Showing papers on "Hardening (metallurgy) published in 2004"

Mechanical Behavior of Materials

Abstract: 1. Stress and strain 2. Elasticity 3. Mechanical tensile testing 4. Strain hardening of metals 5. Plasticity 6. Strain-rate and temperature dependence of flow stress 7. Slip 8. Dislocation geometry and energy 9. Dislocation mechanics 10. Mechanical twinning 11. Hardening mechanisms 12. Discontinuous and inhomogeneous deformation 13. Ductility and fracture 14. Fracture mechanics 15. Viscoelasticity 16. Creep and stress rupture 17. Fatigue 18. Residual stresses 19. Ceramics 20. Polymers 21. Composites 22. Mechanical working Appendix I. Miller indices Appendix II. Stereographic projection.

A physical model of the twinning-induced plasticity effect in a high manganese austenitic steel

Abstract: The steel Fe–22 wt.% Mn–0.6 wt.% C exhibits a low stacking fault energy (SFE) at room temperature. This rather low value promotes mechanical twinning along with strain which is in competition with dislocation gliding, the so called twinning-induced plasticity effect. The proposed modeling of the mechanical behavior introduces the formation of mechanical microtwins in a viscoplasticity framework based on dislocation glide at the mesoscopic scale in the case of a simple tensile test. The important parameter is the mean free path of the dislocations between twins, whose reduction explains the high hardening rate (by a dynamical Hall–Petch-like effect). It takes into account the typical organization of microtwins observed in electron microscopy (geometrical organization by using a twin-slip intersection matrix). To take into account the polycrystalline disorder, the macroscopic flow stress is calculated by assuming that the deformation work is equal in each grain for each strain step. This model gives an intermediate rule between Taylor and Sachs approximations and is simpler to compute than self-consistent methods. The parameters for gliding are first fitted on results at intermediate temperatures (without twinning), and the whole modeling is then correlated at room temperature. The simulated results (microstructure and mechanical properties) are in good agreement with experience.

Strain hardening, strain rate sensitivity, and ductility of nanostructured metals

Abstract: This paper presents an overview of the strain hardening and strain rate hardening behavior of nanostructured and ultrafine-grained metals. The experimental findings obtained in our laboratory are summarized, with some recent data for ultrafine-grained Cu presented as a model case. Due to the diminishing strain hardening capacity and inadequate strain rate hardening, plastic instabilities in the form of inhomogeneous and localized deformation such as necking and shear banding often contribute to the low ductility of nanostructured and ultrafine-grained metals at room temperature (RT). The observed grain size dependence of the strain rate sensitivity is also discussed in terms of its implications for new deformation mechanisms when the grain size is in the nanocrystalline (nc) and ultrafine regime.

Strain hardening and large tensile elongation in ultrahigh-strength nano-twinned copper

Abstract: A high density of growth twins in pure Cu imparts high yield strength while preserving the capacity for efficient dislocation storage, leading to high strain hardening rates at high flow stresses, especially at 77 K. Uniform tensile deformation is stabilized to large plastic strains, resulting in an ultrahigh tensile strength of similar to1 GPa together with an elongation to failure of similar to30%. (C) 2004 American Institute of Physics.

Aluminium Alloys - A Century of Age Hardening

Abstract: A century has elapsed since Alfred Wilm made the accidental discovery of age hardening in an aluminium alloy that became known as Duralumin. His work, and the gradual realization that hardening arose because of the presence of fine precipitates which provided barriers to the motion of dislocations, is a good example of the transition of metallurgy from an art to a science. A brief account is given of the development of age hardenable aluminium alloys and the way that modern experimental techniques allow precipitation processes to be understood on an atomic scale. Some contemporary issues in age hardening are then discussed.

Uniaxial and non-proportionally multiaxial ratcheting of ss304 stainless steel at room temperature: experiments and simulations

Abstract: The uniaxial and non-proportionally multiaxial ratcheting behaviors of SS304 stainless steel at room temperature were initially researched by experiment and then were theoretically described by a cyclic constitutive model in the framework of unified visco-plasticity. The effects of cyclic stress amplitude, mean stress, and their histories on the ratcheting were experimentally investigated under uniaxial and different multiaxial loading paths. The shapes of non-proportional loading paths were linear, circular, elliptical and rhombic, respectively. In the constitutive model, the rate-dependent behavior of the material was reflected by a viscous term; the cyclic flow and cyclic hardening behaviors of the material under asymmetrical stress-controlled cycling were reflected by the evolution rules of kinematic hardening back stress and isotropic deforming resistance, respectively. The effect of loading history on the ratcheting was also considered by introducing two fading memorization functions for maximum inelastic strain amplitude and isotropic deformation resistance, respectively, into the constitutive model. The effect of multiaxial loading path on the ratcheting was reflected by a non-proportional factor defined in this work. The predicting ability of the developed model was proved to be good by comparing the simulations with corresponding experiments.

Banded Microstructure in 2024-T351 and 2524-T351 Aluminum Friction Stir Welds: Part II. Mechanical Characterization

Abstract: A series of micro-mechanical experiments have been performed to quantify how the friction stir welding (FSW) process affects the material response within the periodic bands that have been shown to be a common feature of FSW joints. Micro-mechanical studies employed sectioning of small samples and micro-tensile testing using digital image correlation to quantify the local stress–strain variations in the banded region. Results indicate that the two types of bands in 2024-T351 and 2524-T351 aluminum FSW joints (a) have different hardening rates with the particle-rich bands having the higher strain hardening exponent, (b) exhibit a periodic variation in micro-hardness across the bands and (c) the individual bands in each material have the same initial yield stress.

Modeling Cyclic Deformation of HSLA Steels Using Crystal Plasticity

Abstract: High strength low alloy (HSLA) steels, used in a wide variety of applications as structural components are subjected to cyclic loading during their service lives. Understanding the cyclic deformation behavior of HSLA steels is of importance, since it affects the fatigue life of components. This paper combines experiments with finite element based simulations to develop a crystal plasticity model for prediction of the cyclic deformation behavior of HSLA-50 steels. The experiments involve orientation imaging microscopy (OIM) for microstructural characterization and mechanical testing under uniaxial and stress-strain controlled cyclic loading. The computational models incorporate crystallographic orientation distributions from the OIM data. The crystal plasticity model for bcc materials uses a thermally activated energy theory for plastic flow, sell and latent hardening, kinematic hardening, as well as yield point phenomena. Material parameters are calibrated from experiments using a genetic algorithm based minimization process. The computational model is validated with experiments on stress and strain controlled cyclic loading. The effect of grain orientation distributions and overall loading conditions on the evolution of microstructural stresses and strains are investigated.

On the kinetics of plasma nitriding a martensitic stainless steel type AISI 420

Abstract: Plasma nitriding was employed to treat martensitic stainless steel type AISI 420. The ability to remove the passive film from the surface is an important advantage in this process in order to guarantee a homogeneous surface treatment. The resulting nitrided surface shows the presence of a compound layer and a diffusion zone. The interface between the diffusion zone and the substrate is flat, as a consequence of the high chromium content on the alloy. The precipitation of chromium nitrides is also responsible for the high levels of hardening obtained. A value up to 1350 HV 0.025 constituting a maximum hardness was obtained. Diffusion depth increases with an increase of the nitriding temperature as measured by optical microscopy and verified by the hardness profiles. Calculated diffusion coefficients of nitrogen were very low compared to the data for alpha iron, and allow calculation of the Arrhenius behaviour during the process.