Darren C. Pagan

Local Structure and Short-Range Order in a NiCoCr Solid Solution Alloy

Multielement solid solution alloys are intrinsically disordered on the atomic scale, and many of their advanced properties originate from the local structural characteristics. The local structure of a NiCoCr solid solution alloy is measured with x-ray or neutron total scattering and extended x-ray absorption fine structure (EXAFS) techniques. The atomic pair distribution function analysis does not exhibit an observable structural distortion. However, an EXAFS analysis suggests that the Cr atoms are favorably bonded with Ni and Co in the solid solution alloys. This short-range order (SRO) may make an important contribution to the low values of the electrical and thermal conductivities of the Cr-alloyed solid solutions. In addition, an EXAFS analysis of Ni ion irradiated samples reveals that the degree of SRO in NiCoCr alloys is enhanced after irradiation.

Connecting heterogeneous single slip to diffraction peak evolution in high-energy monochromatic X-ray experiments

A forward modeling diffraction framework is introduced and employed to identify slip system activity in high-energy diffraction microscopy (HEDM) experiments. In the framework, diffraction simulations are conducted on virtual mosaic crystals with orientation gradients consistent with Nyes model of heterogeneous single slip. Simulated diffraction peaks are then compared against experimental measurements to identify slip system activity. Simulation results compared against diffraction data measured in situ from a silicon single-crystal specimen plastically deformed under single-slip conditions indicate that slip system activity can be identified during HEDM experiments.

An experimental system for high temperature X-ray diffraction studies with in situ mechanical loading

An experimental system with in situ thermomechanical loading has been developed to enable high energy synchrotron x-ray diffraction studies of crystalline materials. The system applies and maintains loads of up to 2250 N in uniaxial tension or compression at a frequency of up to 100 Hz. The furnace heats the specimen uniformly up to a maximum temperature of 1200 °C in a variety of atmospheres (oxidizing, inert, reducing) that, combined with in situ mechanical loading, can be used to mimic processing and operating conditions of engineering components. The loaded specimen is reoriented with respect to the incident beam of x-rays using two rotational axes to increase the number of crystal orientations interrogated. The system was used at the Cornell High Energy Synchrotron Source to conduct experiments on single crystal silicon and polycrystalline Low Solvus High Refractory nickel-based superalloy. The data from these experiments provide new insights into how stresses evolve at the crystal scale during thermomechanical loading and complement the development of high-fidelity material models.

Characterization of the crystal structure, kinematics, stresses and rotations in angular granular quartz during compaction

Three-dimensional X-ray diffraction and X-ray computed tomography are used to study the grain-scale response of angular granular materials to understand grain kinematics, stresses and rotations during compaction and to compare the responses of angular grains with those of spherical grains.

Chemical complexity induced local structural distortion in NiCoFeMnCr high-entropy alloy

ABSTRACT In order to study chemical complexity-induced lattice distortion in high-entropy alloys, the static Debye–Waller (D-W) factor of NiCoFeMnCr solid solution alloy is measured with low temperature neutron diffraction, ambient X-ray diffraction, and total scattering methods. The static atomic displacement parameter of the multi-element component alloy at 0 K is 0.035–0.041 Å, which is obvious larger than that of element Ni (∼ 0 Å). The atomic pair distance between individual atoms in the alloy investigated with extended X-ray absorption fine structure (EXAFS) measurements indicates that Mn has a slightly larger bond distance (∼0.4%) with neighbor atoms than that of others. GRAPHICAL ABSTRACT IMPACT STATEMENT The chemical complexity induced local structural disorder in the high entropy alloy is distinguished from the thermal contribution by the combination of neutron and X-ray techniques.

InSitμ@CHESS, a Resource for Studying Structural Materials

High-energy X-ray diffraction (HEXD) experiments, which include real-time measurements of micromechanical material response using in-situ loading and the non-destructive creation of three-dimensional maps of polycrystalline microstructure, are very rapidly replacing traditional macroscopic mechanical tests and forensic metallurgical characterization methods for structural materials. The center for Integrated Simulation and X-ray Interrogation Tools and Training for Micromechanics at the Cornell High Energy Synchrotron Source (InSitμ@CHESS) was created to facilitate the use of HEXD experiments on structural materials; more notably, metallic alloys such as steel, titanium, aluminum, and nickel. The Office of Naval Research (ONR) has financially supported InSitμ, specifically enabling enhanced industrial user support. This article describes the experimental considerations associated with using HEXD on structural materials and, through a set of examples, illustrates how InSitμ addresses these considerations.