The Effect of Cobalt on Martensitic Toughening Parameters in NiAl
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THE EFFECT OF COBALT ON MARTENSITIC TOUGHENING PARAMETERS IN NiAl
SCOTT M. RUSSELL, C. C. LAW AND M. J. BLACKBURN Pratt & Whitney, 400 Main St., East Hartford, CT
06108.
ABSTRACT The martensitic transformation and the effect of cobalt additions on important martensitic toughening parameters are being studied as a means of toughening NiAl alloys. Cobalt additions to NiAl martensite are seen to lower the Ms temperature, reduce the transformation strain anisotropy, and reduce the transformation temperature hysteresis (an indicator of interfacial mobility). Optimization of these parameters should allow martensitic transformation toughening processes to aid in overcoming the ambient temperature brittleness of NiAl alloys. INTRODUCTION NiAl alloys offer advantages in terms of lower density, higher melting temperature and better oxidation resistance relative to current high temperature nickel and cobalt base alloys. These features could translate into significant performance gains in applications such as gas turbine engines. However, current NiAl alloys are brittle at low temperatures and are not suitable for use in engineering structures. This brittleness must be overcome for NiAl alloys to be of practical use. In earlier attempts to ductilize NiAl-base alloys, one of our approaches was to modify the basic slip behavior of NiAl with selected substitutional solutes [1,2], for it is usually considered that the ductility problem in NiAl can be traced to an insufficient number of slip systems to satisfy the Von Mises criterion for plastic flow in polycrystalline aggregates. Although the slip behavior of the material was successfully changed with Mn and Cr additions, the alloys were still brittle [1]. (For further details of this study, refer to the paper by Miracle, et al in these proceedings [2].) Grala [3] and Schulson [4] studied grain refinement in NiAl alloys as a means of ductility enhancement using microalloying and thermomechanical processing, respectively, for a material in fine grain condition generally shows better ductility than in coarse grain condition. However, these approaches succeeded only in lowering the ductile-brittle transition temperature to about 400'C. In contrast, some ductility has been observed in cobalt-modified alloys with lower Al contents [1]. An alloy of Ni-25 a/o Al-25 a/o Co exhibited 10% elongation at room temperature [1]. Such alloys contain a mixture of phases: Ni 3 Al, martensite and NiAl in various proportions depending on the exact composition [1]. On first sight, the ductility in such alloys may be attributed primarily to the presence of the Ni 3 Al phase which is intrinsically more ductile than NiAl. However, it should be noted that considerable ductility can be produced in NiAI crystals as a result of the transformation and/or reorientation of the martensite variants under applied stresses. This is clearly seen by comparing the room temperature ductility of NiAl single crystals of stoichiometric composition, which do not undergo a martensitic transformation, and crystals of lowe
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