Elastic-plastic characterization of nanocrystalline Al-Mg alloy using nanoindentation

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A nanocrystalline Al-Mg alloy synthesized via cryomilling and consolidated by cold isostatic pressing with subsequent extrusion was subjected to nanoindentation testing. The data collected from these tests was subjected to different data analysis techniques to investigate the capabilities of such techniques in full, accurate elastic-plastic mechanical characterization. A commercially available, coarse-grained sample of this same Al-Mg alloy was also tested to investigate these models' capabilities of distinguishing between the two types of material. Finite element analysis was used as a verification mechanism for the property values extracted from the nanocrystalline material, and initial results showed signs of good accuracy of characterization. Additionally, extensive testing was performed in an effort to investigate how the levels of certain test parameters might affect the resulting values of calculated mechanical properties. The test parameters that were varied during experimentation were maximum applied load, loading/unloading rate, and length of hold period at maximum load. Tests were conducted on specimens cut longitudinal and transverse to the extrusion direction of the bulk sample to investigate any effects anisotropy might play in the characterization process. Results show the levels of the studied parameters can hold effect over the measured values of hardness (H), elastic modulus (E), yield stress (σ_y), and strain-hardening exponent (n). This implies that individual materials may require specific levels of testing parameters if accurate values of mechanical properties are to be measured. Extensive comparison of load-depth behavior indicated that longitudinal specimens exhibit greater material non-homogeneity than transverse specimens as expected trends showed greater variation in the former.

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