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A common feature of uniaxial high-temperature tension tests, and to a certain extent compression tests, performed using a Gleeble thermomechanical testing system that employs direct resistance heating for the characterisation of the rheological behaviour of materials is longitudinal and radial thermal gradients. The aim of this article is to experimentally quantify the axial thermal gradients for a given tensile specimen geometry of free cutting steel during heating and deformation, and design a modelling methodology to simulate their influence on the strain distribution as compared to the assumption of isothermal heating and deformation. For this purpose, a feedback algorithm was developed to control the electric current input in a similar manner to that applied by the Gleeble testing system, which implemented via the UAMP user subroutine in ABAQUS for use in electro-thermal simulations of the direct resistance Joule heating used by the Gleeble testing system. The predicted temperature fields were compared with the temperature distributions recorded experimentally along the gauge section of the tensile specimens. Finite element simulations of Gleeble tensile tests were carried out under isothermal conditions and using the temperature distributions calculated by the feedback algorithm for a range of strain rates and temperatures in order to evaluate the difference in predicted stress state. The results show that an isothermal assumption should only be used conservatively in finite element simulation of the Gleeble thermomechanical test employing direct resistance heating to avoid significant errors.

The stability, diffusivity and clustering behaviour of defects in uranium diboride (UB 2) was investigated in light of the potential application as a burnable absorber in nuclear fuel. UB 2 was found to accommodate limited deviations from stoichiometry, which should be a consideration when manufacturing and operating the material. Self-diffusivity of both U and B was found to be sluggish (10 −14 cm 2 /s for B and 10 −19 cm 2 /s for U at 2000 K) and highly anisotropic, with migration along the basal planes being orders of magnitude faster than c-axis migration. The anisotropy of defect migration (both interstitials and vacancies) is predicted to hinder recombination of defects produced by collision cascades, thus limiting the radiation tolerance of the material. Boron and uranium vacancies exhibit a drive to cluster. Boron vacancies in particular, which are mobile on basal planes, are predicted to cluster into strongly bound di-vacancy, which in turn are less mobile. These are then predicted to grow into larger two-dimensional vacancy clusters on the B plane, leading to anisotropic swelling. We provide an analytical expression to predict the stability of these clusters based on purely geometrical considerations. Finally, the accommodation of Li, He and Xe onto vacancy clusters was considered. Li appears to stabilise the structure upon U depletion, while the retention of He and Xe appears to rise with increasing B depletion, through the formation of vacancy clusters.

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Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria-stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450-670 K in 2-0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33-300 m/s, laser powers of 10-30 W, and an applied electric field of 3000 V/cm.

Influence of boron isotope ratio on the thermal conductivity of uranium diboride (UB2) and zirconium diboride (ZrB2)