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The versatile nature of neutron diffraction makes it an ideal tool to undertake novel inquiries into industrial issues. Developing techniques include non-invasive thermometry, real-time tracking of oxidation, monitoring of electrochemical reactions, and large-volume-scanning of microstructural homogeneity. Measurement of residual stress is the technique most frequently used by our industrial clients.
There are a variety of neutron diffraction techniques that are applied to industrial problems:
- Residual stress scanning
- Near-surface stress mapping
- Surface and interface characterization
- Texture mapping
- Volume fraction analysis
- Temperature Measurement
Capabilities for sample conditions:
Infrastructure available on the wider laboratory site at Chalk River is sometimes key to the success of a project. There are facilities for machining of very heavy components, cranes and transport for moving large samples. There are also precision machining capabilities available for the production of custom-made experimental equipment, and innovative solutions to difficulties in a project.
Neutrons can be used to plot three-dimensional maps of internal residual stress, non-destructively in engineering components, such as weldments, pipes, rails, bent plates, turbine blades and turbine discs. The technique can be used to evaluate manufacturing processes such as heat treatment, surface modification, forging, straightening and welding. Data are obtained for a wide variety of materials, including steel, aluminum, titanium, zirconium, nickel-based alloys, ceramics and composites. Typically measurements are made in volume elements of order 1 mm3 (but both larger and smaller volume elements have been successfully applied), with typical precisions of ±10 to ±30 MPa, depending on the material properties.
Determining the thermal evolution of stress in a 3-component (Inconel, Zr-2.5%Nb, & Stainless Steel) rolled joint.
Completely non-destructive and highly accurate: neutron diffraction is used to determine internal residual stresses in crystalline materials (e.g. metals, alloys, ceramics and composites). Neutrons can probe several centimeters beneath the surface-well into joints, internal components and other critical sub-surface regions vital to the performance of materials and engineering components. The spatial resolution (defined by a volume element) ranges from 0.2 to 1,000 mm3. The resolution in lattice strain is of the order 0.01%.
- New Product Development - Information on residual stresses at the design stage can help to optimize the performance and reliability of new products. Prototypes can be evaluated and compared against calculations.
- Process Evaluation - Knowledge of the development of residual stress in components at various stages of production-extrusion, rolling, machining, welding and heat-treating- can be used to optimize processes and improve product reliability and performance.
- Problem Solving - Residual stress data can help in determining the causes of failures so that appropriate remedial action can be taken. With neutron diffraction, accurate measurements can be obtained deep within a joint or below highly corroded or fouled surfaces. Neutrons penetrate right through the surface region, no surface preparation is needed.
The stresses introduced by surface treatments such as shot peening and laser ablation, extend to several millimetres below the surface, with the greatest variations occurring within the first millimetre of the surface. The accurate determination of residual strain by diffraction has traditionally fallen into two spatial regimes:
- surface measurements, using highly attenuated x-rays (typically 1-100 mm)
- measurements at depth, using highly penetrating neutrons (typically 1-30 mm)
A new non-destructive neutron diffraction technique, called Near-Surface Stress Mapping, has been developed to probe continuously from 0.1 mm below the surface to well inside the specimen. Labor-intensive layer removal and re-measurement is not needed.
Surface and Interface characterization
Non-invasive and non-destructive, neutron reflectometry can determine area-averaged chemical composition and roughness of a surface. In addition, the technique is sensitive to variation of chemical composition with depth. If the sample consists of layers of different materials it gives chemical composition and thickness of each layer, as well as the roughness of the interfaces between the layers.
With suitable samples, researchers can achieve excellent resolution: within the overall sensitive depth of up to 300 nm one can often see layers that are only a few atomic layers thick.
Texture is determined from the variation of diffracted neutron intensity versus direction in material, plotted here as a stereographic pole figure. Neutrons can be used in the quantitative analysis of the preferred orientations of crystallites within a material. This helps to identify materials with directionally-enhanced properties, such as corrosion resistance, yield strength, creep resistance, elastic stiffness and thermal expansion. Neutron diffraction averages over the bulk of the texture specimen, providing data of good statistical quality. The technique can be applied to a wide variety of materials, including steel, aluminum, titanium, zirconium, nickel-based alloys, ceramics and composites. Spatial variations of texture can also be evaluated or fully determined depending on the specific problem. |
Volume Fraction Analysis
Neutron diffraction patterns can be analyzed to determine the volume fractions of component materials within a composite specimen, such as graded ceramics, metal-matrix composites or precipitates in alloys. Volume fractions as low as 0.1% can be measured. Data can be acquired as a volume-average of bulk material, or as a non-destructive spatial scan of the interior of a component.
Just as neutrons measure crystal lattice strains from applied or residual stresses, they also detect strains from changes in temperature (e.g. expansion with heating). With the ability of neutrons to penetrate inside complex components and bulk materials, neutron diffraction could be applied to measure temperature non-invasively, for example of the turbine disc in an aeroengine, even while it is operating. Shown here is a PT6 gas turbine engine from Pratt & Whitney Canada Inc., which was the subject of feasibility tests of this novel thermometry method.
Pratt and Whitney Gas Turbine undergoing neutron measurements.
Neutrons can be used to probe materials in several ways, via diffraction, reflection, small angle scattering, or transmission techniques such as radiography or tomography. In each case the technique is non-destructive for the sample.
Neutrons are neutral particles and as so, do not interact readily with matter. Neutrons interact directly with atomic nuclei, which are far smaller than the overall size of atoms. Consequently materials appear to neutrons as mostly empty space. After examination, most specimens are left unaffected and may be returned to service.
Neutrons are very penetrating and can explore material properties up 35 mm inside a steel component, or 300 mm in aluminum.
Because of the penetrating nature of neutrons, material samples that are under investigation, can be housed in almost any environmental conditions of pressure, temperature, humidity etc. The neutron beam can penetrate the wall of the furnace or other environmental equipment, and access the sample inside. Materials can also be subjected to load while being analyzed, tensile or compressive loads up to 50 kN are currently achievable. Challenging environments, such as high electric or magnetic fields, radioactive components, corrosive environments or operating machinery can also be examined. In many cases combinations of environmental conditions can be applied to the sample.
Transient Experimental Conditions
Environmental conditions do not have to be static. Neutrons can gather data on material while it undergoes changes of temperature, strain, electrochemical condition etc. Transient experimental results could show stress, structure or phase composition changes.
Complete neutron diffraction patterns can be acquired as a function of time to investigate the kinetics of processes in engineering materials, such as the growth of precipitates in heat-treated alloys, formation of scale at high temperature and reactions of phases in composites. In vibrating or rotating components, the neutron diffraction pattern can be gated to investigate the response of the material at particular points in each cycle. In some cases the time dependence of strain can also be measured.
