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rate. Thin foils for transmission electron microscopy (TEM) investigation were cut from the deformed samples and thinned using an electrolyte solution consisting of 10% perchloric acid in methanol. A voltage of 4-6 V and current of 25-30 mA was used in the polishing process. RESULTS Figure 1 shows the yield strength versus temperature data for Ni-44Al, Ni-50AI and NiAl0.3Hf. We will refer to the "knee" in the yield strength curve as the temperature (designated as Tk) at which the sharp drop in yield strength is observed. This occurs at a much higher temperature for Ni44A1. The "knee" also corresponds with a change in slip mode from a slip to non-a< 11> dislocation processes. At temperatures below the "knee", deformation occurs mainly through a< 111> slip [10]. While a{ 1121 dominates up to 300 K in Ni-50A1 [11], deformation between 300 K and Tk has been reported to occur either by kinking [12,13] or a{ 1101 slip [2]. This paper will discuss mainly the deformation behavior beyond Tk in all three NiAl alloys described. 2000

S1500

20 - NiAI-0.3Hf - -- - Ni-50AI - -Y- - Ni-44AI -

r1

yI

1000' ~500

S

0 500

1000

1500

Temperature(K) Figure 1: Variation of yield strength with temperature for Ni-50AI, Ni-44AI and Ni-49.7AI-0.3Hf (strain rate=10-4/s).

Studies on off-stoichiometric NiAl [3,9] demonstrate that a{ 1011 glide does indeed contribute to deformation beyond Tk, at least within a definite temperature window. Models detailing the process of decomposition of a dislocations, leading to the slip transition in Ni-44A1, have been presented recently [8,14]. Fraser et al. [71 have reported observation of a dislocations in the deformation microstructure beyond Tk in Ni-50A1, prompting them to conclude that a dislocations do not play an important role in deformation. However, creep studies on Ni-50A1 [15] have provided evidence for a slip and subsequent decomposition of a dislocations into a dislocations, that are eventually observed in the microstructure. The results of TEM investigations of our three alloys beyond Tk are detailed below. Ni-44A1 Figure 2 shows the deformation microstructure in Ni-44A1 strained 1.7% at 800 K (approximately equal to Tk in Ni-44A1). Both a and a dislocations (decomposition products of a< 111> dislocations) are observed in the microstructure. The peculiar line orientation of the KK9.10.2

dislocations has been discussed in part elsewhere [8]. Figure 3 shows the deformation microstructure at 875 K after 0.4% strain. Samples were also studied at 825 K and 850 K. Interestingly, the ratio of a to a dislocations increases steadily with the deformation temperature, indicating that a dislocations indeed multiply and contribute to deformation. From Figure 3, it is clear that the line direction is still the prominent orientation for the a dislocations. This line direction is maintained even after annealing at 950 K, indicating the stability of this particular orientation. Figure 4 shows the post-deformation microstructure of Ni-44A1 strained 2.1% at 1000 K. The 1: a[011] 2: a[1001 3: a[011] 4.