Showing papers on "Hardening (metallurgy) published in 2002"

Ideal pure shear strength of aluminum and copper.

Abstract: Although aluminum has a smaller modulus in [111] shear than that of copper, we find by first-principles calculation that its ideal shear strength is larger because of a more extended deformation range before softening. This fundamental behavior, along with an abnormally high intrinsic stacking fault energy and a different orientation dependence on pressure hardening, are traced to the directional nature of its bonding. By a comparative analysis of ion relaxations and valence charge redistributions in aluminum and copper, we arrive at contrasting descriptions of bonding characteristics in these two metals that can explain their relative strength and deformation behavior.

From dislocation junctions to forest hardening.

Abstract: The mechanisms of dislocation intersection and strain hardening in fcc crystals are examined with emphasis on the process of junction formation and destruction. Large-scale 3D simulations of dislocation dynamics were performed yielding access for the first time to statistically averaged quantities. These simulations provide a parameter-free estimate of the dislocation microstructure strength and of its scaling law. It is shown that forest hardening is dominated by short-range elastic processes and is insensitive to the detail of the dislocation core structure.

A phase-field theory of dislocation dynamics, strain hardening and hysteresis in ductile single crystals

Abstract: A phase-field theory of dislocation dynamics, strain hardening and hysteresis in ductile single crystals is developed. The theory accounts for: an arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations resulting from a piecewise quadratic Peierls potential; the interaction between the dislocations and an applied resolved shear stress field; and the irreversible interactions with short-range obstacles and lattice friction, resulting in hardening, path dependency and hysteresis. A chief advantage of the present theory is that it is analytically tractable, in the sense that the complexity of the calculations may be reduced, with the aid of closed form analytical solutions, to the determination of the value of the phase field at point-obstacle sites. In particular, no numerical grid is required in calculations. The phase-field representation enables complex geometrical and topological transitions in the dislocation ensemble, including dislocation loop nucleation, bow-out, pinching, and the formation of Orowan loops. The theory also permits the consideration of obstacles of varying strengths and dislocation line-energy anisotropy. The theory predicts a range of behaviors which are in qualitative agreement with observation, including: hardening and dislocation multiplication in single slip under monotonic loading; the Bauschinger effect under reverse loading; the fading memory effect, whereby reverse yielding gradually eliminates the influence of previous loading; the evolution of the dislocation density under cycling loading, leading to characteristic ‘butterfly’ curves; and others.

Crystal plasticity model with enhanced hardening by geometrically necessary dislocation accumulation

Abstract: A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation. Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxed Taylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stress equilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this heterogeneity, a geometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides a physically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, a scale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall–Petch effect. Compared to a full-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from a computational point of view for simulating uniaxial tension experiments with various grain sizes.

Strain Hardening at Large Strains as Predicted by Dislocation Based Polycrystal Plasticity Model

Abstract: A recent strain hardening model for late deformation stages (Estrin, Y., Toth, L.S., Molinari, A., and Brechet, Y., Acta Materialia, 1998, A dislocation-based model for all hardening stages in large strain deformation, Vol. 46, pp. 5509-5522) was generalized for the 3D case and for arbitrary strain paths. The model is based on a cellular dislocation arrangement in which a single- phase material is considered as a composite of a hard skeleton of cell walls and soft cell interiors. An important point in the approach is the evolution of the volume fraction of the cell walls which decreases with the deformation and gives rise to a plateau-like behavior (Stage IV) followed by a drop-off (Stage V) of the strain hardening rate observed at large strains. The hardening model was implemented into the viscoplastic self-consistent polycrystal model to predict hardening curves corresponding to different proportional loading paths. The calculated curves were evaluated to elucidate the path dependence of hardening.

An age hardening model for Al–7Si–Mg casting alloys

Abstract: Yield strength (YS) ageing curves have been modelled for A356 and A357 aluminium casting alloys below the solvus temperature of the main hardening precipitate. Predictions are based on the Shercliff and Ashby methodology (Acta Metall. Mater. 38 (1990) 1789) for wrought alloys. Differences between strengthening in wrought and cast Al–Si–Mg alloys are considered. A Brinell hardness to YS conversion incorporating strain hardening has been established to enable YS ageing curves to be predicted with reduced experimental effort.

Identification of elastic-plastic material parameters from pyramidal indentation of thin films

Abstract: The indentation experiment is a popular method for the investigation of mechanical properties of thin films. By application of conventional methods, the hardness and the stiffness of the film material can be determined by limiting the indentation depth to well below the film thickness so that the substrate effects can be eliminated. In this work a new method is proposed, which allows for a determination of the reduced modulus as well as the nonlinear hardening behaviour of both the film and substrate materials. To this end, comparable deep indentations are made on the film/substrate composite to obtain sufficient information on the mechanical properties of both materials. The inverse problem is solved by training neural networks on the basis of finite–element simulations using only the easily measurable hardness and stiffness behaviour as input data. It is shown that the neural networks are very robust against noise in the load and depth. The identification of the material parameters of aluminium films on different substrates results in a significant increase in yield stress and initial work–hardening rate for a reduction of the film thickness from 1.5 to 0.5 µm, while the elastic modulus and the extent of work hardening remain constant.

Development of ultra fine grain structure by martensitic reversion in stainless steel

Abstract: Austenitic stainless steels have good corrosion resistance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce hardening by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chemical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength without impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystallization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystallization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite creates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α′) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α′ reverts to recrystallized austenite γR during annealing at low temperature.

A micromechanical model of hardening, rate sensitivity and thermal softening in BCC single crystals

Abstract: The present paper is concerned with the development of a micromechanical model of the hardening, rate-sensitivity and thermal softening of bcc crystals. In formulating the model, we specifically consider the following unit processes: double-kink formation and thermally activated motion of kinks; the close-range interactions between primary and forest dislocations, leading to the formation of jogs; the percolation motion of dislocations through a random array of forest dislocations introducing short-range obstacles of different strengths; dislocation multiplication due to breeding by double cross-slip; and dislocation pair annihilation. The model is found to capture salient features of the behavior of Ta crystals such as: the dependence of the initial yield point on temperature and strain rate; the presence of a marked stage I of easy glide, specially at low temperatures and high strain rates; the sharp onset of stage II hardening and its tendency to shift towards lower strains, and eventually disappear, as the temperature increases or the strain rate decreases; the parabolic stage II hardening at low strain rates or high temperatures; the stage II softening at high strain rates or low temperatures; the trend towards saturation at high strains; the temperature and strain-rate dependence of the saturation stress; and the orientation dependence of the hardening rate.

Creep strength of magnesium-based alloys

Abstract: The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys.