| In the previous study, the β+γ` alloy containing Ti and C showed a more refined mesh microstructure than the lamellar microstructure. This study revealed that the titanium among alloying elements helped to refine the microstructure. The M_s temperature of NiAl lowered rapidly with an increasing titanium content. This effect therefore contributed to the formation of the fine microsturucture. In the case of a low titanium content(below 2.2at.%), the M_s temperature was above ∼255℃, and when the martensite was heated, the exothermic β`→Ni_5Al₃ transformation appeared first. Because the martensite twins could be inherited during the β`→Ni_5Al₃ transformation, the lamellar β+γ` structure similar to martensite plates was finally formed. However, in the case of a high titanium content, the M_s temperature sufficiently lowered, so the endothermic β`→β transformation could occur before the β`→Ni_5Al₃ transformation. The endothermic transformation extinguished the martensite plates, therefore allowing the mesh microstructure to be independently formed from the martensite plates. |
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