Microstructure-mechanical property relationship to copper alloys with shape memory during thermomechanical treatments
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NTRODUCTION
SYSTEM Cu-Al-Ni alloys with shape memory effect (SME) arouse great study interest because they present an alternative advantage over the Ni-Ti alloys or Cu-Zn and CuZn-Al alloys for a greater number of industrial applications. The advantages of this system consist of the lower cost, when compared with Ni-Ti alloys. In addition to this fact, the thermoelastic properties are more attractive when compared with Cu-Zn, Cu-Zn-Al, or even NiTi alloys. NiTi alloys are used at maximum temperature intervals between 200 °C and 110 °C, while Cu-Zn and Cu-Zn-Al system alloys are used between 180 °C and 100 °C. If these latter ones are used at temperatures above 100 °C, they do not offer longevity. Cu-Al-Ni system alloys become a potential alternative application both for their lower cost and because they present higher thermoelastic temperatures, which can reach 200 °C. However, they have limitations, as all Cu alloys do with SME such as a variation possibility in the form around 4 pct, against the 8 pct for Ni-Ti alloys; great fragility at room temperature; and inconsistency in the long term of the functional properties. Due to those limitations, we can conclude that thermal cycles that are submitted in service enlarge these characteristics and therefore reduce their working life. The fragility that is peculiar to them is associated to phase transformations that involve metastable transformations of the 1 martensite or 1 phases that are decomposed in 2 and NiAl more stable phases of fragile nature. These transformations can occur at room temperature and, besides checking the fragility of the alloys, modify the temperatures of thermoelastic transformation (Ms and As). These limitations frequently come and can be avoided with the addition of a fourth J.L.L. GAMA, Professor, is with the Mechanical Department, CEFET, Maceió/Alagoas/Brazil 57020-510. C.C. DANTAS, Professor, is with the Nuclear Energy Department, UFPE. N.F. QUADROS, R.A.S. FERREIRA and Y.P. YADAVA, Professors, are with the Mechanical Engineering Department, UFPE. Contact e-mail: [email protected] Manuscript submitted April 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
or fifth chemical element with the specific aim of increasing their structural stability, ductility, and the lengthening of their working lifespans. The most advantageous chemical alloy composition from the thermodynamic point of view is the Cu-12Al-4Ni (wt pct). Lower tenors than the habitual ones 14 pct in Al improve the structural stability that minimizes the effect of the instability that is attributed to it. This instability favors the increase in service temperature, Ms, modifying the parameters of SME. Additives such as Ti, B, V, Zr, Be, and others increase the structural stability, making the maintenance of the functional properties possible for a longer time. The addition of Mn improves the ductility. The process of obtaining these alloys for thermomechanical processing can be laborious, because, even in the fabrication, they can precipitate brittle phases comp
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