Creating real-world conditions in the laboratory for studying material behavior

Margaret Koker, CHESS, Cornell University

Even though a metal seems like a strong material, everyone knows a metal paper clip can be broken quite easily by bending it back and forth ten or twenty times.

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Figure 1. View of the diffractometer installed at the A2 experimental station at CHESS. The x-ray beam enters the furnace through the white ceramic window shown at right, and exits to record diffraction data on a large area detector (not shown) to the left. The furnace assembly remains stationary while the specimen is heated and rotated through a variety of diffraction angles.

This type of repetitive use, or cyclic loading, leads to failure of everything from auto components to door hinges to plastic utensils. In the engineering world, understanding failure and predicting failure of parts and materials is important, yet a complete understanding of the processes which lead to failure remains incomplete. In addition to worrying about failure, engineers are also concerned with system efficiency. Many systems, such as automobile engines, run more efficiently at higher operating temperatures. Improving efficiency has enormous technical and economical relevance.

With these demands in mind, an important part of engineering research is to mimic real-world conditions within the laboratory in order to learn more about such phenomena. This means having the capability to apply mechanical loading and control the thermal environment of test specimens during lab-based testing. By reproducing a simulated, but realistic, environment using mechanical testing, both material behavior and component failure can be investigated.

Before something fails, it usually begins to deform. At times, some degree of deformation is even part of the normal use of a component. However, deformation rarely occurs uniformly across a structure, so having a way to probe the variation throughout a body is an important part of the investigation of material behavior. High energy x-ray diffraction is a powerful and nondestructive method for monitoring the behavior of material deep within a deforming metallic specimen. Diffraction methods provide information about the spacing of atoms which can be related to concepts like stress and strain at the microscopic size scale - where fatigue cracks start.

A group of Cornell engineers in the research group of Professor Matt Miller (Mechanical and Aerospace Engineering) has designed and built an instrument (Figure 1) to perform in situ mechanical and thermal loading while recreating the extreme environments some engineering components need to withstand [1]. The group's goal is to predict failure in such materials to validate material models [2]. The system, called a diffractometer, has the ability to align a specimen in a variety of orientations to investigate material response in any direction. Engineering undergraduate Ben Oswald worked with graduate students Jay Schuren and Darren Pagan to design the new sample environment, which includes a furnace that can heat specimens while simultaneously applying cycles of tensile and compressive loading.

The newly designed diffractometer was used at the A2 experimental station at Cornell High Energy Synchrotron Source (CHESS) to study the material used in turbine disks: Low Solvus High Refractory (LSHR) nickel-based superalloys. Diffraction patterns, which provide information about atomic spacing and structure, were collected at high temperature with the applied loading compressive (negative values) or tensile (positive values). Most materials are elastically anisotropic, meaning deformation is dependent on how a plane of atoms within the material is oriented with respect to applied loading. Compiling a large set of diffraction data allows for the calculation of lattice strain in numerous orientations, as shown in Figure 2. Information about microscopic anisotropy is essential for modeling and understanding the properties and failure of multigranular metals.

Figure 2. X-ray diffraction data are reduced to calculate compressive (negative) and tensile (positive) strains inside a LSHR nickel-based superalloy held at 550°C. The macroscopic load is applied along the z axis. Plotted on a sphere to display all known angular dependence, these pole figures show that the microscopic stress-strain response is different for the four crystallographic lattice planes {hkl}s within the superalloy.

With this new equipment comes the possibility to perform a wide range of material behavior investigations. The unique combination of a controlled thermal environment with mechanical loading provides an opportunity for unprecedented in situ studies of failure and fatigue, while also providing a method for material model testing and validation. In the near future, important applications of this instrument will include investigations of material microstructure deformation as a function of load at high temperatures.

References:

[1] B. B. Oswald, J. C. Schuren, D. C. Pagan, and M. P. Miller. An experimental system for high temperature X-ray diffraction studies with in situ mechanical loading. Review of Scientific Instruments, 84:033902, 2013. 

[2] M. P. Miller, J. V. Bernier, J.-S. Park, and A. Kazimirov. Experimental measurement of lattice strain pole figures using synchrotron x-rays. Review of Scientific Instruments, 76:113903, 2005.