J. Quinta da Fonseca

Twinning in structural material with a hexagonal close-packed crystal structure:

The paper aims to provide a brief and comprehensible overview of the current understanding of twin nucleation and growth in structural metallic materials with a hexagonal close-packed crystal structure. It describes possible experimental methods to improve this understanding, which is required to implement twin nucleation and growth in crystal plasticity models using mechanistically meaningful criteria. These aspects are further discussed by presenting results from deformation experiments carried out on a zirconium alloy (ZIRLO™) and Ti–6 wt% Al–4 wt% V (Ti–6Al–4V). It is shown that very significant texture changes after even small levels of plastic deformation can be observed in both materials, strongly suggesting the presence of extensive

Intergranular Stress Evolution in Titanium Studied by Neutron Diffraction and Self-consistent Modelling

Neutron diffraction measurements of intergranular strain development in IMI 125 commercially pure titanium were recorded during uniaxial loading at temperatures between room temperature and 380°C. An elastoplastic self-consistent (EPSC) model was used to elucidate information about the operative deformation modes. Activation of [Formula: See Text] twinning was observed to limit the build up of intergranular stresses at all temperatures.

The kinematics of deformation and the development of substructure in the particle deformation zone

The details of the deformation near undeformable particles influence the recrystallization behaviour of commercial aluminium alloys. It has been shown that the size of the particle deformation zone (PDZ) and the local lattice rotation decrease with particle size and that a uniform distribution of very small closely spaced particles makes the deformation more homogeneous. Although a number of models have been proposed to describe the development of the PDZ, an explanation for this particle size effect remains elusive. In this paper, high resolution digital image correlation was used to map the deformation around particles of different sizes in a model Al-Si alloy. Deformation maps were compared to maps of local lattice rotation obtained using electron back scattered diffraction (EBSD) to elucidate the link between the kinematics and the resultant substructure. These results were also compared to crystal plasticity finite element predictions. The deformation maps revealed that deformation is concentrated in intense slip bands that have a characteristic spacing and which are strongly correlated to the deformation substructure. Whereas particles larger than this characteristic spacing interact strongly with the slip bands, causing large local rotations, smaller particles either interact more weakly or not at all. Since the strain between the bands is only a fraction of that in the bands, the rotations associated with the smaller particles are invariably smaller. Very small, closely spaced particles change the slip band pattern, decreasing the band spacing and decreasing the amount of shear within the bands. CFEM modelling was able to reproduce general features of the particle deformation zone but because it does not predict the development of slip bands and their characteristic spacing it fails to predict an effect of size on the local rotation.

Towards Modelling Intergranular Stress-Corrosion Cracks Using Experimentally Obtained Grain Topologies

Predicting the effects of material aging in view of development of intergranular damage is of particular importance in a number of nuclear installations and especially in structural integrity assessments of critical components in energy generating power plants. Since the damage is initialized on small length scales, detailed multiscale models should be employed to tackle the problem. However, the complexity of such models is high due to the need of incorporating microstructural features. In line of this the research group from Jozef Stefan Institute and The University of Manchester joined forces and knowledge in development of such detailed multiscale models. The basic idea was to pair the knowledge of advanced experimental techniques of The University of Manchester group with the knowledge of advanced microstructure modelling techniques of the group at Jozef Stefan Institute. The presented paper proposes a novel approach for intergranular crack modelling whereby a state-of-the-art X-ray diffraction contrast tomography technique is used to obtain 3D topologies and crystallographic orientations of individual grains in a stainless steel wire and intergranular stress corrosion cracks. As measured topologies and orientations of individual grains are then reconstructed within a finite element model and coupled with advanced constitutive material behaviour: anisotropic elasticity and crystal plasticity. Due to the extreme complexity of grain topologies, transferring this information into the finite element model presents a challenging task. The feasibility of the proposed approach is presented. Difficulties in building a finite element model are discussed. Preliminary results of the analyses are also given.Copyright

Peak broadening anisotropy in deformed face‐centred cubic and hexagonal close‐packed alloys

The broadening of diffraction peaks representing different families of grain orientations has been measured for a number of deformed metals: austenitic stainless steel 316, nickel 200 and the titanium alloy Ti-6Al-4V. These measurements have been compared with predictions that explain differences in broadening in terms of the contrast factor of dislocations via two different approaches. This was done in order to understand the effect the contrast factor has on the results of diffraction peak profile analysis methods and the cause of broadening anisotropy. An approach that considers all grains and orientations to behave similarly was found to be unsuccessful in explaining the large variations of broadening in different peaks. These variations can be explained, and errors reduced, by adopting an approach that uses a polycrystal plasticity model. However, if the plasticity based approach is used to solely calculate changes in the contrast factor, it only partly explains changes in broadening. Instead, factors such as variations in the dislocation density and crystallite size in different orientations, the number of dislocations that are mobile, and the number of edge and screw dislocations need consideration. The way to incorporate these additional factors is difficult, but their contribution to broadening anisotropy can be as important as that of the contrast factor.

