Characterization of Doped Nial by Atom Probe Field Ion Microscopy
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CHARACTERIZATION OF DOPED NiAl BY ATOM PROBE FIELD ION MICROSCOPY RAMAN JAYARAM AND M.K. MILLER, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376. ABSTRACT The atom probe field ion microscope (APFIM) has been used to characterize grain boundaries and matrix in NiAl doped with either boron, carbon or beryllium. Boron was observed to segregate to grain boundaries whereas carbon and beryllium did not. Atom probe analyses of the matrix revealed that the matrix was severely depleted of the solute in the boron- and carbon-doped alloys. Field ion imaging and matrix analyses also revealed ultrafine MB 2 - and MC-type precipitates ranging in size between 2 and 20 nm in diameter in the boron- and carbon-doped alloys. These precipitates occurred in significant number densities. Atom probe analyses of beryllium-doped NiAI did not reveal ultrafine precipitates and was consistent with the fact that almost all the beryllium was in solid solution. The enormous increase in yield stress in the boron- and carbon-doped alloys is predominantly due to a precipitation hardening effect. The small increase in yield stress in beryllium-doped NiAl is due to a mild substitutional solid solution hardening effect. INTRODUCTION The ordered intermetallics such as Ni3 Al and NiAI have been studied extensively on account of their interesting mechanical properties at elevated temperatures. Typically, these materials exhibit an increase in plastic strength with increase in temperature. This behavior is of great significance for technological applications, such as turbine blades, where high creep strength at high temperatures is a critical requirement. NiAI is particularly attractive from that standpoint since it has a lower density and higher melting point than Ni 3AI. A serious drawback of these ordered intermetallics is their brittleness at room temperature. This has been successfully overcome in the case of Ni3AI where the addition of a microalloying element such as boron in excess of 200 ppm results in a significant improvement in room temperature ductility [1,2]. This improvement in ductility has been attributed to boron segregation at grain boundaries which results in the transition from an intergranular to transgranular fracture mode. Similar efforts to ductilize NiAI have not been successful [3]. In the present work, the results of recent atom probe analysis of microalloyed NiAl are summarized. The atom probe field ion microscope (APFIM) combines atomic resolution imaging with single atom detection capability making it a powerful tool for the investigation of grain boundary and matrix chemistry [4]. Alloys of NiAI doped with either boron, carbon or beryllium have been analyzed in the atom probe and the results correlated with tensile property and fracture mode reported elsewhere [3]. EXPERIMENTAL The alloys used in this investigation were prepared by arc melting high purity elemental materials and drop casting into cylindrical copper chill molds. The purity levels of aluminum and nickel were 99.99 wt. % an
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