The structures, the mechanical properties, the high temperature oxidation and sulfidation behavior of the unidirectionally solidified Ni_$3Al(γ^1)$-Ni_3Nb(δ) eutectic alloy were investigated.
Casted eutectic alloy round bar was directionally solidified with rates(R) between 0.6 cm/hr and 12cm/hr at induction furnace maintaining the thermal gradient(G) at solid-liquid interface, 127℃/cm.
The eutectic microstructure was varied according to the growth condition (G/R ratio). When G/R ratio was larger than 127℃ㆍhr/ ㎠ the lamellar structure was formed, and the colony structure was formed, when G/R ratio was smaller than 127℃ㆍhr/㎠. It was found that the interlamellar spacing (λ) was dependent on the growth rate (R) through the relationship, $λ^2$ㆍR = 8.4 × $10^{-13}$㎤/sec.
The results also indicated that high temperature strength in tension to the growth direction of $Ni_3Al-Ni_3Nb$ eutectic alloy was better than that of the known nickel base superalloy B-1900. The microhardness of this eutectic alloy increased with the growth rate increase.
The high temperature oxidation in static air of this eutectic alloy was occurred paraborically. The activation energy of parabolic process was 21 Kcal/mole. High temperature oxidation was accelerated by the presence of $Na_2So_4$.
$Ni_3Al-Ni_3Nb$계 공정조성합금을 일방향 응고시켜 성장조건 변화에 따른 조직변화를 고찰하고, 기계적성질및 고온산화, 부식에 대하여 논하였다.
일방향 응고는, 고상-액상 계면에서의 온도구배 (G) 가 127℃/cm 되도록 하여 0.6cm/hr ~ 12cm/hr의 결정성장속도(R)로 성장시켰다.
일방향 응고된 공정합금의 조직은 계면에서의 온도구배 (G) 와 결정성장속도 (R)와의 비 G/R 값에 따라 다르며, G/R의 값이 127℃ㆍhr/ ㎠보다 클 경우에는 Lamellar 조직이, 작을 경우에는 Colony 조직이 형성되었다. 일방향 응고시킨 시편에 있어서의 Interlamellar spacing (λ)과 결정성장속도 (R) 사이에는 $ λ^2$ㆍR = 8.4 × $10^{-13}$ cm/sec의 관계가 성립하였다.
Lamellar 조직을 갖는 시편의 고온인장강도는 일반주조에 의한 니켈기 초내열합금보다 1000℃에서 약 30%정도 높았으며, 미소 경도는 결정성장속도에 비례하여 증가하였다. 고온산화와 $Na_2SO_4$에 의한 고온부식은 Parabolic rate law를 따라 부식되었으며, 고온산화시 활성화에너지는 21Kcal/mole 이었고, $Na_2SO_4$ 는 고온부식을 촉진시켰다.
