Lattice transformations related to unique mechanical effects
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have T H been numerous E Rreports ofEalloys possessing unique mechanical effects involving shape recovery,
has been reported in such systems as Cu-AI-Ni, 6 NiA1,~ and Cu-Zn, 8 as well as in the much publicized Nithe best known being the "memory" or "shape-memory" tinol alloys (TiNi), and informally alluded to for varieffect in near-equiatomic TiNi alloys. In this case, and ous other systems. Historically, as early as 1932, in the case of other systems and kindred effects, the be- Olander9 noted the striking rubberlike behavior of Auhavior has been related to martensitic transformation, rich Au-Cd martensite, which was followed in the 1950's and the effects will be referred to here generally as by similar initial observations on In-T1,2 Au-Cd,3 Cushape memory effects (SME). It is a general feature of Zn, I~ and Cu-A1-Ni.4 In several Cu-Zn-X ternaries, ordered bcc (B2 or fl') phases to transform martensitreversible pseudoelastic strains of as much as 15 pct can be achieved. ~ Therefore, the plastic deformation ically during cooling, and for this transformation to be recovery behavior of TiNi, discovered in the early quite sensitive to strain. In some systems, such as 1960's at NOL, and often described as "unique," is acthose based on the Cu-Zn/3' phase, the martensitic tually a type of phenomenon which is much more genphase may form thermoelastically, during which proeral than originally or recently suspected. However, cess the martensite plates grow as the temperature is the alloy systems involved in SME behavior are apparlowered, and shrink back upon heating, with little or no ently diverse and in many cases less common. It will hysteresis,x be attempted to present the common threads linking In other systems, a similar form of martensite debehavior in such systems, and to compare with marvelops and grows during continuous application of tensitic alloys which do not exhibit SME. stress, and disappears when the load is removed. This reversible "elastic" (because no permanent strain is involved) martensite results in so-called "rubber-like" THERMOELASTICITY, SUPERELASTICITY, properties, and the phenomenon has been termed AND SHAPE MEMORY "superelasticity" or "pseudoelasticity." Superelastic The occurrence of the various types of SME are inbehavior is known to occur in such martensitic/3' systerrelated, in that several alloy systems exhibit more tems as In-T1,z Au-Cd,3 Cu-AI-Ni, 4 Cu-Zn-Si, and Cuthan one of the effects, depending especially on compoZn-Sn.5 sition and thermomechanical history. For example, in The memory effect is much like the superelastic effect except that upon release of stress the deformation- several systems, on cooling, a thermoelastic martensitic reaction ("slow growth" martensite) is followed induced marteiasite does not revert until activated by by a burst-type martensitic reaction which can be r e heating above some reversion temperature. Such r e lated to superelastic behavior,s Other systems may version, however, carries with it a return to the original (predeformation) shape, provided tha
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