Evolution of grain boundary network topology in 316L austenitic stainless steel during powder hot isostatic pressing

The grain boundary network evolution of 316L austenitic steel powder during its densification by hot isostatic pressing (HIPing) was investigated. While the as-received powder contained a network of random high angle grain boundaries, the fully consolidated specimen had a large fraction of annealing twins, indicating that during densification, the microstructure evolves via recrystallization. By interrupting the HIPing process at different points in time, microstructural changes were tracked quantitatively at every stage using twin boundary fractions, distribution of different types of triple junctions, and the parameters associated with twin related domains (TRDs). Results revealed that, with increase in temperature, (i) the fraction of annealing twins increased steadily, but they mostly were not part of the grain boundary network in the fully consolidated specimen and (ii) the average number of grains within a TRD, the length of longest chain, and twinning polysynthetism increased during HIPing and (iii) the powder characteristics and the HIPing parameters have a strong influence on the development of grain boundary network. Based on the results obtained, possible alterations to the HIPing process are discussed, which could potentially allow twin induced grain boundary engineering

Decomposition of the ?? phase in as-cast and quenched U-Zr alloys

An investigation of the decomposition of the high temperature gamma phase in as-cast and quenched U-Zr alloys was conducted. Differential scanning calorimetry data clearly showed delta reversible arrow g transformations in alloys with <10 wt% Zr while XRD data did not contain any peaks which uniquely identify it's presence. Since delta phase forms via omega transformation, a comparison of the theoretical diffraction patterns for omega and delta revealed that the intensities of the peaks which uniquely identify the existence of delta when alpha-U is present, were either very weak, or were zero in omega, suggesting that the ambiguity can be explained if the phase present in these alloys is omega as opposed to delta. Our data are consistent with the presence of delta and omega in as-cast and quenched U-50Zr alloy, respectively, and (alpha+omega) in rest of the as-cast and quenched alloys. Based on the experimental data, the transformation sequence from gamma phase in U-Zr alloys is proposed.clos

Texture evolution during annealing of hot extruded U-10wt%Zr alloy by in situ neutron diffraction

Texture evolution during annealing of a U-10wt%Zr alloy hot extruded in the (alpha + delta) region was studied by in situ neutron diffraction. The extruded alloy had a lamellar (alpha + delta) microstructure and the initial texture consisted of (100)(alpha), (110)(alpha), and the (0001)(delta) poles oriented along the extrusion direction. The beta phase, after the alpha -> beta transformation, showed no preferred orientation while the gamma phase, after the beta -> gamma transformation, had (110)(gamma) and (111)(gamma) poles oriented along the extrusion direction. After a temperature cycle through the gamma phase, there was a complete loss of texture in the alpha phase while the delta phase retained the initial texture indicating a memory effect

Effect of powder characteristics and oxygen content on modifications to the microstructural topology during hot isostatic pressing of an austenitic steel

The effect of powder size distribution and oxygen content on the extent of multiple twinning and spatial distribution of oxide inclusions in hot isostatic pressed (HIPed) 316L steels was investigated using powders with different characteristics. Modifications to, and differences in their microstructural topology, were tracked quantitatively by evaluating the metrics related to twin related domains (TRDs) on specimens produced by interrupting the HIPing process at various points in time. Results revealed that powder size distribution has a strong effect on the extent of multiple twinning in the fully HIPed microstructure, with specimens produced using narrow distribution showing better statistics (i.e., homogeneously recrystallized) than the ones produced using broad size distribution. The oxide inclusion density in fully HIPed microstructures increased with the amount of oxygen content in the powders while prior particle boundaries (PPBs) were only observed in the specimens that were HIPed using broad powder distribution. More importantly, results clearly revealed that the spatial distribution of the inclusions was strongly affected by the homogeneity of recrystallization. Implications of the results are further discussed in a broader context, emphasizing the importance of utilizing the occurrence of solid state phase transformations during HIPing for controlling the microstructure evolution