주상정 결정립의 NiAl 마르텐사이트 제조에 의한 NiAl/Ni₃Al 일방향응고 합금의 미세조직 제어

Abstract

In the our previous studies about the two-phase NiAl/Ni_3Al alloys, the various microstrucutures of polycrystalline NiAl/Ni_3Al alloys could be obtained by the M_s temperature control of NiAl-martensite including titanium, and the mechanical properties were considerably dependent on their microstructures. In this study, the grain boundaries were tried to align by directional solidification to suppress the inter granular fracture at room temperature or grain boundary sliding at high temperature. To effectively maintain the grain boundary alignment in the course of the heat-treatment of a DS ingot, suppressing recrystallization during solutionizing treatment, which is the preceding process of martensitic transformation of a DS alloy, is very important. Based on the orientation relationship between parent β-NiAl(the B2 structure)and product β`-martensite(the L1_0 structure), the optimum heat-treatment conditions for DS ingot were determined through XRD observation. Consequently, the columnar grained NiAl/Ni_3Al alloys could be achieved for the only Ti-added ternary ingot contrary to the recrystallized binary ingot. And, the columnar grained alloys including Ti could show higher yield strength at high temperature than the usual polycrystalline alloys.