College Park, Maryland      June 6 - 10 , 2004

WP69: Neutron Diffraction Study of Phase Transformations and Mechanical Properties in Ni[AlFe] Alloys

L. Yang (Dept. of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012; Spallation Neutron Source, Oak Ridge National Laboratory), X.-L. Wang (Spallation Neutron Source, Oak Ridge National Laboratory; Metals and Ceramics Division, Oak Ridge National Laboratory), J. A. Fernandez-baca (Condensed Matter Sciences Division, Oak Ridge National Laboratory), A. D. Stoica (Spallation Neutron Source, Oak Ridge National Laboratory), C. T. Liu (Metals and Ceramics Division, Oak Ridge National Laboratory), J. W. Richardson (Intense Pulsed Neutron Source, Argonne National Lab)

Phase transformations in NiAl alloys are complex and have been a subject of extensive experimental and theoretical studies. The martensitic transformation in NiAl alloys involves the formation of long period stacking orders, which can be either B2-2M(3R, L10) or B2-17M(7R) depending on the composition. When magnetic transitional atoms (e.g., Fe, Mn) are added, various types of magnetic properties appear due to the competition of ferromagnetic and antiferromagnetic interactions. The study by Morito et al [1,2] on Ni50[AlxMn50-x] shows a continuous martensitic transformation of B2-10M-14M, and a magnetic phase transition from a paramagnetic to an antiferromagnetic and finally to a spin glass state. It is not yet clear whether or how the martensitic and magnetic phase transformations are related.

Our interest in Ni[AlFe] alloys was prompted by a recent systematic study that reported an unusual softening behavior in mechanical properties [3]. First-principle calculations revealed that when Fe atoms are added, a localized moment of 2.4 mB develops at the Fe site. The calculations further suggest that the magnetic interactions enlarge the atomic size of the Fe atom, which lead to the increase of the lattice parameter and hence the softening effect. If these explanations hold, magnetic interactions may be explored as a new mechanism to alter the mechanical property of metallic materials, which would open up a new way in the design of metallic materials for structural applications.

We have carried out preliminary characterizations of Ni[AlFe] alloys. Magnetic susceptibility measurements confirmed that when Fe replaces Al, a localized moment develops. Upon cooling, a paramagnetic to ferromagnetic transformation was observed, and the system goes to a spin-glass state below 12K. Neutron diffraction data collected at High Flux Isotope Reactor showed an intensity gain at low temperatures, which followed a Q-dependence reminiscent of magnetic scattering. Meanwhile, high-resolution powder diffraction data collected at IPNS revealed extra Bragg peaks below 20 K. Upon warming up, these extra peaks disappear at temperatures above 50K. While the experimental data are still being analyzed, the thermal hystersis and Q-dependence of the extra peaks suggest that they came from a martensitic phase transformation. More systematic studies are underway to determine the structure and the nature of phase transformations in Ni[AlFe] alloys and their relationship with the usual mechanical properties.

This research was supported by Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.

[1] S. Morito, K. Otsuka, Mat. Sci. Engr., A208, 1996, 47-55
[2] S. Morito, T. Kakeshita, K. Hirata and K. Otsuka, Acta Mater., 46, 1998, 5377-5384
[3] C.T. Liu, C.L. Fu, L.M. Pike, and D.S. Easton, Acta Mater., 50, 2002, 3205-3212

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