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Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (σ) and ER changes as a function of the strain (ε). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy.

The microstructural evolution of the CuZnAl shape memory alloys was studied by indirect techniques relating to the atomic migration rate of grain boundaries. Addition elements were used in a Cu-15,5Zn-8,0Al alloy to provide a comparison with the same alloy without microelement additions. The alloys were melted in an induction furnace of 24 kVA. After casting, the bulk samples of the alloys were homogenized. Then they were solution treated and hot-rolled followed by water-quenching to initiate the recrystallization. Finally, annealing produced at different temperature ranges was made in different samples in order to establish a law for the grain growth. Following the heat treatments, all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the same empirical law of grain growth was found for the different alloys and the ln [D-Do] x 1/T diagrams were plotted in order to establish the kinetic behavior. Based on the estimated values of the activation energy, important conclusions were obtained concerning the addition elements. Keywords: microstructural evolution of the CuZnAl, effect of B and Fe on the CuZnAl alloys, kinetics and morphology of CuZnAl. alloys IntrodutionThe alloys of the CuZnAl system with shape memory effect present two problems that hinder their employment on an industrial scale: these are the natural ageing and the grain growth observed during thermomechanical processing. The first degrades the shape memory effect, while the second, observed during thermomechanical processing of the alloy, displaces the temperatures where the thermoelastic transformations are observed. The extent of the ageing and grain growth can be reduced by control of the rate of atomic diffusion of the solute elements from the matrix to the grain boundaries. This control can be obtained indirectly by increase of the activation energy of diffusion, which can be produced through the addition of microelements in the alloy.After academic euphoria concerning the phenomenological aspects of the shape memory effect, researche on the physical metallurgy turned over to the microstructural control of the alloy, in an attempt to eliminating such typical problems as ageing and grain growth. The traditional method for the stabilization of the microstructures was the addition of alloy microelements. For shape memory copper-based alloys, several microelements have been used. Wang. et al.1 verified the influence of zirconium, titanium, boron and iron on the refinement and the stabilization of grains in an alloy of the CuZnAl system. Morris et al. 2,3 verified the influence of simultaneous use of manganese and boron on the thermoelastic effects and the mechanical properties of a CuAlNi system.For any copper alloy system, the problem is the same: determining the amount required of the microelements to be added to the alloy system, while maintaining the ability for plastic forming required for the manufacturing process. In this work, therefore, the effects of the addition of B, Fe ...

The phase transformations during fabrication and aging after cold deformation in three polychrystalline copper alloys of the Cu-Al-Ni system with shape memory effect (SME) were characterized. Some phase transformations were identified with clear repercussion in their mechanical properties during thermomechanical treatments. Around 430 °C, mutual effects of ␤-phase recrystallization and precipitation of ␥ 2 and NiAl phases were observed. Close to 600 °C the dissolution of phase ␣ was observed, beginning transformation into ␤ phase process. Brittle phases such as ␥ 2 and NiAl began to precipitate during a short exposure time at 380 °C, 585 °C, 600 °C, and 700 °C temperatures. The phase transformations were intensified due to the plastic deformation that acted as a driving force for the diffusion processes. The introduction of chemical elements inhibited the grain growth and increased the structural disorder generating an elevation in the hardness property.