Evolution Of Deformation Structures Research Articles

Plastic deformation behavior during unloading in compressive cyclic test of nanocrystalline copper

Deformed coarse-grained polycrystalline metals always unload elastically where permanent dislocation network well-developed in the loading regime hinders the movement of dislocations and allows only the elastic relaxation of stress. Such elastic unloading behavior is, however, unexpected in nanocrystalline metals because the dislocation network cannot effectively form inside nanometer-scale grains. In this work, we report the experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test at room temperature of nanocrystalline Cu. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading test and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation–twin boundaries.

In Situ Observation of the Dislocation Structure Evolution During a Strain Path Change in Copper

The evolution of deformation structures in individual grains embedded in polycrystalline copper specimens during strain path changes is observed in situ by high-resolution reciprocal space mapping with high-energy synchrotron radiation. A large number of individual subgrains is resolved; their behavior during the strain path change is revealed and complemented by the analysis of radial x-ray peak profiles for the entire grain. This allows distinction between two different regimes during the mechanically transient behavior following the strain path change: Below 0.3% strain, the number and orientation of the resolved subgrains change only slightly, while their elastic stresses are significantly altered. This indicates the existence of a microplastic regime during which only the subgrains deform plastically and no yielding of the dislocation walls occurs. After reloading above 0.3% strain, the elastic stresses of individual subgrains are about the same as in unidirectionally deformed reference specimens. They increase only slightly during further straining—accompanied by occasional emergence of new subgrains, abundant orientation changes, and disappearance of existing subgrains.

Martensite shear phase reversion-induced nanograined/ultrafine-grained Fe–16Cr–10Ni alloy: The effect of interstitial alloying elements and degree of austenite stability on phase reversion

We describe here an electron microscopy study of microstructural evolution associated with martensitic shear phase reversion-induced nanograined/ultrafine-grained (NG/UFG) structure in an experimental Fe–16Cr–10Ni alloy with very low interstitial content. The primary objective is to understand and obtain fundamental insights on the influence of degree of austenite stability (Fe–16Cr–10Ni, 301LN, and 301 have different austenite stability index) and interstitial elements (carbon and nitrogen) in terms of phase reversion process, microstructural evolution during reversion annealing, and temperature–time annealing sequence. A relative comparison of Fe–16Cr–10Ni alloy with 301LN and 301 austenitic stainless steels indicated that phase reversion in Fe–16Cr–10Ni occurred by shear mechanism, which is similar to that observed for 301, but is different from the diffusional mechanism in 301LN steel. While the phase reversion in the experimental Fe–16Cr–10Ni alloy and 301 austenitic stainless steel occurred by shear mechanism, there were fundamental differences between these two alloys. The reversed strain-free austenite grains in Fe–16Cr–10Ni alloy were characterized by nearly same crystallographic orientation, where as in 301 steel there was evidence of break-up of martensite laths during reversion annealing resulting in several regions of misoriented austenite grains in 301 steel. Furthermore, a higher phase reversion annealing temperature range (800–900 °C) was required to obtain a fully NG/UFG structure of grain size 200–600 nm. The difference in the phase reversion and the temperature–time sequence in the three stages is explained in terms of Gibbs free energy change that considers Ni equivalent and Cr equivalent instead of Ni/Cr ratio and secondary precipitation of carbides/nitrides. Also, described is the evolution of deformation structures in NG/UFG Fe–16Cr–10Ni alloy. A number of deformation mechanisms occur during tensile straining involving dislocation glide and twinning.

Evolution of deformation structures under varying loading conditions followed in situ by high angular resolution 3DXRD

With high angular resolution three-dimensional X-ray diffraction, individual subgrains are traced in the bulk of a polycrystalline specimen and their dynamics is followed in situ during varying loading conditions. The intensity distribution of single Bragg reflections from an individual grain is analyzed in reciprocal space. It consists of sharp high-intensity peaks arising from subgrains superimposed on a cloud of lower intensity arising from dislocation walls. Individual subgrains can be distinguished by their unique combination of orientation and elastic strain. The responses of polycrystalline copper to different loading conditions are presented: during uninterrupted tensile deformation, formation of subgrains can be observed concurrently with broadening of the Bragg reflection shortly after onset of plastic deformation. With continued tensile deformation, the subgrain structure develops intermittently. When the traction is terminated, stress relaxation occurs and number, size and orientation of subgrains are found to be constant. The subgrain structure freezes and only a minor clean-up of the dislocation structure is observed. When changing the tensile direction after pre-deformation in tension, a systematic correlation between the degree of strain path change and the changes in the dislocation structure quantified by the volume fraction of the subgrains is established. For obtaining the subgrain volume fraction, a new fitting method has been developed for partitioning the contributions of subgrains and dislocation walls.

