Effect of grain shape on environmental embrittlement in Ni 3 Al tested at elevated temperatures
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B.F. Oliver Materials Science and Engineering Department, The University of Tennessee, Knoxville, Tennessee 37996-2200 (Received 7 September 1988; accepted 8 November 1988)
This paper describes the effect of grain shape on environmental embrittlement in boron-doped Ni3Al (24 at. % Al). The alloy showed severe embrittlement when tested at 600 and 760 °C in air. The embrittlement can be alleviated by control of grain shape, and the material with a columnar-grained structure produced by directional levitation zone remelting shows good tensile ductilities when tested in oxidizing environments. The columnar-grained structure with vertical grain boundaries minimizes the normal stress and consequently suppresses nucleation and propagation of cracks along the boundaries.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Polycrystalline Ni3Al is very brittle at ambient temperatures because of intrinsic grain-boundary brittleness.1"4 The ductility of Ni3Al can be dramatically improved by microalloying with a small amount of boron, which has a strong tendency to segregate to the grain boundaries. A number of studies have shown that boron-doped Ni3Al alloys exhibit excellent tensile ductility and ductile transgranular fracture at room temperature.1'2'5"9 A high elongation of more than 50% was obtained in Ni3Al containing 24 at. % Al and doped with 0.5 at. % B (0.1 wt. %). 2 The ductility of Ni3Al alloys is, however, sensitive to test environments at elevated temperatures, with much lower ductilities observed in air than in vacuum.10'11 The maximum loss in ductility, occurring around 600-800 °C, is accompanied by a change in fracture mode from ductile transgranular to brittle intergranular. The environmental embrittlement has been attributed to a dynamic effect involving simultaneously high localized stresses, elevated temperatures, and gaseous oxygen. The embrittlement was also observed in Ni3Al alloys containing 10 and 15 at. % Co tested at 600 and 760 °C in oxidizing environments,12 including air and impure argon.
Two ingots of a boron-doped Ni3Al alloy containing 24 at. % Al and 0.2 at. % B were prepared by arc-melting and drop casting into copper molds, using pure nickel and aluminum metals and a master alloy of Ni-4 wt. % B. The first ingot with a dimension of 2.54 X 1.3 X 13 cm was homogenized for 4 h at 1000 °C and then fabricated into sheets (0.8 mm) by repeated cold rolling with intermediate anneals at 1050 °C. Sheet tensile specimens with a gauge section of 12.7 x 3.2 X 0.7 mm were given a final heat treatment of 30 min/1000 °C, 1 h/1050 °C, or 4 d/1000 °C for recrystallization and grain growth. The specimens have an equiaxed grain structure, with grain size, in terms of linear intercept length, ranging from 20 to 91 ^tm. The second ingot with a dimension of 2.54 cm diameter x 13 cm length was directionally levitation zone remelted (DLZR) at a freezing velocity of 6.4 x 10~6 m/s (0.9 in./h) with the freezing interface rotating at 200 rpm opposite to the general induction-induced fluid flow field.1314 The melting induct
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