Effect of Grain Size on the Mechanical Properties of Multi-Phase Alloys

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The goal of this research is to understand the influence of second-phase particles on the mechanical behavior of selected metals and alloys as their grain sizes are reduced to the smallest nanosized scales that can be achieved with processing methods that produce bulk, artifact-free metals. The scientific approach is as follows: (1) introduce second-phase particles with controlled size, spacing and volume fraction into selected fcc, bcc and hcp metal matrices; (2) reduce the grain size of the matrix to the order of 100- 200 nm and the parameters that define the particle distributions by suitable processing and heat-treatment methods; (3) characterize the grain size distributions and particle distributions using electron microscopy, scanning-probe microscopy, x-ray diffraction and thermal analysis; (4) compare mechanical behavior of multi-phase alloys with matrix metals to reveal effects of second phases on the mechanical properties as the grain size is reduced to nanometer size scales; (5) develop test methods suited for the available sample sizes to optimize the mechanical testing effort. Hardness, automated ball-indentation and miniaturized disk-bend testing are employed to provide an initial screening. Tension and compression testing will be added for selected ranges of the test conditions and microstructural parameters. Task (6) involves analysis and modeling using phenomenological and model-based analyses appropriate for the identification of the underlying deformation mechanisms. A micro-mechanical model for stress-strain behavior mechanisms as a function of grain size should aid in defining strategies for optimizing the properties and performance of this class of alloys. To optimize their mechanical properties and performance, structural metals used for engineering applications are predominately multi-phase alloys rather than pure metals. It is expected that multi-phase alloys will provide these same benefits in nanocrystalline form. The properties of this class of metals and alloys have not been documented by systematic mechanical testing, and the deformation mechanisms as a function of grain size are not understood with any degree of confidence.