A critical test of two theories of non-IPS geometric surface relief effects associated with diffusional phase transforma
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" t e n t " or " v e e : ' or " r o o f g a b l e , " either protruding above or intruding below the plane of polish, M o r e complex f o r m s , still associated with a single c r y s t a l of precipitate, a r e readily found, some having the app e a r a n c e of miniature mountain ranges. ~ In GSR's of the second type, however, the b a s i c "unit of construct i o n " r e m a i n s the plane, though it is possible that as a consequence of complexities in their origin some curved connecting surfaces may also be p r e s e n t . Restricting our attention henceforth to the second types of GSR, and emphasizing the tent-shaped reliefs, t h e r e a r e p r e s e n t l y two different theories of t h e i r o r i ~ m One i s that the p r e s e n c e of p a r t i a l o r full coheremLv a c r o s s the broad f a c e s of precipitate plates (or other reproducibly anisotropic morphologies such as laths and rods) constrains them to undergo a local shape s t r a i n s i m i l a r to that of maxtensite plates with the s a m e crystallography,* even though the atomic *Mar~ens:iewodeces ~ent-shapedrthe~s only wkefl two plates wILt different shear~fo~n bacbto~back
A Critical Test of Two Theories of Non-I PS Geometric Surface Retief Effects Associated with Diffusionat Phase Transformations K. R. KINS~IANAND H, L AARONSON T h e r e a r e two basically different types of r e l i e f effect produced at an initially planar f r e e s u r f a c e by phase t r a n s f o r m a t i o n s . One is the noncrystatlographic rumpling of the s u r f a c e restflting principally f r o m the difference in atomic volume of the m a t r i x and p r e cipitate phases. ~ The other is composed p r e d o m i nantly of one or m o r e planar surfaces and has been t e r m e d genericaIIy " g e o m e t r i c s u r f a c e r e l i e f " (GSR).~ T h e s i m p l e s t and best understood variant of the GSR is the In:mri~mt plane strain (IPS) type~ formed by the undistorted rotation of the t r a n s f o r m e d a r e a of the planar surface about an axis lying within the surface, x'a A single plane thus c o m p r i s e s that portion of this type of C~R which lies within the plate-shaped p r e c i p i t a t e . The IPS relief effect is customarily associated with ma~tensite plates, but has also been found to a c c o m pany transforms*ions, even in substitutional alloys, in which all of the crystallographic r e q u i r e m e n t s of the phenomenologicai theory of m a r t e n s i t e a r e fulfilled but in which the composition of the precipitate differs f r o m that of the m a t r i x even during the e a r l y s t a g e s of growth. T h e s e t r a n s f o r m a t i o n s inctuxle the simple fCC {a)-to-hcp (7') reaction in AI-Ag~ as well as the m o r e complex bee (/~)-to-fccorthorhomobic (al) reaction in Cu-Zn~. The second type of GSR includes all relief morphologies composed of m o r e than one plane and produced by a single plate~ needle or lath. This f o r m of GSR is associated, to the b e s t of our knowledge, only with diffusional phase t r a n s f o r m a
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