High-pressure and high-temperature sintering of nanostructured bulk NiAl materials
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Nanostructured bulk NiAl materials were prepared at high pressure and temperature (0–5.0 GPa and 600–1500 C, respectively). The sintered samples were characterized by x-ray diffraction, scanning electron microscope, density, and indentation hardness measurements. The results show that NiAl nanoparticles may have a compressed surface shell, which may be the reason why NiAl nanomaterials were difficult to densify sintering using conventional methods and why high-pressure sintering was an effective approach. We also observed that B2-structured NiAl could undergo a temperature-dependent phase transition and could be transformed into Al0.9Ni4.22 below 1000 C for the first time. It was interesting to note that Vickers hardness decreased as grain size decreased below 30 nm, indicating that the inverse Hall-Petch effect may be observed in nano-polycrystalline NiAl (n-NiAl) samples. Moreover, a tentative interpretation was developed for high-pressure nanosintering, based on the shell-core model of nanoparticles.
I. INTRODUCTION
NiAl intermetallic compounds with the B2 structure are of great interest as structural materials for aerospace and automotive applications due to their low density, high stiffness, good oxidation resistance, and corrosion resistance at elevated temperature.1,2 However, few reports have described the preparation of nanostructured bulk NiAl materials, mainly due to the particular preparation method of the starting nanopowder by flow-levitation method.3 Therefore, the microstructure and mechanical properties of bulk NiAl nanomaterials remain poorly characterized. We know that the development of sintering is an important milestone for modern technical materials.4 Its final-stage sintering processes are always accompanied by rapid grain growth.5,6 Unless it is controlled, the efforts to produce dense material with nanometer-scale structure (grain size less than 100 nm) and the prospects of conventional pressureless sintering will be seriously hampered. In contrast, high-pressure sintering is an effective method to suppress grain growth and improve densification of samples, and some works have been reported in the literature.7,8 The microstructure of the nanoparticle, especially the surface layer, is always ignored in those works. Palosz et al.9,10 established the core-shell model
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0227 J. Mater. Res., Vol. 24, No. 6, Jun 2009
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to illustrate a nanoparticle with a grain core and a surface shell. There is a general understanding in the material community that nano ceramic powders are always more difficult for sintering than metallic grains,11 and the surface shell of the nanoparticle may play a vital role in sintering. Based on traditional sintering theories, Liao et al.7 have described a high-pressure sintering mechanism of n-TiO2, and they claimed that grain growth is controlled by grain boundaries and pores. More recently, Zhao12,13 and Palosz et al.
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