J. Gil Sevillano

Low energy dislocation structures in highly deformed materials

Abstract Gross low temperature plastic deformation of metals results from the movement of large numbers of dislocations. This movement is characterized by dislocation-dislocation interaction events statistically distributed in time and space and by the continuing trend of the dislocation density to rearrange into low energy configurations. The various approaches proposed to link flow stress and strain hardening with the evolving substructure may be grouped into families, emphasizing one or the other of those aspects. The first examines possible low energy dislocation configurations and derives the observed flow stress and strain hardening from the characteristics of the proposed dislocation architecture. The second phase relates flow stress evolution to the kinetics of dislocation movement, i.e. the build-up of the substructure as resulting from successive interaction events. The two approaches have been developed independently. It is the aim of the present paper to examine to what extent experimental observations fit into the framework of the existing models (the “kinetic” model being somewhat modified by an addition proposed in this paper) and to what extent the models are complementary in covering different aspects of the one same truth or are mutually exclusive.

Room temperature plastic deformation of pearlitic cementite

Abstract Cementite lamellae extracted from a eutectoid carbon steel wire drawn to ϵ = 0.43 show a substructure of intersecting straight bands, probably slip bands, which can be ascribed to (001), (010), (100), (011), (110) or (101) cementite traces. Only the (001) and (010) planes had been previously reported as cementite slip planes. There is extensive evidence in the literature concerning the ability of cementite to sustain plastic deformation but the important question of its potential ductility (capability to sustain any imposed strain) remains unanswered. The necessary condition for a crystal to behave ductilely, i.e. to possess five independent slip systems, would be unfulfilled if the (001) and (010) were the only available slip planes. The observation of four possible new slip planes supports the potential ductility of cementite.

Preface to the viewpoint set on: geometrically necessary dislocations and size dependent plasticity

Abstract Much of the recent effort in developing phenomenological constitutive descriptions of size dependent plasticity for crystalline solids has associated size dependence with geometrically necessary dislocations. However, there are a variety of perspectives on geometrically necessary dislocations and the characterization of their role in size dependent plastic flow phenomena is controversial. Therefore, a Viewpoint Set on this issue, including materials and mechanics points of view, and experimental and theoretical perspectives was deemed timely.

A quantitative assessment of forest-hardening in f.c.c. metals

Abstract Macroplastic deformation results from the long-distance movement of dislocations. In singlephase crystals it implies cutting the dislocation forest traversing the slip plane of the running dislocations and, as a consequence of the non-regular distribution of the “trees”, dislocation loops are left around the harder islands in their slip planes. The dislocation length so stored represents an increment of the obstacle density already present in other non-coplanar slip systems and thus contributes to their work-hardening. This work presents quantitative results on the contribution by forest cutting in a f.c.c. metal upon flow stress and work hardening rate. It has been obtained by computer simulation of dislocation glide through a mixture of punctual and linear obstacles whose strengths reproduce approximately the strength spectrum of a f.c.c. forest as derived by Shoeck and Frydman. Simulations have been conducted for random arrays of obstacles and for more realistic spatial dislocation distributions (cells, subgrains). Both the flow stress (and its temperature and strain rate dependence) and the athermal work-hardening rate so obtained are in good agreement with those measured for f.c.c. polycrystals in experiments covering up to large strains.

Hall-Petch Relationship of a TWIP Steel

The grain size dependence of the tensile properties of a TWIP steel has been determined for a wide range of grain sizes obtained by grain growth after complete recrystallization of cold rolled material. The near-linear stress-strain behaviour typical of either TWIP steels or other materials that deform by twinning has been observed, the work hardening rate being larger for the smaller grain sizes. The Hall-Petch slope increases as a function of strain, from 350 MPa μm1/2 for the yield stress to 630 MPa μm1/2 for the maximum uniform strain in the tensile tests, ε  0.40. Profuse twinning is observed in deformed specimens by means of FIB-ISE.

Two-dimensional sections of the yield locus of a Ti6%Al4%V alloy with a strong transverse-type crystallographic α-texture

Abstract Results obtained from mechanical tests on a Ti6%Al4%V sheet alloy were used to obtain the biaxial normal stress and the in-plane simple shear flow stress sections of its yield locus. The critical resolved shear stress (CRSS) of the different activated slip systems is derived from these experimental results of mechanical tests combined with microstructural observations. A strong asymmetry of the CRSS and some influence on it of the hydrostatic stress state was found for pyramidal 〈 c + a 〉 systems, i.e. a deviation from the Schmid law. Owing to the high sharpness of the texture in this alloy, a quasi-single-crystal approximation of the plastic anisotropy on the rolling plane (RP) and of the yield locus sections measured was made. The good agreement between the simulated and the experimental results encourages the use of the proposed CRSS values for a simulation of polycrystalline texture development and yield locus derivation.

