Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system
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INTRODUCTION
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T H E effect of various microstructures on the high temperature properties of a Ni-Fe-Cr alloy system was studied in part fulfillment of a large experimental program on Nimonic PE16 alloy. 1 The role of different alloying additions in contributing toward the overall creep strength and ductility of Nimonic PE16 composition was highlighted by studying five different alloys of 45 pct Ni-35 pct Fe20 pct Cr base composition. One particular alloy containing Mo and Ti additions showed some unexpected trends in terms of creep strength and creep ductility during the course of the investigation. It is the purpose of this paper to report and to rationalize some of these trends on the basis of known effects of dynamic recrystallization during creep. 2-5 In low stacking fault energy materials, the recovery rate is relatively slow, and sufficient stored energy may be accumulated during creep to cause nucleation and growth of new grains. 2-5 This phenomenon is termed dynamic recrystallization, and its characteristic effect is to produce a rapid increase in creep rate that may or may not be followed by oscillations, which terminates the otherwise normal stage of primary creep, usually before steady state deformation has been obtained/'6 Creep induced dynamic recrystallization has been observed in pure Pb, Ni, and austenite/ It can also occur in complex solid solutions and precipitation strengthened alloys, e.g., Nimonic 108. 7,8 Oscillating creep curves have also been observed in IN738LC (a turbine blade alloy) at low creep stresses and near service temperatures. 9 Dynamic recrystallization during creep commences only when a critical strain is exceeded which varies for a given material with different conditions of deformation. 2,4 The critical strain (e~) for recrystallization decreases with an increasing stress and increasing temperature 4 (Figure 1). At very high creep stresses, however, the critical strain (e~) increases with an increase in the applied stresses (Figure 1). Addition of impurities also increases the strain to recrystallization for a given temperature and stress level (Figure 1). This effect has generally been attributed to the hindrance of grain boundary migration by impurities. 4 Under hotworking conditions, the addition of solute is said to diminish A . K . K O U L and J-P. A. I M M A R I G E O N are Research Officers with Structures and Materials Laboratory, National Aeronautical Establishment, National Research Council, Montreal Road, Ottawa, ON K 1 A 0R6, Canada. Manuscript submitted July 13, 1984. METALLURGICAL TRANSACTIONS A
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