NO DRAWINGS 1194, Inventor: DAIN STEDMAN EVANS Date of filing Complete Specification: 25 Oct., 1968.
Date of Application (No. 50181/67): 3 Nov., 1967.
Complete Specification Published: 10 June, 1970.
Index at acceptance:-.C7 A(A23Y, A237, A239, A24X, A241, A243, A245, A247, A249, A25Y, A253, A255, A257, A259, A260, A263, A266, A269, A27X, A272, A276, A279, A299, ASOY, A30X, A300, A303, A305, A307, A309, A31X, A311, A313, A316, A319, A320, A323, A326, A329, A339, A349, A35Y, A36Y, A360, A362, A364, A366, A369, A37Y, A370, A375, A377, A379, A38X, A381, A383, A385, A387, A389, A409, A41Y, A41X, A410, A414, A416, A418, A42X, A422, A425, A428, A43X, A432, A435, A437, A439, A44Y, A440, A447, A449, A45X, A451, A453, A455, A457, A459, A48Y, A48X, A481, A483, A485, A487, A489, A49X, A491, A493, A595, A497, A499, A50X, A501, A503, A505, A507, A509, A529, A549, A579, A599, A609, A61Y, A61X, A615, A617, A619, A62X, A621, A623, A625, A627, A629, A67X, A670, A671, A672, A673, A674, A675, A677, A679, A68Y, A68X, A681, A682', A683, A685, A686, A687, A688, A689, A69X, A690, A693, A694, A695, A696, A697, A698, A699, A70Y, A70X, 71X, 749, 750, 751, 752, 753, 758); 07 D(8A1, 8A2, 8B, 8C, 8D, 8E, 8G, 8H, 8J, 8Q, 8R, 8S, 8V, 8W, 8Y, 8Z1, 8Z9, 8Z10, 8Z11, 8Z12, 8Z13, 8Z14, Al); 07 U(3, 7L) International Classification: -C 22 c 27/00 COIMPLETE 'SPECOIFlICATION Improvements in or relating to Metal Bodies and their Manufacture We, THE GENERAL ELECTRIC AND ENGLISH ELECTRIC COMPANIES LIMITED, formerly The General Electric Company Limited, of 1 Stanhope Gate, London, W. 1., a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: -
This invention relates to metal bodies of the kind in which the main constituent consists of tungsten, or of molybdenum, or of both tungsten and molybdenum, and more particularly to metal bodies of this kind which 'are capable of being rendered resistant to oxidation at elevated temperatures by the formation, on the surface of the body, of a protective layer derived from the material of the body itself. The invention also relates to oxidation-resistant metal bodies consisting of metal bodies of the said kind which have been provided with such a protective layer, and to the manufacture of such oxidation-resistant bodies.
Tungsten and molybdenum, and alloys of which these metals form the base, are, by virtue [Pi, of their refractory nature and their mechanical properties, suitable for use in the construction of components which are subjected to high temperatures in operation. However, since tungsten and molybdenum are oxidisable by exposure to air at elevated temperatures, the surfaces of such components are liable to suffer severe oxidation in use, resulting in curtailment of the useful life of the components or even in rapid failure of the components in operation.
Tungsten- and/or molybdenum-based metal bodies which are resistant to oxidation are described in Patent Specification No. 11,086,708, these bodies containing, in addition to tungsten and/or molybdenum comprising the main constituent, at least one metal which is preferentially oxidisable with respect to the main constituent and at least one of the metals of Group VIII of the Periodic Table of the elements consisting of cobalt, nickel, ruthenium, rhodium, palladium, and platinum, and having formed upon their surfaces a protective coating consisting of or including a compound containing oxygen and the said preferentially oxidisa4ox )00 21194,2 ble metal derived from a surface layer of the metal body.
We have now found that the oxidation resistance of metal bodies of the type described in the said specification can be improved by the inclusion of certain elements of Groups IVA, VA and VIA of the Periodic Table as constituents of the metal bodies.
