Seminar - Mechanical Behaviour and Testing at Small Length Scales: from Plasticity in Silicon to Brittleness in Copper
By Prof. Vikram Jayaram from the Department of Materials Engineering, Indian Institute of Science, Bangalore
Thursday 8th June 2017, from 12pm-1pm
The strength of engineering systems in the range of ~ 1 micron to 1 mm offers particular challenges in testing and in understanding materials behaviour. Such systems may arise in electronic devices in the form of thin films or MEMS, as protective coatings on high temperature materials or on graded microstructures generated in service through tribological or radiation damage. In recent times, a number of testing methods have been introduced that combine one or more of micromechanical handling, in-situ observation and full field deformation measurements. Such techniques have allowed the detection of behaviour as diverse as plastic flow in small samples of silicon, brittle behaviour in copper, steep R-curves in a high pressure form of silica that undergoes transformation toughening and so on. After a brief review, this talk will focus on 3 examples of work on metallurgical systems in our laboratory. In the first, we describe the use of a new fracture geometry that is stable even in load control to measure the effect of platinum on the toughness of 100 micron thick graded beta NiAl bond coats used on superalloys in aero-engines. This test technique is then adapted to examine fatigue crack growth where small changes in stiffness are the prelude to a sudden jump in crack length. In the final example of small scale testing, it is shown how small cantilevers of aluminium can be crept under bending to allow the determination of the usual creep parameters. This method reduces the volume of material needed for residual life prediction and reveals interesting contrasts with uniaxial behaviour in the primary and tertiary stages.