GB2107560A

GB2107560A - A method for determining the orientation of a crystal

Info

Publication number: GB2107560A
Application number: GB08130347A
Authority: GB
Inventors: Colin Baxter; Edward Philip White; Joseph Douglas
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1981-10-07
Filing date: 1981-10-07
Publication date: 1983-04-27
Also published as: FR2514140A1; DE3236109A1; JPS5875051A

Abstract

Apparatus and method for determining the orientation of a crystal uses the Laue diffraction pattern of X-rays back-scattered from the surface layers of the crystal. To enable the use of a real time X-ray detector, the X-ray beam (12) and the detector axis (22) are arranged at an oblique angle to the surface (13). The detector output is used in a computer to determine the angle of the crystal. <IMAGE>

Description

SPECIFICATION A method for determining the orientation of a crystal This invention relates to apparatus and a method for determining the orientation of a crystal. It is particularly useful in the determination of the orientation of a single crystal object such, for instance, as a cast single crystal blade for a gas turbine engine, but it is also applicable to other objects and to a single crystal forming part only of a complete object.

The physical properties of single crystals are highly anistropic, and consequently the properties of objects such as these mentioned above which include or consist of a single crystal may depend strongly on the orientation of the crystal. It is therefore desirable to be able to measure the orientation of the crystal accurately, and if it is to be useful in a production process the measuring technique must operate in real time and must be simple to use.

The most widely used laboratory method of determining the orientation of a crystal utilised the interpretation of the well-known Laue diffraction patterns. These patterns are formed when a beam of X-rays is diffracted by the crystal in question. The geometry of the atoms within the crystal involved causes diffraction of the beam, and this diffraction produces areas where the scattered beam is reinforced.

These areas provide the Laue pattern referred to.

There are two methods of producing this pattern which are in common use.

In the transmission method the beam passes through a thin section of the crystal, and the need to provide this thin section limits the usefulness of this method. In the alternative back-scatter method the beam is scattered off a surface of the crystal in a rearward direction. The Laue pattern thus produced is usually recorded on a photographic plate, or sometimes a real time detector, surrounding the input beam of X-rays.

The main problems arising when trying to adopt the back-scatter method to a real time system involve this need to arrange the detector around the input beam. Thus there are some kinds of detector where it is impossible to provide a suitably isolated central channel for the passage of the beam. Even if such a channel is provided it tends to mask even more of the central area of the pattern than is the case in photographic techniques, and it is an unfortunate fact that this central area of the pattern contains potentially the greatest density of information.

The present invention provides an apparatus and method which enables this central pattern area to be detected in real time, albeit at the expense of some distortion of the pattern.

According to the present invention apparatus for determining the orientation of a crystal comprises an X-ray generator adapted to produce a collimated beam of X-rays, a specimen holder adapted to hold the crystal so that one surface is exposed to the incident beam of Xrays, and an X-ray detector adapted to detect the X-rays back-scattered by the surface layers of the crystal, the X-ray generator, specimen holder and X-ray detector being positioned and oriented so that the collimated incident beam of X-rays, the removal to the crystal surface at the point of impingement of the beam of X-rays, and the axis of the detector all lie in the same plane and the incident beam of X-rays and axis of the detector make the same angle with the normal.

In a preferred embodiment this angle is approximately 45 , enabling a detector to be used where aperture is not restricted by interference with the X-ray beam or generator.

The X-ray detector may comprise a fluorescent screen or scintillator, an image intensification device and an image scanning device.

The output from the image scanning device may" then be converted into a form suitable for providing an image of the pattern on a video monitor and for input to a computor which will calculate the crystal orientation from the detected Laue pattern.

The invention also comprises a method of determining the orientation of a crystal comprising projecting a collinated beam of X-rays at an oblique angle on to a prepared surface of the crystal, detecting the X-rays back-scattered from the surface about an axis lying at the same oblique angle to the surface and in the same plane as the beam of X-rays and the normal to the surface at the point of impingement of the beam on the surface, and using the detected pattern of back-scattered X-rays to calculate the orientation of the crystal.

The invention will now be particularly described, merely be way of example, with reference to the accompanying drawing which is a diagram of the layout of apparatus in accordance with the invention.

In the drawing, an X-ray generator 10 produces a beam of X-rays which pass through a collimator 11 to emerge as a collinated beam 1 2. In the present instance the generator 10 produces a polychromatic beam of 40kv energy which is collinated to a beam cross section of 1 mm.

The beam 1 2 falls on a reference surface 1 3 of a specimen 14 held in a specimen holder 1 5. In this instance the specimen 14 comprises a turbine blade for a gas turbine engine, and is cast as a single crystal which makes up the complete blade. It is desirable that this crystal should be oriented so that one of its principal axes extends along the longitudinal extent of the blade, and the method and apparatus of the invention are used to check this orientation.

Because of the shape of the blade specimen is relatively easy to locate it in the holder 1 5 which is arranged to grip the root portion in a secure manner. The blade 1 5 is provided with a micrometer at 1 6 which enable it and the blade 1 4 to be rotated about an axis perpendicular to the plane of the drawings so that the angle of the surface 1 3 may be accurately adjusted.