Thus according to the first aspect of the present invention, a metal body of which the main constituent consists of tungsten, or of molybdenum, or of both tungsten and molybdenum, is composed of, or has at least a surface layer composed of, an alloy consisting of, by weight, from 5.% to 20% in aggregate of at least one of the preferantially oxidisable metals chromium, thorium, titanium, hafnium, zirconium, uranium, magnesium, cerium, aluminium, beryllium, from 0.01% to l10% in aggregate of at least one of the metals of Group VIII of the Periodic Table of the elements consisting of cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, from 0.001% to 10% in aggregate of at least one of the elements of the A Subgroups of the Periodic Table consisting of silicon, germanium, tin, phosphorus, arsenic, antimony, bismuth, sulphur, selenium, and tellurium, and the balance tungsten or molybdenum or both tungsten and molybdenum in any relative proportions, the relative proportions of the said preferentially oxidisable metal, Group VIII metal, and A Sub-group element present in the said alloy being such that the metal body is capable of being rendered resistant to oxidation at elevated temperatures by the formation upon the surface of the alloy, when subjected to an oxidising heat treatment in air or moist hydrogen, of an adherent protective layer consisting of oxygen and elements derived from the said alloy.
It is to be understood that the use herein of the term "preferentially oxidisable" with reference to the metals chromium, thorium, titanium, hafnium, zirconium, uranium, mangnesium, cerium, aluminium and beryllium, means that these metals are capable of being oxidised preferentially with respect to the tungsten and/or molybdenum forming the main constituent of the metal body. The preferentially oxidisable metal included in the said alloy forming the metal body or a surface layer thereof is preferably chromium. In some cases the preferentially oxidisable metal (for example when one of the refractory metals titanium, hafnium, or zirconium is employed) when distributed throughout the metal body may have a beneficial effect on the mechanical properties of the metal body as a whole.
The proportion of the Group VIII metal or metals present in the said alloy is preferably from 11% to 5% by weight of the alloy. The preferred Group VIII metal for use in accordance with the invention is palladium.
The nreferred A Sub-zroup elements for inclusion in the said alloy are silicon, germanium, phosphorus, arsenic, sulphur and selenium.
It is to be understood that the presence of accidental impurities, which might be introduced with the starting materials or by contamination during manufacture of the metal body, and which do not have any appreciable adverse effect on the oxidation resistance of the metal body, is not excluded.
If desired, the metal body in accordance with the first aspect of the invention may be wholly composed of an alloy as aforesaid. Alternatively the metal body may consist of a main part composed of tungsten and/or molybdenum, possibly. with one or more additional constituents which may be required for modifying the mechanical or other properties of the body in known manner and which will not have any deleterious effect on the oxidation resistance of a protective surface layer formed thereon, and a surface layer, of any desired thickness, formed of a said alloy. Examples of additional constituents which may be included in the main part of a metal body of the latter type are other refractory metals such as rhenium or tantalum, and additives which might be used in amounts of 1-2% by weight for influencing the grain structure of the base metal in known manner, such as thoria or potassium silicate. A metal body in accordance with the invention having only a surface layer of a said alloy may be advantageous for some applications, in that it enables the metal body as a whole to retain the mechanical properties characteristic of the main constituent, while having improved resistance to oxidation at elevated temperatures.
It will be appreciated that the degree of resistance to oxidation required to be imparted to a metal body in accordance with the invention in any particular case may vary considerably, depending upon the expected conditions of use of the component formed from the metal body, especially upon the temperature to which the component will be heated in operation and the length of time for which the component will be required to be subjected to a high temperature. 'In general, a metal body is to be regarded as "oxidation resistant" at a given 1 temperature if it can be maintained at that temperature for at least 15 minutes with a weight change of less than 10 milligrams per square centimetre of surface area, but in many cases the degree of oxidation resistance re- 1 quired will be very much greater than this.
