WO1990012485A2

WO1990012485A2 - Multiple source evaporation for alloy production

Info

Publication number: WO1990012485A2
Application number: PCT/GB1990/000610
Authority: WO
Inventors: Robert Lewis Bickerdike; Alan William Bishop; David John Bray; Robert William Gardiner; Brian William Viney
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1989-04-21
Filing date: 1990-04-20
Publication date: 1990-11-01
Anticipated expiration: 1991-10-21
Also published as: GB2248457B; GB9121008D0; EP0469050A1; GB8909088D0; GB2248457A; GB2230792A; WO1990012485A3; JPH04505478A

Abstract

An improved method and apparatus for the production of alloys by a process of evaporating and condensing constituent vapours utilises at least two evaporation sources communicating with a conduit which has a single discharge outlet facing a collector. Means are provided to heat the walls of the conduit to a temperature at least as high as the hottest source to suppress condensation and to promote mixing of the respective vapour streams by scattering from the hot walls. In a preferred arrangement sources (1, 2) are mounted vertically, one above the other, in a flue (4) heated by radiant heater (7). The flue radiates heat onto second source (2). The more volatile constituent is evaporated from lower source (1) by heater (6) and its vapour flows upwards past second source (2) into mixing zone (8). Here it mixes with vapour from source (2) and the mixture passes from the flue onto collector (5).

Description

MULTIPLE SOURCE EVAPORATION FOR ALLOY PRODUCTION

This invention relates to improved apparatus and a method for the preparation of alloys by a physical vapour deposition process of the type described in our earlier British Patents GB 1206586 and GB 1265965. The alloy constituents are evaporated from one or more evaporation baths to be condensed upon a temperature controlled collector and the apparatus is operated under vacuum within a vacuum chamber.

The apparatus and method described here are particularly adapted to produce substantial quantities of the desired alloy for engineering purposes. The deposit is of such a thickness and has sufficient structural integrity that it can be removed from the collector for working into sheet, strip or other wrought form and heat-treated before, during or after working to achieve the desired mechanical properties. Alternatively, the deposited alloy may be removed and pulverised for subsequent processing by powder metallurgical techniques, for example when it is desired to obtain articles close in form to the finished product.

Recently there has been growing interest in the possibility of obtaining improved magnesium alloys by creating new compositions achievable through rapid solidification rate (hereinafter referred to as RSR) production processes. Although magnesium is the lightest of the structural metals, its alloys have not found widespread use in aerospace applications, partly because of certain shortcomings in mechanical properties, but principally because of their poor corrosion resistance. In magnesium alloys produced by conventional non-RSR methods, the addition of elements such as aluminium, chromium or silicon which are known to form protective surface films in other alloy systems, is ineffective because of their insufficient solubilities in the magnesium matrix. Under normal equilibrium conditions, the concentration of such additives in solid solution is too low to provide an effective barrier to corrosion.

For the purpose of improving corrosion resistance it is very important that the additives should be assimilated into a solid solution with a uniform electrode potential. If they are allowed to segregate and form precipitates, and if the precipitates and matrix have different electrode potentials, then they effectively behave as tiny Galvanic cells and corrosion resistance is worsened rather than improved.

Rapid solidification and physical vapour deposition techniques provide the means to overcome thermodynamic constraints and achieve compositions which are beyond the scope of the ingot metallurgist by 'freezing' constituent atoms in position before they have the opportunity to migrate and segregate as they would in conventional melt chill processes. These techniques therefore offer a potential solution for improving corrosion resistance in magnesium alloys by increasing the population of corrosion - inhibiting species in the magnesium matrix without forming harmful precipitates.

Physical vapour deposition has several advantages over RSR processing. Firstly, the cooling rate is higher, thereby improving the opportunity for forming solid solutions. Secondly, the choice of potential alloying constituents is considerably widened because whilst certain elements may not be miscible in the molten state, perhaps because of phase differences, their vapours can be mixed intimately. This is particularly true in the case of magnesium because, at the melting temperature of many potentially interesting alloying additions, magnesium has a very high vapour pressure and will quickly evaporate away.

