Electron yttria strengthened Ni-based superalloy sheet in

Electron beam physical
vapor deposition (EB-PVD) technology was used for preparing a Y2O3
dispersion strengthened nickel-based superalloy sheet of 0.15 mm thickness.
X-ray fluorescence spectrometer(XRF), X-ray diffraction(XRD) and scanning
electron microscopy(SEM) were used for performing phase composition,
microstructure investigations and for finding element contents on as-deposited
and heat-treated specimens. The goal was to improve mechanical properties of
the alloy sheet using different heat treatments. In this paper, author has
specified three types of heat treatments that were applied to yttria
strengthened Ni-based superalloy sheet in order to study microstructure
evolution before and after the heat treatments and to evaluate those effects of
heat treatments on mechanical properties of super alloy sheet.

 

Ni-based super alloy
was prepared using large scale EB-PVD of type L5. The diameter of the stainless-steel
substrate was 1000 mm and the distance between the substrate and the
evaporation target was 500 mm. An Y2O3 ingot of diameter
60 mm and two ingots of (Ni-20Cr-1.4Al) this alloy with a diameter of 100mm
were enclosed in a water -cooled crucibles as the vapor sources. Deposition was
performed in high vacuum of about 0.01 Pa using a turbomolecular pump. The
author has specified three different heat treatments to as-deposited samples
which included aging at 800 ? C for 3 hours, for sample T2;
heat treatment at 1100 ? C for 0.5 hours followed by the same at 800
? C for 16 hours, for sample T3; heat treatment at 1100 ? C
for 3 hours, for sample T4. These samples were encapsulated in quartz tubes,
evacuated at 0.01 Pa, before the heat treatment.

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In the results it was
seen that the contents of Y, Cr, O, Al in layer are higher at the substrate
side as compared to the other side, indicating non-homogeneous evaporation at the
beginning of deposition providing excellent protection against corrosive and
oxidizing atmospheres because of the high contents of layers of Cr and Al. It
was observed that high contents of Y2O3 prevents grain
growth by pinning the boundaries and restrains more effectively on the
substrate side. It was also observed that the inter crystalline voids and
micropores formed affects mechanical properties leading to stress
concentration. It was seen that thermal effects occur at 800? C and
1100 ? C. In XRD patterns of samples T1, T2, T3 and T4, in the
graphical representation, there are several parameters that are not seen on the
graph, example ?-Ni and Cr is present in sample T1, but it is not represented
on the graph using the specified symbols. Similarly, the precipitates of sample
T2 are Cr23C6, since there is no such symbolic
representation of Cr23C6 on the XRD patterns, so one
cannot figure it out what exactly is the intensity at a certain angle.

 

In the end, it was
observed from the stress train curve that T2 sample gives ultimate tensile
strength of 1220 MPa. The author mentions that characteristics for columnar
crystals disappears for samples T3 and T4, stating grain growth as one of the
reason of decrease in the strength for these samples, but has not justified it
properly. The conclusion of this article infers that ductility is poor due to
micropores between columnar grains. But using heat treatment, the strength and
ductility can be improved, which the author has observed and justified at 800
? C for 3 hours. At the same time, he has also mentioned in the last
statement when sample T2 is treated at 1100 ? C, the strength is reduced
but ductility is enhanced due to precipitates of Y3Al5O12(YAG).
Overall the author has given relevant information and the experimental data,
except the XRD pattern for sample T2. Despite of some typographical mistakes,
in general the article is easy to read and understand the concept of enhancing
the mechanical properties using heat treatments, and has provided sufficient
references for the same.

 

 

 

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