The degree of oxidation resistance of a metal body at a given temperature, which can be measured by the life of the protective surface layer, that is to say the length of time for 1:
which the metal body can be maintained at that temperature before failure of the protective layer and the consequent onset of rapid oxidation, or by the actual rate of oxidation as indicated by the gain in weight of the body 1:
[10 1l194,6W0 in unit time, is determined by the composition of the alloy forming the body or a surface layer thereof, and by the heat treatment employed for forming the protective layer. Hence the choice of the preferentially oxidisable metal, of the Group VIII metal and of the A Sub-group element used, and of the amounts within the ranges specified above, of these metals and element included in the alloy, and the choice of the heat treatment employed, will be decided by the degree of protection which will be acceptable for a particular application of the metal body when provided with a protective surface layer. In general, the degree of oxidation resistance which a metal body is capable of attaining as a result of a given heat treatment increases with increasing contents of preferentially oxidisable metal, Group VIIHI metal, and A Sub-group element in the said alloy.
We have found that arsenic is a particularly advantageous A Sub-group element for inclusion in the said alloy, and the invention will be further described mainly with reference to metal bodies containing arsenic.
The optimum amount of arsenic for inclusion in the alloy depends to some extent upon the proportions of preferentially oxidisable metal and Group VIII metal present. A preferred range of compositions for arsenic-containing alloys in which the preferentially oxidisable metal and the Group VIII metal are, respectively, chromium and palladium, consists of 51% to 20% chromium, 0.5% to 10%/ palladium, 0.01% to 11.0% arsenic, by weight,'the balance being tungsten and/or molybdenum.
The nature of the heat treatment, in respect of atmosphere (whether air or moist hydrogen), temperature and duration, which is required for forming a protective surface -layer on the metal body depends upon the composition of the alloy forming at least a surface layer of the body to be treated, as well as upon the conditions to which the body will be subjected in use. In some cases a satisfactory protective surface layer can be produced by heating the metal body in air, for example at a temperature in the range of 1100001C to 1500 C, depending upon the composition of the said alloy; in such cases the metal bodies are usually self-protective, that is to say the heat treatment required for forming the protective surface layer can be effected by the initial heating of the metal body in air under normal conditions of use, provided that the body is initially heated in use to a sufficiently high temperature to result in the formation of an adequate protective layer. Examples of metal bodies in accordance with the invention which are thus self-protective at temperatures of 1200 C to 1450 C are those composed of, or having a surface layer composed of, alloys consisting of 5% chromium, 1% to 5% palladium, 0.115% to 11% arsenic, and the balance tungsten,.or consisting of 10%/ chromium,. 1% to 5% palladium, 0.001% to 11% arsenic, arid the balance tungsten, or consisting of 10% chromium, 0.5% to 3% palladium, 0.117% to 1% arsenic, and the balance molybdenum, all percentages quoted being by weight.
The proportions of palladium (or other Group VIII metal) and arsenic '(or other A Sub-group element) required to render the metal bodies self-protective increase with increasing temperature to which the bodies are to be exposed in use: for example, in the cases of molybdenum-based alloys containing 10% chromium, with palladium and arsenic, the proportions of palladium required for achieving self-protective characteristics at 900 C, 1300 1C, 11400 IC, and 1500 iC, respectively, are 0.5%, 0.5% to 0.9%, 1%, and 2% to 3%, by weight, the preferred proportions of arsenic in these alloys being respectively 0.117%, 0J17% to 0.3%, 0.3% and 0.7% to 1.0%, by weight. An additional advantage of the self-protective metal bodies is that they are self-healing, that is to say if the protective surface layer becomes damaged or partially removed, it can be renewed by heating the metal body in air to a temperature similar to that at which the original protective layer was formed, or simply is renewed in actual use of the body at a suitable temperature.
In other cases, where the alloys forming at least a surface layer of the metal bodies are not self-protective, a heat treatment in moist hydrogen is necessary for forming the protective surface layer. In general, alloys with lower contents of preferentially oxidisable metal, or with lower Group VIII metal and arsenic contents, for a given preferentially oxidisable metal content, than those indicated above, require this treatment before being subjected to the temperatures of use in air. Examples of alloys which are not self-protective, but which can be provided with an oxidation-resistant surface layer by heating in moist hydrogen for 24 hours at 13000 C, are alloys consisting of 10% chromium, 0.01% to 0.5% palladium, 0.001% to 0.017% arsenic, by weight, and the balance molybdenum, and alloys -consisting of 5% chromium, 0.01% to 11% palladium, 0.001% to 0.015% arsenic, by weight, and the balance tungsten. In some cases, the oxidation resistance of self-protective alloys can be enhanced, in respect of the temperature and/or the length of time for which the protective surface layer is effective, by subjecting the alloys to pretreatment in moist hydrogen: for example, analloy consisting of 11.0% chromium, 0.5% palladium, 0.1OJ17% arsenic, and the remainder molybdenum, which is self-protective at 900 C., is rendered oxidation-resistant up to 1400 0C. by heat treatment in moist hydrogen.
