Superplasticity in Nanocrystalline Ni 3 Al and Ti Alloys
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Superplasticity in Nanocrystalline Ni3Al and Ti Alloys Sam X. McFadden*, Alla V. Sergueeva*, Tomas Kruml+, Jean-Luc Martin+, and Amiya K. Mukherjee* *Division of Materials Science and Engineering University of California, One Shields Avenue, Davis, CA 95616 + Departement de Physique Ecole Polytechnique Fédérale de Lausanne 1015 Lausanne, Switzerland ABSTRACT The advent of nanocrystalline materials has provided new opportunities to explore grain size dependent phenomenon. Superplasticity is such a grain size dependent phenomenon defined by the ability to attain tensile elongation of 200% or more. Superplasticity in microcrystalline materials has been well characterized. The constitutive equations that describe microcrystalline superplasticity predict enhanced properties for nanocrystalline materials. Enhanced properties in such nanocrystalline material include lower superplastic temperature at constant strain rate, higher superplastic strain rate at constant temperature, and lower flow stresses. Investigations with nanocrystalline Ni3Al and ultra-fine grained Ti-6Al-4V alloy have shown a reduction in the superplastic temperature. However, the flow stresses in these materials are significantly higher than expected. The high flow stresses are accompanied by strong strain hardening. Transmission electron microscopy in situ straining of nanocrystalline Ni3Al has shown that grain boundary sliding and grain rotation occurred during straining. The sliding and rotation decreased with strain. Dislocation activity was observed but was not extensive. There was no observable dislocation storage. The parameters of the generalized constitutive equation for superplasticity for nanocrystalline Ni3Al and Ti-6Al-4V are in reasonable agreement with the parameters for microcrystalline material. The rate parameters suggest that nanocrystalline superplasticity shares common features with microcrystalline superplasticity. In contrast, the observed flow stresses and strong strain hardening indicate that nanocrystalline superplasticity is not a simple extension of microcrystalline behavior scaled to finer grain size. INTRODUCTION Nanocrystalline materials are usually characterized as having a grain size of 100nm or less. Ultra-fine grained materials have grain sizes from 1000nm to 100nm. Superplasticity is defined as tensile deformation of 200% or more. Interest in nanocrystalline superplasticity derives mainly from the grain size dependence of superplastic flow. Superplasticity is often characterized using the generalized constitutive equation p
ε&= A
DGb b σ kT d G
n
B1.3.1
(1)
where ε& is the strain rate, D is the appropriate diffusivity (lattice or grain boundary), G is the shear modulus, b is the Burgers vector, k is the Boltzmann constant, T is the test temperature, d is the grain size, p is the grain size exponent, σ is the applied stress and n is the stress exponent [1]. A large body of data for microcrystalline superplasticity in metals, intermetallics, and ceramics, has shown the grain size exponent p
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