Creep deformation of Ni 3 Al-Mo in situ composites
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INTRODUCTION
H I G H temperature eutectic composites promise significant improvements over conventional nickel-base superalloys in terms of tensile strength, impact strength, creep rupture, and fatigue resistance. 1 The reasons having prevented so far the use of directionally solidified (ds) eutectics mainly are very low solidification rates and excessive property anisotropy. 2 These shortcomings are more or less avoided by ds Ni-AIMo eutectics exhibiting excellent off-axis strength and ductilityfl '4 and allowing planar front solidification at relatively high growth rates. 3 Poor resistance to oxidation 5 and to thermal cycling6 seem, however, to be major disadvantages of this alloy system. Ternary ds Ni-A1-Mo alloys contain two ductile phases, 7(Ni,Cr, A1) or y'(Ni3A1) composing the matrix, and a(Mo) constituting the fibers. Heat-treated alloys, additionally, contain y or y ' matrix precipitates. Compositions generally are located along the monovariant eutectic trough L ~ y + a. Immediately upon solidification, alloys enter into the three phase field y + y ' + a and y ' is precipitated from the parent phases y and t~.7,8 It was recently shown that as-solidified y - a structures can undergo a transformation of the peritecto-eutectoid type below 1130 °C whereby the and a phases are transformed into 6(NiMo) and 7' .7,9 Longterm thermal stability is achieved only in alloys with compositions close to the ternary eutectic y - y ' - a transforming entirely into y ' + a at temperatures below 1280 °C. 8'7 The purpose of this paper is to investigate the role of microstrnctural dimensions and morphology on creep of ds Ni-A1-Mo alloys. Alloy compositions were chosen such as to obtain two-phase y'-a alloys on cooling upon solidification. Solidification conditions were varied in order to cover a wide range of microstructures. Unlike other eutectic systems, variation of solidification rates not only entailed alteration of fiber spacings but also changed fiber morphology and orientation relationship between fibers and matrix. 8 W. FUNK, formerly Graduate Assistant in the D6partement de mat6riaux, Ecole polytechnique f6d~rale de Lausanne, Switzerland, is Senior Scientist at Maschinenfabrik Rieter AG., CH-8406 Winterthur, Switzerland. E. BLANK is Senior Research Associate at Ecole polytechnique f6d6rale de Lausanne, 34 ch. de Bellerive, CH-1007 Lausanne, Switzerland. Manuscript submitted November 21, 1979.
METALLURGICALTRANSACTIONS A
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EXPERIMENTAL PROCEDURES
A. Melting and Solidification Master melts of nominal composition Ni-27 wt pct Mo8 wt pct A1 were made from high-purity elements in alumina crucibles in a vacuum induction furnace. The system was exhausted, backfilled with argon to atmospheric pressure, and the melt was held for about 15 minutes prior to pouring into copper chill molds. The chill-cast alloy bars were placed in 8 mm diameter alumina tubes and directionally solidified in a modified Bridgman furnace, under a constant 250 °C superheat. Four different microstructures, called alloys A, B, C, and D, respecti
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