The preparation of magnesium alloys containing for example aluminium, chromium or silicon by the physical vapour deposition process poses special problems because of the large differences between the vapour pressure of magnesium and the vapour pressures of the additives. The patent specifications mentioned previously disclose that alloy constituents can be evaporated either from a single evaporator or from separate evaporators. It is desirable to use a single evaporator whenever possible both for simplicity and to encourage a uniform composition in the deposit. However, when alloy constituents have vapour pressures which differ by several orders of magnitude, as in the magnesium-chromium system for example, this is no longer practicable.

When separate sources are arranged side-by-side the composition of the deposit is non-uniform across the substrate due to imperfect mixing of the vapour streams. Better mixing can be achieved by increasing the separation of the collector from the sources but this has the effect of lowering the deposition rate.

One way of approaching this problem is to introduce lateral motion between the relative positions of the collector and the sources. In practice it is easier to keep the sources stationary and move the collector, either by rotation or by a reciprocating motion. In this way exposure of different parts of the substrate to the respective sources is equalized. Whilst a mobile collector offers considerable benefit in improving the homogeneity of the deposit, a small degree of local non-uniformity still arises because the deposit is effectively laid down as a series of sub-layers which are alternately rich in one particular constituent. In a structural member even this level of non-uniformity may be critical to its overall strength and could also be bad for corrosion resistance.

Hitherto vapour deposition from multiple sources has either been a compromise between homogeneity in the deposit and the rate of deposition or has required special equipment to effect movement of the substrate relative to the position of the sources. This latter option is complicated by the need to maintain the apparatus under vacuum.

It is an aim of this invention to provide an apparatus and method which facilitates the deposition of homogeneous alloys at a high rate from multiple evaporation sources even when the constituent elements have widely differing vapour pressures. The invention is an apparatus for the production of an alloy by a process of evaporating the alloy constituents under vacuum and condensing the constituent vapours upon a collector, said apparatus comprising: a conduit defining a rising flue which has a discharge opening at its upper end; a collector positioned so as to collect vapours issuing from the discharge opening of the rising flue; a first evaporation crucible located and configured so as to discharge into the rising flue at a position remote from the discharge opening thereof; a second evaporation crucible located within the rising flue at a position intermediate the discharge opening thereof and the first evaporation crucible said second evaporation crucible being configured and disposed such that there is an unobstructed flue passage therearound; a first heating means which heats a charge in the first evaporation crucible said first heating means being controllable so as to maintain the temperature of this charge at a desired level to generate an appreciable vapour pressure; a second heating means which heats a charge in the second evaporation crucible said second heating means being controllable so as to maintain file temperature of this charge at a desired level to generate an appreciable vapour pressure, and a wall heating means to heat the conduit in at least that portion thereof above the second evaporation crucible to a temperature at least as high as that of the-hotter evaporation crucible in order to suppress condensation of vapour on the walls of the conduit and thereby enhance mixing of the respective vapour streams.

In another aspect the invention provides a method for the production of an alloy by a process of evaporating the alloy constituents under vacuum and condensing the constituent vapours upon a collector, said method comprising: evaporating a charge of a first alloy constituent or first alloy constituents of relatively high volatility from a first evaporation crucible by heating this charge to a sufficient temperature to generate an appreciable vapour pressure; evaporating a charge of a further alloy consitituent or futher alloy constituents of relatively low volatility from a second evaporation crucible by heating this charge to a sufficient temperature to generate an appreciable vapour pressure; maintaining a stream of constituent vapours from the respective evaporation crucibles by continued heating of the charges; directing the constituent vapour streams into a rising flue defined by a conduit which houses at least the second evaporation crucible; heating the walls of the conduit in at least that portion thereof above the second evaporation crucible to a temperature at least as high as that of the second evaporation crucible in order to suppress condensation of vapour on the walls of the conduit; mixing the respective vapour streams in the conduit in that portion thereof above the second evaporation crucible, and discharging the mixed vapours from the rising flue to impinge upon a collector positioned above it.

Clearly, for successful operation of the invention the walls of the vessel must be formed from a material inert to the metal vapours with which they come into contact. The choice of material will therefore be determined by the nature of the metals to be evaporated.

The dimensions of the conduit must be such that sufficient scattering occurs by reflection of vapour atoms from its hot walls to promote thorough mixing. If the conduit is too large in relation to the sizes of the respective sources and the flow rates of vapour therefrom, efficient mixing may not take place because the separation distance between the conduit walls and the sources is too great for sufficient scattering to occur.

