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THE EFFECT OF BORON ON THE MICROSTRUCTURE OF

S.R. SCHUON AND V. REHZETS General Metallurgy Dept. 48090-9055

Motors

Research

Ni 2 AITi.

Laboratories,

Warren,

MI

ABSTRACT Ingots with a nominal composition of Ni-34a/o Al-15a/o Ti + O.Ow/o B, 0.05w/o B, 0.1w/o B or 0.3w/o B were prepared and compression tested at a strain rate of 1.7 x 10-7/s at room temperature, 650"C, 870"C, and 1000"C in air. Alloys were single-phase Ni AITi and borides were contained in all compositions containing boron. Th9 compressive yield strength and the ultimate compressive strength of NAITi containing boron is strongly affected by the type and distribution of borides. The formation of TiB at 0.3w/o B increased the strength at room teperature, 650"C, 870"C, ang 1000"C, while formation of a TiB-Ni AITi eutectic at 0.05w/o and 0.lw/o B Crack deflection by TiB is an decreased strength at all temperatures. important strengthening mechanism at room temperature and 650*C. H~wever, at 1000°C, dispersion strengthening may be an important strengthening factor. BACKGROUND interest has grown in NiAI as a Over the past few years, structural material because of its high oxidation resistance and low it an attractive alternative to current density, which may make superalloys. However, NiAI loses strength rapidly above 650°C, but can be strengthened by alloying with titanium to form an ordered BCC structure, Ni AITi, where titanium atoms substitute for alternate aluminum atoms [1]. Adgition of boron to aluminide alloys has been found effective in For example, the improving the ductility or for dispersion strengthening. ductility of polycrystalline Ni3Al and FeAI is improved by the addition of boron which segregates to grain boundaries, increasing the grain boundary the addition of TiB has been used to cohesiveness [2,3]. In Fe 3Al, improve the elevated temperature strength and refine ihe grain size [4]. Based on these results, it appears that the addition of boron to Ni AITi may improve the mechanical properties by forming borides or improvin4 the grain boundary strength. In this investigation, various levels of boron were added to Ni AITi in order to determine the effect of boron on the ductility and strength of the alloy. EXPERIMENTAL PROCEDURE In a previous study [5], ingots (5 cm dia. x 15 cm) with a nominal composition of Ni-34 a/o, Al-15 a/o, Ti + 0.0 w/o B, 0.05 w/o B, 0.1 w/o B, and 0.3 w/o B were prepared from high purity aluminum, nickel, and titanium, and boron was added as Ni-14 w/o B chips in order to minimize boron loss. The compositions were induction melted in a zirconia crucible and an argon atmosphere, cast into steel molds, and annealed for 4 hours in vacuum at 1100°C. Microscopic segregation of aluminum and titanium occurred at all boron levels. Castings had a 1 mm chill zone at the mold-metal interface that was high in aluminum while titanium segregated to the center of the Mat. Res. Soc. Symp. Proc. Vol. 133. c 1989 Materials Research Society

648

casting. The composition of specimens taken in the region center.

Compression s