Zachary C. Cordero

Six decades of the Hall–Petch effect – a survey of grain-size strengthening studies on pure metals

Refining a metal’s grain size can result in dramatic increases in strength, and the magnitude of this strengthening increment can be estimated using the Hall–Petch equation. Since the Hall–Petch equation was proposed, there have been many experimental studies supporting its applicability to pure metals, intermetallics and multi-phase alloys. In this article, we gather the grain-size strengthening data from the Hall–Petch studies on pure metals and use this aggregated data to calculate best estimates of these metals’ Hall–Petch parameters. We also use this aggregated data to re-evaluate the various models developed to physically support the Hall–Petch scaling.

Nucleation and growth of chimney pores during electron-beam additive manufacturing

The nucleation and growth of chimney pores during powder-bed electron-beam additive manufacturing is investigated using in situ infrared thermography and micro-computed tomography. The chimney pores are found to nucleate heterogeneously at dimples on the side surfaces of additively manufactured components, and to grow through a molten-film rupture process. Further, these nucleation and growth processes are found to be strongly influenced by the beam diameter. Several strategies for suppressing the formation of chimney pores are discussed in light of these results.

Effects of Ultrasonic Welding on Nanocrystalline Ag-W Investigated with 30 kV Transmission Kikuchi Diffraction (tKD) and 300 kV STEM SE Imaging

Nanocrystalline alloys have high hardness values [1], excellent wear resistance [2], and other unique mechanical properties that make them ideal for structural applications [3]. However, because these materials have highly non-equilibrium microstructures that tend to coarsen at elevated temperatures [4], and because most welding processes involve a large thermal excursion, it is challenging to retain the structure of nanocrystalline materials when they are being joined. This inability to weld nanocrystalline metals without affecting their structure is a major obstacle that has prevented their widespread use.