Metallographic examination and microprobe X-ray analyses of the microstructure of metal bodies in accordance with the 'first aspect of the invention consisting of tungsten- and molybdenum-based alloys containing chro3.
1jI94AW miinn palladium and arsenic, which bodie have been subjected to a suitable oxidising heat treatment as indicated above, have shown that the protective surface layer includes at least one metallugically stable phase rich in palladium, containing arsenic, and having in solution a high ratio of chromium to tungsten or molybdenum, the palladium-rich phase or phases forming a layer at least partially coating the main body of the alloy which includes grains of tungsten or molybdenum; the palladium-rich phase is also present at the grain boundaries between the tungsten or molybdenum grains in the alloy body. The protective surface layer also contains some chromic oxide, mainly on the surface, and a small amount of tungsten oxide or molybdenum oxide may be present, resulting from oxidation of the tungsten or molybdenrUm taking place in the early stages of the heat treatment, before protection of these metals by the palladium-rich phase is completed.
Thus according to the second aspect of the invention, a metal body of which the rain constituent consists of tungsten and/or molybdenum, and which is resistant to oxidation at elevated temperatures, is composed of, or has at least a surface layer composed of an alloy consisting of the said main constituent, a minor proportion of at least one metal which is capable of being preferentially oxidised with respect to the main constituent, a minor portion of at least one of the metals of Group VIII of the Periodic Table of the elements consisting of cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, and at least one of the elements of the A Subgroups of the Periodic Table consisting of silicon, germanium, tin, phosphorus, arsenic, antimony, bismuth, sulphur,.selenium, and tellurium, in a.proportion in the range of 01)01% to 110% by weight of the said alloy, and thesaid metal body has formed upon the surface of said alloy an adherent protective layer composed of elements derived from the alloy together with oxygen, the main body of the alloy including grains of tungsten and/or molybdenum, and the said protective layer including an oxide of the, or each, said preferentially oxidisable metal and at least one metallurgically stable phase which is rich in the said Group VI H metal or metals, contains the said A Subgroup element or elements, and has in solution a high ratio of the said preferentially oxidisable metal or metals to tungsten and/or molybdenurmn, the said phase or phases forming a layer at least partially coating the said main -body of the alloy and being present at the grain boundaries between the tungsten and/or molybdenum grains in the main alloy body.
The oxidation resistance of the protective surface layer is achieved by the interaction of several factors in the composition of the said alloy, as well as by a snitable heat treatment as aoresaid. There must be a sufficient propore tion of the group V metal present in the surface layer to ensure that at least one phase e rich in this metal is present at all temperatures up to the maximum temperature to which the 70 metal body is to be subjected in use, and for obtaining a self-protective metal body the proportion of the Group VIII metal present must be such as to form the said phase or phases in a sufficient amount to provide a substantially complete coating on the main body of the alloy. It is evident that a Group VIII metalrich phase which has a reduced concentration of tungsten and/or molybdenum as compared with the alloy as a whole, serves to protect 80 the individual tungsten and/or molybdenum grains from oxidation, and that the preferentially oxidisable metal present in this phase sacrifically protects the tungsten and/or molybdenum content of the said phase. In the 85 cases of alloys containing chromium and palladitarthe proportions of these metals present in the protective phase or phases are such that the oamentraton of tngsten and/or molybdenum in this phase is reduced, by the presence 9( of these metals to 5% to 10% by weight These effects are enhanced by the inclusion of an A Sub-group element as aforesaid in the alloy forming the metal body or surface layer thereof. For example, in the cases of alloys contaming chromium, palladum and arsenic, the arsenic is preferetially, with respect to tungsten and/or molybdenum, dissolved in the palladium-rich phase, and further increases the ratio of chromium to tungsten and/or molybdenurn in this phase by suppressing the solubility of tungsten and/or molybdenum therein; if the ratio of chromium to tungsten and/or molybdenum is already relatively high, only a relatively small addition of arsenic is necessary 105 to produce the required low concentration of tungsten and/or molybdenum in the protective phase: hence self-protective alloys can be obtained with smaller proportions of arsenic, the higher the proportion of chromium present, as 110 has been indicated above. It is believed that an additional effect of the arsenic present in these alloys is to increase the ability of the palladium-rich phase to wet the tungsten and/or molybdenum grains, thus faciliating 115 the formation of a coating of this phase on the grains.