Measuring and predicting the effects of residual stresses on crack propagation

This article presents the first part of a study on the interaction between residual stresses and crack driving force. Blunt notched CT specimens were pre-strained to introduce residual stresses at the notch, where a crack is subsequently introduced. FE modelling is used to model the specimen preload and pre-cracking. Modelling predictions are validated by two different methods. The total predicted surface residual strains are compared to image correlation measurements. The predicted residual strains were measured using neutron diffraction, both before and after fatigue cracking. The residual strain profiles show good agreement with the 3D FE model in the far field but the peak strains measured near the notch are smaller those predicted. This is a result of the low spatial resolution of the technique.

Determination and interpretation of texture evolution during deformation of a zirconium alloy

Worldwide, crystal plasticity models are currently developed to predict texture development during processing of material. Such models require a precise knowledge of the active deformation mechanisms. The activation energy for certain deformation modes will change with temperature and also depend on the chemistry of the alloy as well as the microstructure. Deformation mechanisms were studied in ZIRLO™ during room and high-temperature uniaxial compression testing. Materials with a strong crystallographic basal texture and a more random texture due to β-quenching were investigated with the aim of establishing the effect of temperature, microstructure, and texture on the active deformation modes during the initial stages of deformation. First, specimens were strained at room temperature, 180°C and 300°C to 2 % and 5 % or 10 % total strain and subsequently analyzed by Electron Back Scatter Diffraction (EBSD) to determine the texture evolution. It was found that a dramatic texture change was observed for all testing temperatures in the strongly textured specimen after only 5 % total strain, which can only be understood in terms of tensile twinning of {10 1 ¯ 2 } ⟨ 1 ¯ 0 1 1 ⟩ being active mainly at room temperature and compressive twinning of {11 2 ¯ 2 } ⟨ 1 ¯ 1 ¯ 2 3 ⟩ being operational at room and elevated temperature. The β-quenched specimens did not show any evidence of texture change when strained to 10 %. In-situ intergranular strains were measured by time-of-flight neutron diffraction during continuous compressive loading. This information enabled the development of a crystal plasticity finite element model (CPFEM), which was subsequently used to predict the stress state in individual grains. It was found that in the strongly textured material the spread of intergranular strain in the {0002} grain family (normal pointing towards the ND direction) results in some grains being in compression even though the mean stresses are tensile, which could explain the activation of the observed compressive twinning. The crystal plasticity model also demonstrated that the observed texture changes in the strongly textured material, including those at high temperature, cannot be explained by slip alone even when ⟨c+a⟩ slip is considered. In addition, the model showed that the dramatic difference in yield strength of the two conditions studied here cannot be solely attributed to the difference in texture but that grain size plays an important role.

Influence of Sn on Deformation Mechanisms During Room Temperature Compression of Binary Zr–Sn Alloys

Role of Sn on the deformation mechanisms of Zr was investigated using in situ neutron diffraction and complementary electron microscopy techniques. Binary Zr-Sn alloys having fully recrystallized microstructure and typical rolling texture were subjected to in situ loading and diffraction experiments along the rolling direction of the sample. Significant twinning activity was observed and the twins were observed to be {101 ̅2}〈101 ̅1〉 type tensile twins. Critical stress for the twin nucleation and the extent of twinning were found to be strongly influenced by the Sn content. Critical plastic strain for the nucleation of twining, however, was observed to be weakly dependent on the Sn content. Results indicate significant plastic slip activity to be a necessary condition for the onset of twinning. http://mc04.manuscriptcentral.com/astm-stp STP: Selected Technical Papers

Intergranular Strains in Pre-Strained and Welded Pipes

Neutron diffraction has been used to investigate the weld residual stresses and the intergranular residual strains in butt-welded 316H pipes. Measurements have been made on pipes subjected to varying degrees of plastic pre-straining before welding, in order to assess the effects of plastic strain on the weld residual stresses and the intergranular strains in the material. The intergranular strains following plastic deformation will also be affected by the annealing effect of the welding. Pipes were initially prepared with plastic strain of 0, 10, 15, 20 and 25% plastic deformation. Thereafter, the pipes were cut in half and welded with a circumferential butt-weld. Bar specimens were extracted from the remote end of the 0, 10, 15, 20 and 25% pre-strained and welded pipes. Cross-weld bar specimens were also machined from the 0 and 20% pre-strained and welded pipes. Neutron diffraction measurements were made at ENGIN-X, ISIS and FRM-II, Munich. The aim of this paper is to evaluate the intergranular strains developed after pre-straining from measurements made in remote bar specimens from the remote-end of the pipes. The annealing effect of the welding cycle on the intergranular strains is also studied, with measurements done at several points on cross-weld bar specimens, obtaining the strain response of different hkl lattice planes. The results show that the {200} and {220} planes are at the extremes of response during loading. Furthermore, the welding thermal cycling relaxed the intergranular strains from the prior plastic deformation.

Assessment of Defects under Combined Primary and Residual Stresses

Residual stresses can provide a significant element of the crack driving force for defects in welded components. Structural integrity assessment methods are available, such as the R6 defect assessment procedure [1], which provide detailed guidance for the assessment of such defects under the combined influence of primary and residual stresses. However, in some circumstances these methods may be unduly conservative due, in part, to an over-estimation of the crack driving force due to the residual stress, KJ s. This over-estimation can lead to a pessimistic view of actual safety margins for welded components and premature replacement or repair strategies.