Vapor deposition apparatus and method

Details Vapor deposition apparatus and method Vapor deposition apparatus and method

1993-04-21

1994-09-20

Rosenberg, Peter D.

Alloys or metallic compositions

Processes

75 1029, 266206, C22C 100, C21B 1110

Patent

active

053487036

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an improved apparatus and method for the production of alloys by a physical vapour deposition process generally as described in our earlier British Patents GB 1206586 and GB 1265965. In such a process, the apparatus is operated under vacuum within a vacuum chamber and the alloy constituents are evaporated from one or more evaporation baths before being caused to condense upon a temperature-controlled collector.
The apparatus and method described here are suitable for producing alloys in substantial quantities and with sufficient structural integrity that the deposits may be removed from the collector intact. The deposit can then be worked into sheet, strip or other wrought form and heat-treated to achieve the desired mechanical properties. As with conventionally-cast alloys, the deposited alloy may be heat-treated before, during or after working. Alternatively, the deposited alloy may be removed and pulverized for subsequent powder metallurgical techniques, for example when it is desired to produce an article close in form to the intended final shape.
2. Discussion of Prior Art
Recently, interest has grown in the possibility of obtaining improved magnesium alloys by creating new compositions using rapid solidification rate (hereinafter referred to as RSR) production processes. Although magnesium is the lightest of the structural metals, its alloys have yet to find widespread use in aerospace applications, partly because of certain shortcomings in their 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 is ineffective in improving the corrosion resistance even though such additives are known to form protective surface films in other alloy systems. This inefficacy is due to their poor solubilities in the magnesium matrix: Under normal equilibrium conditions the concentration of such additives in solid solution is tool low to provide an effective barrier to corrosion.
In magnesium alloys it is generally believed that corrosion-inhibiting additives should be incorporated in the form of solid solutions so that a uniform electrode potential is maintained throughout the alloy. If the additives are allowed to segregate and form precipitates, and if the precipitates have a different electrode potential from the matrix, then they effectively behave as tiny Galvanic cells and corrosion resistance is worsened rather than improved.
Rapid solidification techniques including vapour deposition provide the means to overcome equilibrium 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 ingot processes. These techniques therefore offer a possible route to improved corrosion resistance in magnesium alloys by allowing an increase in the population of corrosion-inhibiting species in the magnesium alloy without forming harmful precipitates.
Physical vapour deposition is favoured over other forms of RSR processing for a number of reasons. Firstly, the cooling rate is very much higher in physical vapour deposition and thereby increases the likelihood of formation of solid solutions. Secondly, physical vapour deposition offers a considerable choice of potential alloying constituents since the candidate elements are raised to the vapour state, thereby ensuring miscibility between different constituents. By contrast, other forms of RSR processing are limited in the possible combinations of alloying constituents which they can offer to those elements which are miscible in the molten state. This is a particularly important consideration in the case of magnesium since, at the melting temperature of many potentially interesting alloying additions, magnesium has a very high vapour pressure and hence evaporates very

REFERENCES:
patent: 3607222 (1971-09-01), Kennedy
patent: 4517027 (1985-05-01), Bickerdike
patent: 4548670 (1985-10-01), Pinkhasov
patent: 4626410 (1986-12-01), Miura
patent: 4708190 (1987-11-01), Hack

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