Microscale mechanical experiments at elevated temperatures: System development and material characterization
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Date
2016-01-01
Type of Work
Department
Mechanical Engineering
Program
Engineering, Mechanical
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This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
Distribution Rights granted to UMBC by the author.
Distribution Rights granted to UMBC by the author.
Abstract
Mechanical testing and characterization across length scales provides many unique insights into material behaviors that are not evident by only observing the macro-scale responses. The microscale testing technique has been applied to many material processing methods, such as surface treated materials, thin foils, and welds. This local characterization serves to provide information on the material behavior to further understanding of the process. Microscale evaluation of materials across different test conditions is still in its infancy. In this work, the design, development and validation of an elevated temperature microsample testing system is presented. Microsamples in this work have a footprint of 3 mm x 1 mm and a cross sectional gage area of 250 µm x 250 µm. Experiments were conducted across temperatures from 25°C to 600°C. Methods for digital image correlation were developed to measure strain of the microsamples across temperatures. The system was found to successfully measure material properties including the coefficient of thermal expansion, elastic modulus, yield strength, ultimate tensile strength and strain to failure across temperatures. At these temperatures, the widely used ?+? alloy, Ti-6Al-4V was used as a reference material to validate the developed system. An oxidation analysis was conducted to verify the ability of the system to measure unoxidized material behavior. Mechanical tests across temperatures showed agreement with macroscale literature responses of the same alloy. Ti-5111 (5Al-1SN-1V-1Zr-.8Mo), a near-? titanium alloy of interest to the U.S. Navy was also investigated at temperatures between 25°C and 600°C. The microsamples were extracted from a friction stir weld of Ti-5111, which enabled the examination of two grain morphologies: the unaffected base metal, and the weld region. At room temperature, the mechanical responses of base metal Ti-5111 was found to be strongly affected by the length scales of the microstructural features within the sample gage length. This was shown to be due to the sizes of colonies within the samples. This led to reduced ductility and a change in failure mechanism. This highlighted the importance of understanding the three dimensional grain size and morphology when performing microscale tension experiments. The stir zone showed less variation in material properties due to the refined equiaxed microstructure of the weld. Validation of the developed system showed that material properties including coefficient of thermal expansion, elastic modulus, yield strength, ultimate tensile strength and strain to failure could successfully be measured at elevated temperatures on the micro scale. Elevated temperature examination of base metal Ti-5111 showed a softening affect resulting in reduced strength and increased ductility. A plateau in strength between 400°C and 600°C was observed corresponding to the material's transition to an athermal regime below diffusion temperatures. The refined microstructures of Ti-6Al-4V and FSW Ti-5111 showed reduced strength at elevated temperatures, but the strain to failure was limited due to void growth leading to rupture.