Date: Thursday, Nov 10th
Presenter: Materials Science, Oregon State University
Halsey Ostergaard - Fatigue and Creep Crack Growth Rates and Mechanism Assessment in Wrought and Additively Manufactured Alloy 718
Alloy 718 is a Nickel-Iron-Chrome alloy used extensively in turbine systems at service temperatures up to ~650C. At these high service temperatures, there is a strong interaction between fatigue processes, creep processes, and environmental processes. The large stress concentration present ahead of a crack tip enables fast diffusion of oxygen ahead of the crack, particularly along grain boundaries, which are progressively oxidized, embrittled, then cracked. The stress concentration accelerates creep deformation ahead of the crack as well. Evidence is shown for this environmental attack, called Stress Assisted Grain Boundary Oxidation (SAGBO), being the dominant fatigue and creep crack advance mechanism at high temperature and low load frequencies in wrought material.
Additive alloy 718 parts produced by the Direct Metal Laser Sintering process are now routinely reaching near-wrought tensile properties, but possess a unique epitaxial microstructure and build defects that interact with fatigue processes, often leading to much worse fatigue properties compared to wrought material. Preliminary fatigue crack growth characterization has been completed, and further experiments are planned to assess the interaction of microstructure and additive manufacturing defects with the high temperature crack growth mechanisms identified in wrought material.
Halsey Ostergaard recently completed a MS in Mechanical Engineering from OSU, focusing on blade cutting as a fracture mechanics process. He has recently been accepted into PhD program at the University of New South Wales in Sydney, Australia, where he plans to continue his current work researching fatigue in additively manufactured metals.
Austin Fox - Crystallographic texture in Bi-based piezoelectric thin films
Bi-based and other Pb-free piezoelectrics remain an area of interest in both bulk and thin film embodiments. Thin films, in particular, have not been optimized to a degree that would allow their use in commercial devices. Here we report on Bi(Na,K)TiO3-based thin films deposited via chemical solution deposition on Pt/Si substrates. Polycrystalline thin film Bi(Na,K)TiO3 exhibits a d33,f up to 130 pm/V with a dielectric constant in the range of 500-700 and loss as low as 1% at low frequency depending on the composition. In other piezoelectric materials such as Pb(Zr,Ti)O3 it has been shown that crystallographic ordering can enhance electrical properties due to the polarization effect. Various processing methods were investigated to enhance crystallographic texture and in turn enhance electromechanical properties. Crystallographic texture was analyzed with X-Ray diffraction and electromechanical properties were characterized via double beam laser interferometry and piezoelectric force microscopy.
Austin Fox received his B.S. degree in ceramic engineering with a minor in glass science from Alfred University, Alfred, NY, in 2011. He has been a Ph.D. candidate in materials science working with the Multi-Functional Thin Films Group at Oregon State University since 2012. He has been working on texture morphology relationships for ferroelectric materials and the development of lead-free ferroelectric thin films and ceramics. His main area of interest is processingstructureproperty relationships in lead-free thin films deposited by CSD methods.