An electron microscopy study of the precipitation of rhenium in the B2 nickel aluminide
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Precipitation of rhenium in /3-NiAl was studied by analytical and high resolution electron microscopy. Extended solid solutions were created by solidification processing, and the precipitation and hardening behavior were studied. Evidence suggests that rhenium is initially precipitated as disks parallel to {100} of /3-NiAl. Subsequent growth produces either a rod morphology or a pair of twin-related rods that form the shape of a butterfly. The twin plane of the butterfly is {1011} and this plane is nearly parallel to {110}_ofj3-NiAl. A twinning transformation given by Kx = {Toil}, K2 = {T013}, 771 = (1012), and T]2 = (3032) was determined for the butterfly-shaped particles. All of the precipitates exhibited an orientation relationship consisting of parallel close-packed planes and directions, i.e., (101//(0001) and [111]//[1210]. Performing the twinning transformation on a rhenium precipitate produces a variant rather than a new orientation relationship. After elevated temperature aging, a rod morphology was observed with the rod axes aligned parallel with either (12 1) or (131) of the /3-NiAl matrix. A total of 24 different variants are possible based upon the observed orientation relationship and the two observed growth directions.
I. INTRODUCTION The B2 nickel-aluminide, /3-NiAl, offers many engineering advantages in the design of high temperature gas turbine blades, and a review of these considerations has been published by Darolia.1 Two approaches using either intermetallic or refractory metal precipitates to improve the high temperature strength of /3-NiAl have been explored. Significant improvements in elevated temperature strength were attained through the formation of Heusler precipitates (Ni2AlX, where X is Hf, Nb, Ti, Zr, V, or Ta), and strengths comparable to the Mar M200 nickel-based superalloys were attained. However, these Heusler alloys exhibited reduced ductility at room temperature. A viable alternative is the addition of refractory metals that form simple eutectics with /3-NiAl, such as Cr, Mo, W, or Re. These materials offer both the possibility of improved creep resistance and ductile phase toughening. Of the refractory elements that form simple eutectics with /3— NiAl, only rhenium exhibits both an appreciable room temperature ductility and elevated temperature strength.2'3 At room temperature rhenium exhibits a 6% tensile elongation,2 and evidence of crack bridging by ductile rhenium fibers has been reported for the eutectic microstructure.4 Although improvements in the room temperature fracture toughness of the eutectic are expected to be small, since the volume of rhenium is only 2% at the eutectic composition,5 a higher fiber loading may be obtained by composite engineering. 2524 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 10, Oct 1993 Downloaded: 08 Apr 2015
Initial studies by Vedula et al.6 show only a modest improvement to the steady state creep rate of /?-NiAl mechanically alloyed with rhenium via hot extrusion. In this study the rhenium particle size was large, and th
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