Ultra-high mechanical damping during superelastic bending of micro pillars and micro beams in Cu-Al-Ni shape memory alloys

Abstract

Mechanical spectroscopy techniques and internal friction measurements have historically offered a key contribution to the understanding of fundamental mechanisms of defects mobility and phase transformations in many kinds of materials. Nowadays, at the beginning of the millennium, the apparition of the new paradigm of nanotechnology reveals new unforeseen properties of the materials at the nano-scale. Nevertheless, instrumented nanoindentation appears as a new mechanical spectroscopy technique to approach the measure of internal friction at the nano-scale through the direct evaluation of the loss factor η. Indeed, nano compression tests on micro and nano pillars recently measured ultra-high mechanical damping in several Cu-based shape memory alloys (SMA). A step forward is given in the present work, approaching the design of potential micro dampers of SMA working in bending and torsion modes. Several micro devices were milled by focused ion beam (FIB) technique, on [001] oriented single crystals of a Cu-Al-Ni SMA. Ultra-high mechanical damping is demonstrated during superelastic bending of square section micro pillars by in-situ testing inside the scanning electron microscope, using a flat apex 60º-conical indenter in off-axis mode. Then, a series of micro beams and micro cantilevers were designed and milled by FIB, and tested using instrumented nanoindentation equipment by applying the load in bending mode as well as in a mixed bending-torsion mode during off-axis tests. In all cases, the internal friction, or mechanical damping per cycle and per unit of volume, was calculated from the measured load-displacement curve. Superelastic cycling during bending of micro pillars, micro beams and micro cantilevers, exhibits stable and reproducible ultra-high mechanical damping with a loss factor η > 0.1. These results paved the road for designing micro-dampers of Cu-based SMA, able to work in different mechanical modes of compression, bending and torsion, opening the way for many applications of mechanical damping at the nano-scale.