Conveniently the constituent evaporated from the lowermost evaporation crucible is the most volatile and is also the principal alloy constituent. Difficulties arising from unwanted condensation of vapour in undesired parts of the apparatus and reverse flow of vapour streams in the cooler regions are thus minimised.

Advantageously the flue is constructed in a number of longitudinal sections. This not only simplifies assembly of the apparatus between each operation by allowing easier access to the or each evaporation crucible within the flue passage, but also facilitates use of different materials in the different sections. For example, the lowermost section can be formed from a thermally insulating material to minimize conduction of heat downwards from the hottest regions of the

5 apparatus to the lowermost evaporation crucible. This is especially important when the lowermost evaporation crucible contains a source of material which is particularly volatile.

Although the inventors know of no theroretical reasons why the evaporation sources cannot be mounted side-by-side using an

10 arrangement of passages or nozzles to direct their vapour flows to a common mixing chamber, practical considerations favour the vertical configuration. Firstly, the general direction of vapour flow is inherently upward by virtue of the fact that the vapour evolves from a molten metal bath. To divert the flow from this generally upward path

15 requires the intervention of guide means which would need to be heated to a temperature at least as high as the hottest evaporation crucible, for the reasons given above. Thus extra energy is expended in keeping additional components hot. Secondly, because the vertical configuration is the most compact it can be accommodated in a smaller

20 vacuum chamber.

Another advantage of the invention compared to known physical vapour deposition apparatus is that the deposited alloy has a more uniform composition across the substrate without recourse to undesirably low rates of deposition or to translation of the collector

25 across the field of deposition. The equipment has the capability to produce homogenous alloys at a deposition rate of several millimetres per hour or even higher. The drawback of using very high rates of deposition is the occurrence of an unacceptable degree of porosity in the product so in practice the rate is optimised to achieve a sensible

30 microstructure.

By careful control of the vapour flow rates from the respective evaporators it is possible to deposit many constituents with wide differences between their vapour pressures.

The invention will now be described by way of example with

35 reference to the following drawings, in which: Figure 1 is a schematic diagram of the apparatus, and Figure 2 is a vertical section through a preferred form of the apparatus with some detail omitted from the right hand side for clarity. Referring now to Figure 1, reference numeral 1 denotes a lower evaporation source positioned at the bottom of a chimney 4 within which there is disposed a second evaporation source 2. Facing the mouth of chimney 4 is a collector 5 which receives the mixed vapours emanating therefrom. Sources 1 and 2 each have an associated heater 6, 7. The lower heater 6 operates directly to heat the walls of a crucible which houses the lower evaporation source 1 whilst the second heater 7 operates to heat up the walls of the chimney which in turn radiates heat to the second evaporation source 2. Thus vapour is generated from the second source whilst satisfying the condition that the chimney walls should be at least as hot as the hottest source.

Vapour from source 1 is supplied to the chimney 4 and flows past source 2 into a mixing zone 8 where it mixes with vapour from source 2. The mixing zone is simply that portion of the chimney above source 2 where both species of vapour coexist. Collisions between vapour stream 1 and the walls of the chimney or the second source 2 act so as to randomize movement of the vapour atoms and this assists thorough mixing with the vapour from source 2. Similarly, collisions between the vapour stream from source 2 and the walls of the chimney encourage intimate mixing by imparting random movement to the vapour atoms. The mixed vapour stream emerging from the mouth of the chimney is therefore substantially homogeneous and a deposit with uniform microstructure is formed on the collector 5.

Figure 2 shows a preferred form of the apparatus in vertical section with detail omitted from the right hand side for clarity. The arrangement depicted here has been developed particularly for the deposition of magnesium alloyed with lower vapour pressure constituents such as aluminium, chromium or silicon.

