Grain boundary strengtheners like Carbon, Boron and Zirconium segregate to grain boundaries by forming carbides and borides, to strengthen them but how they effect the melting temperature of superalloys (reason behind it ?)
In the overwhelming majority of cases, the addition of various elements to the base metal reduces the melting point of the resulting alloy. This is because in the state diagrams of most binary, ternary and more systems of substances, there are regions with a melting point lower than that of individual components - eutectics. Moreover, this is true not only for metals, but also for non-metallic substances. Thus, as a rule, a pure substance has the maximum melting point among all alloys based on it, and the more alloying elements, the lower the melting point of the alloy.
Two Ni-based SX superalloys containing different additions of boron (0 and 0.02 wt%) were directionally solidified using Bridgman technique, and their as-cast and heat-treated microstructures were investigated. Following conclusions can be drawn.1)
The addition of 0.02 wt% boron increased the volume fraction of (γ + γ′) eutectic, resulted in the formation of script-like M3B2 boride and reduced the amount of micropores in as-cast microstructure.
2)
The addition of minor boron was found to markedly decrease the liquidus and solidus temperatures and the incipient melting point.
3)
The volume fraction and size of γ′ phase as well as γ channel width were not changed by the minor boron addition.
4)
Script-like M3B2 boride was found to absorb strengthening elements Cr, Mo, W and Re and serve as the initiation sites of fracture, which was detrimental to the high temperature creep properties of Ni-based single crystal superalloys.
Dear Shubhansu Singh
I have borrow the following paragraph from the article by the Yunsong Zhao et.al, titled "Influence of minor boron on the microstructures of a second generation Ni-based single crystal superalloy" to reflect the authors explanations for the role of boron addition on reducing the incipient melting temperature of polycrystalline superalloys.
Article Influence of minor boron on the microstructures of a second ...
"Some studies in directionally solidified polycrystalline superalloys showed that the M3B2 boride generally precipitated at the grain boundaries or interdendritic regions. Script-like M3B2 boride was formed at the final solidification stage and in the front of (γ+γ′) eutectic. Seo et al.[14]found that the limited solubility of Cr, Mo, and W in the γ′phase and the strong positive segregation tendency of boron in Ni-based alloys made those elements enriched in the residual liquid during (γ+γ′)eutectic reaction and eventually led to the formation of boride in the final residual liquid. The M3B2 boride was reported to melt in a temperature range from 1200 °C to 1220 °C, and then it could promote the melting of (γ+γ′) eutectic[15]. Thus, as a phase with low melting point, M3B2 boride significantly reduced the incipient melting temperature."
Hope it is helpful.
Best regards
@Safwan-al-qawabah thanks for your response
can your direct me to the work which you have mentioned, specially for the 4th point
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