Mechanical behaviors of chromium- or niobium-modified Ni 47.5 Al 25 Fe 27.5 alloy at elevated temperature
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I. INTRODUCTION
THE NiAl intermetallic compound has great potential as a structural material for high-temperature applications. However, its poor ductility at low temperatures and lack of strength at high temperatures limit its application. These problems must be solved before the NiAl alloy can be commercialized. Microstructural control is well known to improve the ductility and toughness of this alloy. Alloying with the substitute elements changes its crystal structure and promotes extra slip systems, so this approach is commonly used. Macroalloying by the addition of a third element, Fe, can modify the microstructures of NiAl from that of a single-phase state to a mixture of three phases, (fcc), (ordered fcc, L12 structure), and (ordered bcc, B2 structure), and improve the ductility at room temperature.[1–6] This improvement demonstrates that the thin ductile -phase layer which surrounds the relatively brittle phase inhibits the nucleation of cracks and facilitates plastic deformation. In our previous investigation on the Ni75–xAl25Fex alloys, the value of x is varied from 20 to 40 at. pct; the Ni47.5Al25Fe27.5 alloy has the optimal combination of roomtemperature tensile properties of any of these alloys.[2] Accordingly, this alloy was chosen as the master alloy for further study. Additionally, Ishida et al. investigated the NiAl-X alloys[4] and found that the third element of Fe or Cr preferred to form the phase rather than and phases, and Nb preferred to form the phase rather than the and phases. This work used Cr or Nb as the fourth element and added it to the Ni47.5Al25Fe27.5 alloy. The formation of and phases in Ni47.03Al24.75Fe27.23X1 alloys (X Cr or Nb) had a similar effect. Adding Cr increased the volume fraction of the phase in interdendritic regions and the ductility at room temperature.[7] Adding Nb reduced the volume fraction of interdendritic regions, but caused the preCHUN-HUEI TSAU, Associate Professor, is with the Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 111, R.O.C. Taiwan. Contact e-mail: [email protected] JIEN-WEI YEH, Professor, is with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, R.O.C. Taiwan. JASON SHIAN-CHING JANG, Professor, is with the Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, R.O.C. Taiwan. Manuscript submitted July 16, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
cipitates to remain in the -matrix in the interdendritic regions during annealing, so the strength was maintained.[8] This work investigated the microstructural evolution and elevatedtemperature tensile properties of ternary Ni47.5Al25Fe27.5 alloys, and the effect of Cr or Nb on quaternary Ni47.03Al24.75Fe27.23X1 alloys. II. EXPERIMENTAL METHODS Three experimental alloys of Ni47.5Al25Fe27.5, Ni47.03Al24.75Fe27.23Cr1, and Ni47.03Al24.75Fe27.23Nb1 were adopted herein. Table I lists the nominal compositions and the actual compositions of th
