Available at: http://digitalcommons.calpoly.edu/theses/1196
Date of Award
MS in Engineering - Materials Engineering
When testing forgings for specifications, it was found that some parts did not meet the requirements for mechanical properties. This triggered an investigation into two of the parts from the lot that did not meet specification. The ultimate reason for failure was due to lower than necessary yield strength and ultimate tensile strength values, as well as unwanted variability between regions of the part. Therefore, samples of the regions were tensile tested to determine the differences that existed in yield strength, ultimate tensile strength, and elongation. After tensile testing, quantitative metallography and fractography were conducted to identify aspects of the microstructure and fracture surfaces that may have caused the variability. Three aspects of the microstructure that were identified as characteristics that may affect the mechanical properties were: grain size, Widmanstätten colony size, and volume fraction of the β phase. Based on measurements it was determined that a smaller Widmanstätten colony size found to be roughly 120 microns/colony was associated with a larger yield strength and UTS than larger colony sizes of roughly 170 microns/ colony. Grain size also played a role with smaller grain sizes of roughly 1550 microns/grain being associated with a higher yield strength and UTS than the larger grains of roughly 2000 microns/grain. Fractography also suggested that the presence of interlamellar decohesion and trans-lamellar failure may have created sites of further crack initiation, resulting in a lower ultimate tensile strength. These differences were theorized to be caused by a temperature gradient created during the heat treatment that created non-uniform cooling rates, resulting in the differences in microstructural characteristics.
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