Recrystallisation behaviour of a fully austenitic Nb-stabilised stainless steel

We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during 1 h isochronal annealing up to 1100°C using scanning electron microscopy. This steel grade is used as cladding material in Advanced Gas-cooled fission reactors, due to its resistance to thermal creep and oxidation. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure at 850°C. A number of twins are observed in the vicinity of deformation bands prior to the start of recrystallisation. New Nb(C, N) particles form gradually in the microstructure, and the particle dispersion presents a maximum volume fraction of 2.7% at 930°C. At higher temperatures, the smaller particles become unstable and gradually dissolve in the matrix. Consequently, the Zener pinning pressure exerted on the grain boundaries is progressively released, triggering the growth of the austenite grains up to an average size of ∼47 μm at 1100°C. The observed temperature window for recrystallisation and grain growth can be predicted by a unified model based primarily on the migration of high- and low-angle grain boundaries. Lay Description: Austenitic stainless steel containing high percentage of chromium and nickel is currently used as fuel cladding material in the British Advanced Gas-cooled Reactors (AGR). This material has been chosen because of its high resistance to thermal creep and corrosion, both enhanced by the presence of a fine dispersion of carbonitrides precipitated during the cladding thermomechanical processing. During the time spent in the reactor core, few fuel cladding elements can become susceptible to local chromium depletion at grain boundaries, which is ascribed to the time evolution of the microstructural damage caused by the neutron bombardment in the reactor core. This depletion might increase the susceptibility of this steel to intergranular corrosion attacks during medium-to-long term storage of spent fuel elements in water ponds. The severity of the local chromium depletion depends not only on the irradiation conditions, but also on the grain boundary geometry. We have investigated the recovery, recrystallisation and grain growth of AGR stainless steel during 1 h annealing at selected temperatures relevant for the thermomechanical processing of the steel claddings, focusing on the formation and evolution of grain boundaries and second phases. These two features play a key role in the progression of the neutron damage and the subsequent development of local chromium depletion during reactor service operations. A deep understanding of the mechanisms and conditions behind their formation during the thermomechanical processing of the cladding material and their interaction with each other constitutes the foundation to evaluate, and potentially mitigate, the effect of irradiation on the cladding material.</p

On the role of aluminium in segregation and banding in multiphase steel

Despite considerable attention in the literature on the origin and effects of chemical segregation in steels and the resultant effect on microstructure, the focus has been mainly on carbon and manganese, with minimum attention paid to other alloying additions, and none on the role of aluminium. Therefore, a systematic study of the effect of aluminium on segregation and banding in multiphase steel is presented here. It is shown that Al segregates preferentially to the solid during solidification, in contrast to the other elements which segregate preferentially to the liquid. In addition to the segregation and the resultant continuous band at the centre-line, it has been demonstrated that interdendritic segregation during casting leads to microstructural banding. Furthermore a new numerical model is presented for predicting the effect of carbon, aluminium, manganese, silicon and chromium on the austenite to ferrite transformation temperature, Ae3

Mechanical properties and fracture behaviour of ODS Steel Friction Stir Welds at variable temperatures

We have assessed the microstructure and the temperature-dependent mechanical behaviour of five bead-on-plate friction stir welds of Oxide Dispersion Strengthened (ODS) steel, produced using systematic changes to the tool rotation and traverse speed. Friction stir welding can potentially retain the fine dispersion of nanoparticles, and therefore also the high-temperature strength and radiation damage resistance of these materials. Tensile testing was carried out on the MA956 base material at a range of temperatures, from room temperature up to 750 °C. The mechanical properties of the welds were investigated via tensile testing at room temperature and at 500 °C, together with micro-hardness testing. The welds exhibited similar strength and ductility to the base material at both testing temperatures as welding caused a partial loss of particle strengthening, alongside an increase in grain boundary strengthening due to a greatly refined grain size in the stir zones. The micro-hardness data revealed a trend of increasing hardness with increasing tool traverse speed or decreasing rotation speed. This was attributed to the smaller grain size and lower nanoparticle number density in the welds created with these parameters. At 500 °C, the yield stress and ultimate tensile stress of the base material and the welds decreased, due to a progressive reduction in both the Orowan-type particle strengthening and the grain boundary strengthening.</p

In-situ TEM investigation of nano-scale helium bubble evolution in tantalum-doped tungsten at 800 °C

The aim of this work is to probe the helium-induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W-5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33 × 1016 He+/cm2. The presence of radiation-induced dislocation loops was not observed at this temperature. The low density of the incipient bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3 × 1014He+/cm2. The experiments conducted at 800°C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the bubble, thus hindering helium bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose ≥0.3 dpa, a progressive transition from a spherical to a faceted shape of the bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, &gt;80% of the bubbles in W were of the faceted type with the facet planes of {110}.</p