Mechanisms of Formation of Submicron Grain Structures by Severe Deformation

Keynote Presentation in Int. Symposium on Fundamentals of Deformation and Annealing, Manchester, September, 2006.The grain refinement mechanisms operating during severe deformation processes, like ECAE, are discussed using data obtained from model Al-alloys. Refinement occurs predominantly by orientation splitting, micro and macroshear banding, and the geometric requirement for high angle boundary area to increase with strain. The deformation structure evolution is affected strongly by the processing route, die geometry, and material parameters. Both cell bands and microshear bands show strong alignment with the dies shear plane. Shear bands are promoted by changes in the shear plane orientation each extrusion cycle. At high strains a steady state is approached, where the grain size converges with the subgrain size, controlled by dynamic boundary migration.

Texture and Deformation Structure Evolution during Rolling of Individual Grains of Columnar Grain Nickel

Texture and Deformation Structure Evolution during Rolling of Individual Grains of Columnar Grain Nickel

Investigations of texture formation in geomaterials by neutron diffraction with high pressure chambers

Abstract This paper suggests a classification of texture in quartzitic rocks. Possible mechanisms of texture formation and their relation to tectonic processes are discussed. A scheme for the experimental varification of some models describing the evolution of deformation structures in geological materials in dependence on various erxternal conditions (high pressures, temperatures, various loadings, etc.) acting on a sample is proposed. To investigate “in situ” the mechanisms of texture formation in rocks, high pressure devices are under construction. They will be built of a special Ti − Zr alloy, which has zero coherent scattering lenght and therefore is well suited for neutron diffraction investigations. Two different devices are proposed. The first one allows neutron diffraction measurements of sample volumes up to 4 cm3, a hydrostatic pressure of 1, 5 GPa, a temperature of 300° C, and uniaxial compression up to 50 kN. Pressure temperature and axial load are measured inside the chamber. Besides during ...

Mechanism of formation and mechanical behavior of tilt grain boundaries

It is shown for NaCl crystals that the damping character of dislocation unpinning, motion, and multiplication during different tests (at stress rates σ ≃ 10 −4 to 35 MPa s −1 and in the temperature range T ≃ (10 −3 to 0.945) T m , T m melting point) inevitably leads to consecutive formation of subgrain walls, cells, and grain boundaries under growing σ. The scaling behavior of dislocation mean paths at multiplication, their spacing in slip lines and various high density dislocation structures versus the reciprocal of applied stress confirms the prevailing role of dislocation cross-slip, climb, and the Orowan bowing mechanisms in the evolution of deformation structures in different crystal classes

Deformation dynamics at low and ambient temperatures

The variation of tensile yield stress at a constant strain rate as a function of temperature for well-annealed pure metals show, with increasing temperatures, a rather sharp drop in yield stress (low temperature regime), followed by the intermediate temperature regime where yield stress decreases more slowly (and the ratio of yield stress to shear modulus remains more or less constant), which in turn is followed by the high temperature regime where the yield stress drops again rather sharply. The paper discusses the phenomenological framework for studying deformation dynamics in the low and intermediate temperature regimes. The approach adopted is the well-known state variable approach, where the evolutionary nature of deformation structure is described by one or more structure variables such that the current values of mechanical variables and structure variables together completely define the current state of deformation. A critical analysis of experimental results available suggest that at least for deformation at low strain rates, stress-rate is probably not a state variable of deformation. Thus deformation is most conveniently studied in terms of TASRA (thermally activated strain rate analysis) where the stress, plastic strain rate, temperature and structure are interrelated through a Gibb’s free energy for thermal activation by an Arrhenius equation. The stress-dependence of Gibb’s free energy and its maximum value then form the basis of identifying the rate-controlling obstacles. The need for careful experimentation and systematic analysis is illustrated by the example of low temperature deformation of hard hep metals. Modelling for the evolution of deformation structure is also touched upon.