It is known that certain alloys under certain conditions can undergo very large amounts of deformation without failure, the phenomenon being known as superplasticity and characterised by a high strain rate sensitivity index in the material as a result of which the normal tendency of a stretched specimen to undergo preferential local deformation ("necking") is suppressed. Such large deformations are moreover possible at relatively low stresses so that the forming or shaping of superplastic alloys can be performed more simply and cheaply than is possible with even highly ductile materials which do not exhibit the phenomenon. As a convenient numerical criterion of the presence of superplasticity, it may be taken that a superplastic material will show a strain rate sensitivity ("m"-value) of at least 0.3 and a uniaxial tensile elongation at temperature of at least 200%, "m"-value being defined by the relationship .sigma.=.eta. .epsilon..sup.m where .sigma. represents flow stress, .eta. a constant, .epsilon. strain rate and m strain rate sensitivity index.
No known aluminium-base alloy can be superplastically deformed other than the Al-Cu entectic composition which contains 33% copper and has neither the low density nor the good corrosion resistance characteristic of aluminium alloys. A known alloy of 22% Al and 78% Zn is also superplastically deformable, but because of its high density and poor creep and corrosion resistance has not found commercial acceptance.