Impact of Friction Stir Welding on the microstructure of ODS steel

We have assessed the impact of the welding parameters on the nano-sized oxide dispersion and the grain size in the matrix of an ODS steel after friction stir welding. Our results, based on combined small angle neutron scattering and electron microscopy, reveal a decrease in the volume fraction of the particles smaller than 80 nm in the welds, mainly due to particle agglomeration. The increase in tool rotation speed or decrease in transverse speed leads to a higher reduction in nano-sized particle fraction, and additionally to the occurrence of particle melting. The dependence of the average grain size in the matrix on the particle volume fraction follows a Zener pinning-type relationship. This result points to the principal role that the particles have in pinning grain boundary movement, and consequently in controlling the grain size during welding.</p

A comparative three-dimensional neutron depolarization study on RCrO4 oxides (R=Y, Er, Tm, Yb)

Three-dimensional neutron depolarization experiments have been performed on RCrO4 (R=Y, Er, Tm, Yb) powder samples in order to gain insight into their magnetic domain structure in the submicrometer range. The temperature evolution of both the average domain size and the net magnetization of each compound has been studied for different applied magnetic fields. The largest average domain size at zero external magnetic field was found in YbCrO4. The effect of an applied magnetic field on the magnetic domain structure is relatively small in ErCrO4 and TmCrO4, when compared to YCrO4 and YbCrO4 where the average domain size even surpasses the average particle size determined by Scanning Electron Microscopy studies.</p

Synthesis and sintering of ZrC1-x powders with variable stoichiometry (0

ZrC is a potential candidate for high temperature nuclear applications, such as nuclear fuel cladding on TRISO fuels and as a coating on conventional clad in the third and fourth generation of fission nuclear power plants due to its refractoriness, chemical stability, high thermal conductivity, irradiation tolerance as well as low activation under neutron irradiation [1]. The variation in C content in ZrC1-x is known to produce a significant variation in physical properties, such as thermal and electrical conductivity over the range x=0.0-0.2, and a non-monotonic change in lattice parameter that peaks in the same range of stoichiometry (figure 1.) We have investigated the evolution of the distribution of C vacancies with the C content. Usually, ZrC is prepared from ZrO2 followed by a carbothermal reduction process at high temperatures under an inert atmosphere. Recently, a work in our group showed that ZrC1-x for stoichiometry x\u3c0.2 have a non-negligible amount of O, and could be therefore considered oxy-carbides rather than carbides [2]. This may indicate that the previously reported ZrC could also contain O, since that earlier work dates back to the 1970s. Please click Additional Files below to see the full abstract

Formation of lower bainite in a high carbon steel – an in-situ synchrotron XRD study

The microstructural evolution of and simultaneous dimensional changes in high-carbonSAE 52100 bearing steel were monitored continuously during austempering for 120 min atselected temperatures in the range of 210 C-270 C, and also during its subsequenttempering to 340 C for an additional 120 min, via high-energy X-ray diffraction in real timeand in-situ dilatometry. The austenite-to-bainitic ferrite transformation induces lattice defectsand internal lattice stresses that increase with austempering time and at lower austemperingtemperatures. These changes are evidenced by the increase in the full-width halfmaximumof the relevant reflections in X-ray diffraction. The lattice parameter of bainiticferrite takes its highest value during the early stages of austempering, and then graduallydecreases as the transformation progresses. This observation points to an initial state ofcarbon supersaturation in the ferritic lattice that is likely reducing due to carbon segregationclose to dislocations, fine carbide precipitation within the bainitic ferrite, and carbon partitioninginto the surrounding austenite. The carbon partitioning into austenite is evidencedin particular at the higher austempering temperatures of 240 C and 270 C, at which there isa noticeable increase in the lattice parameter of the remaining austenite at longer times. Thedimensions of the bearing steel specimens are governed by the volume change due to theformation of bainitic ferrite during austempering and by the relaxation of its lattice distortionduring tempering at 340 C in the absence of further phase transformation

Additive friction stir processing and hybrid metal additive manufacturing of high melting point materials: A review

Ever since the beginning of 4th industrial revolution, metal additive manufacturing has revolutionized the paradigm of printing high melting point materials. In this context, this paper reviews experimental and computational aspects of friction stir processing and hybrid techniques applied for metal additive manufacturing of high melting point materials like steel, and titanium alloys. Initially, friction stir processing working principle has been discussed. Secondly, friction stir processing is compared with other severe plastic deformation techniques and summarized their advantages, disadvantages and applications in a tabular form. Then based on the state-of-the-art of literature, additive friction stir processing and hybrid metal additive manufacturing processes are discussed for high melting point materials and results have been presented with respect to their microstructural developments, mechanical behavior, etc. Finally, gaps are highlighted for high melting point materials that shows importance of selecting process parameters, tooling capacity, computational analysis, mathematical modelling, etc., and presented these as future scope of work