GB2085028A

GB2085028A - Platinum-based Alloys

Info

Publication number: GB2085028A
Application number: GB8100433A
Authority: GB
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1980-01-25
Filing date: 1981-01-08
Publication date: 1982-04-21
Also published as: GB2085028B

Abstract

A metallic material suitable for use under rigorous or hostile conditions, such as in the handling of molten glass or the preparation of X- ray fluorescence spectroscopy specimens, comprises a grain- stabilised alloy of gold and platinum containing not greater than 10 wt% gold. The grain-stabilising additive may be an oxide, carbide nitride or silicide of an element more reactive than Au or Pt during its formation. Examples of these reactive elements are Se, Y, Th, Zr, Hf, Ti, Al or a lanthanide, but the preferred stabiliser is ZrO2 or ThO2.

Description

SPECIFICATION Improved Apparatus This invention relates to metallic material suitable for use under rigorous or hostile conditions, particularly conditions which are liable to promote creep, embrittlement, contamination, corrosion and the like in the material.

There are numerous working operations which require the use of metallic apparatus and which, by their nature, are conducted under conditions which are rigorous and/or hostile to the material from which the apparatus is made. Apparatus which typically is subject to such conditions includes apparatus for handling molten glass and apparatus for use in the preparation of X-ray fluorescence spectroscopy specimens.

Apparatus for handling molten glass, particularly in the production of glass fibre, in which molten glass typically is contained in a reservoir or "bushing" having a baseplate equipped with perforations or orifices, which may be either jetted orjetless, through which the glass flows to form the fibres, is generally made from a platinum-rhodium alloy. Such bushings are typically operated at temperatures in the region of 1200-1 4000C and it has been found that platinum-rhodium alloys are necessary to give the required strength at these high temperatures particularly over weeks or even months of continuous service. Strength is, however not the only consideration.For example, molten glass is extremely corrosive and tends to attack the bushing material; moreover, cooling fins, typically made from silver because of its high heat conductivity, are normally provided beneath the baseplate and silver tends to contaminate the bushing material at these temperatures, leading to premature failure. A further disadvantage of platinum-rhodium alloys, at least when used for the manufacture of those parts of the baseplate from which issues molten glass, is that they have an affinity for the molten glass. Consequently, if a fibre breaks, glass issuing from the orifice tends to wet the outer surface of the orifice and cause "flooding"; that is to say, the molten glass spreads over the underside of the baseplate and/or the external surface of the jet, if fitted, rather than readily forming a fresh fibre.

One solution to this problem, applicable to jetless baseplates only (that is, where the orifices are defined merely by holes drilled or otherwise formed directly through the baseplate) has been proposed in British patent number 1242921.

Accordcng oo the spdcification of this pateno, the baseplate (referred to as "die plate") is characterised in that the "main body" of the plate is made of an alloy of rhodium and platinum and also optionally iridium and the "side" of the plate (that is, that part of the plate from which glass issues) is faced with an alloy of platinum and gold with optional further additions of rhodium, iridium, copper and/or palladium. The platinum/rhodium alloy imparts to the die plate the necessary strength while the platinum/gold alloy has good non-wetting characteristics which ensures good parting of the molten glass from the circular holes as well as minimising the possibility of flooding.A difficulty encountered with this arrangement, however, is that, under service coriditions, the gold tends to diffuse into the platinum/rhodium alloy with consequent gold depletion in the facing alloy, causing a lowering of the non-wetting characteristics.

A similar solution, this time for jetted baseplates, is described in British Patent Specification No. 1049517. Here, the bushings themselves, or the "tip plate" (that is, the jetted baseplate) or at least the terminai end of the tips, is constructed from an alloy of platinum and 1050% by weight of gold. There is a trade-off between lowering alloy melting point on one hand and increasing non-wetting properties on the other, and further the hardness increases to a peak and then decreases, as the gold content is increased. The optimum gold content generally is considered to be in the range 3050% by weight.A disadvantage of such alloys, however, is that, when formed into bushings, or at least the baseplates thereof, they are unacceptably weak in service and, at the elevated operating temperatures encountered in glass fibre manufacture, the bushings or baseplates tend to sag due to creep.

