Micro residual stresses typically result from the difference between the thermal expansion of the different phases in a composite or other multiphase material. After cooling from the processing temperature, the thermal expansion mismatch can lead to large residual microstresses being created between the grains. The magnitude of these stresses can contribute to the toughening of a composite or lead to microcracking upon thermal cycling. Measurement of the micro residual stress state as a function of temperature can provide: (1) a direct determination of the bond strength between phases in a composite; (2) a measure of the degree of microcracking; and (3) the temperature at which each phase is stress free.

Because thermal neutrons are highly penetrating, they are used to measure the specimen's volume average microstress state, free from any surface effects. The full diffraction pattern is measured over a few hours and used to analyze the microstresses. Rietveld whole pattern refinement techniques are typically used to fit the lattice parameters and volume fraction of the constituent phases for each sample as well as data from strain free constituents. Analysis of the shifts in lattice constants yields the strains.

Features:

• Monochromatic beam
• Thirty-two Helium-3 detectors with soller slits
• Pattern from 10 to 130 degrees 2-theta
• High resolution
• Non-ambient capabilities
  • High-temperature furnace
  • Low-temperature cryostat
• Automated data collection
• Rietveld analysis codes
• Powder diffraction analysis codes

Acknowledgment

The Neutron Powder Diffractometer is located at the High Flux Isotope Reactor (HFIR) at ORNL and managed by the Neutron Scattering Group, Solid State Division, ORNL. HFIR is supported by DOE-Energy Research. The Neutron Scattering Group is supported by DOE-ER, Division of Materials Sciences.