As indicated above, in some cases two or more palladium-rich phases, of slightly differing co=posiion, are formed, but these phases 120 are compatible with one another and have the same total effect as a single such phase in respect of the protection of the tungsten and/or molybdenum grains.
When a metal body in accordance with the 125 second aspect of the invention is subjected to oxidising conditions, the chromium (or other preferentially oxidisable metal) present in the palladium (or other Group VIII) metal)rich phase is oxidised progressively, and as this 130 1,194,600 oxidation takes place the metallic chromium in this phase is replenished by chromium derived from the main body of the alloy, this process continuing until both the palladiumrich phase and the main body of the alloy are depleted of metallic chromium to such an extent that the oxidation resistance of the palladium-rich phase can no longer be maintained, and the protective surface layer ultimately fails.
Some of the metal bodies in accordance with the second aspect of the invention, while having good oxidation resistance at temperatures in the range of 1200 C. to 1450 C. or 1500 C, show poor resistance at temperatures below 1200 C. In the cases of bodies containing arsenic, this characteristic appears to be dependent upon the arsenic content, a higher proportion of arsenic being required to render an alloy resistant to oxidation at the lower temperatures than is needed to render an alloy of the same composition, apart from the arsenic content, oxidation-resistant at temperatures above 12000 C. For example, a tungsten-based alloy containing 10%/ of chromium and 1% of palladium, by weight, requires 0.25% of arsenic to render it oxidation-resistant at temperatures in the range of 800 C. to 1200 iC., and in the case of a molybdenum-based alloy containing:10% of chromium and 3% of palladium an arsenic content of 1% imparts oxidation resistance at temperatures from 4O00C. to 1500 0G. In general the molybdenum-based alloys are less subject to low temperature failure than the tungsten-based alloys.
An additional advantage of the metal bodies of the invention is that the protective surface layers formed thereon are resistant to thermal cycling, that is to say the surface layers remain stable and adherent when the metal bodies are subjected to rapid fluctuations of temperature within limits varying with different alloy compositions, the degree of resistance to thermal cycling of any given alloy depending to some extent upon the maximum temperature to which it is heated during the cycles.
The initial metal body may be formed in a desired shape, prior to the formation of the protective surface layer, by any known convenient method. A body composed wholly of an alloy as aforesaid may be produced by arc melting the alloy constituents together in the required proportions and casting or, preferably, by pressing a mixture of powdered constituents in the required relative proportions to form a compact and sintering the compact, suitably at temperatures of 13000 C. to 1400 01C. Arsenic may be introduced into the mixture in elemental powder form, or in the form of a master alloy with palladium or any other Group VIII metal to be included in the alloy. All the powder constituents of the mixture preferably consist of particles of sizes smaller than five microns.
Alternatively, for the manufacture of a metal body having only a surface layer composed of the said alloy, a shaped body may be formed from tungsten and/or molybdenum, with or without any desired additional constituents for modifying the mechanical or other 70 properties of the body, referred to above, either by arc-melting and casting the metal or metals, or by pressing and sintering metal powder, and the required preferentially oxidisable metal,. Group VIII metal, and A Subgroup element may then be deposited upon the surface of the prefabricated metal body, for example by vapour deposition or electroplating, in known manner, the body then being heated to cause the deposited elements to diffuse into 80 the surface layer of the underlying base metal or alloy to form a layer of the alloy capable of being provided with a protective surface layer in accordance with the invention.
