An EBSD study of the deformation of service-aged 316 austenitic steel

Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297K and 823K (24 °C and 550 °C)at strain rates 3.5x10-3 to 4 x 10-7 s-1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains 0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10-5 s-1 all metrics were insensitive to deformation temperature, mode (tension vs. compression) and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both aged and unaged material. Lattice misorientations build up with strain faster in aged steel than in un-aged material and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements and good agreement established between the two techniques of assessing plastic strain in aged 316 steel

Recrystallization and grain growth in single tungsten fiber-reinforced tungsten composites

High heat fluxes in future fusion reactors pose big challenges on the materials of plasma-facing components due to restoration processes occurring at high temperatures. Tungsten is considered most suitable as plasma-facing material. To overcome its inherent brittleness at low temperatures, tungsten fiber-reinforced tungsten composites are developed which contain ductile, potassium-doped, drawn tungsten wires in an undeformed tungsten matrix. Such composites show pseudo-ductile behavior, an improved toughness and a more controlled fracture compared to undeformed tungsten. Model systems containing a single fiber either without any interlayer or with an yttria interlayer between fiber and matrix are annealed and characterized by electron backscatter diffraction (EBSD) in order to investigate their thermal stability. The restoration process in wire and matrix differ from each other: Recrystallization followed by grain growth occurs in the deformation structure of the wire. Grain growth is the sole mechanism affecting the undeformed matrix. An yttria interlayer between fiber and matrix is supposed to separate the differently restoring microstructures from each other and thereby preserve the improved mechanical properties of the composite. The investigation focuses on characterizing the as-processed condition and the microstructural changes after annealing at 1450 °C for either four days or two weeks. After two weeks of annealing, grains in the region or the vicinity of the wire have coarsened so much that former fiber and matrix cannot be distinguished any longer; not even in a model composite with a 1 μm thick yttria interlayer