Metal treatment – Stock – Aluminum base
Patent
1992-06-11
1993-11-02
Roy, Upendra
Metal treatment
Stock
Aluminum base
420549, C22F 100
Patent
active
052580814
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a particular form of heat treatment for aluminium-lithium alloys, that is those alloys based on aluminium which include lithium as a deliberate alloying addition rather than a trace impurity. Practical aluminium-lithium alloys include strengthening ingredients additional to the lithium such as copper, magnesium or zinc. The heat treatment is intended for use on such alloys in certain product forms and/or tempers to improve fracture toughness or ductility particularly in the short transverse direction. The term "short transverse direction" is a term of art applied in respect of plate or sheet material to specify the axis of cross-section through the thickness of the material and used also in respect of other product forms such as extrusions and forgings to identify a cross-grain orientation.
BACKGROUND OF THE INVENTION
Aluminium-lithium alloys based on the aluminium-lithium-copper and aluminium-lithium-copper-magnesium systems have been developed to the stage where they are currently being considered for large-scale commercial use on the next generations of civil and military aircraft. The attractiveness of such alloys as replacements for established non lithium-containing aluminium alloy lies in their reduced density and increased stiffness but widespread application of these materials in aerospace structures will be dependent upon attainment of a satisfactory combination of many properties. The aluminium-lithium-copper-magnesium alloy registered internationally under the designation 8090 provides reduced density and increased stiffness in combination with strength, fracture toughness, corrosion resistance, fatigue resistance and ease of production at a level far in advance of the first aluminium-lithium alloys. Nevertheless there remains a perceived problem with regard to current aluminium-lithium alloys in regard to low fracture toughness in the short transverse direction. It might be that a low fracture toughness in the short transverse direction. It might be that a low fracture toughness in this axis presents no real barrier to use of the alloys in normal applications because the materials will not be subjected to usage which presents a stress on the axis but it remains something of a barrier to confidence in the new materials and might conceivably affect service life in some situations. The 8090 alloy for example, when aged to yield a tensile strength of 500 MPa or more which is typical of the modern high strength aerospace 7000 series alloys in the T76 condition, can exhibit low levels of fracture toughness in the short transverse direction typically 11 or 12 MPa (m).sup.1/2 as against 18 to 20 MP (m).sup.1/2 for the 7000 material whilst fracture toughness in other orientations of the 8090 alloy is more than acceptable.
The problem or perceived problem is not new-found and various tentative explanations have been advanced previously in the prior art. It is known that fracture in the short transverse plane (whether crack growth occurs in the longitudinal direction or the transverse direction orthagonal to the applied stress) occurs along grain boundaries and is brittle in nature showing little evidence of local ductility in those materials exhibiting low short transverse fracture toughness. The tentative explanations already made in the open literature embrace the following possibilities: localisation of the plastic strain at grain boundaries; grain boundary embrittlement by traces of hydrogen or low melting point metallic elements such as sodium, potassium or calcium; and the formation of large phases at the grain boundaries containing lithium, copper and possibly magnesium. This invention provides a convenient solution to the problem and studies made in relation to the invention indicate that these previously proposed explanations do not go the root of the matter though some of them relate to phenomena which will make some degree of contribution to the problem in certain circumstances.
Those present day aluminium-lithium alloys which are produced by the ingot metallurgy
REFERENCES:
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patent: 4806174 (1989-02-01), Cho et al.
patent: 4844750 (1989-07-01), Cho et al.
patent: 4861391 (1989-08-01), Rioja et al.
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Aerospace, vol. 16, No. 5, May 1989, pp. 18-23 Peel et al. "The present status of the development and application of Aluminum-Lithium Alloys 8090 and 8091".
Lynch Stanley P.
Peel Christopher J.
Roy Upendra
The Secretary of State for Defence in Her Britannic Majesty's Go
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