Structure and initial precipitation in a rapidly solidified nickel superalloy
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1. INTRODUCTION
Much
enthusiasm has recently been generated by the announcement that significant improvements in high temperature mechanical properties can be achieved by the manufacture of components through consolidation of rapidly solidified superalloy powd e r s .l'e It has also been reported that the l a r g e s c a l e production of such powders is now possible using a helium gas atomization technique,1 which g i v e s a quench rate between 104 and 106 K. s-1. Previously, s i m i l a r quench r a t e s had been obtained in the production of l a r g e , i r r e g u l a r powders u s i n g the melt extraction process. 3 However, the quotation of a definite quench rate can be misleading, as shown by r e c e n t work relating process variables and gaseous atmosphere effects t o the quench rate of steels and superalloys a f t e r melt extraction and melt spinning.4 Whilst the quench rate from above the liquidus to about 900°C can be in the region of 105 K "s-1 on the rotating solid substrate, subsequent quench r a t e s t o ambient temperature depend entirely on atmosphere and can be as low as 102 K- s-1 in vacuum (for w i r e s of 30 ~m diam). A s i m i l a r reduction in quench rate may be present in gas atomization when the powder has left the r e g i o n of high velocity gas flow. This variation in the solid s t a t e cooling rate may influence the a g i n g response of rapidly solidified alloys. The subsequent processing of these alloys, typically by hot isostatic p r e s s i n g or hot canning extrusion, will also markedly change the final microstructure. It is necessary therefore t o determine whether t h e r e are any fundamental differences in the a g i n g characteristics of rapidly quenched superalloys and those produced by conventional m e a n s . In general, t h e r e are t h r e e main effects of rapid solidification which may influence a g i n g behavior: i) the development of a s m a l l g r a i n size (~ 1 tzm), J. V. WOODis Lecturer, Department of Materials, The Open University, WaltonHall, Milton Keynes, P. F. MILLS is Research Student, University of Cambridge, J. K. BINGHAM is Scientist, General Electricity Company, and J. V. BEE is Research Fellow, Department of Metallurgy and Materials Science, University ofCambridge, Cambridge, England. Manuscript submitted September 11, 1978. METALLURGICAL TRANSACTIONS A
ii) the reduction in s c a l e and quantity of solute segregation, and iii) an i n c r e a s e in the e x c e s s vacancy supersaturation. These effects are found throughout the material, even in regions which display markedly different solidification structures. T h e s e structures, observed in alloys b a s e d on the F e - N i - C r fcc system, have been described previously.~,6 T h r e e distinct m i c r o structures were identified: a) nonsegregated, homogeneous grains, b) a low a n g l e cellular* structure, and *The terminology relatingto solidification structures conflicts with similar terms pertaining tosolid state transformations. Noattempt has been made here to resolve this conf
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