Abstract
A rotary X-ray anode has a support body and a focal track formed on the support body. The support body and the focal track are produced as a composite by powder metallurgy. The support body is formed from molybdenum or a molybdenum-based alloy and the focal track is formed from tungsten or a tungsten-based alloy. Here, in the conclusively heat-treated rotary X-ray anode, at least one portion of the focal track is located in a non-recrystallized and/or in a partially recrystallized structure.
Claims
1. A rotary X-ray anode, comprising: a powder-metallurgically produced composite formed of a support body and a focal track on said support body; said support body being formed of molybdenum or a molybdenum-based alloy; said focal track being formed of a tungsten-rhenium alloy having a rhenium proportion in a range of 5-10% by weight; and wherein, in a conclusively heat-treated rotary X-ray anode, at least one portion of said focal track is present in a non-recrystallized or a partially recrystallized structure and having a hardness of ≧350 HV 30.
2. The rotary X-ray anode according to claim 1, wherein said at least one portion of said focal track has, in a direction perpendicular to a focal track plane, a preferential texturing in a <111> direction with a texture coefficient TC.sub.(222) of ≧4 determinable by way of X-ray diffraction and a preferential texturing in a <001> direction with a texture coefficient TC.sub.(200) of ≧5 determinable by way of X-ray diffraction.
3. The rotary X-ray anode according to claim 1, wherein the following relationship for the texture coefficients TC.sub.(222) and TC.sub.(310) determinable by way of X-ray diffraction is satisfied for the portion of the focal track perpendicular to the focal track plane:
TC.sub.(222)/TC.sub.(310)≧5.
4. The rotary X-ray anode according to claim 1, wherein the at least one portion of said focal track is present in a partially recrystallized structure.
5. The rotary X-ray anode according to claim 4, wherein: crystal grains formed in the partially recrystallized structure by new grain formation are surrounded by a deformation structure; and in terms of a cross-sectional area through the partially recrystallized structure, the crystal grains have an areal proportion in a range of 10% to 80%.
6. The rotary X-ray anode according to claim 1, wherein the at least one portion of said focal track has a mean small-angle grain boundary spacing of ≦10 μm; wherein the mean small-angle grain boundary spacing can be determined by a measurement process in which grain boundaries, grain boundary portions and small-angle grain boundaries with a grain boundary angle of ≧5° are determined on a radial cross-sectional area running perpendicular to said focal track plane in a region of the at least one portion of the focal track; to determine the mean small-angle grain boundary spacing parallel to the focal track plane, a group of lines which runs parallel to the cross-sectional area and is made up of lines each running parallel to the focal track plane and at a spacing of in each case 17.2 μm in relation to one another is placed into the grain boundary pattern thereby obtained, respectively the spacings between in each case two mutually adjacent intersections between the respective line and lines of the grain boundary pattern are determined on the individual lines, and the mean value of these spacings is determined as the mean small-angle grain boundary spacing parallel to the focal track plane; to determine the mean small-angle grain boundary spacing perpendicular to the focal track plane, a group of lines which runs parallel to the cross-sectional area and is made up of lines each running perpendicular to the focal track plane and at a spacing of in each case 17.2 μm in relation to one another is placed into the grain boundary pattern obtained, respectively the spacings between in each case two mutually adjacent intersections between the respective line and lines of the grain boundary pattern are determined on the individual lines, and the mean value of these spacings is determined as the mean small-angle grain boundary spacing perpendicular to the focal track plane; and the mean small-angle grain boundary spacing is determined as a geometric mean value of the mean small-angle grain boundary spacing parallel to the focal track plane and of the mean small-angle grain boundary spacing perpendicular to the focal track plane.
7. The rotary X-ray anode according to claim 1, wherein said at least one portion of said focal track has a preferential texturing in a <101> direction in directions parallel to a plane of said focal track plane.
8. The rotary X-ray anode according to claim 1, wherein at least one portion of said support body is present in a non-recrystallized or partially recrystallized structure.
9. The rotary X-ray anode according to claim 8, wherein the at least one portion of said support body has a hardness of ≧230 HV 10.
10. The rotary X-ray anode according to claim 8, wherein: said at least one portion of said support body has a preferential texturing in a <111> direction and in a <001> direction perpendicular to the focal track plane; and/or said at least one portion of said support body has a preferential texturing in the <101> direction in directions parallel to said focal track plane.
