Microstructure and Mechanical Properties of cast, Homogenized and aged NiAl Single Crystal Containing Hf
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MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CAST,Hf HOMOGENIZED AND AGED NiAI SINGLE CRYSTAL CONTAINING I. E. LOCCI°, R. DICKERSON', R. R. BOWMAN', J. D. WHITTENBERGER', M. V. NATHAL" AND R. DAROLIA'" "NASA-Lewis Research Center, Cleveland, OH 44135 "'GE Aircraft Engines, Cincinnati, OH 45215.
ABSTRACT Small additions of Hf to [001] oriented single crystal NiAl are shown to be effective in improving high temperature creep strength. The presence of Hf-rich second phases and/or solid solution strengthening are responsible for the improved behavior observed at high temperatures. In the as-cast condition, large Hf-rich interdendritic regions were found. Homogenization heat-treatments (1590 K for 50 hours) substantially reduced this interdendritic segregation. TEM observations of the as-homogenized microstructure revealed fine G-phase (Ni16Hf6SiT)
precipitates, with plate or cuboidal shapes. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G-phase. Aging the homogenized material at 1300 K resulted in the formation of Heusler precipitates ([Y-Ni:AIHf), preferentially nucleating at the G-phase sites. These Heusler precipitates were more stable at this or higher temperatures and coarsened at the expense of the less stable G-phase. Post-test analyses of compression tested specimens, conducted at 1200, 1300 and 1400 K, revealed extensive changes in the distribution and size of the second phases. Deformation at 1300 K appears to occur by two distinct mechanisms: at high strain rates the stress exponent is g 4 while at slower rates (< 10"%a) a much higher exponent (~ 12) was found. Testing at 1300 K of specimens over-aged at 1400 K reduced the creep resistance of the alloy which suggests a contribution by precipitation strengthening to the overall strength of the alloy. INTRODUCTION Binary NiAI alloys, regarded as potential candidates for high temperature applications14, require improvement in room temperature ductility or toughness as well as high temperature creep resistance to compete with today's superalloys. Ternary additions, especially from group IVA and VA elements have been shown to provide significant strengthening at high temperatures.!6 Vedula et al.• suggested that Hf additions to polycrystalline NiAI would be effective; however subsequent testings of a polycrystalline small grain sized Ni-47AI-0.SHf (at.%) alloy failed to confirm their results. Because grain boundary sliding could contribute to the creep rate, it is possible that the observed weakness in the latter study could be circumvented through the use of single crystals.13 Hence a study of the elevated temperature deformation properties of Hf modified NiAI single crystals was initiated to characterize the phases, their stability and their possible influence on the high temperature strength of NiA! alloys. EXPERIMENTAL Single crystal [001] oriented NiAI alloys containing 0.3 or 1.0 at.% Hf substituted for AI were prepared at General Electric Aircraft Engines (GEAE) by directional solidification using
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