice strain in nanocrystalline silver seems to have contributions from dislocations over and above the contribution from excess volume of grain boundaries associated with vacancies and vacancy clusters.

Electrostriction in elastic dielectrics undergoing large deformation

Abstract: We develop a thermodynamic model of electrostriction for elastic dielectrics capable of large deformation. The model reproduces the classical equations of state for dielectrics at small deformation, but shows that some electrostrictive effects negligible at small deformation may become pronounced at large deformation. The model is then specialized to account for recent experiments with an elastomer, where the electric displacement is linear in the electric field when the strain of the elastomer is held fixed, but the permittivity changes appreciably when the strain changes. Our model couples this quasilinear dielectric behavior with nonlinear elastic behavior. We explore the consequence of the model by deriving conditions under which the deformation-dependent permittivity suppresses electromechanical instability.

Flow behavior and microstructures of superalloy 718 during high temperature deformation

Abstract: Flow behavior and microstructures of superalloy 718 were investigated by hot compression tests performed at temperatures ranging from 950 to 1100 °C with strain rates of 10−3 to 1 s−1. The dependence of the peak stress on deformation temperature and strain rate can be expressed by a hyperbolic-sine type equation. The activation energy for superalloy 718 is determined to be 443.2 kJ mol−1. A power exponent relationship between the peak strain and the Z parameter is obtained. Microstructure analysis shows that the dynamically recrystallized grain size is inversely proportional to the Z parameter. The nucleation mechanisms of DRX are closely related to the value of Z parameter. Under low Z conditions, DRX nucleation and development are mainly assisted by the formation of twins near the original grain boundaries.

Crack initiation mechanism of extruded AZ31 magnesium alloy in the very high cycle fatigue regime

Abstract: Ultrasonic fatigue testing as well as conventional fatigue testing has been conducted on commercial extruded AZ31 magnesium alloy. The S-N curve for this alloy appears to have a continuous decreasing trend in the very high cycle regime. Fatigue strength at 109 cycles is 88.7 +/- 4.1 MPa. The ratio of endurance limit at one billion cycles to the tensile strength (sigma(-1)/sigma(b)) is 0.301. Fatigue failure mainly originated from the specimen surface or near surface, where deformation twins were found after cyclic loading. Fatigue cracks were observed along twin bands. Based on cyclic deformation irreversibility caused by twinning, fatigue damage mechanisms are proposed in terms of surface/near surface crack initiation. (C) 2008 Elsevier B.V. All rights reserved.

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles

Abstract: Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the sphere’s diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation. Nanocrystalline materials usually exhibit high strength and their deformation caused by stress is limited. Nanocrystalline CdS with spherical and hierarchical shell geometry is shown not only to withstand extreme stresses, but also to deform considerably before failure.

Atomistic Modeling of Materials Failure

Abstract: Basics of Atomistic, Continuum and Multiscale Methods.- Basic Atomistic Modeling.- Basic Continuum Mechanics.- Atomistic Elasticity: Linking Atoms and Continuum.- Multiscale Modeling and Simulation Methods.- Material Deformation and Failure.- Deformation and Dynamical Failure of Brittle Materials.- Deformation and Fracture of Ductile Materials.- Deformation and Fracture Mechanics of Geometrically Confined Materials.

The continuous strength method

Abstract: Many of the principal concepts that underpin current metallic structural design codes were developed on the basis of bilinear (elastic, perfectly-plastic) material behaviour; such material behaviour lends itself to the concept of section classification. The continuous strength method represents an alternative treatment to cross-section classification, which is based on a continuous relationship between slenderness and (inelastic) local buckling and a rational exploitation of strain hardening. The development and application of the continuous strength method to structural steel design is described herein. Materials that exhibit a high degree of nonlinearity and strain hardening, such as aluminium, stainless steel and some high-strength steels, fit less appropriately into the framework of cross-section classification, and generally benefit to a greater extent from the continuous strength method. The method provides better agreement with test results in comparison to existing design codes, and offers increas...