Continuous dynamic recrystallization of extruded NiAl polycrystals during the superplastic deformation process
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I. INTRODUCTION
INTERMETALLIC compound NiAl with B2 crystal structure is regarded as a potential candidate of hightemperature structural materials, because it offers a wide range of attractive mechanical, chemical, and physical properties, such as high melting point, low density, good thermal conductivity, and high stiffness.[1,2] However, it exhibits inherently brittle fracture and zero ductility at ambient temperatures, and its use as an engineering material is restricted in many cases by its poor fracture resistance and limited fabricability. In an attempt to improve the ductility of single-phase NiAl, the tensile behavior of NiAl has been investigated at wide ranges of temperatures and strain rates using polycrystals[3,4] and single crystals.[5,6,7] In addition, the deformation of NiAl by compression has been studied.[8,9,10] Some interesting results were disclosed by these studies. For NiAl single crystals with various kinds of orientation, the anomalously large tensile elongation almost like superplastic deformation has been observed to take place at a very limited temperature regime of 0.35 to 0.40 Tm. For NiAl polycrystals, NiAl or Ni-rich NiAl alloys underwent an obvious brittle-to-ductile transition (BDT) at about 400 °C and exhibited superplasticity at temperatures ranging from 1000 °C to 1100 °C under initial strain rates of 1.67 104 to 1.67 102 s1.[4,11–14] There was substantial evidence indicating that details of the dislocation structure play a central role in determining the deformation behavior of NiAl.[1,2] At room temperature and below, NiAl deformed almost exclusively by the motion of 100 dislocations whose Burgers vector gave only three independent slip systems. Some authors have argued that the phenomenon of BDT occurring at intermediate temperatures originated from the onset of a diffusion-assisted deformation process, such as dislocation climb.[10] The superplasticity observed at high temperatures for NiAl was ascribed to the process of dynamic recovery and recrystallization, but the authors did not definitely elucidate the role of dislocations during the superplastic deformation process.[11] Furthermore, the superplasticity for XINGHAO DU, Ph.D. and BAOLIN WU, Professor, are with the Department of Materials Engineering, Shenyang Institute of Aeronautical Engineering, Shenyang 110034, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted February 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Ni-rich NiAl alloys has been recently investigated by Lin et al.[12,13,14] They found that the initial strain rates for Ni-rich NiAl alloys to deform superplastically fell in the range of 103 to 104 s1 and the maximum tensile elongation to fracture increased with the Ni content.[12,13,14] In the present study, we discovered that extruded NiAl exhibited superplastic deformation even at high strain rate (101 s1) at high temperatures, which could be used directly by conventional industry, such as forging. This discovery makes it possible for the compon
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