Creep Deformation of Ultrafine Grained Ni 75 B 17 Si 8
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CRBBP DEFORMATION OF ULTRAFINB GRAINED NiK 7 B1 7 Sie GILLES NUSSBAUM AND DIETER G. AST Materials Science and Engineering, Bard Hall, Cornell University, Ithaca NY, 14853-1501. ABSTRACT The creep behavior of the fine grained, intermetallic superplastic alloy of overall composition Ni7 sB1 7 Si9 prepared by crystallizing an amorphous precursor was studied in the temperature range between 500 and 0 615 C, the stress range between 36 and 800 MPa, and for grain sizes between 6.8 and 1.12*l0- 5 mm. The strain rates measured varied between 10-7 and 6*10-5 sec- 1 . Two different creep deformation mechanisms were observed. At low applied stresses, the material deformed in a diffusion controlled mode, with l proportional to or'. No dislocations developed in the grains, and the activation energy for creep was 4 eV. This value is higher than the self-diffusion of Ni in Ni (the principal constituent in the alloy) in agreement with observation in other Ni based alloy systems. At high applied stresses, a second mechanism became rate limiting. The activation energy of this process is about 0.6 eV and dislocations develop in the interior of grains when this mechanism operates. The transition between the two regimes is grain size and temperature dependent and occurs at about 400 MPa at a deformation temperature of 550 0 C and a grain size of 6.8 10-5 mm. An analysis of the grain size dependence of creep strain rate as well as its absolute value indicates that the low stress regime is Coble type creep. INTRODUCTION In a polycrystalline material, different deformation modes can contribute to creep, such as glide and climb of dislocations, subgrain formation, grain boundaries (GB) sliding, point defect diffusion and twinning. If the grain size is very small and the temperature above half the melting temperature, such processes can readily occur and the material may behave in a superplastic manner [1], even when the alloy is an inherently brittle intermetallic alloy. Fine grained intermetallic alloys can be prepared conveniently by the controlled crystallization of amorphous alloys. This preparation method was chosen by S. Reusswig et al [1], who investigated the deformation of fine grained NisB1 7 Si 8 with a bimodal grain size distribution in tensile tests at strain rates between 5*10-4 and 10-2 s-i. They showed that this alloy behaved in a superplastic manner at elevated temperatures but who did not determine activation energies. To explain the observed stress sensitivity of .5, and the presence of dislocations in large grains only, these authors assumed a mixed mode deformation, with diffusional creep dominating in small grains, and dislocation deformation in large grains. This paper extends this previous study towards much lower strain rates. Special care was taken in crystallizing the material, in order to obtain a single valued grain size distribution. Creep tests were conducted over as wide a temperature range as possible, between 500ýTý615 0C, in order to derive a value for the activation energy. When accommodation occurs by b
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