All of the above-described procedures for 85 manufacturing the initial metal body must, of course, be carried out in such a manner that oxidation of the tungsten and/or molybdenum is substantially avoided.
In some cases, where the metal body is 90 wholly composed of an alloy as aforesaid and is formed by a sintering technique, the heat treatment for producing the protective surface layer thereon may be carried out simultaneously with the sintering step, by sintering the 95 metal compact in moist hydrogen or in any other atmosphere suitable for forming a protective surface layer on the particular alloy employed, and by continuing the sintering process for a sufficient length of time to produce 100 the requisite protective layer. This procedure is only applicable when the sintering temperature required for the alloy concerned is also suitable for the formation of a satisfactory protective layer, and in general when the sintering ternperature is at least as high as the temperature to which the metal body will be subjected in use. If desired, an additional heat treatment, at a higher temperature if necessary, can be carried out after completion of the sintering; 110 alternatively if desired, any protective, or partiallv protective, layer formed during sintering can be removed, and the metal body can then be subiected to a further heat treatment for producing the required protective layer. If the i15 protective layer formed during sintering is removed during fabrication, or if the metal body is Droduced by a process other than sintering or has only a surface layer formed of the requ;red alloy, then a suitable heat treatment as 120 indicated above is carried out for the production of a protective surface layer.
Metal bodies in accordance with the invention which have been formed by arc melting are preferably subjected to a homogenising 125 heat treatment, for example for 100 hours, which may also incorporate the heat treatment for forming the protective surface layer. Sintered metal bodies may be similarly homogenised if desired, especially those consisting 130 of molybdenumrn-based alloys. It is usually preferred to carry out such homogenising treatment at least partially at the same temperature as that to which the metal body is subsequently to be subjected in use or during oxidation resistance tests, in order to produce the metallurgical structure characteristic of the temperature of test and to avoid the occurrence of changes in constitution of the alloy during such l0 subsequent heating. In the case of sintered tungsten-based alloys, however, a homogenising treatment should only be applied to alloys containing relatively high proportions of arsenic, at least 0.025% by weight, since such treatment has a marked deleterious effect on oxidation resistance in the cases of tungsten based alloys containing smaller proportions of arsenic.
Some specific examples of metal bodies in accordance with the invention, which we have produced and tested for oxidation resistance, are listed in the following Table. In all the examples, the metal bodies were wholly composed of sintered alloys of tungsten or molybdenurn, as indicated in the column headed Base metal", with chromium, palladium and arsenic: the proportions of these three elements given in the next three columns, which are in weight percentages of the total weight of the alloy, are those which were included in the initial powder mixture, the proportions actually present in the final alloy possibly being slightly different as a result of loss of constituents during sintering.
The powdered constituents employed for the production of the alloy bodies of the examples were all of particle sizes approximately one micron, the arsenic being incorporated in the powder mixture with palladium, in the form of a 62%/, palladium - 38%/ arsenic master alloy in Examples 1 and 2, and as a 69%palladium - 311%/ arsenic master alloy in the rest of the examples the compositions of these master alloys are by weight. All the metal bodies of the examples were prepared by the following method:Immediately prior to the preparation of the alloy, the tungsten or molybdenum powder was given a reducing treatment, to remove any oxide present, by heating in hydrogen for 2 hours at 750 C; the tungsten or molybdenum powder was then thoroughly mixed with the chromium and palladium-arsenic powders by ball-milling under acetone for one hour, after which time the aoetone was decanted off and the powder mixture dried over a water bath.
- The dry powder was introduced into a rubber - bag of the desired shape of the body to be produced, the bag was evacuated and sealed, and was subjected to hydrostatic pressure of tons per square inch in glycerol. The cornm- pact was then removed from the bag and sintered by heating in moist hydrogen (dew point about 20C.) for 24 hours, at the temperature indicated in the Table for each example. Any further heat treatment to which the metal bodies were subjected after sintering is also indicated in the Table, in the next column to the sintering temperature.
