E.C. Oliver

Time-resolved and orientation-dependent electric-field-induced strains in lead zirconate titanate ceramics

Electric-field-induced lattice strains in a tetragonal ferroelectric lead zirconate titanate bulk ceramic are characterized under application of subcoercive cyclic electric fields using neutron diffraction and a stroboscopic data collection technique. At a driving electric field equal to half of the coercive field, the field-induced lattice strains are found to be a function of orientation with the greatest electric-field-induced strain coefficient of 680pm∕V in crystal orientations such that the 211 pole is parallel to the electric field. A time dependence of the 111 strain was also observed. Suggestions as to the nature of these dependences are discussed.

Neutron diffraction study of extruded magnesium during cyclic and elevated temperature loading

Neutron diffraction has been used to study the progress of deformation twinning and intergranular strain evolution in extruded magnesium during cyclic and monotonic loading at two temperatures. Differences in the intergranular strains generated during tensile and compressive tests are attributed to the operation of twinning in compression. Twinning activity is reduced relative to slip at higher temperature, leading to greater similarities between tension and compression. During cyclic loading, a distinct Bauschinger effect is observed after each compressive loading stage. The origin of this effect is identified as the reversal of twinning during unloading and subsequent tensile loading.

Geometry Effects when Controlling Residual Stresses in Friction Stir Welds by Mechanical Tensioning

Finite element modelling has proved to be an effective tool for the investigation of trends effected by changing welding conditions. This is especially important in mechanical tensioning of friction stir welds because of the large number of parameters involved. In this paper, an FE model is used to examine the effectiveness of the mechanical tensioning technique for controlling residual stresses in FSWs by the investigation of trends caused by changes to the welding parameters. Comparisons between different geometries, traverse speeds, and welding off-axis angle all produced consistent results, and showed that the peak stresses are most strongly influenced by both the local tensioning and heat input, and not by the more global welding conditions. The results also showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~50% of the room temperature yield stress, tensile stresses were replaced by compressive residual stresses within the weld.

Evolution of Interphase Stress in Zr-2.5%Nb during Deformation

A series of neutron diffraction in-situ uniaxial compressive deformation tests have been carried out on textured Zr-2.5Nb. Test samples were prepared from the hot rolled plate with loading axes variously parallel to each of the plate rolling (RD), transverse (TD) and normal directions (ND). The evolution of the lattice parameters in both phases were measured in all three principal directions for each case of loading axis. The average phase strains determined by Rietveld refinement showed an anisotropic yielding dependent on sample orientation relative to the parent plate. The g-phase yielded first at stresses around 330-420MPa and transfered load to the く-phase. The く-phase yielded at stresses about 400-500MPa causing reverse load transfer to the g-phase. A Finite Element Model (FEM) was used to simulate the stress strain behaviors of the phases. Despite the simplicity of this model, a very good agreement was obtained between the experimental results and the FEM calculations.

FE Modelling of Mechanical Tensioning for Controlling Residual Stresses in Friction Stir Welds

Although Friction Stir Welding (FSW) avoids many of the problems encountered when fusion welding high strength Al-alloys, it can still result in substantial residual stresses that have a detrimental impact on service life. An FE model has been developed to investigate the effectives of the mechanical tensioning technique for controlling residual stresses in FSWs. The model purely considered the heat input and the mechanical effects of the tool were ignored. Variables, such as tensioning level, heat input, and plate geometry, have been studied. Good general agreement was found between modelling results and residual stress measurements, justifying the assumption that the stress development is dominated by the thermal field. The results showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~ 50% of the room temperature yield stress, tensile were replaced by compressive residual stresses within the weld.

Evolution of lattice strains in three dimensions during in situ compression of textured Zircaloy-2

The evolution of lattice strains in Zircaloy-2 was investigated in situ by time-of-flight neutron diffraction during uni-axial compression in three principal directions, normal, transverse and rolling. The material is a warm-worked Zircaloy-2 slab with basal plane normals mostly concentrated in ND. Lattice strains relative to the undeformed sample were measured by neutron diffraction during cyclic compression and unloading for three test directions and three scattering vector directions. Intensity variations of the reflections were monitored to assess the grain rotations. Substantial tensile twinning was inferred from the evolution of {0002} lattice strains and the intensity of prismatic and basal poles. The spread of lattice strains in the test direction (amongst different crystal orientations) is much greater in compression than in tension. The sources leading to this asymmetry were ascribed, through preliminary simulations using an elasto-plastic self-consistent model, to the compressive load, the occu...

Effect of Residual Stresses on Mechanical Properties of Duplex Stainless Steel Studied by Diffraction and Self-Consistent Modelling

The aim of this work is to study the influence of residual stresses on the properties of textured duplex stainless steel (DSS). The properties of both phases in DSS were studied using Xray diffraction whilst external load was applied “in situ” to the sample. The interpretation of experimental data is based on the diffraction elastic constants calculated by the self-consistent model taking into account the anisotropy of the studied material. Carrying out measurements in both compression and tension by using neutron diffraction, important differences in the evolution of lattice strains were noticed. An elastoplastic model is used to predict the evolution of the internal stresses during loading and to identify critical resolved shear stresses and strain hardening parameters of the material. The influence of the initial residual stresses on the yield stresses of the phases is considered. The difference between tensile and compressive behaviour of the steel is explained when the initial stresses (measured in the as received non-loaded sample by diffraction methods) are taken into account in model calculations. The yield stresses in each phase of the studied steel have been experimentally determined and successfully compared with the results of the elastoplastic self-consistent model.

Effects of texture and anisotropy on intergranular stress development in zirconium

The influence of texture and anisotropy on the generation of intergranular stresses in clock-rolled zirconium is investigated using neutron diffraction and elastoplastic self-consistent modelling. Comparison between experimental data and model calculations indicates that the operation mainly of prismatic and basal slip explains the trends in intergranular stress evolution during in-plane tensile and through-thickness compressive deformation, whilst twinning plays a significant role during in-plane compression.

In Situ Neutron Diffraction Studies of the Pseudoelastic-Like Behaviour of Hydrostatically Extruded Mg-Al-Zn Alloy

In-situ neutron diffraction has been used to study the pseudoelastic-like behaviour of hydrostatically extruded AZ31 magnesium alloy during stress-strain cycles in compression and tension along the extrusion direction. It has been confirmed that the activation of reversal twinning processes during unloading is responsible for the macroscopically observed hysteresis effect. Moreover, neutron diffraction data reveals the existence of high tensile stresses in grains which have just experienced significant twinning activity prior to the start of the unload cycle. It is thus proposed that this tensile stresses provides the necessary driving force for the activation of untwinning in already twinned grains.

An Analysis of Lattice Strain due to Disclination Dipole Walls in Fe–Pd Martensite

The martensite structure in Fe– Pd is shown to possess disclination dipole walls, which induce a locally fluctuating residual stress field. In a polycrystal, the residual stress averaged over each (former austenite) grain vanishes, since the average transformation strain is the same for all grains. Thus, the stress fluctuations within each grain contribute to diffraction line broadening. A simple method is developed to evaluate the average of the square of the residual strain, which is directly related to the line broadening. The method consists of evaluating the elastic energy produced by the domain walls. The structural parameters, such as disclination dipole wall spacing, are also discussed in terms of the elastic energy.