The heterogeneous nature of slip in ice single crystals deformed under torsion

Ice single crystals were deformed under torsion and dislocation arrangements were analyzed by synchrotron topography at ESRF (European Synchrotron Radiation Facility). Profile analysis of the topographs revealed the scale-invariant character of the dislocation arrangement with long-range correlations. Dislocation density gradients are shown to be slightly anti-correlated as the intensity profile is similar to an anti-persistent random walk-like signal. This analysis reveals the influence of internal stresses on dislocation arrangement up to the sample scale. Similar observations in reversed torsion experiments, together with strong hardening behaviour, allow a mechanism of cross-slip of basal dislocations on prismatic planes to be suggested for interpretation of local dislocation interaction behaviour.

On the Yield and Flow Stress of Lamellar Pearlite

ABSTRACT The experimental (macroscopic) values for the yield and flow stress of fully pearlitic eutectoid carbon steel may be represented by a linear function of either, the inverse of the true interlamellar spacing or its square root. The latter representation determines, however, a negative zero-intercept ordinate in the case of the yield stress. Different theoretical models may be contemplated for the critical resolved shear stress (CRSS) of pearlitic ferrite. Most of them give way to inverse proportionality relationships between the CRSS and either, the apparent interlamellar spacing on the slip plane considered or its square root. A quantitative comparison between the results of non pile-up models and the experimental values of the yield stress is made in this paper, making use of the value of an orientation factor appropiate to pearlitic ferrite obtained previously (Gil Sevillano, Van Houtte and Aernoudt, 1978). Experimental values for the flow stress of pearlite after large strains are also interpreted in this way. The paper stresses the plausibility of non pile-up models against the more widely accepted pile-up models for the yield and flow stress of pearlite.

Kinetics of Recrystallization and Grain Growth of Cold Rolled TWIP Steel

Hot rolled, laboratory-cast, TWIP steel samples (5.4 mm thick) of 22% Mn - 0.6% C (in mass-%) were cold rolled to different reductions (from 40 % to 70 %) and subsequently isothermally annealed for various times at temperatures ranging from 450º C to 1100º C. The evolution of recrystallization and grain growth was followed by control of the softening kinetics complemented by metallographic, OIM and microtexture observations. A map of the recovery, recrystallization and grain growth in the temperature-time space was obtained. In all instances, the grain size at the end of recrystallization was very fine, D ≤ 2 µm and larger grain sizes were the result of grain growth. A range of grain sizes 2 µm ≤ D ≤ 50 µm was covered by the grain growth experiments. A phenomenological grain growth equation that is useful for the annealing control of this steel was derived from the measurements.

Heterogeneous deformation and internal stresses developed in BCC wires by axisymmetric elongation

BCC wires macroscopically deformed by axisymmetric elongation (wire drawing) develop an intense <011> fibre texture and exhibit a characteristic non-uniform deformation of the grains evident in transverse sections (grain curling or “Van Gogh sky structure”). The extraordinary grain morphology induced by the <011> fibre texture is also accompanied by a peculiar constant strain hardening rate in single-phase BCC wires (exponentially increasing in case of BCC containing composite wires) that allows to reach very high strengths. Here we present a calculation of the elastoplastic axial elongation of such an aggregate of BCC grains with the ideal <011> fibre texture, using a slip-gradient dependent large-strain crystal plasticity constitutive equation incorporated into a finite element method (FEM) code, i.e., with proper account of the influence of the evolving shape and size of individual grains and of the local grain interactions. The results reproduce well the observed macroscopic behaviour (linear flow stress-strain curve at large strains) and the peculiar mesoscopic structural changes (grain curling in transverse sections). The simulation is focused on the analysis of strain and dislocation density heterogeneities and on the building up of mesoscopic (inter- and intra-granular) internal stresses during deformation. The computed average transverse tensile stresses acting normal to the axially oriented {100} planes approximately parallel to the boundaries of the flattened grains is close to 0.3 times the tensile flow stress of the aggregate, in good agreement with previous calculations based on the Taylor-Bishop-Hill model or on elasticplastic self-consistent calculations and with available neutron diffraction measurements. Such a high level of internal tensile stresses explains the well-known tendency of high strength BCC wires to fail by longitudinal splitting.