Christopher J. Marvel

Connecting Phase Stability to the Grain Growth Behavior of Ni-W Alloys

Nanocrystalline materials are beneficial due to their promising mechanical properties. However, selecting proper alloy compositions to limit unwanted grain growth has become a challenge because of increasing difficulties in understanding nanocrystalline thermodynamics and kinetics. There is a strong dependence on accurate phase diagrams to correctly predict at which compositions certain thermal stability mechanisms are most likely active. Regardless of the stability mechanism, proper knowledge of the phase diagram is paramount to choose the best alloy composition.

Phase diagram of carbon-nickel-tungsten: A superatom model

Carbon solubility in face-centered cubic Ni-W alloys and the phase diagram of C-Ni-W are investigated by means of first principle calculations and semi-grand canonical Monte Carlo simulations. With density functional theory (DFT) total energies as fitting data, we build a superatom model for efficient simulation. Multi-histogram analysis is utilized to predict free energies for different compositions and temperatures. By comparing free energies of competing phases, we are able to predict carbon solubility and phase diagrams of C-Ni-W at different temperatures. A simple ideal mixing approximation gives qualitatively similar predictions.

A Grain Boundary “TTT” – “Tribute to Thomas” !

I had the good fortune to study electron microscopy at UC Berkeley during my graduate studies under the expert guidance of Gareth Thomas. He was a true master of the electron microscope and had an incredible gift for applying electron microscopy to solve critical problems. My year-long fellowship under Gareth was a pivotal experience in my life and had a profound influence on my conception of how materials science research should be conducted. I am deeply grateful to Gareth for his influence on my development as a materials scientist and I truly believe that he deserves recognition for his positive impact on an entire generation of materials scientists such as myself. I would like to honour Gareth by highlighting a few of my research endeavours in which electron microscopy has played a crucial role.

Microstructural Analysis of a Silver-Plated Trombone

The joint was sectioned, mounted, and polished with standard metallographic procedures. Due to the fact that several different materials were used in this joint, it was not possible to reveal all microstructures simultaneously with a single etchant. The metallographic sample, shown in Fig. 3, prepared from the location shown in Fig. 2, was initially etched with a potassium dichromate solution: 2 g K2Cr2O7, 8 mL H2SO4, 4 drops HCl, 125 mL H2O. The sample was viewed under crossed polars to reveal the brass microstructure (Regions 1, 3, and 5 in Fig. 4). The lead–tin solder of the brace joint was severely attacked by the potassium dichromate. The sample was re-polished and imaged in the as-polished condition, Fig. 5. To reveal the microstructure of the silver braze (Region 6, Fig. 4), the specimen was etched with a modified Stewart’s reagent: A solution of 50 mL 3% hydrogen peroxide and 50 mL ammonium hydroxide diluted with 25 mL of water. The as-prepared samples were examined under a light microscope (LM), and a scanning electron microscope (SEM) using energy dispersive spectroscopy (EDS) to qualitatively identify composition of the various components.