Applications

Macro residual stresses extend over a long range relative to the scale of the specimen's microstructure. These stresses originate from differential plastic flow, differential cooling rates, or phase transformations with volume changes. Macro residual stresses are created, for example, by welding, forging, casting, rolling, machining, surface treatments, or heat treating. These stresses are often high at the surface of a material or at specific locations within the specimen. Their variation across a surface or through thickness can be critical to material performance. Research areas include studies of the development of residual stress as a function of the rate and method of material removal or resulting from changes in a manufacturing process. Other studies include determination of residual stresses in surface treated materials for assessing the effectiveness of the surface treatment in introducing beneficial compressive stresses at the surface.

Microstresses typically originate from the differential thermal expansion between phases or differential plastic flow between grains. For example, microstresses that develop in composites primarily result from differences in the respective thermal expansion coefficients. Studies of ceramic composites include characterizing the stress as a function of fiber or whisker loading, coatings, and processing, and can be determined at various temperatures.

Typically, researchers seek to know the residual stresses (macro and micro) and texture for a given set of samples. Each set of samples possesses its own geometry as well as the particular region of interest. X-ray work usually focuses on the investigation of surfaces of samples as x-rays provide information from the first 50 micrometers of most metals and ceramics. For example, usage of coatings and thin films has increased over the past decade. These coatings often posess large residual stresses as well as texture which can both be well characterized with x-ray diffraction. Machined surfaces also require characterization as parts often fail from machining induced flaws. Grazing incidence x-ray diffraction can be used to obtain the residual stress profile in the near surface region. Here the depth of penetration is controlled by the angle of incidence. Finally, curved surfaces, especially cylinders, are a common engineering shape. Interpretable x-ray diffraction of these surfaces requires the use of parallel beam and near parallel beam optics. These techniques are available through the RSUC.

Pole figures and texture are frequently used to optimize processing parameters and to predict the properties and performance of a textured material. Diffraction methods are widely accepted as the most general method of quantifying the crystallographic texture of materials. Properties of materials are heavily dependent on their texture, often a result of the manufacturing processes that include rolling, extrusion hot pressing, preferred crystalline growth, epitaxy, and vapor deposition. Texture affects such important design and processing properties as yield strength, corrosion resistance, formability, thermal transport, magnetic and electric properties.

For more about the RSUC facilities and applications read "Materials Under Stress: ORNL's Measures for Helping Industry"