NOVEL TECHNIQUES TO ASSESS ENVIRONMENTALLY-ASSISTED CRACKING IN A NICKEL-BASED SUPERALLOY

A number of drawbacks are associated with conventional mechanical testing methodologies for nickel-based superalloys: (i) a substantial amount of material is needed, which exacerbates cost, (ii) thus for alloy development purposes - when the quantity of material is often limited - it may not be possible to do the necessary range of tests required to assess fully the properties, (iii) it does not always provide sufficient insight into the mechanisms occurring locally, especially on the length-scales of the grains. Here, an electro-thermal-mechanical testing (ETMT) system is employed between room temperature and 900 degrees C which circumvents some of these difficulties. Miniature test specimens of small cross-section of 1 x 1 mm(2) are employed; this allows for rapid mechanical property assessment from small quantities of material. It is demonstrated that time-dependent damage mechanisms in a polycrystalline nickel-base superalloy can be studied. Slow strain rate testing at high temperature in both oxidizing and inert environments is used to assess the susceptibility of the alloy to environmentally-assisted cracking. Micro-mechanical tests using micro-cantilever bend tests - are conducted at both virgin grain boundaries and at oxidized secondary crack-tips, thus providing insight into the mechanisms of fracture during environmentally-assisted cracking. These novel methods for mechanically assessment - combined with high resolution characterization techniques - allow the susceptibility of a nickel-based superalloy to environmentally-assisted cracking to be assessed.