AFRC_CORE 312: Materials Characterisation by ETMT

The Electrical Thermal Mechanical Testing (ETMT) machine has been used in conjunction with the Digital Image Correlation (DIC) system to form the basis of a testing methodology which sought to better understand the micro-mechanical properties of Inconel 718 at elevated temperatures. It was whilst performing these tests that several undesired problems were discovered which were creating misleading artefacts in the ETMT results, most notably revolving around use of the ETMT with precipitation hardened materials. The main problem discovered by DIC involved strain localisation taking place out with that of the heated zone of test specimens installed in the ETMT resulting in systematic failed tests. This was not known previously when the ETMT was used without the DIC. It was thought that this problem could be fixed by altering the shape of the test samples. As a part of this project a literature review was carried out to form the basis of an investigation into the type of sample geometries used by other users of the ETMT, specifically looking at how the sample geometries could be used with precipitation hardened materials, such as Inconel 718, and to demonstrate the AFRC’s investigation into finding a suitable sample geometry for testing of a precipitation hardened material.
When analysing the stress relaxation curves from the thermo-mechanical study of Inconel 718, it was seen that the standard hyperbolic shape applicable to most stress-relaxation curves was not applicable to Inconel 718 for the temperature range between 620°C and 770°C. An extremely repeatable curve shape was seen however it is yet to be characterised such that a constitutive equation of the stress-relaxation behaviour for Inconel 718 can be established. The curve was dissected into several parts and the influences of strain and cooling rate from 980 to test temperature on the shapes of the various parts of the curve was studied.
Thermophysical properties such as electrical resistivity of Ti-10V-2Fe-3Al (titanium 10-2-3), and Inconel 718 as a function of temperature have been studied using an electrical thermal mechanical testing (ETMT) machine.
A miniature sample geometry has been chosen to evaluate electrical resistivity as a function of temperature under various heating and cooling rates. The obtained results are used to estimate phase transformations and transus temperatures of each phase in both alloys. A good agreement has been achieved between the transus temperature gained by ETMT system and those reported in literature.