11. The rotary X-ray anode according to claim 8, wherein said at least one portion of said support body has an elongation at break of ≧2.5% at room temperature.
12. The rotary X-ray anode according to claim 1, wherein said support body is formed of a molybdenum-based alloy having further alloying constituents including at least one alloying constituent selected from the group consisting of Ti, Zr and Hf, and at least one alloying constituent selected from the group consisting of C and N.
13. A method of generating X-ray radiation which comprises providing a rotary X-ray anode according to claim 1 in an X-ray tube and generating the X-ray radiation therewith.
14. A method of producing a rotary X-ray anode, the method which comprises: providing a starting body produced as a composite by pressing and sintering corresponding starting powders, the starting body having a support body portion made of molybdenum or a molybdenum-based mixture and a focal track portion, formed on the support body portion, made of a tungsten-rhenium mixture with a rhenium proportion of 5-10% by weight; forging the starting body; and subjecting the body to a heat treatment during the forging step, after the forging step, or during and after the forging step, to form a rotary X-ray anode according to claim 1; adjusting a temperature of the heat treatment and a processing time of the heat treatment such that, in the finally and conclusively heat-treated rotary X-ray anode, at least one portion of the focal track obtained from the focal track portion is present in a non-recrystallized and/or in a partially recrystallized structure and has a hardness of ≧350 HV 30.
15. The method according to claim 14, which comprises carrying out the heat treatment at temperatures in a range of 1300° C.-1500° C.
16. The method according to claim 14, wherein the forged body has a degree of deformation in a range of 20% to 60% after completion of the forging step.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
(1) FIGS. 1A-1C show schematic illustrations for visualizing different degrees of recrystallization;
(2) FIG. 2 shows a schematic graph for visualizing the hardness profile depending on the temperature of a heat treatment;
(3) FIG. 3 shows a schematic cross-sectional view of a rotary X-ray anode;
(4) FIGS. 4A-4D show a schematic illustration for visualizing an EBSD analysis;
(5) FIGS. 5A-5C show inverse pole figures of the focal track of a rotary X-ray anode according to the invention along different directions;
(6) FIG. 6 shows an inverse pole figure of a focal track which was applied by means of CVD; and
(7) FIG. 7 shows an inverse pole figure of a focal track applied by vacuum plasma spraying.
DETAILED DESCRIPTION OF THE INVENTION
(8) The following explanation of FIGS. 1A-1C and 2 reveals criteria which can be used to distinguish a non-recrystallized structure, a partially recrystallized structure and a (completely) recrystallized structure from one another. Furthermore, parameters which can be used to state the degree of recrystallization are explained on the basis of these figures. These explanations apply both with respect to the focal track and with respect to the support body. FIGS. 1A-1C schematically show (greatly enlarged) structures as can be represented, for example, in an electron micrograph of a correspondingly prepared abraded surface, in particular in the course of an EBSD analysis (EBSD: Electron Backscatter Diffraction). A suitable process for sample preparation, a suitable measurement arrangement and a suitable measurement process will be explained with reference to FIGS. 4A to 4D. As is known in the specialist field, the grain boundaries or grain boundary portions (and also if appropriate the small-angle grain boundaries) and the dislocations can be made visible in such an electron micrograph. To this end, it is necessary to specify a minimum angle of rotation beyond which a grain boundary is indicated. In FIGS. 1A to 1C, it is assumed (apart from the section shown separately in FIG. 1B) that a minimum angle of rotation of 15° has been specified, so that the profile of the large-angle grain boundaries (or grain boundary portions) is visible. FIG. 2 schematically shows, proceeding from a starting hardness -AH- obtained in the course of the powder metallurgy production after the forging process (starting hardness -AH- of the deformation structure), the dependency of the hardness on the temperature -T- of a subsequent heat treatment (stress relief annealing), which is carried out for a predetermined period of time -t-, for example for a period of time of one hour. If the heat treatment is carried out for a longer predetermined period of time, the step shown in FIG. 2 shifts more to the left (i.e. toward lower temperatures), whereas it shifts more to the right (i.e. toward higher temperatures) in the case of a shorter period of time.
