Under the AIM program, AFRL researchers have sought to measure the single-crystal mechanical properties, such as the critical resolved shear stresses and strain hardening rates, of micro- and nanoscale samples extracted from relevantly processed structural alloys (see figure). Scientists are currently developing direct methods to automatically and rapidly characterize both the mechanical response of relevant microstructural elements and the stochastic nature of material property variation to establish the mechanical properties of a material’s representative volume elements (RVE).

It is essential for scientists building continuum models to quickly determine the mechanical properties of RVEs in order to quantify the inherent variability in material properties, the observed variability in experimental measurements, and the uncertainty in predicted properties. They can then establish “confidence metrics” for the data they incorporate into the designer’s knowledge base. Without such confidence, scientists can add new materials (or old materials in new applications) to the knowledge base only after extremely difficult and costly testing.

The new characterization method uses FIB milling to isolate and prepare single-crystal mechanical test specimens from individual grains, or precipitates, of a conventionally processed alloy. Scientists then move the prepared specimens to a conventional nanoindenter device outfitted with a flatpunch indentation tip. The nanoindenter imposes uniaxial compression on the microsamples and records highfidelity load-displacement measurements as the samples deform. With the development of this novel mechanical behavior test capability, researchers now envisage sampling the local mechanical effects of material microstructure and statistically incorporating these results in improved constitutive response surfaces, which could be used in simulations of critical component features.

Dr. Dennis M. Dimiduk, Dr. Michael D. Uchic, and Dr. Peter S. Meltzer (Anteon Corporation), of the Air Force Research Laboratory’s Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document ML-H-04-10.