Processing and High Temperature Deformation of Nb 3 Al
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PROCESSING AND HIGH TEMPERATURE
DEFORMATION OF Nb 3 Al
Y.MURAYAMA, T.KUMAGAI* AND S.HANADA Institute for Materials Research, Tohoku University, Sendai 980, Japan *Now at National Research Institute for Metals, Tokyo 153, Japan ABSTRACT Alloys based on the ordered intermetallic Nb3Al were fabricated by powder metallurgy processing, infiltration processing and clad-chip extrusion processing. By controlling processing variables, single phase Nb AI and two phase Nb3AI-Nb solid solution alloys were obtained with various microstructures. The single phase Nb Al deforms by {001 ) slip above 1400K and stress-strain curves exhibit deformation softening after showing a high peak stress. Transmission electron microscopic observations revealed that dislocations are dissociated into two partial dislocations which bound a stacking fault and the dissociation width between the partial dislocations depends on aluminum content in Nb3 Al. The deformation softening was found to be caused by dynamic recrystallization. The iwo phase alloy shows similar stress-strain curves to the single phase alloys. However, sub-boundaries and dislocation networks were observed without recrystallized grains. INTRODUCTION Nb 3 Al, with the A 15 structure, is a candidate as a high temperature structure material for use above 1500K. It has a high melting temperature, 2333K, low specific gravity, 7.26Mg/m 3 , and high strength at elevated temperatures[I]. However, an arc-melted button with a near-stoichiometric composition is easily cracked during cooling after solidification because of severe brittleness[2]. Therefore, it is difficult to fabricate structural parts of the near-stoichiometric NbA by ingot metallurgy. Cracking in an arc-melted button is suppressed with decreasing Al content and sound castings can be produced in Nb-rich NbAI alloys consisting of Nb3Al and Nb solid solution(Nbss)[2]. Microstructural observations have revealed that the solidification structure of these castings is very coarse and inhomogeneous, that is, the Nbss phase exists inhomogeneously at grain boundaries and within grains of Nb3 AI[2,3]. Moreover, this structure is affected by solidification conditions. Thus, sample preparation due to ingot metallurgy is not appropriate for experiments to investigate mechanical properties of Nb 3 AI alloys at elevated temperatures as a function of composition in single phase Nb 3 A or in alloys containing Nbs. Consequently, the need to develop other process in which cracking can be suppressed and microstructure can be controlled is apparent. Fabrication processes of Nb3Al as a superconducting material have been extensively studied[4-12]. In this study three compound fabrication processes, i.e., the powder metallurgy process, the infiltration process and the clad-chip extrusion process, are investigated to obtain structure-controlled Nb 3A alloys without micro- and macrocracks. Compressive properties of the obtained alloys at high temperatures are reported. POWDER METALLURGY PROCESS Nb/Al composites for superconducting Nb 3AI wires have b
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