(9) FIG. 1A shows a pure deformation structure as is obtained, for example, after a forging operation (which is carried out in the course of the powder metallurgy production). As is known in the specialist field, such a deformation structure has no clear grain boundaries circulating corresponding crystal grains. Instead, what can merely be identified are grain boundary portions -2- which each have an open beginning and/or an open end. To some extent, here (depending on the degree of deformation during the forging operation) portions of the grain boundaries of the original grains of the sintered body can also be identified. Furthermore, the deformation (forging operation) forms dislocations -4-, which are represented by the symbol “⊥” in FIGS. 1A and 1B, and new grain boundary portions -2-. The original grains of the sintered body are, if they can still be identified, greatly squashed and distorted on account of the deformation. Furthermore, the deformation structure has a substructure, which can be made visible using an EBSD analysis of the respective abraded surface with a relatively small minimum angle of rotation being set. This substructure of the deformation structure will be explained below with reference to FIG. 1B. With an increasing degree of deformation, the original grain boundaries (of the grains of the sintered body) disappear in certain portions or even entirely. The intensity and frequency of these typical features of the deformation structure depend inter alia on the (material) composition and the degree of deformation. In particular, it is to be taken into account that, with an increasing degree of deformation, small-angle grain boundary portions arise increasingly and also the frequency of large-angle grain boundary portions increases. A determination of the mean grain size, which is regularly effected in the case of uniform microstructures in accordance with the standard ASTM E 112-96, is not possible since (at least in the case of a minimum angle of rotation of 15°) only grain boundary portions can be identified.
(10) Recovery processes which increase with an increasing temperature generally proceed in the deformation structure. For such recovery processes, which can be identified for example from disappearance and/or ordering of dislocations, no activation energy is required. These recovery processes lead to a decrease in hardness. In this range -EH- of the recovery processes (range up to T.sub.1 in FIG. 2), the hardness decreases continuously with an increasing temperature, the slope in this range -EH- being relatively flat (cf. FIG. 2). Above a specific temperature -T.sub.1-, the activation energy required for new grain formation in the course of the recrystallization can be applied. This temperature -T.sub.1- is dependent inter alia on the composition and the degree of deformation of the deformation structure and also on the duration of the heat treatment carried out in each case. If recrystallization occurs, there is (firstly) a partially recrystallized structure. FIG. 1B shows a partially recrystallized structure having a number of crystal grains -6- formed by new grain formation. The crystal grains (or crystallites) -6- each have circumferential grain boundaries -8-, which can be represented for example in an electron micrograph of a correspondingly prepared abraded surface, in particular using an EBSD analysis (EBSD: Electron Backscatter Diffraction). The remaining proportion (or the proportion surrounding the crystal grains -6-) of the partially recrystallized structure is still present in the deformation structure. On account of the new grain formation and in some cases on account of recovery processes, the dislocations -4- which arise in the deformation structure disappear increasingly.
(11) As has already been mentioned, a further feature of the deformation structure is that it has a substructure. Such a substructure can be made visible using an EBSD analysis by specifying a relatively small minimum angle of rotation, for example by a minimum angle of rotation of 5° (or possibly also an even smaller angle). In this way, the small-angle grain boundaries -9- which form the substructure can also be identified in addition to the large-angle grain boundaries (grain boundary portions -2- and circumferential grain boundaries -8-). This is shown in FIG. 1B in the bottom box, in which a section of the structure shown in the box above is illustrated on an enlarged scale. The small-angle grain boundaries -9- of the substructure are shown in this illustration as relatively thin lines. As can be seen on the basis of this illustration, the large-angle grain boundaries of the grain boundary portions -2- are to some extent also continued by small-angle grain boundaries -9-. The crystal grains -6- formed by new grain formation are in this case free from the substructure. In the case of the rotary X-ray anode according to the invention, the substructure -9- of the deformation structure has in particular a fine-grained form.