The lower evaporator comprises a mild steel crucible 10 standing on an alumina support ring 11 which acts so as to minimize conduction of heat away from the crucible to support table 12. The crucible is sealed with a gasket 30 and screw-top lid 31, both made from mild steel. Disposed in the centre of the lid and gasket assembly is a graphite nozzle 32 the purpose of which is to control the flow of magnesium vapour from the crudble. The nozzle, gasket and crucible lid are optional features which may be required for evaporating certain elements from the lower crudble. Earlier experiments with magnesium, for example, had demonstrated the presence of an oxide layer on the molten metal surface which hindered evaporation of the metal. When the oxide was present the evaporation rate was unacceptably low. Increasing the temperature had little effect until a point was reached when the oxide was dispersed to the periphery of the crucible, leaving a clean pool of molten metal in the centre. At this temperature the evaporation rate was too high. This difficulty was eventually overcome by operating the lower evaporator at a temperature suffidentiy high that the magnesium surface was dear of oxide and by interposing a nozzle at the mouth of the source to reduce the flow rate to the desired level. The required evaporation temperature for magnesium is in the range 700-800°C.

Support table 12 is recessed at 13 and 14 to provide positive engagement for the support ring 11 and lower heater support 15, respectively. Lower heater support 15 is an alumina tube having slots 16 therein through which are threaded heater element 17. The heater element 17 comprises a single strip of tantalum metal wound in the form of a helix around lower crudble 10. To reduce radiation losses the heater is surrounded by three stainless steel screens 18. Molybdenum disc 19 is then placed over the crudble and screens to reduce the amount of heat radiated from top heater 27 down to the mild steel crudble 10.

After passing through the nozzle 32 the vapour enters vertical chimney 40 which, in practice, is formed in three sections. Lower section 41 is made from alumina which has a poor thermal conductivity and therefore helps to insulate the first evaporation source from the higher temperatures prevailing at the second evaporation source. The middle section-42 and top section 43 are formed of graphite for reasons which will become apparent below. Crudble 20 holding the second evaporation source is located at the junction between the middle section 42 and the top section 43 of the chimney. It rests on a graphite support ring 21 which contains a number of holes to f adlitate passage of magnesium vapour from the lower source past the σudble 20 into the top section of the chimney. The top section of the chimney defines a mixing zone 44. Here the vapour from the lower source mixes with vapour from the second source before flowing out of the chimney and condensing on a temperature controlled collector 50. Different materials may be employed for the crudble 20 depending on the material to be evaporated. An alumina crucible is suitable for the evaporation of aluminium or chromium, whereas a vitreous carbon crudble is used for the evaporation of silicon.

A second radiant heater 27 is used to heat the chimney in the ^' vidnity of the second evaporation source. Heat radiated from the chimney is then used in turn to heat the second evaporation source to generate vapour therefrom. Successful operation of the invention depends on maintaining the temperature of the chimney at least as high as that of the second evaporator otherwise vapour condenses on the chimney walls. These difficulties are conveniently overcome by using the arrangement outlined here where a radiant heat source is used to heat the chimney which then radiates heat onto the crudble 20.

The reasons for choosing graphite as the material for the middle and top sections of the chimney are two-fold: Firstly, graphite has a high thermal conductivity and a high emissivity which allows rapid transfer of heat to the inside wall of the chimney and thence to the crucible 20 by radiation. Secondly, graphite is physically and chemically stable up to 3000°C in vacuum and can be easily machined to shape.

The upper heater 27 is also a single tantalum strip wound in a helix through slots 26 in tubular alumina heater supports 25. These upper heater supports are coupled to their lower counterparts 15 through the agency of spigots 24 which protrude through the molybdenum disc 19. The spigots 24 are dimensioned to be a loose fit in the bores of the alumina tubes. The operating temperature of the second evaporator will obviously depend on the material being evaporated. For example, aluminium and chromium require a temperature of 1300°C to achieve a satisfactory evaporation rate whilst silicon requires a temperature of about 1550°C. This figure represents the maximum feasible operating temperature of the apparatus as described because at higher temperatures the tantalum was susceptible to chemical interaction with the support material. By using a different combination of heater and support material the operating temperature range may be extended.

Since the chimney is much hotter in operation than the mild steel crudble 10, more radiation screens 28 are provided to surround it, the number of screens required being dependent on the evaporation temperature of the second source. In practice, nine screens are used for the evaporation of chromium whilst twelve screens are required for silicon. The three inner screens are made from molybdenum which does not melt until 2610°C and has a very low vapour pressure. The remaining screens are shielded from the heat source to be sufficiently cool to be formed from stainless steel without danger of contamination of the deposit.

Finally, five horizontally-disposed annular screens of molybdenum 29 are placed over the screens 28 to cover the heater 27 and shield the collector 50 from excessive radiant heating, without restricting the flow of vapour. Only one screen 29 is shown for darity.

