No abstract

The mechanisms of formation and microstructure of pack aluminized diffusion coatings depend on a wide range of parameters. These impact coatings differently depending on the processing mode selected (out-of-pack or in-pack) and are still the subject of debate. The need to systematize data on the influence of processing mode on the mechanisms of the formation of aluminized coatings was the motivation for this study, which reviews the influence of processing mode based on an analysis of experimental data for aluminized coatings processed in-pack and out-of-pack on a Ni alloy with Al at unit activity at 800 °C [low temperature with high Al activity (LTHA)] and 1100 °C [high temperature with high Al activity (HTHA)]. The results show that processing mode determined the amount of Al deposited at the surface and that the formation of a larger halide-depleted zone during out-of-pack diffusion accounted for the reduced deposition of Al. Processing temperature and Al concentration at the surface of the alloy determined the mobility of the elements in the coating and, consequently, the coating thickness and dominant diffusion mechanisms. Aluminization with the specimen immersed in the pack resulted in a preferential inward diffusion of Al for both processing temperatures. In contrast, coatings processed by out-of-pack diffusion showed a dependence on processing temperature. When a HTHA process was used, the coatings were thinner and formed in a two-stage diffusion process, while a LTHA process failed to form aluminide diffusion coatings.

In this article the delamination phenomena, which occurs in the X70 steel during fracture process from the Charpy impact tests, was investigated. Microstructure, microtexture and Taylor factor map analyses were performed by scanning electron microscopy, light microscopy and electron backscatter diffraction, respectively. In a cross-section of two fractured surface regions namely, in the perpendicular and parallel regions to the propagation fracture direction where delamination occurred, it was possible to notice that delamination showed a key role in the anisotropy of impact toughness between the L-T and T-L orientations. The results also revealed that the cause of the delamination can be attributed to the presence of microstructural banding and elongated ferrite grains aligned in a rolling direction. The presence of (100)[011] and (111)[110] crystallographic orientations, with Taylor factors close to 2.7 and 4.5, respectively, were identified in the delamination region. They contribute to the occurrence of cleavage delamination during the fracture process.

Aluminized coatings on Ni based alloys greatly contribute to achieve process efficiency at higher operating temperatures. The present study characterized Pd-modified and unmodified aluminized coatings and compared with those of Pt-modified coatings regarding the mechanisms of formation and oxidation performance. The results show that Pd reduces the driving force for diffusion of Al during coating formation, increases outward diffusion of Ni and reduces diffusion of alloying elements (Cr and Ti) into the intermetallic layer. In contrast, Pt increases the driving force for diffusion of Al and the mobility of Al in the intermetallic layer of the aluminized coating. These characteristics have a direct impact on oxidation at 1000 °C that showed that Pd reduced the rate of θ-Al 2 O 3 → α-Al 2 O 3 transformation, accounting for higher density of voids at the interface β-(NiAl)/θ-Al 2 O 3 , diffusion of oxygen into the coating, spalling and faster degradation of coatings.