Rafting in Superalloys
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Rafting in Superalloys
FRANK R.N. NABARRO
The phenomenon of rafting in superalloys is described, with particular reference to modem superalloys with a high volume fraction of the particulate y' phase. It is shown that in the elastic regime, the thermodynamic driving force for rafting is proportional to the applied stress, to the difference between the lattice parameters of the y matrix and the y' particles, and to the difference of their elastic constants. A qualitative argument gives the sign of this driving force, which agrees with that determined by Pineau for a single isolated particle. Drawing on the work of Pollock and Argon and of Socrate and Parks, it is shown that after a plastic strain of the sample of order 2 • 10 4, the driving force is proportional to the product of the applied stress and the lattice misfit, in agreement with the results of the calculations of Socrate and Parks. The rate of rafting is controlled by the diffusion of alloying elements. Here, the tendency of large atoms to move from regions of high hydrostatic pressure to those of low may outweigh the influence of concentration gradients. The deformation of the sample directly produced by rafting is small, of order 4.5 • 10 -4. The rafted structure is resistant to creep under low stresses at high temperatures. Under most experimental conditions at relatively high stresses, rafting accelerates creep; this effect may be less pronounced at the small strains acceptable under operational conditions. FRANK R.N. NABARRO is Professor Emeritus at the University of Witwatersrand and in the Division of Materials Science and Technology, South African Council for Scientific and Industrial Research. He received his early degrees from Oxford and his D.Sc. degree in metallurgy from Birmingham University. In 1949, he joined A.H. Cottrell in Birmingham and then worked with S.C. Hunter. He moved to the University of Witwatersrand in 1953. His research has concentrated mainly on dislocations, both within materials science and in other fields. He has published the Theory of Crystal Dislocations and has edited nine volumes of Dislocations in Solids. He is a fellow of the Royal Society of London and past president of the Royal Society of South Africa and has three honorary degrees.
The Institute of Metals Lecture was established in 1921, at which time the Institute of Metals Division was the only professional division within the American Institute of Mining and Metallurgical Engineers. It has been given annually since 1922 by distinguished people from this country and abroad. Beginning in 1973 and thereafter, the person selected to deliver the lecture will be known as the "Institute of Metals Division Lecturer and R.F, Mehl Medalist" for that year.
I.
INTRODUCTION
A typical modem superalloy for first stage turbine blades is a single crystal containing almost cubical particles of an ordered phase stacked in a very close approximation to simple cubic packing in a disordered alloy matrix (Figure 1). The ordered phase is called y' and is based on the L12 stru
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