In an attempt to overcome this disadvantage, we investigated the effect of adding rhodium to binary gold/platinum alloys but we found that, although the desired creep resistance was thus obtained, the alloys were extremely difficult to work, to the extent that it was impossible, from a production point of view to fabricate jetted baseplates using the preferred method.Methods which can, in general, be used to fabricate jetted baseplates include a) pressing, in which small indentations are pressed into a baseplate at the positions where jets are to be located and platinum alloy is melted and applied drop by drop to each indentation, the resulting build-up being drilled to form an orifice; b) welding, in which jets are pre-manufactured and welded into holes formed in the baseplate; and c) coining followed by deep drawing, in which a baseplate is coined to provide a relatively greater thickness of alloy (a boss) in those regions where jets are to be formed, the baseplate is then deep-drawn in those regions and finally the resulting solid jets are punched through and smoothed to provide the orifices. As an alternative to deep drawing, punching may be used both to form the jet from the boss and to form an aperture in the jet.We prefer to use the coining/deep drawing or punching method because it isa relatively straightforward mechanical procedure carried out at room temperatures.

Turning now to the preparation of X-ray fluorescence spectroscopy specimens, it is the normal practice in the preparation of such specimens to melt the test material with a flux such as borax and pour the melt into a casting dish. Alternatively, the test material and flux are melted in the casting dish. Themelt is then cooled slowly to avoid stresses so that a substantially homogeneous sample results in the form of a "bead" or a "button".

It is important that the material used in the fabrication of the casting dish should exhibit at least some of the following properties:-- good hot strength, high resistance to the effects of thermal cycling, good workability, a low tendency to be wetted by the molten test material/flux mixture and a high resistance to grain boundary contamination.Considering these in order: (i) hot strength is required because it is important, from the point of view of accuracy in analysis, for the bead to have an essentially flat surface and, whereas this can always be achieved by grinding and/or polishing after removal of the bead from the casting dish, it is clearly more convenient for the bead to be capable of being used in the "as cooled" condition, which requires that the casting dish should accurately retain its shape at elevated temperatures; (ii) the greater the resistance to thermal cycling the less often will the casting dish require to be refabricated; (iii) good workability is not of absolute necessity but is a significant convenience in fabricating casting dishes; (iv) non-wetting is important in order to facilitate the removal of the cooled sample from the casting dish and (v) grain boundary contamination is the primary cause of failure of casting dishes, particularly since samples are often being prepared from unknown materials.

Unfortunately, hitherto it has proved to be impossible adequately to satisfy all of these requirements. For example, alloys of gold with platinum exhibit the non-wetting property but do not have adequate hot strength. Addition of rhodium increases the hot strength but decreases the workability and additionally, at higher rhodium concentraTions, the resistance to thermal cycling. None of the alloys in current usage exhibits adequate resistance to grain boundary contamination.

We have now found that the shortcomings of prior art materials from which is fabricated apparatus for use under rigorous or hostile conditions may be overcome by fabricating the apparatus from a grain stabilised alloy of gold and platinum or one or more platinum group metals.

According to the present invention, therefore, we provide a grain stabilised alloy of gold and platinum or one or more of the platinum group metals.

The alloy should contain not less than about 2% by weight and not more than about 10% by weight of gold. A preferred range is 3 to 8%, for example 5%.

Surprisingly, we have found that such an alloy is particularly suitable for use in apparatus which, in addition to being capable of withstanding rigorous or hostile conditions, is required to have a high resistance to wetting by molten materials, and we believe that adequate wetting resistance is achieved in such low-gold alloys partly by the presence of the grain stabilising agent.

We have found that apparatus made from alloys according to the invention is particularly useful for handling molten glass, for example in the production of glass fibre, and for the preparation of X-ray fluorescence spectroscopy specimens.

