Mechanical behavior of cast single phase alloys solidified from undercooled melts
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recent study of the mechanical behavior of undercooled AISI 4340 low alloy steel, a definite improvement in fatigue strength, fracture toughness and Charpy V-notch impact strength was established with undercooling. 1 This improvement was attributed t o the combined effect of enhanced alloy homogeneity and sulfide inclusion refinement with undercooling. The work reported herein follows the same investigation pattern for t h r e e other single-phase alloys which were practically sulfur free so that the role of sulfide inclusions and of t h e i r modification with undercooling could be neglected. T h e s e alloys are: nickel30 wt pct copper, iron-25 wt pct nickel and nickel-10 wt pct cobalt. The modification of cast microstructure of the f i r s t two alloys with undercooling was reported previously .2,3 EXPERIMENTAL PROCEDURE S e v e r a l ingots of these alloys (0.0254 m in diam × 0.10 m long) were undercooled by various amounts p r i o r t o nucleation of the solid, following the proc e d u r e described previously.2'S Two s e r i e s of ingots of nickel-30 wt pct copper were prepared. In one s e r i e s , ingots were undercooled 0, 50, 125, 150, 175, 200, and 240 K, and were solidified at an a v e r a g e cooling rate of 1 K/s by adjusting the induction power input at the onset of nucleation at the appropriate level. In another s e r i e s , ingots were undercooled by 45, 100, 180, 200, and 250 K, and were solidified at 5 K/s. Iron-25 wt pct n i c k e l ingots were undercooled 0, 20, 50, 100, 150, 200, and 230 K and were solidified at about 3 K/s. Ingots of nickel-10 wt pct cobalt were undercooled 0, 50, 120, and about 200 K, and were solidified at r a t e s ranging between 2 and 4 K/s. Test bars were machined from a s - c a s t m a t e r i a l and tested in tension, r e v e r s e d bending fatigue and, in some c a s e s , in Charpy V-notch impact. Metallographic specimens were etched with dilute Marble's and Rosenhain's reagents. G. D. MERZ and T . Z. KATrAMIS are Graduate Assistant and Professor, respectively, Department of Metallurgy, Institute of Materials Science, University of Connecticut, Storrs, CT 06268. Manuscript submitted July 19, 1976. METALLURGICAL
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RESULTS AND DISCUSSION Nickel-30 Wt Pct Copper The variation of secondary dendrite a r m spacing w i t h undercooling and cooling rate is illustrated in Fig. 1 up to an undercooling of about 170 K. Beyond this undercooling the dendritic growth is replaced by spherical growth with one sphere corresponding to one grain, and Fig. 1 then illustrates the dependence of g r a i n or sphere diameter on undercooling. For each cooling rate t h e r e is a unique c u r v e r e p r e senting dendrite a r m spacing or g r a i n d i a m e t e r v s undercooling, which is continuous a c r o s s the c r i t i c a l undercooling value of 170 K. This simply indicates that the mechanism which controls the establishment of dendrite a r m spacing for undercoolings lower than 170 K is the same as that which controls g r a i n diameter for undercoolings abo
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