(12) With an increasing recrystallization, which increases with the temperature (and also the time) of the heat treatment, the hardness decreases greatly (cf. FIG. 2). In FIG. 2, above the temperature -T.sub.1- the previously flatly falling graph passes over into a region with a steeply falling slope. The transition region between the flatly falling portion and the steeply falling portion of the graph, in particular the point with the greatest curvature, is referred to as the recrystallization threshold -RKS- (cf. FIG. 2). With an increasing degree of recrystallization, the crystal grains which have already been formed by new grain formation are enlarged, further crystal grains are formed by new grain formation and the deformation structure disappears increasingly. In particular, the deformation structure is increasingly “consumed” by the crystal grains formed by new grain formation. With a further increasing degree of recrystallization, the grain boundaries of the crystal grains formed by new grain formation collide, and finally also fill (at least largely) the remaining interstices. In this stage, the crystal growth slows down again, and in FIG. 2 the slope of the graph flattens out. What is reached is a state in which the recrystallization is completed to an extent of 99%, in particular in which the crystal grains formed by new grain formation have an areal proportion of 99% with respect to a cross-sectional area through the structure. The recrystallization temperature, which in FIG. 2 corresponds to -T.sub.2- (in FIG. 2, the duration of the heat treatment is one hour), is defined in this case in such a way that, after a heat treatment of one hour at this recrystallization temperature, the recrystallization is completed to an extent of 99%. The region -RK-, which extends beginning from the temperature -T.sub.1- up to the recrystallization temperature -T.sub.2-, is referred to as the recrystallization region, since recrystallization processes proceed therewithin to a considerable extent. Finally, the graph passes over into a region -EB-, in which it no longer falls or falls only in a very flat manner. In this region, although grain growth still occurs, no recrystallization takes place or recrystallization takes place only to a very small extent (in particular of the remaining one percent of the structure).
(13) FIG. 1C shows an idealized, completely recrystallized structure. The grain boundaries of the crystal grains formed by new grain formation directly adjoin one another. The original deformation structure has completely disappeared. Here, FIG. 1C shows the “ideal case” of a completely recrystallized structure, since the grain boundaries adjoin one another in each case along their entire direction of extent.
(14) FIG. 3 schematically shows the structure of a rotary X-ray anode -10-, which is formed with rotational symmetry in relation to an axis of rotational symmetry -12-. The rotary X-ray anode -10- has a plate-shaped support body -14-, which can be mounted on a corresponding shaft. An annular focal track -16- is applied on the top side of the support body -14- and, in the embodiment illustrated, has a frustoconical form (a flat cone). The focal track -16- covers at least a region of the support body -14- which, during use, is traversed by an electron beam. In general, the focal track -16- covers a region of the support body which is larger than that of the track of the electron beam. The outer form and the structure of the rotary X-ray anode -10- can differ from the rotary X-ray anode shown, as is known in the specialist field. As is apparent with reference to FIG. 3, the (macroscopic) proportion of the non-recrystallized and/or partially recrystallized structure (both for the focal track and for the support body) can generally be established by virtue of the fact that a radial (i.e. running through the axis of rotational symmetry -12-) cross-sectional area running perpendicular to the focal track plane is examined as to which regions are present in a non-recrystallized and/or in a partially recrystallized structure.
(15) Hereinbelow, an EBSD analysis (EBSD: Electron Backscatter Diffraction) which can be carried out with a scanning electron microscope is explained with reference to FIGS. 4A to 4D. In the course of such an EBSD analysis, a characterization of the respective structure can be carried out on a microscopic level. In particular, in the course of such an EBSD analysis, the fine-grained nature of the respective structure can be determined, the occurrence and the extent of substructures can be ascertained, the proportion of the crystal grains formed by new grain formation in a partially recrystallized structure can be determined and also preferential texturings which arise in the structure can be determined. To this end, in the course of the sample preparation, a cross-sectional area running radially and perpendicular to the focal track plane (corresponds to the cross-sectional area shown in FIG. 3) through the rotary X-ray anode is produced. A corresponding abraded surface is prepared in particular by embedding, abrading, polishing and etching at least one portion of the obtained cross-sectional area of the rotary X-ray anode, with the surface then also being subjected to ion polishing (to remove the deformation structure formed by the abrasion process on the surface). Here, the abraded surface to be examined can be chosen in particular in such a way that it comprises a portion of the focal track and a portion of the support body of the rotary X-ray anode, so that both portions can be examined. The measurement arrangement is such that the electron beam impinges on the prepared abraded surface at an angle of 20°. In the case of the scanning electron microscope (in the present case: Carl Zeiss “Ultra 55 plus”), the spacing between the electron source (in the present case: field emission cathode) and the sample is 16.2 mm and the spacing between the sample and the EBSD camera (in the present case: “DigiView IV”) is 16 mm. The information provided between parentheses relates in each case to the types of appliance used by the applicant, where in principle other types of appliance which make the described functions possible can also be used in a corresponding manner. The acceleration voltage is 20 kV, a 50-fold magnification is set and the spacing between the individual pixels on the sample which are scanned in succession is 4 μm.
