Assessment of Pipe Coupling by Using the Recovery of Stress-Induced Martensites in Superelastic Cu-11.8Al-0.6Be-0.5Nb Alloy

Superelastic Cu-11.8wt.%Al-0.6wt.%Be-0.5wt.%Nb shape memory alloy was tested under loading and unloading tensile tests at different temperatures and different strain rates of 5 × 10⁻⁴ s⁻¹ and 1 × 10⁻⁵ s⁻¹. The results of tests performed at 243 and 298 K demonstrate that this Cu-Al-Be alloy exhibits an appreciable amount of residual strain at room temperature, reaching strain levels as high as high as 4% when tested at 243 K for an imposed deformation of 7%. The residual stress associated with retained martensite is reverted by heating at the new austenite start (A _s) temperature, which is 60 °C higher than that of the initial alloy. In addition, with increasing temperature, the critical stress for the austenite → martensite transformation increases. Tube joining experiments show that the reversion of retained martensite is very effective to make pipe coupler fittings. The reversion of stress-induced martensite results in a coupling with a pull-out force equivalent to that obtainable by conventional shape memory effect.