·Table of Contents
·Materials Characterization and testing
Developments in Real Time Crystallographic TechniquesNick Townsend
X-innovations ltd, Bristol, UK
Consequently all single crystal components had and still have a mandatory 100 % inspection requirement for primary crystal orientation.
In the period where component numbers were relatively small this inspection was carried out using a Laue back scatter technique, where a thin beam of polychromatic x-rays is incident on the surface of the component. A diffracted pattern of bright spots is scattered back towards the x-ray source, and an x-ray film (wet film or Polaroid) interposed between the source and the sample detects the spot pattern. This is then analysed by hand using a Greninger chart.
Whilst this technique is workable, it requires considerable skill to interpret the results, is expensive in terms of consumable materials, is time consuming and is open to operator mis-interpretation of the pattern.
For these reasons it was decided that the technique was not suitable for large volume production and that a system was required with the following key features:
1.1 Be able to achieve accurate and repeatable measurements of crystal primary orientation.
1.2 Allow high volume production workloads.
1.3 Have low operating costs.
1.4 Minimise, as far as possible, the required operator skill levels.
1.5 Be robust enough to operate in a shop floor environment.
Early in the development programme a further requirement was included, namely the ability to measure relative orientations of surface breaking crystal grains.
These were the key development goals of the programme and the result was the birth of the SCORPIO system.
2.1 The film has been replaced by a high sensitivity real-time x-ray detector.
2.2 The geometry has been changed from 'true' back reflection to an inclined geometry of between 56° and 90°
2.3 An image processing system and software package has been added.
2.4 A component handling system has been added.
In operation, a prepared component is placed on a component jig, and the x-ray beam, incident on the component, scatters onto the real-time detector. A pattern of spots is produced in real-time, which is a distorted representation of the traditional 'true' back scatter Laue image. This pattern is produced in the form of a video image and is passed to the image processing computer. A series of algorithms, including image averaging or integration, is used enhance the quality of the image, which is then passed to the analysis part of the package. At this stage, a computer generated pattern representing the spot positions is overlaid on the diffraction image. The overlay pattern is centred on the diffraction pattern by the operator, and a further operator controlled adjustment allows rotation of the overlay until it lines up exactly with the diffraction pattern. The system then calculates the main orientation angles (gamma, delta and kappa) and the derived angles (rho, alpha and theta) for a primary orientation.
The system is also capable of measuring relative orientation or 'R' value for a surface breaking grain. In this mode the blade is positioned on a motorised jig which allows the relevant component surface to be scanned across the beam in two directions (both orthogonal to the beam axis). A reduced level of image processing is applied to the resulting diffraction pattern to give an image with adequate noise reduction without excessive motion blur. The operator scans the component across the beam and, by monitoring the resulting diffraction pattern, is able to position the x-ray spot accurately on either side of a grain boundary. The system then allows the pattern to be analysed in each position in the same manner as for primary orientation and calculates the orientation of the grain relative to the remainder of the component. Thus the severity of the misorientation of the grain can be assessed.
3.1 ACCURACY AND REPEATABILITY
The system typically gives repeatability of better than 30' of a degree and accuracy of better than 1°
The system allows typically one thousand primary orientations to be measured in a day.
The elimination of all film consumable gives the system very low operating costs.
3.4 SKILL REQUIREMENTS
The measurement of primary orientations particularly requires relatively low skill levels since a few simple key strokes are used to match the patterns. Operators rapidly achieve high levels of expertise in using the software.
The SCORPIO systems have been in use with the original developers for more than fifteen years and have been used to make many millions of analyses. Systems are also In use by a number of precision casting facilities around the world, all of which are achieving very high reliability figures.
Nevertheless there has been substantial progress in the single crystal manufacturing process which is having a knock on effect on the requirements for orientation measurement.
The key change which has come to single crystal manufacturing is that it was originally a 'top end' technique, used only on the most critical components. It is now penetrating downwards and being used or considered for a much wider range of components in a much wider range of applications. The result of this is that orientation measurement systems must cope with:
It was therefore necessary to establish the changes required to the original SCORPIO concept to allow it to cope with current requirements.
One of the main constraints on the early system is the relatively cramped layout of detector source and collimator (see fig 1.) which prevents the introduction of large components (the 'crunch' effect). It was established that the system had to be modified to incorporate the following features:
In addition to this, to cope with a wider range of alloys and surface treatments, a further constraint is added:
Inclined geometry, integral open detector.
Zero degree geometry, integral open detector.
Figures 2 and 3 show the two different configurations.
|Fig 2:||Fig 3:|
The zero degree geometry has substantial attractions in that it allows the working distance to be varied continuously without affecting the position at which the beam strikes the component. Secondly, this configuration gives the best capability in terms of accommodating large component sizes.
Unfortunately, as a result of the basic physics a clearer spot pattern can be achieved at the inclined geometry. (This is the result of the 'form factor' which is an angular dependant scattering intensity variable).
Trial systems were built using each system configuration and spot patterns were obtained in each case. Whilst both designs worked, it was decided to adopt the inclined geometry for the following reasons:
Figures 4 and 5 illustrate the final system design.
|Fig 4: Scorpio II System||Fig 5: Scorpio II System|
In summary the key features of the SCORPIO II system are:
6.1 It retains the rugged proven track record of the earlier SCORPIO analysis method..
6.2 It has enhanced detector capability giving clearer diffraction spot patterns, aiding analysis of components with poor surface finish.
6.3 It has an increased working distance to facilitate measurement of large components.
6.4 It has redesigned mechanical configuration to eliminate access constraints for large components.
6.5 Software now runs under Windows.
7.1 Fully digital detector system.
7.2 Zero angle or close to zero angle geometry.
7.3 Capability to operate with minimal surface preparation.
7.4 An option of operator controlled or automatic analysis.
The above paper provides outline information only regarding the development of the SCORPIO system. Further information can be obtained from the author: firstname.lastname@example.org Tel: +44 177 9412291
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