Observation of a New Creep Regime in Poiycrystallitne Ni-50(at.%) Al Intermetallic Alloy
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OBSERVATION OF A NEW CREEP REGIME IN POLYCRYSTALLME Ni-50 (at.%)Al INITRETALLIC ALLOY
S. V. Raj and Serene C. Farmer NASA Lewis Research Center, MS 49-1, Cleveland, OH 44135. ABSTRACT Constant load creep tests were conducted on fine-grained ( - 23 Mm) polycrystalline Ni-50(at.%) Al in the temperature range 1000 - 1400 K. Powerlaw creep with an average stress exponent, n, of 6.6 and an average activation energy, Qc, of about 300 kJ mol-1 was observed above 25 MPa, while n = 2 and Qc = 95 kJ mol- 1 for a < 25 MPa. Primary creep was observed in both regions thereby signifying dislocation activity during the initial period of the test. Preliminary experiments with coarse-grained Ni-50A1 suggested that the mechanism in the n = 2 region is dependent on grain size. Transmission electron microscopy observations of the deformed specimens revealed dislocation tangles, dipoles, loops and networks in the power-law creep regime. The B&rgers vector was determined to be with the dislocations lying on the (100) and (110) planes. Although well-defined subgrains were not always observed, there was a greater tendency towards subgrain formation at stresses above 25 MPa. The deformation icrostctwures were inhcmcgeneous in the n = 2 creep regime and many grains did not reveal any dislocation activity. The observed characteristics of the low stress region suggest the dominance of an accoorrodated grain boundary sliding mechanism. INTRODUJCION Stoichicmetric Ni-50(at.%)Al is being developed as a potential aircraft engine material to replace superalloys. Certain components, such as turbine blades and vanes, are subjected to high stresses and high temperatures during
use so that good creep properties are an important design requirement. As a result, an understanding of the mechanisms governing creep of NiAl is essential in order to design alloys with superior creep properties. Earlier investigations on the creep behavior of NiAl have been largely restricted to the power-law creep regime [1-5] and no data exist at lower stresses, where other processes, such as diffusion creep, may dominate. This paper reports the observation of a new mechanism in Ni-50AI at low stresses. EXPERIMENIAL Pre-alloyed vacuum atomized Ni-50Al powder was procured from Hoimogeeous Metals, Inc., Clayville, NY. The powders were sealed under vacuum in mild steel cans and extruded at 1400 K using a ratio of 16:1. The average linear intercept equiaxed grain size, d, was 22.9 ± 0.7Mm at the 95% confidence interval. Compression specimens of nominal dimensions 6.4 mm in diameter and 12.7 mm long were machined from the extruded rods with the compression axis parallel to the extrusion direction. The specimens were tested under constant load between 1000 and 1400 K with the initial stresses varying between 3.0 and 52.0 MPa. Additional data were obtained from stress increase tests. Microstructural observations were conducted on selected tested specimens using optical and transmission electron microscopy (TEM). RESUULS AND DISCUSSION Two distinct creep regimes were observed at
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