Volume 62, Issue 8

In the 3D microstructural analysis of a digital twin of porosity in the sintered steel Astaloy CrA , pore shape, average pore size as well as pore distribution will be analyzed. Porosity plays a major role in powder-metallurgical materials since it greatly impacts the mechanical properties of these materials and therefore represents a key parameter in their characterization. Based on the robot-assisted automated serial sectioning and imaging ( RASI ) system of the Federal Institute for Materials Research and Testing (BAM, Bundesanstalt für Materialforschung und Prüfung ) in Berlin, the technique of metallographic serial sectioning will be used to image the microstructure and reconstruct a digital 3D twin from the stack of images obtained. Compared with an individual 2D microsection, the quantitative microstructural analysis of this 3D twin will enable more accurate conclusions on the shape, size and distribution of pores. This paper will detail the key steps in 3D microstructural analysis, including the metallographic preparation routine, the imaging technique, image alignment as well as the segmentation of pores. After the methodology has been described, the results of the quantitative microstructural analysis will be presented and the validity of quantitative parameters of 3D and 2D images will be compared and discussed. The analysis of more than 10,000 pores revealed a correlation between pore shape and pore size. It was also found that a 2D representation of the material surface is insufficient for a precise quantitative characterization of porosity.

In this paper, the nucleation mechanism of recrystallized cubic orientation grains on the surface rolling direction-transverse direction (RD-TD) and cross-section rolling direction-normal direction (RD-ND) of Ni5W alloy substrates under a strain level of 5.1 was investigated using the quasi-in-situ electron backscatter diffraction technique. The results indicate that during the nucleation stage, a pronounced shear band is formed on the surface RD-TD of the Ni5W alloy substrate with a strain level of 5.1, where recrystallized grains preferentially nucleate near the shear band, exhibiting a more significant recrystallization nucleation phenomenon compared to the cross-section RD-ND. Moreover, it was found that the recrystallized cubic grains in the Ni5W alloy substrate with a strain level of 5.1 do not originate from the initial rolling cubic grains, but are formed through twinning of S-oriented grains, further clarifying the source of recrystallized cubic grains. It lays a theoretical foundation for the large-scale production of Ni5W alloy substrates and provides support for the practical application of second-generation high-temperature superconducting coated conductors, which is of strong scientific significance and social value.

Surface damage to axial shims was reported from the final assembly department of an original equipment manufacturer (OEM) of large gas turbine engines. The damage reportedly occurred upon casing assembly, namely during the “wedding” of the compressor exhaust diffuser with the outer casing. The subject parts were new and never saw any engine service. The subject shims were made of a low-alloy quenched and tempered steel. Their surface hardness was considered too low for this particular application. To rectify the problem, the shims received a physical vapor deposition (PVD) coating using CrN. It was determined that the metallurgical root cause of the surface damage was what is called the eggshell effect. The very hard PVD coating could not be supported by the relatively soft core of the low-alloy steel, causing cracking of the coating and subsequent galling of the substrate when the two heavy casings to be mated in final assembly touched each other hard.