The structure of NiTiCu shape memory alloys
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THE
occurrence of the shape memory effect (SME) in equiatomic and near equiatomic fl-NiTi alloys has been widely recognized and investigated.~-~ It has been shown that this effect is displayed in alloys undergoing a martensitic transformation and that it is associated with deformation either of the low-temperature phase, or at a temperature w h e r e this can be s t r e s s induced.6-9 In e i t h e r case shape memory is shown on heating when reversion t o the parent austenitic phase occurs, which is well established as being the o r d e r e d bcc, C s C 1 , structure./'~°-~2 The effect of ternary alloying elements has been little explored in t e r m s e i t h e r of t h e i r effect on the characteristics of any SME displayed, or on the high or low temperature phase structures. In the only reported studies ~s,~4 the effect of substitutions of up t o 2 a t . pct of Ni or T i by Co, Fe, A1, Au, Mn and Z r was explored, and it was demonstrated that SME was retained. However it has recently been shown~-~7 that substitution of Ni by up to 35 wt pct Cu l e a d s to martensitic alloys which still exhibit the SME. When d e a l ing with such l a r g e compositional deviations from the studied binary it cannot be assumed that the structures of the phases and the details of the transformation remain constant. Consequently the present study was undertaken t o investigate and delineate the structural parameters of both high and low temperature phases as a function of copper content, by a combination of electron microscopy and X - r a y diffraction m e a s u r e ments. 2. EXPERIMENTAL A r g o n a r c melted buttons of the alloy compositions shown in T a b l e I were vacuum induction remelted in a graphite crucible to form rods 8 mm diameter. T h e s e were canned in stainless steel, hot swaged (800°C) t o 5 m m d l a m , when the canning was r e m o v e d R. H. BRICKNELL, formerly Visiting Scientist at Brown Boveri ResearchCenter o n leave of absence from Cambridge University,
England, is now at General Electric Corporate Researchand Development, Schenectady, NY 12301,and K. N. MELTON and O. MERCIER are Research Scientists at the Brown Boveri Research Center, CH-5 401 B a d e n , Switzerland. Manuscript submitted August 10, 1978. METALLURGICAL
TRANSACTIONS A
before final hot swaging t o 3 mm d i a m . Sample preparation was completed by a 1 h anneal in a r g o n at 950°C and air cooling. D i s c s suitable for electron microscopy were prepared u s i n g a jet polishing technique from sections cut perpendicular t o the rod axis with a diamond saw. For the X - r a y diffraction s t u d y , d i s c s were machined directly from the a s - c a s t alloys in o r d e r to minimize texture effects. T h e s e d i s c s were ground and finally electropolished p r i o r to examination at room temperature or 155°C on a Phillips Diffractom e t e r u s i n g CuK~ radiation. The temperatures of the s t a r t of the martensite transformation (M s ) for each of the alloys used is also shown in T a b l e I, and was determined by an AC electr
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