Elevated and Room Temperature Mechanical Behavior of Boron Doped NiAl Allots
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ELEVATED AND ROOM TEMPERATURE MECHANICAL BEHAVIOR OF BORON DOPED NIAI ALLOYS
YI TAN+, TETSUMORI SHINODA+รท, YOSHINAO MISHIMA, AND TOMOO SUZUKI Precision and Intelligence Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan. *Department of Metallurgical Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan. +On leave from Department of Materials Engineering, Dailian University of Technology, Daillan, China. ++On leave from Hitachi Research Laboratory, Hitachi Ltd., Saiwal-cho, Hitachi-shi, Ibaraki 317, Japan
ABSTRACT The effects of boron addition on defect hardening at room temperature and on high temperature creep properties are investigated in the B2 NIAI intermetallic compound. It is found that boron addition Is effective In Increasing the room temperature hardness on the Ni-rich side but has no effect on the Al-rich side of stoichiometry. These observations are attributed to interstitial dissolution of boron in Ni-rich NiAl and due to a lack of solubility, and consequently an enrichment at grain boundaries on the Al-rich side. The similar effect is found for high temperature creep resistance of NiAl by boron addition, where It is increased at Ni-rich side but is unaffected at Al-rich side of offstoiciometry.
INTRODUCTION An Intermetallic compound NiAl is of Berthollide type having a high melting temperature and a wide compositional range for the B2 ordered structure. It is consequently expected to be a candidate as a heat resisting material by itself. However, efforts in designing this compound for this purpose has not been extensive mainly due to such drawbacks as poor room temperature ductility and insufficient high temperature strength. It has been shown that the defect structure in NiAl as the composition deviates from stoichlometry differs between the Ni-rich and the Al-rich side, where it consists of anti-structure defects In the former and vacancies on Ni-site in the latter[l]. Accordingly, various properties of the compound show characteristic change as the composition deviates from stoichlometry. For example, it has been shown by Vedula and Khadkikar[21 that the room temperature strength normalized by shear modulus is twice as high on the Al-rich side than on the Ni-rich side when compared at identical deviations from stoichlometry. Although there have been several studies on the high temperature creep properties of NIAI[3-7], it should be recognized that the effect of deviation from stoichiometry, which Is very important in practice, has not been systematically investigated. The low temperature brittle nature of NIAl is characterized by grain boundary fracture[8,91. As one of the efforts to improve room temperature ductility of the compound, boron addition has been attempted [101 because the method was successful in eliminating the grain boundary brittleness in NI3Al. However, there has been no work to correlate the effect of boron Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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addition on the mechanical proper
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