Influence of aging on transformation characteristics in shape memory CuZnAl alloys
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I. INTRODUCTION
THE martensitic transformation is a diffusionless phase transition in the solid state in which the atom distribution of the martensitic lattice is inherited from that of the austenite phase prior to the transformation.[1] The thermoelastic martensitic transformations are responsible for the shape change obtained in the shape memory alloys (SMAs). By a special training process in these alloys, it is possible to obtain a macroscopic shape change repeatedly during the martensitic transformation and reverse transformation on cooling and heating. In this respect, two-way shape-memory effect (TWME) refers to the reversible, spontaneous shape change from a hot austenitic shape to a cold martensitic shape. The TWME is only obtained after particular thermomechanical treatments (training), which generally consist of thermomechanical cycling through the transformation region in order to “memorize” the martensitic shape.[2] In the noble-metal alloys, the high-temperature-matrix phase is disordered bcc (A2) at elevated temperatures. The bcc phase undergoes two ordering transitions upon cooling. The first transition occuring on cooling is a first-nearestneighbor (nn) ordering reaction which results in a B2 (CsCl type) superlattice, designated as b2. Further cooling induces the next-nearest-neighbor (nnn) ordering and the crystal structure has been suggested to become DO3 (Fe3Al type) superlattice, designated as b1, or L21 (Cu2MnAl type) superlattice, designated as b3.[3,4] However, Wu et al.[5,6] have proposed that the perfect DO3 superlattice with binary superlattice sites would not be expected in the off-stoichiometric CuZnAl alloy, and the resulting structure due to the nnn ordering could be L21 superlattice. The transformation from disordered A2 to B2 parent phase occurs during the process NEJDET KAYALI, Assistant ¨ Professor, RAS¸IT ZENGIN, Research Assistant, and OSMAN ADIGUZEL, Professor, are with the Department of Physics, Firat University, 23169 Elazig, Turkey. Manuscript submitted November 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
of quenching in water,[7] whereas the transition from B2 to L21 can be suppressed by water quenching after solution treatment.[8] The degree of ordering can therefore be changed by prolonged aging in parent or martensite phase. It has been reported that this change in the ordering degree due to aging leads to the variation of martensite structure.[9,10] One of the primary requirements of any SMA to be used in temperature controlled devices is that the shape-recovery temperature should lie within the prescribed limits and remain unaltered under service conditions. The shape recovery observed in these alloys is associated with the reverse transformation of the martensite to the parent phase. However, it is well known that Cu-based shape-memory alloys are susceptible to postquench aging and cycling effects. These effects in ternary CuZnAl alloys have been extensively investigated.[10–14] But, both the results and the interpretations of the aging effects remain controver
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