The surface 1 3 is proposed before the blade specimen 14 is repeated in the holder 1 5 by guiding it accurately past and into an accurate predetermined relationship to the location surfaces used to define the portion of the blade.

When the blade specimen has been mounted in the holder 1 5 the adjustment 1 6 is used to rotate the surface 1 3 until the angle and between the beam 1 2 and the removal 1 7 to the surface 1 3 at the point of impingement of the beam is at a predetermined value. We prefer to make the angle 45 , for reasons referred to below. (It should be noted that because the beam 1 2 is of finite cross section there is no true point of impingement, but the centre of the beam can be used to define the point).

The true pattern of X-rays back-scattered from the surface layers 1 3 is detected by a real time detector 1 8. It will be appreciated that because the beam 1 2 impinges obliquely on the surface 13, the scattered X-rays will not strictly comprise back-scatter in the sense that they travel in a reverse direction to that of the beam 1 2. Nevetheless, this term is used to distinguish from the "transmission' type of scatter which is not used in the present invention.

The detector 1 8 is a composite device cavity of a promenent screen or distributor 19, bridge interpriter 20 and an image scanning device in the form of a charge-coupled device 21. Operation of the detector is simple in principle in that the screen 1 9 glows under the impact of X-rays, the image interpriter 20 interprites the light thus produced and the image reasoning device 21 converts the interified image into an electrical output signal.

It will be seen that in the apparatus of the invention the detector 1 8 is mounted so that its axis 22, lying normal to the screen 1 9 and passing through its centre, passes through the point of impingement of the beam 1 2 on the surface 1 3 and makes an angle a with the normal 1 7 which is the same as that between the beam 12 and the normal 1 7. These three lines of the beam 12, and 1 7 and axis 22 also lie in the same plane. This geometry will be recognised as one in which a notional reflection of the beam 1 2 would impinge on the centre of the screen 1 9 at right angles to it.Since the beam 1 2 is diffracted and not reflected this is not entirely the case in this apparatus, but it will be seen that the centre of the true pattern will fall on the centre of the screen 19, and that because the detector 1 8 is angularly displaced from the generator 10 there is no need to make provision for the beam 1 2 to pass through the detector.

It will also be noted that with the preferred value of the angle at 45 , these provided the 'radius' of the generator 10 is less than the distance from the point of impingement of the beam 1 2 to the screen 19, the size of the detector 1 8 is not limited by any interference between the detector and the generator. It will be appreciated that this is true of any angle a greater than 45 , but as the angle a increases the distortion of the Laue pattern and, more importantly, of the individual 'spots' which make up the pattern, also increases and it is desirable therefore to limit the angle to 45 or thereabouts.

The output for the detector 1 8 therefore comprises an electrical signal representative of the distorted Laue pattern falling on the face of the detector. This is then processed in an analyser 23 into a form suitable as an input to a video monitor 24 and a computer 25. The monitor 24 enables the operator to see that the apparatus is producing a satisfactory pattern and, if necessary, to calculate the orientation, but usually this calculation will be performed in the computer 25 where output is a readout of the characteristic angles defining the orientation of the crystal in the specimen 14.

It will be seen that this apparatus and method provides a way in which the orientation of a single crystal object can be determined quickly and with apparatus which could be used by a relatively unskilled operator. It should be understood that the same apparatus could be used for any specimen which incorporates a single crystal, even if this crystal does not form the complete specimen.

Claims

1. Apparatus for determining the orientation of a crystal comprising an X-ray generator adapted to produce a collimated incident beam of X-rays, a specimen holder adapted to hold the crystal so that one surface is exposed to the incident beam of X-rays, and an X-ray detector adapted to detect the X-rays backscattered by the surface layers of the crystal, the X-ray generator, specimen holder and X ray detector being positioned and oriented so that the collimated incident beam of X-rays, the normal to the crystal surface at the point of impingement of the incident beam of X rays, and the axis of the detector intersect at the point of impingement and all lie in the same plane, and the incident beam of X-rays and axis of the detector make the same angle with the normal.

2. Apparatus as claimed in claim 1 and in which said angle between the normal and the beam of X-rays and detector axis respectively 45".

3. Apparatus as claimed in claim 1 and in which said detector comprises a fluorescent screen, an image intensifier and an image scanning device.

4. Apparatus as claimed in claim 3 and comprising an analyser adapted to modify the output of the image scanning device into a form suitable to be input to a video monitor or a computer.

5. Apparatus as claimed in claim 4 and comprising a computer adapted to determine the orientation of the crystal from the output of the analyser.

6. A method of determining the orientation of a crystal comprising projecting a collinated beam of X-rays at an oblique angle on to a prepared surface of the crystal, detecting the X-rays back-scattered from the surface about an axis lying at the same oblique angle to the surface and in the same plane as the beam of X-rays and the normal to the surface at the point of impingement of the beam on the surface, and using the detected pattern of back-scattered X-rays to calculate the orientation of the crystal.

7. Apparatus for determining the orientation of a crystal substantially as hereinbefore particularly described with reference to the accompanying drawing.