Microstructure Control of DS NiAl/Ni_3Al Alloy by Fabrication of Columnar-Grained NiAl Martensite

摘要

We have reported that the various microstructures of polycrystalline NiAl/Ni_3Al alloys can be obtained by heat-treatment using the M_s temperature control of NiAl martensite, and that the mechanical properties are considerably dependent on their microstructures. In this study, we tried to align the grain boundaries by directional solidification to suppress the intergranular fracture at room temperature or grain boundary sliding at high temperature or both. 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), we determined the heat-treatment condition for the DS ingot. Consequently, columnar-grained NiAl/Ni_3Al alloys could be achieved after additional aging treatment, and they could show a higher yield strength than the polycrystalline alloys at high temperature.