Advanced Shape Memory Alloys Processed by Powder Metallurgy** By Rau Âl B. PØrez-Sµez, Vicente Recarte, María L. Nó, Oscar A. Ruano, and Jose  San Juan* In recent years there has been an increasing attention to the development and research of so-called ªsmart materialsº. These new kind of materials appeared as a result of the syn- thesis of structural and functional materials combined with the integration of a control mechanism. [1] Some of the most in- teresting materials in this class are shape memory alloys (SMA). This is due to the unique and special functional prop- erties exhibited by SMA: one-way shape memory effect, two- way shape memory effect, constraint recovery thereby creating a high (recovery) force, pseudoelasticity, and high damping capacity. [2] Several SMA families can be found, such as Ni±Ti, Cu±Zn±Al, Cu±Al±Ni, Fe±Mn±Si,... The properties of these alloys can be changed and controlled by means of the thermomechanical processing of the material, so that they ex- hibit the desired properties. Nowadays, Ti±Ni and Cu±Zn±Al are the most used families in practical applications. However, they cannot be used for high temperature applications (above 100 C). [3] For that reason there has lately been an increasing effort in the development of alloys overcoming these limita- tions. Cu±Al±Ni are outstanding members of the SMA family, not only due to the fact that they show as good shape memo- ry properties as Cu±Zn±Al alloys, but also because they are thermally more stable in high temperature ranges. [3,4] Never- theless, some properties of Cu±Al±Ni alloys require further elucidation to enhance their suitability for practical applica- tion. Among these improveable properties the low ductility stands out, in some cases intergranular fracture can occur when the deformation is below 1 %. Stress concentrations in the grain boundaries seem to be the cause of intergranular fractures in these Cu±Al±Ni alloys. It has been put forward that this behavior depends on four different parameters at least: [5,6] a) the large elastic anisotropy, b) the large grain size, c) the large orientation dependence of the transformation strain, and d) the grain boundary segregation. In order to suppress the intergranular fracture and to im- prove the ductility of these alloys, the stress concentration in the grain boundaries must be controlledÐeither by develop- ment of high-textured alloys or development of fine grain al- loys. [5] The high texture enables the accommodation of stress among adjacent grains and, as a consequence, the stress con- centration in the grain boundaries decreases. Alternatively, the stress concentration decrease is due to the grain-size re- finement, which produces smaller stress among adjacent grains. Several techniques are suitable for producing the grain-size refinement: addition of other elements in small quantities, [7] melt spinning, [8±10] and powder metallurgy. [11,12] Finally, powder metallurgy is a relatively new technique in this area, and no attempts to use it in the development of this kind of alloys had been made, in spite of its promising capabilities. [14,15] Thus, taking into account these facts, a new production process of Cu±Al±Ni SMA by powder metallurgy is developed in this work. The first part of the present paper contains a detailed description of each stage of the process, whereas the second one is devoted to the characterization of thermomechanical and fracture properties of the obtained materials. Finally, a comparison of these properties with those of single crystals and polycrystals of the same kind of alloys but obtained by classical methods, is also performed. The process used in the present work is shown schemati- cally in Figure 1. It starts with the pre-alloying of the initial products: 99.99 % Cu, 99.99 % Al, and 99.97 % Ni in an Argon atmosphere. The alloy composition is chosen taking into ac- count how it affects the kind of martensite obtained. [16] In this particular case a composition of Cu±13.1 Al±3.16 Ni (wt.-%) was selected in order to obtain b¢ martensite, which is the most suitable one for achieving the best memory effect prop- erties. The pre-alloyed melt is atomized by Argon at a pres- sure of 2.3 MPa, in this case, a vertical atomizer LEYBOLD VIGA 2S has been used. [17] Once the powder has been ob- tained, the compaction is carried out by hot isostatic pressing (HIP). A pressure of 140 MPa at 850 C was applied during 2 h in an ABB Autoclave systems Inc. QIH-3 device. After the compaction, the product from HIP was cut in 5 mm thick plates and hot-rolled until 0.8 mm in several COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2000, 2, No. 1±2 49 ± [*] Prof. J. San Juan, Dr. R. B. PØrez-Sµez, Dr. V. Recarte Departamento Física de la Materia Condensada Facultad de Ciencias, Universidad del País Vasco Apdo. 644-48080 Bilbao (Spain) Prof. M. L. Nó Departamento Física Aplicada II Facultad de Ciencias, Universidad del País Vasco Apdo. 644-48080 Bilbao (Spain) Prof. O. A. Ruano Centro Nacional de Investigaciones Metalrgicas Gregorio del Amo 8 28040 Madrid (Spain) [**] This work has been carried out with the financial support of the Spanish ªComisión Interministerial de Ciencia y Tecnologíaº (CICYT) in the framework of the ªPlan Nacional de Materia- lesº (Project number MAT 92-0353) and the ªUniversidad del País Vascoº (Projects number UPV EB049/95 and EA 127/ 97). The authors would like to express their gratitude to Prof. M. Torralba, Dr. M. Lieblich, and Dr. G. Caruana of the ªCen- tro Nacional de Investigaciones Metalrgicasº for their co- operation in the elaboration of the samples. 1438-1656/00/0102-0049 $ 17.50+.50/0