(16) The individual pixels -17- are in this case arranged in equilateral triangles in relation to one another, the length of a side of a triangle corresponding in each case to the grid spacing -18- of 4 μm (cf. FIG. 4A). The information for an individual pixel -17- originates here from a volume from the respective sample which has a surface with a diameter of 50 nm (nanometers) and a depth of 50 nm. The information for a pixel is then represented in the form of a hexagon -19- (shown with a dashed line in FIG. 4A), the sides of which in each case form the perpendicular bisectors between the relevant pixel -17- and the (six) pixels -17- located closest in each case. The examined sample area -21- measures in particular 1700 μm by 1700 μm. As shown in FIG. 4B, it comprises in the present case, in a top half, a focal track portion -22- (in cross section) measuring approximately 850 by 1700 μm.sup.2 and, in the bottom half, a support body portion -24- (in cross section) measuring approximately 850 by 1700 μm.sup.2. The interface -26- (between the focal track and the support body) here runs parallel to the focal track plane and centrally through the examined sample area -21- (in each case parallel to the sides thereof). Furthermore, it runs parallel to the radial direction -RD- (cf. e.g. direction -RD- in FIGS. 3, 4B). As is explained above with reference to FIG. 4A, the examined sample area -21- is scanned with a grid of 4 μm.
(17) To determine the mean grain boundary spacing (or small-angle grain boundary spacing), grain boundaries and grain boundary portions having a grain boundary angle of greater than or equal to a minimum angle of rotation within the examined sample area -21- are made visible using the EBSD analysis. In the present case, a minimum angle of rotation of 15° is set in the scanning electron microscope to determine the mean grain boundary spacing. The examined portion of the rotary X-ray anode in this case has an (overall) degree of deformation of 60%. Here, it is to be taken into consideration that, on account of the high hardness of the focal track, the (local) degree of deformation of the focal track per se is lower, whereas the (local) degree of deformation of the support body is higher at least in certain portions. In particular, the degree of deformation of the support body increases away from the focal track in a direction perpendicular to the focal track plane toward the bottom. Accordingly, the result of the examination is dependent respectively on the (overall) degree of deformation of the examined portion and also on the position of the examined sample area -21-. On account of the explained position of the examined sample area -21- in the region of the interface -26-, both the examined focal track portion -22- and the examined support body portion -24- are spaced apart from the interface -26- by less than 1 mm (this is relevant in particular in terms of the support body, in which different degrees of deformation arise depending on the height, i.e. in a direction parallel to the axis of rotational symmetry). The scanning electron microscope determines and represents, within the examined sample area -21-, grain boundaries or grain boundary portions between two grid points -17- whenever a difference in orientation of the respective lattice of ≧15° is determined between the two grid points -17- (if a different minimum angle of rotation is set, the latter is significant). The difference in orientation used in each case is the smallest angle which is required to transfer the respective crystal lattices present at the respective grid points -17- to be compared into one another. This process is carried out for each grid point -17- in respect of all grid points surrounding it (i.e. in each case in respect of six surrounding grid points). FIG. 4A shows, by way of example, a grain boundary portion -20-. A grain boundary pattern -32- which is formed in the case of a partially recrystallized structure (given a minimum angle of rotation of 15°) by grain boundary portions and circumferential grain boundaries is thereby obtained within the examined sample area -21-. This is represented schematically in FIGS. 4C and 4D for a section -28- of the focal track. If a minimum angle of rotation of 5° is set, the small-angle grain boundaries of the substructure can also be made visible in addition (these are not shown in FIGS. 4C and 4D).
