Elevated temperature creep properties of NiAl eryomilled with and without Y 2 O 3
- PDF / 5,261,144 Bytes
- 16 Pages / 584 x 793 pts Page_size
- 55 Downloads / 166 Views
Michael J. Luton Exxon Research and Engineering, Annadale, New Jersey 08801 (Received 27 June 1994; accepted 30 January 1995)
The creep properties of lots of NiAl cryomilled with and without Y2O3 have been determined in compression and tension. Although identical cryomilling procedures were used, differences in composition were found between the lot ground with 0.5 vol % yttria and the lot ground without Y 2 O 3 . Compression testing between 1000 and 1300 K yielded similar creep strengths for both materials, while tensile creep rupture testing indicated that the yttria-containing alloy was slightly stronger than the Y2O3-free version. Both compression and tensile testing showed two deformation regimes; whereas the stress state did not affect the high stress exponent (n ~ 10) mechanism, the low stress exponent regime n was ~ 6 in tension and ~ 2 in compression. The strengths in tension were somewhat less than those measured in compression, but the estimated activation energies (2) of ~600 kJ/mol for tensile testing were closer to the previously measured values ( - 7 0 0 kJ/mol) for NiAl-AIN and very different from the Q's of 400 and 200 kJ/mol for compression tests in the high and low stress exponent regimes, respectively. A Larson-Miller comparison indicated that cyromilling can produce an alloy with long-term, high-temperature strength at least equal to conventional superalloys.
I. INTRODUCTION High energy milling of the B2 crystal structure intermetallic NiAl in liquid N has been identified as an effective technique to impart high temperature strength in this aluminide.1"10 Such processing, generically termed cryomilling, 1112 leads to the formation of a thin mantle containing extremely fine A1N precipitates around each NiAl powder particle.13 During subsequent elevated temperature densification procedures (for example, hot isostatic pressing or extrusion), powder particles are welded together to yield a cellularlike microstructure, where AIN-enriched cell walls encircle nitride deficient cell interiors. Typically the AIN-reinforced aluminide exhibits a factor of 6 increase in creep strength over that demonstrated by polycrystalline NiAl.10 While the reproducibility of the cryomilling process in terms of mechanical strength has been verified by milling different heats of prealloyed NiAl powder5 and multiple trials on the same heat of powder,9 the oxidation resistance of various batches of NiAl-AIN has been problematic. For instance, the first lot of AIN-reinforced material, which was milled with a 0.5 wt. % Y 2 O 3 addition, was shown to have excellent oxidation properties14;
a) Currently
at Max-Planck-Institut fur Metallforschung, Institut fur Werkstoffwissenschaft, Stuttgart, Germany. J. Mater. Res., Vol. 10, No. 5, May 1995
http://journals.cambridge.org
Downloaded: 17 Mar 2015
indeed, the behavior of this alloy nearly equaled the best third element doped NiAl. However, subsequent cryomilling,5 which was accomplished without yttria, yielded materials with decidedly poorer oxidation resistance than the in
