Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy
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I.
INTRODUCTION
THE commercial applications of Cu-based shape memory alloys have been restricted because of their poor workability in fabrication and liability to postquench aging effects. In comparison with CuZnAl alloys, CuAlNi alloys have a better thermal stability and a higher operating temperature; hence, the alloys are potential candidates for the high-temperature shape memory alloys which are of practical concern. However, the polycrystalline CuAlNi alloys are severely brittle and liable to intergranular cracking in fabrication due to the large grain size, high elastic anistropy, and presence of brittle g2 (Cu9Al4) phase. Adding some alloying elements such as Mn, Ti, and B to the alloys and altering the content of Al and Ni can significantly improve their ductility and properly modify their operating Consequently, CuAlNiMnTi(B) temperatures.[1–5] pentatomic alloys have been designed and developed. Though the workability and mechanical properties of the alloys are greatly improved, however, some recent investigations have revealed that the thermal stability of the alloys is not as desirable as expected.[3,6–8] In order to solve the problems resulting from the undesired phase metastability and to further improve the performance of the alloys, it is necessary to study the kinetics of the aging process and to clarify the origins of the aging effects in the alloys. The aging effects in ternary CuAlNi as well as CuZnAl shape memory alloys have been extensively studied.[9–16] However, the origins of the complex aging effects in the
alloys are not well understood. It is known that the postquench aging process, either in the state of parent or martensite phase, is a complicated thermally activated diffusional process during which atom rearrangement occurs, assisted by the migration of vacancies. It is believed that the changes in the state of atomic order will play a key part in the aging effects.[13,15] However, as regards the kinetic evolution of the state of atomic order during the aging process, there is scarce literature available. Moreover, there have even been some conflicting interpretations of the changes in the state of atomic order during either parent or martensite phase aging.[10,15] This is, to a great extent, attributed to the difficulty in determining the state of atomic order in the coarse-grained ternary alloys with conventional methods. Though thin film electron diffraction and powder X-ray diffraction (XRD) methods can be employed, the aging behaviors of the thin film or powder samples are not exactly the same as in the bulk samples. Fortunately, the pentatomic alloy makes it feasible to study the kinetic evolutions of the state of atomic order in bulk samples at ambient temperatures, in both the parent and martensite phase state, by using conventional techniques. Most recently, we have studied the reverse transformation sequences during heating in the CuAlNiMnTi alloy.[17] In the present work, the microstructural evolutions and the effects on the performance of the shape memory alloy during is
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