1000 to 1200 K time-dependent compressive deformation of single-crystalline and polycrystalline B2 Ni-40Al
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I.
INTRODUCTION
T H E B2 crystal structure aluminide NiA1 is currently being studied as a potential replacement material for conventional Ni-base superalloys because of its much higher melting temperature (1911 K for Ni-50AI*) and *All compositions are given in terms of atomic percent.
inherent high-temperature oxidation and hot corrosion resistance. Unfortunately, the creep resistance of polycrystalline NiA1 is quite low; [1,2,31therefore, alloying for solid solution strengthening or precipitation of a second phase [4-91 and composite technology [~~ are currently being investigated. Another possible method to induce high-temperature strength in NiA1 might be the use of single crystals. Such an approach seems reasonable, since deformation in NiA1 usually occurs on {011}(100) and occasionally on {001}(100) slip s y s t e m s . [17,18A91 Therefore, (100)-oriented crystals should be more deformation resistant than non-(100) orientations simply due to the low resolved shear stress on (100) dislocations. This behavior has been borne out at temperatures less than 600 K, I2~ where deformation is observed to occur by either (111) slip [20'21'22] or by kinking. I21"23-251Greater than 600 K, however, (001)oriented NiA1 is reported to deform by either {110}(011) s l i p [21'26'271 o r a combination of glide and climb of dislocations with a (100) Burgers v e c t o r . [24'25'281
Concurrent with the change in modes of deformation in (100)-oriented NiA1 crystals, there is a significant decrease in yield strength with increasing temperature. For example, Pascoe and Newey, t211 who compared the 0.2 pct yield strengths of (100) and (110) NiA1 single crystals tested at a strain rate of 2.2 x 1 0 - 4 s - 1 , found (1) below 850 K, the (100) orientation was stronger; (2) between 850 K and 1050 K, the (110) orientation was slighly more deformation resistant; and (3) above 1050 K, both orientations appeared to have comparable strengths. However, contrary to this observation, Strutt and co-workers[26,27[ reported that the creep strength of [ 100] single crystals far exceeded that of [110] or [111] single crystals over the temperature range of 1073 to 1373 K. The objective of the present study was to re-examine the 1000 to 1200 K time-dependent deformation of [100] and non-[100[ orientated single crystals over a relatively large range of strain rates and compare their properties with those of a polycrystalline version. This work was undertaken on materials containing 40 at. pct A1 as (1) the technical difficulties in growing single crystals are significantly lessened by the lower solidification temperature of Ni-40A1 (--1800 K) in comparison to Ni-50A1 and (2) little data exist on the properties of this composition. II.
J. D A N I E L W H I T T E N B E R G E R and R.D. NOEBE, Materials Engineers, are with N A S A Lewis Research Center, Cleveland, OH 44133. C.L. CULLERS, Graduate Student, is with the School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA 30332. K.S. K U M A R , Senior Scientist, is with Martin M
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