Accordingly the present invention also includes apparatus for use in handling molten glass and for the preparation of X-ray fluorescence spectroscopy specimens, respectively, the apparatus comprising a grain stabilised alloy of gold and platinum or one or more of the platinum group metals. In the handling of molten glass, the invention is especially applicable to the production of glass fibre because the alloy has a high resistance to wetting by molten glass. By "platinum group metals" we mean platinum, rhodium, palladium, ruthenium and iridium.

A grain stabilised alloy which we prefer to use is grain stabilised 5% gold-platinum.

Conveniently, the grain stabilising agent is in the form of a very fine dispersion of particles dispersed in a host matrix of the gold/platinum group metals. Preferably the paticles are in the form of an oxide, carbide, nitride or silicide of an element which is relatively more reactive, under the conditions of forming the oxide, carbide, nitride or silicide, than the gold and platinum group metal of the host matrix. Mixed compounds such as carbonitrides may also be used.

Examples of such relatively more reactive elements include scandium, yitrium, thorium, zirconium, hafnium, titanium, aluminium and the lanthanides. We prefer to use zirconium as its oxide, (i.e. zirconia); thoria may also be preferred.

The concentration of grain stabilising agent need not generally, at least when made by the preferred method, as hereinafter described, be greater than 0.5% by weight, preferably less than 0.1% by weight. An example of a grain stabilised material suitable for fabricating apparatus according to the invention is 5% gold-platinum containing 0.08% by weight of zirconia.

Grain stabilised material for use in apparatus according to the invention is preferably made, for best results, by the method described in our British Patent Number 1280815. According to that patent, a method of making a grain stabilised metal or alloy comprises spraying a starting material containing a metallic first material and a minor amount of a second metallic material through an atmosphere in which the second material reacts preferentially relative to the first material to form at least one stable metal compound, directing the sprayed starting material in molten condition on to a target to form an ingot, removing the ingot from the target and, thereafter, densifying the ingot by mechanical working. In the case of preparing a grain stabilised alloy, the metallic host material (for example, platinum/gold alloy) need not necessarily be prepared before spraying. If required, spraying can be accomplished using a mixture of metal powders (that is, the constituents of the alloy) which will alloy when fused.

A grain stabilised metal or alloy so prepared has particles of dispersed phase in the sub-micron size range and a relatively fine host grain structure which are determined to a large extent by the dimensions of the molten particles of the sprayed jet of such particles. Examination of such alloys and metals indicates a dispersion of the dispersed phase in the submicron particle size range, that is, too small for resolution using an optical microscope and typically in the range 200 1 oooA. Since the reactive constituent from which the dispersed phase is formed is molten at the same time as the host material, the reactive constituent solidifies to form the dispersed phase under conditions which approach thermodynamic equilibrium.Thus, where the dispersed phase is an oxide for example, and if the host material has any tendency to reduce the oxide, as to some extent all metals do, this tendency is satisfied in the molten condition, so that no further reactions occur occur at temperatures bellow the melting point.

Grain stabilised metals or alloys so prepared generally contain a small but detectable amount of the second metallic material in the unreacted state. A preferred maximum amount is 0.04% by weight but lower amounts are desirable, since the unreacted material tends to accumulate along the grain boundaries of the host material where it is liable to be oxidised under high-temperature conditions of use, leading to zones of potential weakness. However, the conditions of forming the sprayed ingot are such that 7580% conversion of the second metallic material is readily achieved which yields, in a grain stabilised metal or alloy containing 0.1% by weight of dispersed phase, a maximum of 0.025% by weight of unreacted material which is comfortably below the preferred maximum amount.

The dispersed phase contained in alloys and metal may be in the form of an oxide, carbide, nitride, or silicide or mixtures thereof and, for the reasons stated above, such dispersed phases possess high stabilities even in a metal matrix.

Additional grain stabilisation is achieved by the dissolved absorbed or entrapped gas films which are associated with the molten spray when it impinges on the target or on previously deposited metallic material and which are, thereafter, permanently entrapped within the metallic matrix.