(18) Hereinbelow, the determination of the mean grain boundary spacing of the focal track material parallel to the focal track plane will be explained. To determine the grain boundary spacing of the focal track material, in each case only the focal track portion -22- measuring approximately 850 by 1700 μm.sup.2 of the examined sample area -21- is evaluated. Here, in the process explained in the present case, the mean grain boundary spacing is determined along the direction -RD-, i.e. along a direction running parallel to the focal track plane (or to the interface -26- in FIG. 4B) and substantially radially. To this end, a group -34- of 98 lines each having a length of 1700 μm and a relative spacing of 17.2 μm (1700 μm/99) is placed into the grain boundary pattern -32- within the examined sample area -21- (which has an area of 1700×1700 μm.sup.2). In FIG. 4C, this is shown schematically for a section -28- of the focal track placed within the examined focal track portion -22-. The group of lines -34- here runs parallel to the examined surface (or cross-sectional area) and the individual lines each run parallel to the direction -RD-. Respectively the spacings between in each case two mutually adjacent intersections between the respective line and lines of the grain boundary pattern -32- are determined on the individual lines. In the regions in which the end of a line does not form an intersection with a line of the grain boundary pattern -32- (i.e. forms an open end because it reaches the boundary of the examined focal track portion -22-), the length of the portion from the line end up to the first intersection with a line of the grain boundary pattern -32- is evaluated as half a crystal grain. The frequency of the various spacings which were determined within the focal track portion -22- (approximately 850×1700 μm.sup.2) is evaluated, and then a mean value of the spacings is formed (corresponds to the sum total of the detected spacings divided by the number of measured spacings). The process described for determining the mean grain boundary spacing is also referred to as “Intercept Length”. The determination of the mean grain boundary spacing perpendicular to the focal track plane, i.e. along the direction -ND-, is effected correspondingly within the focal track portion -22-. In turn, a group -36- of (again 98) lines is placed into the grain boundary pattern -32-. The group of lines -36- here runs parallel to the examined surface (or cross-sectional area) and the individual lines each run parallel to the direction -ND-. This is shown schematically for the section -28- in turn in FIG. 4D. The spacings are evaluated in a manner corresponding to that explained above. In this way, it is possible to indicate a measure of the fine-grained nature of the structure which is formed from (large-angle) grain boundaries and (large-angle) grain boundary portions. The mean grain boundary spacing parallel to the focal track plane is in this case generally greater than the mean grain boundary spacing perpendicular to the focal track plane. This effect is brought about by the action of force perpendicular to the focal track plane during the forging operation. The mean grain boundary spacing d can then be determined from the mean grain boundary spacing parallel to the focal track plane d.sub.p and the mean grain boundary spacing perpendicular to the focal track plane d.sub.s, as is apparent on the basis of the following equation:
d=√{square root over (d.sub.p×d.sub.s)}
(19) In a corresponding manner, the determination of the mean (small-angle) grain boundary spacing of the portion of the focal track parallel and also perpendicular to the focal track plane can be carried out stating a minimum angle of rotation of 5°. The mean small-angle grain boundary spacing can then be determined therefrom in turn in accordance with the formula indicated above. By stating a minimum angle of rotation of 5°, the small-angle grain boundaries of the substructure (which is present in the deformation structure) are additionally taken into consideration. In this way, it is possible to indicate a measure of the fine-grained nature of the structure which is formed from (large-angle) grain boundaries, (large-angle) grain boundary portions and small-angle grain boundaries.
(20) The degree of recrystallization can be determined on a microscopic level by virtue of the fact that the areal proportion of the crystal grains formed by new grain formation (relative to the total area of the examined portion) is determined in a microsection, as shown schematically for example in FIGS. 1A-1C. This determination can be effected in turn with a scanning electron microscope during an EBSD analysis. In this respect, reference is made to the measurement arrangement and sample preparation explained above with reference to FIGS. 4A to 4D and the measurement process explained. The minimum angle of rotation stated here is in particular an angle of ≧15°, so that the profile of the large-angle grain boundaries can be seen. In this way, it is possible to determine in particular the circumferential grain boundaries of the crystal grains formed by new grain formation and also the (large-angle) grain boundary portions. Furthermore, in addition the same region can also be examined stating a minimum angle of rotation of ≧5° (or another small value for the minimum angle of rotation) in order to check whether the individual crystal grains are crystal grains formed by new grain formation (these do not have a substructure). Then, the ratio of the area of the crystal grains formed by new grain formation relative to the total area examined is determined.
(21) Furthermore, the degree of recrystallization can also be estimated on the basis of the hardness. This can be effected, for example, by virtue of the fact that, after the forging operation, a plurality of samples produced in the same way are each subjected to heat treatments for a predetermined duration at a respectively different temperature (if appropriate, in addition or as an alternative the duration of the heat treatment can also be varied). A hardness measurement is then carried out on the samples at an identical position in each case (within the sample). Thus, substantially the course of the curve shown in FIG. 2 can be traced, and it is possible to establish the region of the curve in which the respective sample lies. As explained above, work is preferably performed within the region -TB- around the recrystallization threshold -RKS- (the region -TB- in FIG. 2 being shown schematically by the circle with dashed lines around the recrystallization threshold -RKS-).