The last three columns of the Table are concerned with the oxidation resistance tests carried out on the metal bodies of the examples, these columns showing, respectively, the temperature at which each sample was heated during the test in air flowing at the rate of one foot per second, the life of the protective surface layer on the body, and the mean oxidation rate during the life indicated, expressed as milligrams increase in weight per square centimetre of surface area per hour, where this has been determined. In the "Life" column, the symbol ">" preceding the figure indicates that the test was terminated after this time without failure of the protective surface layer, the absence of this symbol indicating that the protective layer failed after the length of time specified.
In most cases the metal bodies of the examples were self-protective, as is indicated by the fact that they were not given any further heat treatment after sintering (or sintering and homogenising) and prior to the oxidation test, the protective surface layer being formed spontaneously, in these cases, by heating in air at the temperature at which the test was carried out, at the commencement of the test: where no further treatment is specified, it is to be understood that the samples were tested in the as-sintered condition. In the cases of samples of molybdenum-based alloys which were subjected to a homogenising treatment, the surface layer formed during this treatment was removed before the oxidation test was carried out, so that the self-protective properties of the sample could be evaluated by the test. In I the case of all the self-protective alloy bodies tested at 113000C. or above, the protective surface layer was formed within two or three minutes of commencement of the heating at the test temperature; at lower temperatures the I formation of the protective layer took slightly longer.
1;194,600 -4 TABLE
Oxidation resistance tests Life Mean Composition of alloy of oxidation Sintering Test protective rate Base % % % tempera- tempera- layer (Mg/cm2/ Example metal Cr Pd As ture Further heat treatment ture (hrs.) hour) 0.1 0.015 0.055 0.001 0.001 0.001 0.015 0.015 0.1 0.25 0.25 0.46 0.46 0.03 1300 C.
homogenised for 100 hours in moist hydrogen at 1300 C. surface layer removed, then heated for 24 hours in moist hydrogen at 1300 C.
204 >432 >24 >196 >24 >175 >24 0.11 3.17 0.029 0.41 0.013 0.0019 0.193 0.0030 0.168 PIIs Q W W W W W W W W W W W W Mo TABLE
Oxidation resistance tests Life Mean Composition of alloy of oxidation Sintering Test protective rate Base % % % tempera- tempera- layer (Mg/cm2/ Example metal Cr Pd As ture Further heat treatment ture (hrs.) hour) 14 Mo 10 0.5 0.17 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. surface layer 1300 C. 48 removed, then heated for 24 hours in moist hydrogen at 1300 C.
Mo 10 0.93 0.3 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. then for 24 hours at 1400 C. 1400 C. 30(failed at 1450 -C.) 16 Mo 10 0.93 0.3 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C., then for 16 hours at 1450 C. surface layer removed, then 1450 C. 1 heated for 10 hours in moist hydrogen at 1450 C.
17 Mo 10 2 0.6 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. then for 8 hours at 1500 C. surface 1500 C. 38 removed, then heated for 1 hour in moist hydrogen at 1500 C.
I.I o6 %0 TABLE
Oxidation resistance tests Life Mean Composition of alloy of oxidation Sintering Test protective rate Base % % % tempera- tempera- layer (Mg/cm2/ Example metal Cr Pd As ture Further heat treatment ture (hrs.) hour) 18 Mo 10 3 1 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C., 400 C. <1400 19 Mo 10 3 1 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. 700 C. <686 Mo 10 3 1 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. 1000 C. <2105 0.005 21 Mo 10 3 1 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. 1200 C. 124 0.16 22 Mo 10 3 1 1300 C. homogenised for 10 hours in moist hydrogen at 1500 C. 1500 C. 14 23 Mo 10 3 0.1 1300 C. homogenised for 100 hours in moist hydrogen at 1300 C. 13000 C. 87 24 Mo 20 1 0.3 13000 C. - 13000 C. 202 _ N 11l94,00 The above Table shows the effects, on the degree of oxidation resistance of metal bodies according to the invention, at varying temperatures, of variations in the arsenic content of the alloys, of variation of the chromium content in tungsten-based alloys and, in the cases of molybdenum-based alloys, of variations in the palladium content in conjunction with variations in the arsenic content. The alloys of examples 13, 14, 16 and 117, which were pretreated in moist hydrogen, as indicated, were found to be non self-protective at the test temperatures used.