Rapid solidification route aluminium alloys containing lithium

Details Rapid solidification route aluminium alloys containing lithium Rapid solidification route aluminium alloys containing lithium

1989-04-13

1991-11-19

Andrews, Melvyn J.

Alloys or metallic compositions

Aluminum base

Titanium, zirconium, hafnium, vanadium, niobium, or tantalum...

148437, 148438, 148440, C22C 2100, C22C 2106

Patent

active

050664573

DESCRIPTION:

BRIEF SUMMARY
This invention relates to aluminium based alloys containing lithium, made by the rapid solidification rate (RSR) route.
It is well known that lithium can be included in an aluminium alloy to reduce its density and increase its modulus of elasticity. Much work has been performed in this area on materials made by RSR routes as well as the more conventional wrought ingot route. RSR routes to the production of high modulus low density alloys based on aluminium-lithium offer at least the three following potential attractions: ingot-derived coarse metallic second phases that can act as sites for crack initiation and corrosion: homogenising slip than are phases incorporated via the ingot route, so improving ductility and toughness; and; the wrought ingot route (by avoidance of segregation limitations) thereby offering the prospect of more significant reductions in density and increases in modulus and strength.
This invention concerns especially the dispersoid phase aspect of the RSR route aluminium-lithium art. Several dispersoid-forming additions have been investigated previously. One prior art investigation looked at the effects of 0.2 to 0.6 weight percent additions of manganese, chromium, iron, cobalt, titanium and zirconium on an aluminium--3 weight percent lithium alloy. Other additions which we know to have been investigated previously (in a variety of documents) are as follows (all proportions being by weight percent): 1 to 7 silicon; 0.2 titanium; 0.4 chromium; 0.2 to 3 manganese; 0.5 iron; 0.2-1 cobalt; 0.04 yttrium; and 0.2 to 1 zirconium.
The problem is one of developing a RSR route aluminium-lithium alloy having a good balance of those properties desirable especially for aerospace structural application, such desirable properties including; strength, high modulus, ductility and fracture toughness together with low density; and the present invention tackles this problem by providing new additives for such alloys, that resist coarsening in the aluminium-base matrix at elevated temperatures of the level that is likely to be experienced in solution treatment, in artificial ageing or in service.
The invention comprises an alloy formed by a rapid solidification rate process being based on aluminium and containing lithium together with X where X is one or more of the elements selected from a group of refractory metal elements consisting of niobium, molybdenum, hafnium, tantalum and tungsten.
Note that all compositions given hereinafter are expressed in proportions by weight.
X is preferably one of said elements rather than more than one, and X is preferably present in the alloy in proportion within the range 0.2 to 5.0 percent in order to achieve satisfactory effect without side effect or too great an increase in density.
Lithium is preferably present in the alloy in proportion within the range 1 to 5 percent. If lithium present in much greater proportion it would be likely to render the alloy overly brittle.
It has been found that the inclusion of such a refractory metal element in an aluminium-lithium base alloy confers improved strength to same (as reflected in the microhardness of RSR splats) and improved thermal stability (as reflected in the change of microhardness with exposure to temperatures representative of solution treatment temperature and artificial ageing temperatures). It is believed that these improved properties are consequent partly upon dispersoids formed within the aluminium matrix by the refractory metal element. It is expected that this benefit will not be significantly diminished by the presence in the base alloy of those ingredients other than lithium which have been added to aluminium-lithium alloys for strengthening purposes etc. A dispersion of the aluminium-refractory metal element compounds act to inhibit grain growth and thus help to retain the fine grain size inherent in RSR alloys. This fine grain size is important for the development of high strength and ductility. Solution heat treatment, quenching and ageing of the alloy results in the precipitation of aluminium-lithium b

REFERENCES:
patent: 4804423 (1989-02-01), Dubast

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