(22) Within the context of determining the degree of recrystallization, it is generally to be taken into consideration that extended recovery processes take place in the case of certain materials (e.g. in the case of molybdenum and molybdenum alloys). According to a notion which is sometimes represented, these recovery processes can also lead to nuclei for new grain formation. Where new grain formation takes place from these nuclei, within the context of this description this type of new grain formation is also encompassed by the term recrystallization. If extended recovery processes occur, the graph in FIG. 2 already falls to a greater extent in the region of the recovery processes -EH-, and the recrystallization threshold can shift toward higher temperatures. At least in the region -EB-, in which the structure is recrystallized, the graph then again runs in a manner corresponding to that in the case of a material without extended recovery processes. In particular, in terms of quality there is a deviation, as shown schematically in FIG. 2 by the dashed line. In the case of molybdenum-based alloys, this effect is additionally superposed by the formation of particles, which can likewise have an effect on the specific curve profile. In terms of quality, however, the curve profile is always substantially as shown in FIG. 2.
(23) The text which follows explains the production of a rotary X-ray anode according to the invention according to one embodiment of the present invention. Firstly, the starting powders for the support body are mixed and also the starting powders for the focal track are mixed. The starting powders for the support body are chosen in such a manner that what is obtained for the support body (apart from impurities) is a composition of 0.5% by weight Ti, 0.08% by weight zirconium, 0.01-0.04% by weight carbon, less than 0.03% by weight oxygen and the remaining proportion molybdenum (after the conclusion of all heat treatments carried out as part of the powder metallurgy production) (i.e. TZM). Furthermore, the starting powders are chosen in such a manner that what is obtained for the focal track (apart from impurities) is a composition of 10% by weight rhenium and 90% by weight tungsten. The starting powders are pressed as a composite with 400 tons (corresponds to 4*10.sup.5 kg) per rotary X-ray anode. Then, the body obtained is sintered at temperatures in the range of 2000° C.-2300° C. for 2 to 24 hours. The starting body (sintered body) obtained after the sintering has in particular a relative density of 94%. The starting body obtained after the sintering is forged at temperatures in the range of 1300° C. to 1500° C. (preferably at 1300° C.), with the body having a degree of deformation in the range of 20-60% (preferably of 60%) after the forging step. After the forging step, the body is subjected to a heat treatment at temperatures in the range of 1300° C. to 1500° C. (preferably at 1400° C.) for 2 to 10 hours. Where ranges are indicated within the context of this exemplary embodiment, good results can be achieved respectively for various combinations within the respective region. Whereas the parameters indicated for the pressing step and for the sintering step are less critical for the properties according to the invention of the focal track (and substantially also for the described advantageous properties of the support body), the temperatures during the forging step and during the subsequent heat treatment in particular have an effect on the properties of the focal track (in particular on the degree of recrystallization thereof). In particular, particularly good results are achieved given the temperature values indicated with preference for the forging step and for the step of the subsequent heat treatment (given the degree of deformation indicated with preference of 60%).
(24) In the case of rotary X-ray anodes which were produced according to the exemplary embodiment explained above, it was possible to achieve a hardness of 450 HV 30 for the focal track and a hardness of 315 HV 10 for the support body. The hardness measurements here are to be carried out on a cross-sectional area running through the axis of rotational symmetry. In the case of the support body, it was further possible to achieve a 0.2% elongation limit R.sub.p 0.2 of 650 MPa (megapascals) and an elongation at break A of 5% at room temperature. In this respect, a sample running radially in the support body is to be used as the measurement sample. Method B described in DIN EN ISO 6892-1 and based on the stress rate is to be employed as the measurement process. In comparison to this, hardnesses of at most 220 HV 10 and also lower elongation limits are typically achieved in the case of conventional support bodies produced by powder metallurgy (except for special alloys and materials reinforced with additional particles).
