1200 to 1400 K Slow Strain Rate Compressive Properties of NiAl/Ni 2 AlTi-Base Materials
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1200 to 1400 K SLOW STRAIN RATE COMPRESSIVE PROPERTIES OF NiAl/Ni 2 AlTi-BASE MATERIALS 3
3 2 1 J. Daniel Whittenberger , R.K. Viswanadham , S.K. Mannan and K.S. Kumar 21NASA Lewis Research Center, Cleveland, OH 44135; Multi-Metals 715 E. Gray St., Louisville, KY 40202; 3 Martin Marietta Laboratories, 1450 S. Rolling Rd. Baltimore, MD21227-3898.
ABSTRACT Owing to their superior strength in comparison to other single phase intermetallics, NiAl-NiAlTi alloys have potential for the matrix material in high temperature composites. An investigation of two compositions, Ni-40A1lOTi and Ni-45Al-5Ti with and without 20 vol. pct. TiB 2 in the form of -lfm diameter particles, was initiated with materials produced by the Martin Marietta XDTM process and compacted by hot pressing. The as fabricated materials were fully dense and polycrystalline where the grain sizes measured -8,m for Ti-poor and about 15tpm for the Ti-rich unreinforced materials but could not be determined for ei her TiB 2 -containing composite. Elevated temperature compression testing was conducted to approximately 8 percent deformation between 1200 and 1400 K with strain rates varying from -10- 4 to 10-7 s-1. While the majority of the tests exhibited diffuse yielding over approximately one percent strain followed by negative strain hardening, a few experiments resulted in steady state behavior where deformation continued under a constant stress. The flow strengths on yielding of both forms of Ni40Al-1OTi were higher than those for the Ni-45Al-5Ti versions. Although for each matrix composition the addition of 20 vol. pct. TiB2 decreased the strength at the higher strain rates in comparison to the TiB 2 -free forms, the particles do provide reinforcement during slow deformation conditions. Optical microscopy of tested specimens revealed that these materials are generally quite brittle as numerous longitudinal and transverse cracks were found irrespective of the type of stress-strain behavior. INTRODUCTION Recent work [1,2] has shown that the creep resistance of binary B2 crystal structure NiAl could be improved at elevated temperature by particulate strengthening. Composite materials, containing from 0 to 30 vol. pct. of nominally 1pm diameter TiB2 particles in the intermetallic matrix, were produced by the Martin Marietta Corporation's XDTM process [3]. Such composites demonstrated significant strength increases, approaching 3 fold for the 20 vol pct materials, in comparison to the unreinforced aluminide. In addition this behavior was accomplished without deleterious side effects as the grain boundaries and particle-matrix interfaces were intact after compressive deformation to 10 percent or more strain. The following study describes an attempt to apply XDTM technology for the fabrication of NiAl-Ni 2 AlTi materials with improved creep properties. This class of materials is of interest as Polvani, Wen-Shian Tzeng and Strutt [4] showed in 1976 that these intermetallics can have creep resistances approaching those for the Ni-base superalloys. However the inher
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