Although the apparatus has been particularly described with reference to the deposition of binary magnesium alloys, it is equally well suited to otiier alloy systems where one of the major constituents has a much higher vapour-pressure than the other alloying ingredients, such as zinc alloys. It is also envisaged that more than two evaporation sources could i§ sue into a common mixing chamber. One means of accomplishing this would be to sustain a temperature gradient in the middle section of the chimney by differential heating which increases towards its mouth and to provide intermediate supports capable of supporting a third or further crudbles. Many other modifications will be apparent to those skilled in the art without departing from the scope of the following daims.

Claims

1. Apparatus for the production of an alloy by a process of evaporating the alloy constituents under vacuum and condensing the constituent vapours upon a collector, said apparatus comprising: a conduit defining a rising flue which has a discharge opening at its upper end; a collector positioned so as to collect vapours issuing from the discharge opening of the rising flue; a first evaporation crudble located and configured so as to discharge into the rising flue at a position remote from the discharge opening thereof; a second evaporation crudble located within the rising flue at a position intermediate the discharge opening thereof and the first evaporation crudble said second evaporation crudble being configured and disposed such that there is an unobstructed flue passage therearound; a first heating means which heats a charge in the first evaporation crudble said first heating means being controllable so as to maintain the temperature of this charge at a desired level to generate an appredable vapour pressure; a second heating means which heats a charge in the second evaporation crudble said second heating means being controllable so as to maintain the temperature of this charge at a desired level to generate an appredable vapour pressure, and a wall heating means to heat the conduit in at least that portion thereof above the second evaporation crucible to a temperature at least as high as that of the hotter evaporation crudble in order to suppress condensation of vapour on the walls of the conduit and thereby enhance mixing of the respective vapour streams.

2. Apparatus as daimed in daim 1 wherein the wall heating means serves also to heat the second evaporation crudble by causing radiant emission from the walls of the conduit.

3. Apparatus as daimed in daim 1 or claim 2 wherein the first heating means is controllable to maintain an appredable vapour pressure of the most volatile constituent or constituents and the second heating means is controllable to maintain an appredable vapour pressure of a further constituent or further constituents, the first and second heating means being controllable to maintain a temperature gradient in the rising flue which promotes flow of the respective vapour streams towards the discharge opening.

4. Apparatus as daimed in daim 3 wherein the flue is constructed in longitudinal sections in which the lowermost section is formed from a thermally insulating material to minimise conduction of heat from the hottest regions of the apparatus to the first evaporation σudble.

5. Apparatus as daimed in any preceding claim wherein the evaporation crudble containing the most volatile constituent or constituents has a nozzle to control the flow rate of vapour emanating therefrom.

6. A method for the production of an alloy by a process of evaporating the alloy constituents under vacuum and condensing the constituent vapours upon a collector, said method comprising: evaporating a charge of a first alloy constituent or first alloy constituents of relatively high volatility from a first evaporation crudble by heating this charge to a sufficient temperature to generate an appredable vapour pressure; evaporating a charge of a further alloy consitituent or futher alloy constituents of relatively low volatility from a second evaportation crudble by heating this charge to a suffident temperature to generate an appredable vapour pressure; maintaining a stream of constituent vapours from the respective evaporation crudbles by continued heating of the charges; directing the constituent vapour streams into a rising flue defined by a conduit which houses at least the second evaporation crudble; heating the walls of the conduit in at least that portion thereof above the second evaporation crudble to a temperature at least as high as that of the second evaporation crudble in order to suppress condensation of vapour on the walls of the conduit; mixing the respective vapour streams in the conduit in that portion thereof above the second evaporation crucible, and discharging the mixed vapours from the rising flue to impinge upon a colledor positioned above it.

7. A method as claimed in daim 6 comprising indirectly heating the charge in the second evaporation crudble by radiant emission from the heated walls of the conduit.

8. A method as claimed in daim 6 or daim 7 comprising evaporating the constituents from evaporation crudbles positioned one above the other and maintaining a temperature gradient in the rising flue to promote flow of vapour towards the collector by controlling the temperature of the first and second evaporation crucibles such that the second evaporation crucible experiences a higher temperature than the first evaporation crudble.