Which is better: a DBTT as a point or as a temperature range? What factors affect it?
Based on my research, I noticed that the DBTT of some aluminide coatings is a point, while for others, the hardness gradually reduces as temperature increases, resulting in a DBTT that spans a range. I want to analyze the factors that cause this effect and understand Which type is better for service applications?
In some Ni-aluminide samples with a DBTT at a specific point, the hardness of the specimen increases before reaching the DBTT. How can this fact be explained? Is there a relationship between having a DBTT at a specific point and the observed increase in hardness before it?
Dear Hora
The ductile-brittle transition temperature for diffusion coatings of Fe-, Ni-, or Ti-aluminides is typically a range of temperatures rather than a single point. It marks the shift where these materials transition from ductile (deformable) to brittle (prone to cracking) behavior.
The exact DBTT can vary based on factors such as alloy composition, processing methods, and coating thickness.
i hope this answer is useful to clear the ambiguity in the field you are looking for.
s.Sh Hashemi thank you so much, I want to know the exact effects of alloy composition. How does this dependence relate to different intermetallics in the coating? for example, that Ni-aluminide with just Ni2Al3 can have DBTT as a point compared to the Fe alumnide which has range of different interetallics with different precentage of Fe. Does it also depend on whether the structure is substitutional or interstitial?
Additionally, is the dependence on thickness only related to crack presence and length, or are other factors important?
Finally, what exactly happens during the DBTT in these coatings? Does improved ductility result from changes in slip systems or dislocation behavior?
I am not excel in this subject, I just to sole this puzzle in my mind. Thank you again
I found these results for Ni aluminide coating, I was wondering if you provide some similar for Fe-aluminide and Ti-aluminide coatings. Thank you
Why does NiAl's strength increase before the DBTT?
This occurs due to stoichiometry deviations and aluminum consumption during oxidation, which harden the material.
Is the DBTT of Ni-aluminides a point?
For NiAl, the DBTT is around 760°C but can vary due to factors like stoichiometry deviations, thickness(cracks), strain rate, grain boundary strengthening, and the solubility of elements like Cr.
What improves ductility at the DBTT—slip systems or dislocations?
In Ni-Aluminide coating, Ductility improvement at the DBTT is linked to dislocation movement, particularly (100) dislocations. The activation energy for the ductile-to-brittle transition is tied to vacancy diffusion, where vacancy movement influences the material's behavior. Additionally, some researchers suggest that the DBTT is more of a phase transition than simple thermal activation, as it also relates to stress handling at grain boundaries.
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