(25) Accordingly, these results show that considerably higher hardnesses (of the focal track and also of the support body) and higher elongation limits (at least in the case of the support body) are achieved in the case of the rotary X-ray anodes according to the invention than in the case of rotary X-ray anodes produced conventionally by powder metallurgy. Furthermore, these investigations show that sufficient ductilization of the support body material can be achieved by a heat treatment, following the forging operation, at temperatures in the region of the recrystallization threshold (of the support body material). In the case of such a “gentle” ductilization (i.e. heat treatment at relatively low temperatures), there is the simultaneous effect that the structure of the focal track continues to remain very fine-grained. The ductilization achieved can be identified in particular on the basis of the values obtained for the elongation at break A at room temperature. In the case of a sample which has not been heat-treated, the elongation at break of the (pressed, sintered and forged) support body material is typically ≦1%. The ductilization can avoid a situation where the rotary X-ray anodes are brittle and fragile.
(26) On rotary X-ray anodes formed according to the invention, the focal track was examined at the end of its service life. In this case, it was possible to determine that cracks are diverted in each case along the grain boundaries of the fine-grain structure and therefore repeatedly change the direction of propagation. On account of this crack diversion along the fine-grained structure, the propagation of cracks deep into the focal track is avoided. It was also possible to observe a uniformly distributed crack pattern with uniformly formed cracks on the surface of the focal track at the end of its service life. By contrast, on comparative rotary X-ray anodes in which the focal track was produced by vacuum plasma spraying, the crystals of the focal track have a columnar form and are oriented perpendicular to the focal track plane. A crack consequently propagates along the grain boundaries deep into the focal track (and if appropriate down to the support body).
(27) To investigate the texture of the focal track and of the support body, a rotary X-ray anode as explained above with reference to FIGS. 4A to 4D was prepared as the sample to be examined. The rotary X-ray anode here was formed according to the invention. The focal track had (apart from impurities) a composition of 90% by weight tungsten and 10% by weight rhenium, whereas the support body (apart from impurities) had a composition of 0.5% by weight Ti, 0.08% by weight zirconium, 0.01-0.04% by weight carbon, less than 0.03% by weight oxygen and the remaining proportion molybdenum. The measurement arrangement too corresponds to the arrangement explained above. In the measurement process, the settings explained above with reference to FIGS. 4A to 4D were used, insofar as these are applicable or are to be performed for determining the texture. The inverse pole figures obtained in the course of the EBSD analysis of the focal track are shown in FIGS. 5A-5C. In this respect, the macroscopic directions perpendicular to one another, -ND-, which runs perpendicular to the focal track plane in the respectively examined region, -RD-, which runs substantially radially and parallel to the focal track plane, and also -TD-, which runs tangentially and parallel to the focal track plane, were defined in relation to the focal track (these directions are drawn in for visualization in FIG. 3). In the forging operation during the process for producing the associated rotary X-ray anode, the force acted perpendicular to the focal track plane (i.e. along the direction -ND-). FIG. 5A shows the inverse pole figure of the focal track in the direction -ND-, FIG. 5B shows the inverse pole figure in the direction -RD- and FIG. 5C shows the inverse pole figure in the direction -TD-. The pronounced preferential texturing in the <111> direction and the <001> direction along the direction -ND- can be identified with reference to FIG. 5A. Furthermore, the (less) pronounced preferential texturing in the <101> direction along the directions -RD- and -TD- can be identified with reference to FIGS. 5B and 5C. Corresponding results were achieved for the texture of the support body which was determined in the outer region of the rotary X-ray anode. In particular, a pronounced preferential texturing in the <111> direction and the <001> direction along the direction -ND- and also a (somewhat less) pronounced preferential texturing in the <101> direction along the directions -RD- and -TD- were measured.
(28) For comparison, correspondingly prepared samples of a focal track made of pure tungsten and applied by a CVD process (cf. FIG. 6) and of a focal track produced by vacuum plasma spraying (cf. FIG. 7) and made of a tungsten-rhenium alloy (tungsten proportion: 90% by weight, rhenium proportion: 10% by weight) were investigated in respect of their texture. FIG. 6 in this respect shows the inverse pole figure in the direction -TD-. As is apparent with reference to FIG. 6, the focal track applied by CVD coating has a preferential texturing in the <111> direction along the direction -TD-. FIG. 7 shows the inverse pole figure in the direction -ND-. As is apparent with reference to FIG. 7, the focal track produced by vacuum plasma spraying has a pronounced preferential texturing in the <001> direction along the direction -ND-.