Grain stabilised metals or alloys used in this invention are characterised not only by the sub micron particle size range of the dispersed phase, as discussed above, but also by the highly-aligned grain structure, the individual grains being plate like (in the case of strip or sheet) or needle-like (in the case of wire) and having a high aspect ratio.

Typically, the aspect ratio is in excess of 10:1 although figures in excess of 20:1 or even 50:1 are not uncommon and are to be preferred.

A grain stabilising agent which is in a very fine degree of dispersion is highly effective and it follows that a relative low concentration is required in order to achieve the desired stabilising effect. This is especially beneficial since any detrimental effects, such as impaired ductility, working characteristics and electrical properties, which would generally be expected in grain stabilised or dispersion strengthened materials produce by other methods and wherein the dispersed phase would therefore be present in higher concentrations, are avoided. We have found, in fact, that apparatus according to the present invention is considerably easier to fabricate than apparatus made from, say conventional creep-resistant gold/platinumcontaining alloys and, moreover, such fabrication may be carried out at room temperatures.

The advantages to be gained by using apparatus according to the present invention under rigorous or hostile conditions include: (i) greater creep resistance than conventional gold-containing alloys; (ii) freedom from grain growth and embrittlement under thermal cycling; (iii) enhanced resistance to progressive intergranular contamination due to the stable fine grain structure; (iv) relative cheapness compared with platinum-rhodium alloys and (v) high resistance to wetting by molten glass and other materials.

In addition, alloys according to the invention are sufficiently workable at room temperature to enable apparatus, for example jetted baseplates, to be readily fabricated therefrom by pressing, for example.

Claims

1. A grain stabilised alloy which has a high resistance to wetting by molten materials and which contains gold and one or more of the platinum group metals and a grain stabilising agent.

2. An alloy according to clam 1 containing from 2 to 10% by weight of gold.

3. An alloy according to claim 1 containing from 3 to 8% by weight of gold.

4. An alloy according to any preceding claim in which the grain stabilising agent is in the form of a fine dispersion of particles comprising an oxide, carbide, nitride or silicide of an element which is more reactive, under the conditions of forming the oxide, carbide, nitride or silicide, than the gold and the platinum group metals or a mixed compound containing such an element.

5. An alloy according to claim 4 in which the said element is scandium, yttrium, thorium, zirconium, hafnium, titanium, aluminium or a lanthanide.

6. An alloy according to claim 4 in which the grain stabilising agent comprises zirconia or thoria.

7. An alloy according to any preceding claim in which the concentration of grain stabilising agent is not greater than 0.5% by weight.

8. An alloy according to claim 7 in which the concentration of grain stabilising agent is not greater than 0.1% by weight.

9. An alloy according to any preceding claim consisting essentially, apart from impurities, of 5% by weight gold, balance platinum, containing 0.08% by weight of zirconia.

10. A grain stabilised alloy according to any of claims 4 to 9 in which the particles of grain stabilising agent are in the submicron size range.

1 An alloy according to claim 10 in which the particles have a size range of 200-1 0ooA.

12. An alloy according to any of claims 4 to 11 in which the particles of grain stabilising agent have a highly-aligned grain structure and an aspect ratio in excess of 10:1.

1 3. Apparatus for use in handling molten glass made from an alloy as claimed in any of claims 1 to 12.

1 4. Apparatus for the preparation of X-ray fluorescence spectroscopy specimens made from an alloy as claimed in any of claims 1 to 12.

15. A process for formation of an alloy as claimed in claim 1, in which a molten mixture or alloy of gold and platinum group metal is sprayed together with a minor amount of a molten further metallic material through an atmosphere in which the further material reacts preferentially relative to the mixture or alloy to form a stable metal compound, directing the sprayed material on to a target to form an ingot, removing the ingot from the target and, thereafter, densifying the ingot by mechanical working, the stable metal compound constituting the grain stabilising agent and being formed and solidifying under conditions which approach thermodynamic equilibrium.