This paper reports the use of multi-spot melt strategy coupled with smaller layer thickness to additively manufacture Y2O3/γ-TiAl nanocomposite. In contrast to the hatch melt, the multi-spot melt strategy results in a lower fraction of γ and B2-phase, a smaller lamellar spacing of 208±72 nm with straight interface between α2/γ, and a more uniform distribution of nanoparticles with a finer size of 90±38 nm. Twins can form in the equiaxed γ grains and within γ lamellae; this applies to both the multi-spot and hatch melt samples. Twins within the γ lamellae can propagate across the twin interface but terminate at the γ/α2 interface. A good combination of 556±11 MPa (tensile strength) and 17.0±3.1% (ductility) at 800 °C with 16.5±0.3 MPa√m (room-temperature fracture toughness) is achieved in the as-built condition. Quantitative microscopy confirms a homogeneous microstructure within the X-Y plane for the multi-spot sample, whilst the use of smaller layer thickness helps to reduce the microstructure degradation due to thermal cycling. In terms of the Y2O3 nanoparticles, both the rod-like Y2O3 with monoclinic and the near spherical Y2O3 with the cubic crystal system are identified using transmission electron microscopy (TEM). High-resolution TEM reveals that the Y2O3/TiAl interface is clean and free of interfacial reactions, exhibiting either a semi-coherent or coherent type interface, suggesting a strong interfacial bonding.

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO2 environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.

When polycrystalline metals and their alloys are used at high temperature, creep deformation leads to changes in their internal state. The change in internal state manifests itself in many ways, but the two ways that concern us in this review are (i) the creation of internal stress arising from the strain incompatibility between grains and/or the formation of cell/sub-grain structures and (ii) a change in the material resistance. This review aims to provide a clear separation of these two concepts by exploring the origin of each term and how it is associated with the creep deformation mechanism. Experimental techniques used to measure the internal stress and internal resistance over different length-scales are critically reviewed. It is demonstrated that the interpretation of the measured values requires knowledge of the dominant creep deformation mechanism. Finally, the concluding comments provide a summary of the key messages delivered in this review and highlight the challenges that remain to be addressed.

The role of misfit stress in kinematic hardening under reversed straining of a Type 316H austenitic stainless steel has been investigated by using neutron diffraction combined with in situ deformation. Initial misfit stresses, often referred to an intergranular internal stresses, were created by the tensile pre-straining at high temperature. The misfit stresses at the length-scale of grain families, measured by neutron diffraction, were shown to be a function of the magnitude of the tensile pre-strain. The pre-strained specimens were further subjected to either continued (tensile) straining or reversed (compressive) straining at room temperature. In situ neutron diffraction measurements were undertaken to monitor the change of the misfit stresses during loading. The macroscopic stress–strain behaviour was used to derive isotropic and kinematic hardening stresses developed in the pre-strained specimens. Results show that the change of the transient softening stress towards a zero value is accompanied by a decrease in the change of the misfit stresses. A multi-scale self-consistent model has been developed to assist in understanding the measured change of the misfit stresses when subjecting the material to strain reversal. An important conclusion is that the origin of the kinematic hardening of Type 316H austenitic stainless steel arises from the misfit stress between grains.

In situ neutron diffraction combined with the incremental deformation at room temperature has been used to provide a measure of the internal stress and internal resistance generated by prior inelastic deformation at high temperature in an austenitic stainless steel. Interactions between the internal stress and internal resistance are considered explicitly by using the proposed measurement technique. The magnitude of the intergranular internal stress is found to be a function of the total inelastic strain created by prior high temperature deformation. The deviation from linearity observed in the lattice strain response is used to derive the microscopic internal resistance, but a crystal plasticity model is required to infer the absolute value. The macroscopic internal resistance is shown to be consistent with Taylor hardening. A refined internal state concept is proposed based on the Kocks–Mecking model to provide a further step to predict the inelastic deformation.

A preliminary sensitivity examination of the ductility exhaustion based creep damage prediction model, currently used in the R5 high temperature assessment procedure, showed that material property inputs had significant effects on damage prediction. In the present work, the link between the microstructural factors and the susceptibility to inter-granular high temperature creep failure is considered. The latter was judged to be associated with the low creep ductility. Here, the longitudinal section of a creep specimen and the fracture surface were examined. Auger electron spectroscopy was used to investigate the grain boundary composition in this specimen, which failed after a creep test of 1038h at 550°C under a triaxial stress state. The present results demonstrate that there is a possibility to correlate the susceptibility to high temperature inter-granular fracture from the low temperature fracture investigations. Finally, the susceptibility of the pre-treated 316H stainless steel to inter-granular high temperature failure and the contribution to the creep damage model are briefly discussed.

This paper reviews factors that contribute to stress relief and reheat cracking in weldments. In particular we consider the influence of thermo-mechanical history on the relaxation of triaxial residual stresses, typical of those observed in the 316H austenitic stainless steel thick section attachment welds was considered. 316H austenitic stainless steel parent material has been subject variously to thermo-mechanical pre-treatments and a critical microstructure encountered within the heat affected zone of the thick section weldments has been simulated. Through thickness residual stress fields induced by a spray water quenching technique and their relaxation as a result of high temperature exposure have been characterized using neutron diffraction. The rates of relaxation are correlated with microstructure. The susceptibility of grain boundary to brittle fracture at a temperature of -196 degrees C is shown to be a function of inter-granular M23C6 carbides and phosphorous segregation at grain boundaries. The influences of the thermo-mechanical history on creep deformation resistance and the potential susceptibility to grain boundary creep cavitation are discussed.

In this study, the residual stress generation and relaxation in 316H stainless steel subjected to systematically selected thermo-mechanical pre-treatments have been investigated by neutron diffraction. These pre-treatments were designed to reflect the thermo-mechanical history that parent material could be subjected to deformation during a welding cycle. The neutron diffraction measurements were carried out by using neutron spallation sources at the Rutherford Appleton Laboratory and at the Paul Scherrer Institute. The underlying principle of neutron diffraction macro- and micro-stress measurement was briefly reviewed. The derived lattice strain, residual stress and the rate of relaxation following thermal exposure at a temperature of 550 degrees C are shown to be a function of microstructure. The results are discussed in terms of the role of microstructure on the magnitude of quenching introduced residual stress fields and creep deformation behaviour encountered during stress relaxation at high temperature.