Changes in Creep Deformation Mechanisms in AlN Particulate Strengthened NiAl
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CHANGES IN CREEP DEFORMATION MECHANISMS IN AIN PARTICULATE STRENGTHENED NiAl T.R. BIELER, J.D. WHITTENBERGER* and M.J. LUTON** Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824 *NASA-Lewis Research Center, Cleveland OH 44135 ** EXXON Research and Engineering, Annandale, NJ 08801 ABSTRACT AIN particulate strengthened NiAl containing about 10vol% AIN was investigated in stress change experiments above, below, and through a transition stress near 80 MPa. All creep transients were normal, and incubation periods were observed Stress change in all stress drops near or below the transition stress. experiments indicated that the microstructure is very stable, and provided similar values for steady state strain rate as the monotonic tests. Analysis of the stress change data reconfirmed the existence of several deformation mechanisms in this material. INTRODUCTION Compression Creep of the NiAI-AlN particle strengthened material made by cryomilling has shown properties that are among the best obtained for NiAl based alloys [1-3]. The microstructure is unusual since the particles are heterogeneously dispersed throughout the matrix in mantle regions surrounding particleThough the power law creep behavior of monotonically free cores of NiAl. deformed specimens has been characterized on the basis of empirical power law creep, little has been established about the rate limiting deformation mechanisms In particular, this material exhibits a responsible for the creep deformation. change in the stress exponent from about 5 to about 11, at a stress between 60 and 80 MPa. The change in the stress exponent is insensitive to temperature between 1000 and 1400 K, so it may be related to physical microstructure rather than thermally activated processes. Stress change experiments were done on one lot of samples to investigate the deformation mechanisms and compare the results of this test procedure with those determined by monotonic tests. EXPERIMENTAL PROCEDURES The NiAI-AlN composite was prepared by ball milling a slurry of NiAl and The powder Y,O powders in a liquid nitrogen medium (cryomilling) at Exxon. charge contained 99.5 wt% Ni-51AI (-325 mesh, from Homogeneous Metals, Inc.) and Following 16 hours of 0.5 wt% YO0 (87.7 MPa " . . ",•
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Figure 3 Effect of temperature and stress cycling on an AIN particulate reinforced NiAl deformed at 1350 K. (a) Imposed stress profile and the resultant strain rate - strain behavior; the break in the curve at about 1.5% strain was due to an excursion to room temperature, (b) creep curve during a stress drop experiment above the transition stress, and (cd) creep curve during a stress drop experiment through the transition stress. In evaluating the threshol
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