Thin foil electron microscope observations on NiTiCu shape memory alloys
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THE near-equiatomic NiTi alloy is one of the best known materials exhibiting the shape memory effect, in which apparent plastic deformation by several percent of the low temperature martensitic phase can be recovered completely upon heating and reverting to the high temperature austenitic phase. Since the earliest electron microscope studies of such alloys, several unusual observations have been reported, which to some extent have confused the understanding of the mechanism of the memory effect itself. Firstly, several structures of the low temperature martensitic phase have been determined >3 which are not found in X-ray measurements of bulk material. 4~ Secondly, extra spots in selected area electron diffraction patterns of the high temperature/3 phase have been reported to o c c u r . 2'5-9 Several explanations have been proposed to account for these extra spots, such as superlattice formation, 3,5 the occurrence of a rhombohedral transition phase on cooling to temperatures near M,, 3 and the intersection of rel-rods, 64 which may result from low frequency transverse acoustic waves. 7-9 In addition to these extra spots and possibly associated with them, a diffuse streaking between lattice reflections is observed and is usually explained as being a consequence of either rel-rods 7,8 or planes. 8,m Two further unusual effects have been reported in the bright field images of the high temperature ,8 phase. A mottling of the image is seen, especially near the Bragg condition,Sa~ 13and secondly, "ghost" martensite images are observed, s,8,u It has generally been reported that these effects increase in magnitude as the temperature is lowered toward M s. Such observations have led to speculation concerning the relationship between the effects and the martensitic transformation, and it is not clear whether they are prerequisite for this phase change, or if they arise from a totally unrelated cause. R. H. BRICKNELL, formerly Visting Scientist at the Brown Boveri Research Centre, CH5405 Baden, Switzerland, is now at General Electric CR&D, PO Box 8, Schenectady, NY 12301; K. N. MELTON, formerly Research Scientist at the Brown Boveri Research Centre, is now Group Leader at Brown Boveri Central Metallurgical Lab, Dept. ZLM, CH5401 Baden, Switzerland. Manuscript submitted December 12, 1979.
Furthermore, it is not established which of these premartensitic effects arise from changes in the/3 phase, and which, if any, from a phase transformation prior to that to martensite. Some or all of these diffraction effects have also been observed in the fl phase of other alloys systems showing a thermoelastic martensite transformation, again at temperatures approaching M s on cooling, 14-21although the details vary somewhat. However, NiTi seems to be unique in that a change in electrical resistivity is found p r i o r to M s , = 24 with a simultaneous specific heat anomaly, 25 and damping peak. 26 It has recently been shown 27-29that the shape memory effect can be retained in alloys in which up to half the Ni atoms in NiTi are replaced b
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