Please consider attending the following: MATERIALS ENGINEERING SEMINAR "Analysis of Sample Size Effects on Strength of Metals and Alloys Through Micropillar Compression Testing" By Indrani Biswas Purdue MSE Ph.D. Preliminary Exam Advisor: Professor Kevin P. Trumble ABSTRACT Yield strength of metals and alloys has long been known as a material property with little or no dependence on specimen size. However, through micropillar compression testing (MCT) it was revealed that, strength of micron-sized single crystal pillars was much larger than of bulk specimens and scaled with the pillar diameter. The paper analyses the theory of dislocation starvation; which assumes increased dislocation annihilation from sample surface as the reason for such high strengths. It was found that the model predicts near zero dislocation densities shortly after the onset of plasticity which is unrealistic. Polycrystals under MCT showed unexpected weakening when the sample size was few times the grain diameter. Two competing theories for weakening have been compared. The first attributes it to occurrence of grain boundary sliding and the second assumes that surface grains provide lower resistance to dislocation motion than those in the interior. Higher fraction of surface to bulk grains in smaller samples is probably responsible for the loss in strength. The analysis presented was that since surface grains are under less constraint, they are also more prone to sliding. So, the weakening can be a combined effect of the two mechanisms. Simulation of polycrystal deformation with different surface to volume ratio was suggested to validate the hypothesis. Date: Monday, December 4, 2017 Time: 12:30 P.M. Place: ARMS 3115